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Main results of the European PASSAM project on severe accident source term mitigation
Abstract
The PASSAM (Passive and Active Systems on Severe Accident source term Mitigation) project was launched in the frame of the 7th framework programme of the European Commission. Coordinated by
IRSN, this four year project (2013–2016) involved nine partners from six countries: IRSN, EDF and university of Lorraine (France); CIEMAT and CSIC (Spain); PSI (Switzerland); RSE (Italy); VTT (Finland) and
AREVA GmbH (Germany).
It was mainly an R&D project of experimental nature aimed at investigating phenomena that might enhance source term mitigation in case of a severe accident in a Nuclear Power Plant (NPP). Both existing systems (i.e., water scrubbing and sand bed filters plus metallic pre-filters) and innovative ones (i.e., high pressure sprays, electrostatic precipitators, acoustic agglomerators and, advanced zeolites and combined wet-dry filtration systems), were experimentally studied in conditions as close as possible from those anticipated for severe accidents.
This paper presents the main experimental results of the project which represent a significant extension of the current database on these existing or innovative mitigation systems. Application of some of
these data for improving existing models or developing new ones should eventually enhance the capability of modelling Severe Accident Management measures and developing improved guidelines
... Consequently, several R&D projects with an aim to reduce uncertainties in source term calculations were launched including limited focus on pool scrubbing related experimental and analytical activities, e.g. EC-CESAM (Nowack et al., 2018), EC-PASSAM (Albiol et al., 2018) and OECD/NEA THAI-3 (NEA, 2021). As part of these projects, focus was either on experiments covering a specific pool scrubbing relevant process or scenario, such as aerosol and iodine reentrainment in OECD/NEA THAI-3 and Filtered Containment Venting Systems (FCVS) relevant processes in EC-PASSAM or on codes by assessing integral prediction capability of a particular severe accident code (EC-CESAM). ...
... Considering the fact that organic iodine is difficult to be retained, many NPPs worldwide consider installing FCVS systems with capability to retain organic iodine ((OECD/NEA, 2014); Albiol et al., 2018). In this context, IPRESCA project partner RASA has developed two types of silver zeolites (AgX®) and (AgR®) as a radioiodine adsorbent. ...
... In the WP3.2 dedicated to the study of bubble hydrodynamic modelling in LP codes; a benchmark has been conducted with the LP codes. Experimental data comes from a test conducted by RSE in the frame work of PASSAM project (Albiol et al., 2018). The objective for the benchmark was to assess the current modelling status of the LP codes regarding the bubble hydrodynamic description as the existing pool scrubbing codes were developed mostly in the early 1990. ...
ool scrubbing is a major topic in water cooled nuclear reactor technology as it is one of the means for mitigating the source-term to the environment during a severe accident. Pool scrubbing phenomena include coupled interactions between bubble hydrodynamics, aerosols and gaseous radionuclides retention mechanisms under a broad range of thermal–hydraulic conditions as per accident scenarios. Modeling pool scrubbing in some relevant accident scenarios has shown to be affected by substantial uncertainties.
In this context, IPRESCA (Integration of Pool scrubbing Research to Enhance Source-term CAlculations) project aims to promote a better integration of international research activities related to pool scrubbing by providing support in experimental research to broaden the current knowledge and database, and by supporting analytical research to facilitate systematic validation and model enhancement of the existing pool scrubbing codes based on different modelling approaches, e.g. lumped parameter, computational fluid dynamics, mechanistic. The project consortium includes>30 organisations from 15 countries involving research institutes, universities, TSOs, and industry. For IPRESCA activities, partners join the project with in-kind contributions. IPRESCA operates under NUGENIA Technical Area 2/SARNET (Severe Accident) – Sub Technical Area 2.4 (Source-term).
The present paper provides an introduction and overview of the IPRESCA project, including main outcomes and key ongoing and planned activities. New insights obtained from code benchmarks and consolidation of experimental database are discussed. Application of these experimental (“new data”) and analytical results shall support in improvement and further development of pool scrubbing models towards reduction in uncertainties for source term calculations.
... The aforementioned codes have been validated with numerous experimental results on decontamination factors during pool scrubbing, such as LACE-ESPAÑA [13], POSEIDON [14], ACE [5], PASSAM [15], etc. With the aforementioned computer codes, comparative studies on modeling of the pool scrubbing and sensitivity analyses [5,6] The computer codes for the pool scrubbing, however, have been developed to be as conservative as possible; for example, MELCOR [12] uses 0.7 cm as a default value for the average equivalent diameter of the bubbles to provide conservative decontamination factors [5]. ...
... where t e (ϕ j , θ) : exposure time of the surface of the bubble with ϕ j moving between θ j,k and θ j,k+1 , calculated as the following equation : (15) θ j,k : discretized cylindrical polar coordinates on the local surface of a bubble with ϕ j satisfying : ...
... The results are due to that the vapor velocities are proportional to steam flux transferring into the bubbles, and the amount of steam flux is proportional to the surface area of the bubbles.In the case of deposition velocities due to Brownian diffusion, they decrease from 2.1E-01 cm/sec to 9.6E-02 cm/sec as the average mean mass diameter of aerosol particles increases from 5.5E-05 cm to 7.2E-04 cm. In addition, they decrease as the bubble equivalent diameter increases since they are inversely proportional to the exposure time of the moving surface, which is proportional to the bubble equivalent diameter, as shown in Eq.(15). However, they are not significant compared to the values of the centrifugal deposition velocities as shown in J o u r n a l P r e -p r o o f ...
In this paper, from a review of the size distribution of the bubbles during pool scrubbing obtained from experiments by EPRI, we apply the bubble size distributions to analyses on the decontamination factors of pool scrubbing via I-COSTA (In-Containment Source Term Analysis). We perform sensitivity studies of the bubble size on the various mechanisms of deposition of aerosol particles in pool scrubbing. We also perform sensitivity studies on the size distributions of the bubbles depending on the diameters at the nozzle exit, the molecular weights of non-condensable gases in the carrier gases, and the steam fractions of the carrier gases. We then perform analyses of LACE-ESPAÑA experiments and compare the numerical results to those from SPARC-90 and experimental results in order to show the effect of the bubble size distributions.
... Among these subjects, experiments related to pool scrubbing are summarized as the following six sections, these results can be used for understanding the source term behaviors in pool and may be helpful for the FCVS development (Albiol et al., 2018;Lind et al., 2015a). ...
... The tests revealed that the DF for aerosols grew with the Weber number (We is the ratio between inertia and surface tension forces) up to approximately 300. The saturation ratio (S is the ratio between steam pressure in the carrier gas and saturation pressure at the pool temperature) indicated that most scrubbing was caused by mechanical phenomena, such as inertial impaction (Albiol et al., 2018). In addition, the effect of submergence was investigated (Herranz et al., 1997), and the results showed that the DF could be fitted with a formula: DF = e b Á e aS . ...
... As for the pool scrubbing tests, the iodine mainly in inorganic form (I 2 ) was released from the solution due to the pH decrease when the water pool was unbuffered. When the water pool was buffered, the HOI has high contribution to the global iodine release from the solution, but no direct iodine release was measured on the May-pack (Albiol et al., 2018). However, mechanical entrainment could result in the significant release of iodine species, such as PS4 in Table 8, under evaporating conditions or boiling conditions, which would occur during a real accident. ...
A filtered containment venting system (FCVS) is one of the important severe accident mitigation systems designed to prevent containment failure due to potential overpressure during accidents. Especially, many research projects related to FCVS have been initialized after Fukushima Daiichi nuclear accident. To demonstrate the functionality and filtration performance of various types of FCVSs with wet scrubbers, confirmatory experiments were performed on test facilities. To sort out the main outcomes of those experiments, this paper summarizes the major experimental works performed in test facilities from the perspective of filtration efficiency and thermal-hydraulic characteristics. The investigated experimental results showed that there is a variety in retention efficiency among different designs of wet type FCVSs and different tests conditions. The overall decontamination factor (DF) for aerosol is usually higher than 3000, the DF for elemental iodine (>100), and for the organic iodine (>10) are lower, which are still sufficient for most countries’ regulatory requirements, and the DF for CH3I can be improved by efficient zeolite module. Along with the experimental studies, some numerical simulation works based on thermal-hydraulic system codes are also summarized. To an extent, thermal-hydraulic codes could predict the long-term operating characteristics of prototypical FCVSs under postulated severe accident conditions, which is complementary to the experiments. This review is expected to provide extensive information to help the optimization and further innovation of FCVSs.
... The main priority of source term research, particularly fostered since the Fukushima accident, has been the prevention and/or mitigation of SA consequences (i.e., radioactivity release to the environment). This has been articulated in a number of EC and NUGENIA projects: FP7/PASSAM (research on passive and active systems for source term mitigation) [Albiol et al. 2018], H2020/FASTNET (emergency preparedness and response) [Rocchi, 2019], and the ongoing projects H2020/MUSA (uncertainties in SA) [Herranz, 2020] and NUGENIA/IPRESCA (pool scrubbing) [Gupta, 2017]. The remaining research needs, though, were openly discussed under the frame of CSNI/WGAMA [NEA, 2020] and, as a consequence, in the coming months the OECD projects ESTER and THEMIS, coordinated by IRSN and Becker Technologies respectively, will be launched with the participation of a good number of TA2 members. ...
... The project has already produced a number of outcomes: a critical assessment of the pool scrubbing database, which discussed the tests qualification for model validation and identified further experimental needs; test matrices on hydrodynamics, aerosols and iodine experiments have been conducted based on the existing experimental database and new tests are being proposed; insights into bubble hydrodynamics have been gained by conducting benchmarks based on LP and CFD approaches. LP benchmark performed based on a RSE test ( Figure 6) [Albiol, 2018], indicate that codes are far from describing the actual bubble morphology or swarm velocity. CFD benchmark revealed that modelling results are still too scattered as to be considered a reliable approach for bubble hydrodynamics analyses. ...
No matter how unlikely they are, severe accidents (SA) may happen and, then, the best management possible of an incredibly complex scenario is needed. Both preventive and
mitigating actions are articulated in Severe Accident Management Guidelines (SAMGs), whose development requires an in-depth knowledge of both SA phenomena and management measures (i.e., efficiency, modes of implementation, side effects, etc.). As a consequence, investigation on SA has been ongoing for decades.
Even though SA research started with the development of nuclear power, it was after the Three Mile Island accident, in 1979, when it took off and drew major attention among
nuclear technology scientists. However, in the 1990’s such significant momentum faded away to some extent and most research focused on the international PHEBUS-FP project
[Clement, 2013]. In the first years of this century (2004), the search for an efficient use of the still available technical and human resources ended up with the launch of a Severe
Accident Research NETwork (SARNET) under the 6th and 7th FrameWork Programmes (FWP) of the European Commission (EC) from 2004 to 2013 [Van Dorsselaere, 2015].
It was coordinated by IRSN and mostly participated in by European institutions, although prominent Asian and American organizations in SA research also joined the network.
SARNET self-sustainability was achieved through integration in the NUGENIA European association in 2013, which had been created two years before to foster R&D on fission technology of Generation II and III reactors. Since then, SARNET turned into what is known as NUGENIA Technical Area 2 (NUGENIA/TA2) of the Sustainable Nuclear Energy Technology Platform (SNETP).
The paper presents the major developments and achievements on SA research accomplished in the last five years by NUGENIA.
https://www.revistanuclear.es/seguridad/nugenia-ta2-achievements-in-severe-accidents-research-2015-2020/
... -Turbulent deposition: Flow will be highly turbulent in the jet region and when it spreads as a two-phase flow in the bundle. Very complex flow structures are generated by the high velocity jet and interaction with the flooded tube bundle (Betschart, 2015;Albiol et al., 2018). Despite this, in the flooded bundle turbulent deposition may be of only limited significance with respect to inertial and droplet effects in the near field. ...
... In the near field of the flooded bundle, two interacting phenomena were ranked as "high significance" and "low knowledge": -Inertial impaction -Retention by droplets Impaction itself is well-understood. However, the tube bundle geometry is complicated and the flow field of the two-phase flow generated at the tube breach exit is difficult to determine (Betschart, 2015;Albiol et al., 2018). Retention by droplets and impaction are concurrent in the jet region where the high velocity gas jet is discharged from the tube break into the steam generator tube bundle surrounded by water. ...
A steam-generator-tube rupture (SGTR) in a pressurized-water reactor is a design-basis accident with which plants have been designed to cope. Its specific significance is in its potential to bypass the containment thereby providing a pathway for radioactivity release to the environment. In combination with significant core damage, uncontrolled SGTR may lead to accidental release of radioactivity in the form of aerosol particles and gaseous compounds.
In this paper, we describe the phenomena identification and ranking tables (PIRT) prepared as an international collaboration for uncontrolled SGTR in connection with core damage. This PIRT is limited to fission-product transport and retention in the steam generator and release to the auxiliary building, i.e., the source term. The phenomena affecting the fission-product retention in the steam generator and release to the environment were divided into those relevant in a dry steam-generator secondary side, and those relevant in a flooded secondary side. Rankings were given to 17 phenomena in the dry and 16 phenomena in the flooded secondary side regarding their: i) impact on the source term, ii) the availability of experimental data applicable to SGTR severe-accident conditions, and iii) the availability of codes/models to describe the phenomenon under the conditions of SGTR severe accidents.
The SGTR boundary conditions and the considered phenomena are briefly described. The results are given in the form of ranking tables separately for the dry and flooded steam generator secondary side. The most important phenomena and their interactions with other phenomena are discussed with references to existing data and models. The most important uncertain phenomena in the dry steam generator scenario are particle bounce and fragmentation; whereas inertial impaction and retention by droplets in the jet region are the most uncertain ones in the flooded configuration. The need for improved knowledge is, though, tempered by overall high aerosol retention in the latter scenario. The applicability of the existing data and models for SGTR accident analysis is considered in the discussion.
... radioactive materials 8 , adsorbents that are easily regenerable hold significant value in most other scenarios. ...
Efficiently capturing radioactive methyl iodide (CH3I), present at low concentrations in the high-temperature off-gas of nuclear facilities, poses a significant challenge. Here we present two strategies for CH3I adsorption at elevated temperatures using a unified azolate-based metal-organic framework, MFU-4l. The primary strategy leverages counter anions in MFU-4l as nucleophiles, engaging in metathesis reactions with CH3I. The results uncover a direct positive correlation between CH3I breakthrough uptakes and the nucleophilicity of the counter anions. Notably, the optimal variant featuring SCN⁻ as the counter anion achieves a CH3I capacity of 0.41 g g⁻¹ at 150 °C under 0.01 bar, surpassing all previously reported adsorbents evaluated under identical conditions. Moreover, this capacity can be easily restored through ion exchange. The secondary strategy incorporates coordinatively unsaturated Cu(I) sites into MFU-4l, enabling non-dissociative chemisorption for CH3I at 150 °C. This modified adsorbent outperforms traditional materials and can be regenerated with polar organic solvents. Beyond achieving a high CH3I adsorption capacity, our study offers profound insights into CH3I capture strategies viable for practically relevant high-temperature scenarios.
... Several experiments have been conducted to verify the effectiveness of such facilities. The PASSAM project (2013-2016 (Albiol, 2018) involved a series of experiments with the aim of investigating unsolved problems associated with source term events, such as the hydrodynamic characteristics observed under the conditions predicted in a steam generator tube rupture (SGTR) event , the hydrodynamic characterization of the pool considering aerosol retention, and the aerosol retention rate at the jet injection regime. The experimental results showed that the aerosol was removed by droplet entrainment inside of the jet. ...
The fission product filtration capability of pool scrubbing is an uncertain factor in severe accident risk analysis. Limited discussions on aerosol transportation phenomena exist due to insufficient understanding of aerosol mass transfer from single bubbles and bubble plume behavior. This study aims to investigate aerosol transportation during pool scrubbing by integrating individual bubble mass transfer into the overall bubble plume.
Decontamination factor (DF) measurements were conducted for various nozzle submergences, followed by analysis of bubble diameter distributions in the corresponding bubble plume. Findings revealed that the MELCOR model overestimates bubble break-up, contrary to the expected positive impact on DF. To address this discrepancy, a computational fluid dynamics simulation was employed to analyze mass transfer coefficients considering different bubble diameters and aerosol properties. By integrating the mass transfer of single bubbles, the discrepancy in the DF tendencies was attributed to the underestimation of aerosol mass transfer coefficients in individual bubbles.
... Post the accident of Fukushima, many follow-up actions were taken notably considering the implementing and enhancing of FCVSs [1,5,[17][18][19][20]. Therefore, research programs on improving safety measures resumed and programs within the international framework were launched, in order to better characterize the mechanism of pool scrubbing [21][22][23][24][25][26][27][28][29][30]. ...
Pool scrubbing has shown potential efficiency to reduce the release of fission products, especially in aerosol forms, into the environment. Considering the large test conditions where pool scrubbing might be encountered, there is still a lack of systematic analysis of this phenomenon, especially its dependence on different hydrodynamic regimes. Experimental work was carried out, where hydrodynamic and decontamination factor measurements were performed. Caesium iodide aerosols were injected into the TYFON facility for different flow regimes (0.08 ≤ We ≤ 15 600) by varying both injection flowrate Qinj and nozzle size D0, in which their effects were examined. The Weber number (We) has shown to be capable of characterizing the decontamination factor, when comparing this work with data from literature. Moreover, the different flow regimes induced different sensitivities of aerosol removal mechanisms, where a minimum scrubbing is observed in the transition between bubbly and jet regime. The effect of pool submergence was also investigated in case of bubbly and jet regimes, in which the contribution of residence time and inertial impaction to pool scrubbing was shown.
... Recently, after Fukushima, a number of experimental programs were carried out under the framework of the European Union (EU) Passive and Active Systems on Severe Accident Source Term Mitigation (PASSAM) project. 7 It should be mentioned that no data have been found in open literature for some experimental work conducted, like the iodine and aerosol testing facility Jod-und Aerosol Versuchsanlage (JAVA) (1989)(1990) The individual program review paid singular attention to the following aspects: representativeness of the initial and boundary conditions, reliability of the experimental techniques used and the experimental protocol adopted, data accuracy, and the method used to derive the value of the decontamination factor (DF). The high complexity of the potential effect of facility scale was also considered, but as a part of the global assessment. ...
The removal of aerosol particles and vapors in gas bubbles moving through a water pool is known to be an efficient means to reduce source term to the environment during severe accidents, as happened in Fukushima Daiichi. This trapping, called pool scrubbing, entails a complex phenomenology in which hydrodynamics, thermal hydraulics, and aerosol physics strongly affect each other and determine the net transfer of radioactivity coming out from the aqueous pond. More than 20 experimental programs have addressed this issue since the early 1980s, but few of them did it in a systematic and representative way. This paper thoroughly reviews the entire pool scrubbing database until 2016 and assesses the adequacy of the experimental setup, representativeness of boundary conditions, weaknesses in decontamination factor derivation, data uncertainties, and some other aspects to finally synthesize a reduced number of experiments that could be used as an experimental matrix for the validation of pool scrubbing models. More than 500 tests were reviewed and classified as Qualified for Validation, Useful for Understanding, or Not Useful; less than 15% of these experiments are considered in the proposed validation matrix due to different reasons. Major insights and remaining needs are also highlighted. This work was conducted under the framework of the Integration of Pool Scrubbing Research to Enhance Source-Term Calculations, or the IPRESCA project, led by Becker Technologies, in the framework of the Sustainable Nuclear Energy Technology Platform/Nuclear Generation II & III Alliance/Technical Area 2.
... Prevention of containment over-pressurization during off normal condition by using filtered venting line in containment as one of the practicable solutions could reduce radiation consequences to the environment. The interest in filtered containment venting system development has been motivate several countries to establish research group for evaluation and management of severe accident to initiate project on systems capable to mitigate severe accident with radionuclide release (Albiol et al., 2018). In 1988, a report was published by the Nuclear Energy Agency Committee on the Safety of Nuclear Installations (NEA-CSNI) on employing filtered venting system and its advantages (CSNI Report, 1988). ...
To mitigate radioactive material release in nuclear facilities, some effective approaches have been suggested such as using filtered venting system (FCVS). For a nuclear containment, the FCVS reduces the overall environmental contamination. This work uses MELCOR for severe accident analysis, ORIGEN for isotope production and depletion calculation and HotSpot for health physics analysis to evaluate the progression of radionuclide release and containment structural stability during station blackout accident (SBO). In this analysis, the performance of a containment with a filter venting system is evaluated. The overall goals are assessment of FCVS effects on radionuclide release into environment and to evaluate containment integrity. In this research, three scenarios of radionuclide releases are considered. In the unmitigated case, no filtered venting system is employed. For the mitigated accident, a proper filtered venting path is considered for decontamination of the released material, and in the unfiltered case, there is a venting path but no filter is provided to absorb radionuclides. Also, the radiation dose produced after the radionuclide release based on the site meteorological condition is estimated. The main results include study of mass and decay heat of deposited radionuclides on filter and fission products release. This research proves, using FCVS saves public health and containment safety during SBO by mitigation mechanism.
... The quantification of how much gaseous iodine could be released from the MF and the sand filter to the environment is thus crucial for the source term determination. This quantification has been performed within the MIRE (2013-2019) and PASSAM (Albiol et al., 2017(Albiol et al., , 2018(Albiol et al., ) (2013(Albiol et al., -2016 projects in which IOx and CsI aerosol stability on such devices has been checked. As IOx aerosol size was found to be smaller (MMAD <0.5 μm (Dickinson et al., 2014)) than (1) the aerosol size used to qualify the aerosols DF in FUSCHIA project and (2) the iodine multicomponent aerosol size quantified in PHEBUS-FPT0/1 containment (MMAD >1 μm (Kissane, 2008)), DF of IOx on the sand filter has thus been reevaluated using a simulant (impaction experiment). ...
The decomposition of deposited CsI and iodine oxides (IOx) aerosols deposited on sand bed and on a metallic filter has been pursued under irradiation at 120 °C. On the sand, it was found that CsI aerosols are decomposed by the irradiation and the temperature effect into an inorganic specie assumed to be molecular iodine. The temperature effect might be due to surface effects of the sand grains as CsI aerosols are usually stable at this temperature. IOx aerosols are expected to be well retained by the sand filter (DF > 100) as long as the sand height is high enough (>60 cm). IOx are also decomposed by the temperature and irradiation. The humidity is also expected to have a significant influence on their decomposition. On the metallic filter, despite both species are expected to be significantly decomposed, very few gaseous iodine release from the filter was observed. The gaseous iodine species produced by the aerosols decomposition is assumed to be quickly adsorbed on the steel wires even if a slow desorption is also expected.
... Almost all the investigations have been carried out in a bare pool, i.e., with only an injection nozzle and no other submerged structures. A few investigations have been conducted in geometries relevant to different wet scrubbers in filtered containment venting systems (FCVS) reflecting the specific features of the FCVS (Albiol et al., 2018). These have been recently reviewed by Dong and Yang (2019). ...
The international EU-SGTR and ARTIST projects investigated the transport of fission products in the form of aerosols during SGTR severe accidents. The major finding of the two projects was that there was significant retention of aerosols in the steam generator secondary side, and that the retention was increased by more than an order of magnitude if the secondary side of the steam generator was flooded with water. Furthermore, the experiments with the flooded secondary side showed that the aerosol particle retention was significantly increased due to the presence of submerged structures, i.e., the tube bundle, as compared to an empty pool. The increased aerosol retention was attributed to the interactions of the high velocity gas jet discharged from the tube break with the dense bundle of the steam generator tubes. Under these conditions, the two-phase flow is very complex due to the high gas velocities and complicated geometry of the steam generator secondary side.
To determine the effect of the tube bundle on aerosol retention, hydrodynamic characteristics of an empty pool and a flooded steam generator secondary side were measured in a facility equipped with wire-mesh sensors. The facility was equipped with either a tube bundle consisting of 221 steam generator tubes, or with a single tube in the center of the facility. The flow development could be followed by making the measurements at different distances between the gas injection and the measurement point, and using different flow rates. The facility was operated at close to ambient conditions.
Void fraction, bubble size distributions, gas phase velocity as well as interfacial area concentration were determined based on the wire-mesh sensor data. In addition, the penetration depth of the initial large gas bubble into the channel was studied for the closest break-sensor distances. The investigations show distinct differences between the flow characteristics in the single tube geometry and in the tube bundle. As compared to the single tube, the flow was more confined in the tube bundle due to the interactions of the flow with the tubes. The interfacial area between the liquid and gas phase is larger with the tube bundle than with the single tube and also the bubble size distributions show distinct differences between the two geometries.
... Hereafter, the main results are summarized. Detailed results are available elsewhere [22][23][24][25][26][27][28]. ...
... The performance of the innovative WESP in comparison to other existing and innovative filtration methods has been discussed in Albiol et al. (2015) and a wider discussion will be given in Albiol et al. (2018). ...
An innovative method to filter gaseous iodine based on a wet electrostatic precipitator (WESP) was developed and tested. It is characterized by an ozone feed located before the WESP inlet to oxidize gaseous iodine into iodine oxide particles. Tests were carried out with titanium dioxide particles, first, and then with gaseous molecular iodine (I2) and methyl iodide (CH3I). The applied electric voltage between the electrodes, the total flow rate inside the WESP, and the injection of water droplets before the WESP were varied. The filtration efficiency based on the number of particles was calculated from ELPI and CPC measurements at the inlet and the outlet of the WESP. To determine the mass filtration efficiency for iodine, ICP-MS analyses were performed. Once the operation parameters were optimised for the tested conditions, the WESP ensured a mass filtration efficiency of 99.9% and a particle number based filtration efficiency up to 99.9% against molecular iodine. The efficiency for the filtration of methyl iodide was not as high. The filtration efficiency data are presented covering the particle size range of 0.04–2 µm.
During severe accidents in nuclear power plants, filtered containment venting system is foreseen to be employed once the containment pressure increases above a pre-set value called venting pressure. Ag-zeolite filters are applied in filtered containment venting systems to retain iodine and organic iodides in the gas phase. In this work, the applicability of Ag-zeolites to not only retain gas phase iodine species, but to also catalyze hydrogen recombination has been experimentally investigated under challenging high humidity conditions. Tests were performed in the medium-scale facility using two Ag-zeolites, one of them designed to both retain gas phase iodine species and recombine hydrogen, the other one designed to only retain gas phase iodine species. Experiments studied the effect of residence time and the carrier gas mixture (steam, N 2 or air) on the retention of organic iodine, represented in the tests by CH 3 I, and hydrogen recombination rate with the two Ag-zeolites. The experiments were carried out under the conditions expected in the containment during severe accidents, however, considering practical limitations. The effects of pressure and the presence of contaminant gases (CO, N 2 O) were investigated in additional tests not included in this study. The steam fraction in the tests varied between 32% and 90%, air fraction was 0%, 5% or 19%, and hydrogen content either 2.5% or 5%. Nitrogen made up the balance for the gas atmosphere. Gas residence time in the zeolite bed was either 100 m or 200 m. Both zeolites showed high retention of CH 3 I under all the gas atmospheres as long as the residence time in the reaction chamber was 200 m. CH 3 I retention was lower when the residence time was reduced to 100 m. Hydrogen recombination was more dependent on the gas atmosphere, as expected. The effect of the gas atmosphere on the hydrogen recombination and retention is discussed.
The role of pool scrubbing in attenuating radioactivity release after severe accidents has been explored extensively. It is known that the scrubbing efficiency is largely determined by the hydrodynamic phenomenology in pools. The aerosol gas forms large globules at the nozzle exit, which subsequently break up to a swarm of stable bubbles, where the change of bubble size can reach over two orders. Furthermore, with the increase of flow rates, the injection regime changes from globule to jet characterized by a continuous gas structure. The flow field in the pool can be divided into injection and rise (swarm) two zones according to the gas-liquid interface morphology. In different zones, the scrubbing is governed by different mechanisms such as inertial impact, diffusion and gravity, whereby bubble shape, size and velocity in addition to particle size are major influential parameters. So far, numerical analysis of pool scrubbing is routinely based on system codes, which rely on empirical correlations for the determination of these parameters. More recently, owing to the increasing availability of computational resources, the knowledge is improved through three-dimensional computational fluid hydrodynamics simulations. Nevertheless, the morphology and regime change represents still a challenge. Due to the limitation of closures, the conventional two-fluid model is generally effective for bubble size smaller than the cell size, while interface-tracking (capturing) methods demands dozens of cells per bubble size. The present work aims to capture the complex hydrodynamic process in the pool scrubbing with a hybrid multi-field two-fluid model. By comparing with experimental data, the results are shown to be promising, which provides a versatile framework for the investigation of particle scrubbing in the future.
A pool scrubber is often used as a wet-type design to mitigate the consequence of a severe nuclear accident. While studies indicated higher decontamination performance of a deeper pool, utilizing a very tall pool can be problematic due to potential structural stability and water backflow issues. This study proposes, as an alternative to a single pool system, a pool scrubber system composed of serially connected multiple pools with lower heights. Since large fraction of aerosol removal takes place in the injection region, serially connected pool scrubber system is expected to enhance the overall decontamination capability of a pool scrubber system. To support the analysis of the proposed system's decontamination capability, a new computer model was developed in the study to describe the bubble size dependent effect on aerosol removal including the effect of pool residence time. The accuracy of the new model was examined against experimental data for its validation. The proposed scrubber system composed of serially connected multiple shorter pools is found to have much improved decontamination performance over the current single pool system design
In severe nuclear power plant accidents, when the containment is in a serious challenging state, the gas mixture in the containment can be injected into the spent fuel pool through the multihole injector by the containment depressurization measure, to reduce the risk of containment overpressure failure and the release of radioactivity to the environment. The pool scrubbing efficiency of aerosol under the jet regime is studied on the small-scale aerosol pool scrubbing facility, focusing on the influence of mass flux, steam fraction, submergence, particle diameter, and pool initial temperature on the aerosol decontamination factor (DF). The results show that under the jet regime, the DF value is significantly greater than that in the bubble regime and the effect of jet flow on the mechanism of steam condensation and aerosol removal of the rising zone is weak under the conditions explored. DF increases with the increase of mass flux owing to the droplet interception and inertial collision aerosol removal mechanisms. Because the high pool temperature weakens the aerosol removal by steam condensation, DF decreases with the increase of initial pool temperature under the conditions explored. Based on the experimental data and the analysis of the removal mechanism under the jet regime, an empirical model of aerosol DF considering mass flux, steam fraction, pool temperature, submergence, and particle size is established and verified by the international experiments. The proposed model can be used to calculate DF in the process of containment overpressure discharge.
Pool scrubbing with bubble swarm generated by gas jet is an effective technique for aerosol retention at severe accidents, owing to large interfacial area and long residence time. Correct understanding of the process and thus enhancing its efficiency relies on analysis of the hydrodynamic behaviour of the gas, since it affects particle removal mechanisms directly. The objective of the present work is to explore the gas jet structure in detail by means of VOF interface-capturing method and additional techniques for tracking bubble characteristics and trajectories. The main findings are: (a) The breakup of globules in the injection zone becomes significant at high gas flow rates and has a great contribution in particle removal; (b) The increase of bubble size and velocity with the injection velocity will promote the inertial and centrifugal deposition of aerosol particles; (c) However, the coalescence probability of rising bubbles is found to increase with the gas flow rate, which may influence particle retention by re-enclosing particles from liquid film and reducing surface area; (d) Furthermore, the reduction in bubble residence time as they rise through the pool is unfavourable for particle removal. Nevertheless, liquid recirculation originated from violent interaction between the gas jet and the pool surface as well as swarm effects helps to prolong the residence of bubbles. The effect of gas flow rates on the decontamination factor is found to be associated with a variety of gas–liquid hydrodynamic phenomena. The proposed numerical approach is capable of acquiring detailed local information that is required for model development. Both the time-averaged spatial distribution of void fraction and the instantaneous size/rise velocity of individual bubbles obtained from the simulation conform to the experimental data. In the next step it will be extended to include aerosol particles.
In the late stage of a severe loss-of-coolant accident, the pressure in the containment building of a nuclear power plant could rise beyond the design limits and thus endanger its structural integrity. Therefore, to avoid pressure failure, it may be necessary to perform con-trolled venting of the containment. During the event of an accident, a large amount of fission and activation products are released into the containment as airborne particles (aerosols). These particles are filtered during the venting process, usually with the help of wet filters, in order to keep risks to the surrounding environment to a minimum. Consequently, the knowledge of the retention processes in a water reservoir (pool scrubbing) is of central im-portance for such filtered containment venting systems (FCVS) and for reactor concepts in which water reservoirs are used for pressure reduction (e.g. condensation chamber of a BWR).
Investigations on pool scrubbing are carried out in the SAAB test facility at the Juelich Research Centre. SAAB is a unique large scale facility with the ability to perform a great var-iation of experiments using various measurement tools. The influence of numerous parameters, such as the height of the water pool, solubility of aerosols and concentration on the retention capacity, is investigated by means of separate effect studies on both insoluble and soluble particles. This paper gives a detailed overview over the facility and includes example results of the first test series with soluble particles including cesium iodine (CsI).
Wet scrubber-based filtered containment venting systems (FCVSs) are commonly deployed at nuclear power plants, owing to the high degree of aerosol, iodine, and organic iodine retention that is possible through the pool scrubbing action that takes place during their operation. However, after a FCVS has been operating during a severe accident, the pool water will become extremely contaminated with previously-captured radionuclides, and the bubbling action of gases passing through the pool, could result in the re-entrainment of contaminated liquid droplets into the gas stream. This would produce a secondary source term during the late phases of an accident. As such, a set of experiments have been conducted in order to evaluate the aerosol re-entrainment rates and size distribution from a system prototypical of a pool-type FCVS. Entrainment rates were measured by tracking the carry-over of a NaCl tracer into a set of liquid traps, while the size of the re-entrained droplets was measured with a phase Doppler anemometer. The experiment employed a prototypical venturi nozzle and gas injection conditions to examine the influence of gas flow rate, liquid level, and pool water temperature. Entrainment factors between 5×10⁻⁵ and 1×10⁻³ were measured, and the water droplet count median diameters were between 3 and 13 µm, and fairly wide particle size distributions, with mass mean diameters were on the order of 40–140 µm. These experimental results help the assessment of the potential magnitude of the late phase source term hazards posed by the fission products retained in pool-type FCVS (secondary release), and the potential loading on the secondary dry filter stage (metal fiber filters) employed in FCVSs after the primary pool scrubbing stage. The secondary metal fiber filters should have a high efficiency in capturing water droplets produced through this mechanism, given the droplet’s relatively large size.
The importance of fuel chemistry has been revivaled since Fukushima Daiichi Nuclear Power Station (FDNPS) accident. The inspection and analysis of damaged three units, which had been operated in March 11, 2011, showed large differences in the accident progression sequence for these units, because operators attempted to prevent or mitigate the accident progression of each unit by all means possible. Characteristics of fuel debris are considered to be largely influenced by the difference in the sequence and, hence, deviated from those predicted from prototypic accident scenarios, which had been mainly identified from the analysis of Three Mile Island-2 accident and the following sim-tests, For the proper improvement of our knowledge on severe accident, including nonprototypic conditions, one has to start improving the phenomenology of fuel/core degradation and fission product behavior. Advances in the chemistry are the most essential approach. The present review attempts to focus on the recent updates and remaining concerns after the FDNPS accident.
Tests at three different scales were conducted in order to understand the mechanisms of pool scrubbing under a wide range of two-phase fluid dynamic conditions with a special focus on rapid depressurization caused by venting. In the small-scale separate effect test, high resolution two-phase flow measurement techniques utilizing equipment such as a high-speed digital camera, wire mesh sensor and PIV were applied to capture the behaviors of a single bubble (deformation, rising velocity and aerosol tracks) and the evolution of two-phase flow structures (bubble globule breakup, bubble sizes, bubble shapes and swarm rising). In the large-scale integral effect test, the dependence of the aerosol removal efficiency on submergence and pool temperature was measured under constant pressure and depressurized conditions. Attention was also paid to aerosol materials with different particle size distributions, hydrophilicity and solubility from the viewpoint of their importance for aerosol removal efficiency. To clarify relationships between individual phenomena and combined phenomena observed in two tests, the mid-scale integral effect test was undertaken. In this paper, we overviewed existing analytical models of pool scrubbing, their deficiencies to be improved, the purpose of this experimental project, an outline of experimental approaches and findings obtained so far.
EXECUTIVE SUMMARY
Background
Nuclear aerosol investigations began in the late 1960s and early 1970s. The progress made both in experimental studies and in code development is attested to by the publication of three CSNI-sponsored State-of-the-Art Reports (SOAR) on nuclear aerosols since 1979. Initially, fast reactor safety was the main concern of the work, as reflected in the content of the first aerosol SOAR. The TMI accident in 1979 motivated an interest in LWR source terms and resulted in the production of a supplement to the first SOAR which concentrated on LWR aerosol issues. In contrast to the fast reactor situation, the large quantity of steam present in the LWR containment atmosphere following an accident tends to make aerosol particles more compact, reducing the modelling problem of defining shape factors. On the other hand, two new effects had to be modelled: deposition due to steam condensation onto surfaces (diffusiophoresis) and particle growth by steam condensation onto the particles themselves.
The second SOAR dealt with primary-system fission-product release and transport. This SOAR included a survey of the models available in the literature for homogeneous and heterogeneous nucleation, for vapour condensation on aerosols, and for aerosol agglomeration, and noted that these and other models had been incorporated in circuit codes.
The last workshop on aerosols organised by the OECD was held in Cologne in June 1998. One recommendation made at this workshop was to prepare a SOAR on aerosol behaviour in both the primary circuit and in containment.
Objectives
Following the recommendation of the Cologne workshop, the CSNI mandated the preparation of a SOAR with four main objectives:
• To assess the status of existing experimental data and analytical capabilities required for predicting aerosol source terms from LWR accidents
• To address the strengths and weaknesses of the codes used to predict aerosol behaviour in the reactor coolant circuit and containment, and provide validation results and uncertainties, in particular for plant applications,
• To summarise findings from recent ISP exercises on nuclear aerosols and related thermal-hydraulic behaviour in the reactor coolant circuit and containment.
• To identify safety-relevant pending issues and to propose research activities that will reduce gaps in the understanding of relevant processes and deficiencies in modelling capabilities.
Scope
The Nuclear Aerosol Writing Group met several times over a three-year period from 2003 to 2006 to prepare the current SOAR, which provides a survey of issues surrounding the prediction of nuclear aerosol behaviour in the primary system and containment. It should be noted that the subject matter of the report is limited to nuclear aerosols and does not extend to other LWR accident source term topics that do not have a direct bearing on aerosol behaviour.
The structure of this SOAR is in line with the objectives listed above. The introduction is followed by a background chapter discussing the fundamentals of aerosol behaviour with bibliography of the pertinent literature. Chapter 3 is devoted to the topics of aerosol formation/growth, transport, and retention and release. Chapter 4 provides descriptions of codes for modelling circuit and containment aerosol behaviour, and Chapter 5 describes the experiments used for their validation. Chapters 6 and 7 provide examples of validation results, and plant applications of source term calculations. Chapter 8 summarises the findings of the Cologne Workshop and progress made since, and provides recommendations for future work.
Results and their significance
The theoretical, experimental and modelling studies presented in this SOAR summarise the status and current understanding of a wide range of nuclear aerosol topics. The significance of these results is that they allow for the identification of existing gaps in code capability and experimental data that prevent accurate predictions of the source term. A general conclusion from this document is that there still exist a number of items for which additional work is required. However, the status of aerosol codes and experimental data-bases has improved substantively since the publication of previous SOARS.
At the time when the 1994 SOAR on circuit aerosols was written, plant analysis codes, (as distinct from special-purpose research tools) were relatively crude, and their validation base was largely confined to tests using simulant materials. Now, while these codes still use lumped-parameter descriptions of the thermal hydraulics that can place some constraints on the accuracy of their predictions, they include a more complete calculation of gas-phase chemistry, interactions with particles and surfaces in the primary circuit, and a better treatment of coupling between aerosol dynamics and thermal hydraulics in the containment. Models are also available for aerosol interactions with engineered safety features such as suppression pools in BWRs, and complex structures such as steam generators in PWRs.
The experimental database for development and validation of aerosol codes has benefited from more accurate and extensive instrumentation to measure a wider range of phenomena (e.g., the ability to measure airborne water). Numerous analytical tests have been performed, and are continuing, to address specific phenomena such as resuspension, revaporisation, and trapping in complex structures. Large-and small-scale containment aerosol experiments have also been performed and analysed in international programmes, notably the KAEVER and VANAM tests. Perhaps the most significant advance on the experimental side is the availability of integral experiments in the PHÉBUS facility on fission product and structural material release and transport. These tests demonstrate the complex inter-linkage between different phenomena and have refined our understanding of nuclear aerosols, particularly in the circuit, where we are now able to refer to measured particle sizes and compositions. PHÉBUS has also provided information on specific phenomena such as revaporisation. The PHÉBUS integral experiments are now complete but post-test analyses and interpretation of the results continues. Data from aerosol-related experimental programmes such as ARTIST and THAI also still require analysis.
A large number of Probabilistic Safety Analyses (PSA2) plant studies have been performed around the world, frequently involving aspects of aerosol behaviour. This report provides some examples, including sensitivity studies that demonstrate the impact of aerosol-related processes, however few such analyses have been published. The aerosol community is therefore not always fully briefed on the risk-relevance of the numerous phenomena currently under study. The question of risk significance deserves a more concerted effort, in aerosol physics as in other relevant fields. This will require additional probabilistic analysis for new plant designs and evaluation of their sensitivity to various models and parameters.
It is expected that increasingly stringent safety standards and new power reactor designs will generate aerosol-related safety questions that will require experimental capabilities, analytical tools, and the expertise to use both intelligently. Aerosol-related computational and modelling capability is now being utilised to address industrial problems, and major environmental issues such as climate change are producing a body of knowledge that should be integrated with that developed within the reactor safety community. The CSNI structure appears well suited to developing structures and mechanisms for stimulating such cross-fertilisation.
Conclusions and recommendations
A number of conclusions and recommendations have been made throughout the text of this document. The most important are summarised briefly below subdivided into three categories
Generic Issues
Code-users’ workshop on plant analysis: The CSNI workshop recommended in 1998 that a meeting on the use of severe-accident codes in plant calculations for source term estimation, including codes used by utilities, be held. More than eight years after formulation of this recommendation it has still not been acted upon. There remains a need to harmonise user practices with respect to plant analyses in order to reduce divergence in results. The objective would ideally be to produce online “best practice” guidelines for the major codes.
Shape factors: Models describing aerosol dynamics generally assume spherical, fully dense particles but nuclear aerosols are often neither, particularly those originating from core melt sequences, or accident scenarios in which large parts of containment have low humidity. The importance of shape factors has been demonstrated in uncertainty and sensitivity analyses. Although such parametric studies can be performed for accident scenarios, the difficulty is in knowing what constitutes a “reasonable” variation of the shape factors for representative conditions. At the minimum, it is recommended that measurements of two diameters (e.g., aerodynamic and volume-equivalent diameter) of aerosols from prototypical integral experiments be obtained to determine the dynamic shape factor.
Reactor Coolant System Issues
Analytical support by the use of CFD Codes: CFD treatment of coupled aerosol/flow phenomena is not universal because of the complexity of the phenomena and geometries, and the effort required for implementation. At present, efforts are being made to simulate particle transport and deposition in complex reactor structures with CFD, however more work needs to be done this treatment becomes satisfactorily accurate.
Mechanical Resuspension: Existing models are inadequate to evaluate the safety impact of aerosol resuspension in the primary circuit, which is both design- and scenario-dependent. Experiments are required for the development and validation of models to: a) account for the porosity of a deposit, which affects the mass and size distribution of resuspended material and b) simulate the effect of shock and vibrations (alone and in combination with flow increases) on resuspension. There is also limited work on the resuspension of wet deposits or wet deposits which have dried in situ. It is recommended that scoping calculations be performed to assess the potential impact of this behaviour on the source term. Finally, it has to be said that the consequences for a severe accident of a light-water reactor due to resuspension from the RCS varies depending on the scenario, i.e., bypass or non-bypass sequences. For non-bypass sequences, the influence of this resuspension on the potential source term may well be insignificant within a few hours. However, improvement of resuspension modelling with respect to bypass sequences is clearly very desirable.
Deposition in singularities and complex structures: No significant advances appear to have been made since 1998 in the experimental investigation or the modelling of deposition in singularities such as changes in cross section or bends. Some code improvements have been made by including the best-available models from published literature.
Significant progress has been made with respect to deposition in steam generators (SGs). Most probabilistic risk assessments (PRAs) and severe accident codes assume that a significant fraction of fission products flowing through an non-isolated break in a SG escapes to the environment. This may not be the case however, as demonstrated in several recent experimental programmes. SGTR was the first European project (2000-2002) to improve understanding in a systematic way of possible retention mechanisms in tubes and in the complex structures of the secondary side of a SG. In particular, the PSAERO and HORIZON experiments from Finland were conducted to study in-tube retention whereas retention in the SG bundle has been investigated in PECA-SGTR and the ongoing ARTIST experiments (respectively in Spain and Switzerland). In addition, certain modelling efforts to develop a correlation for the retention in the bundle were initiated. Thorough interpretation of the data from the above experiments is incomplete and modelling efforts continue. It is expected that the main issues regarding SGTR will be answered after termination of these activities.
Particle break-up in highly turbulent flows: Highly turbulent flow inside a tube may induce break-up of particles due to (i) impaction on the walls, (ii) strong shear in the flow or (iii) the vena contracta and shock wave at the tube exit. This phenomenon was observed, e.g., in Phase I of the ARTIST tests as well as in supplementary experiments. However, to determine whether aerosol particles may break up in severe accidents would require more information on the structure and the nature of the bonding forces between primary particles for severe accident aerosols as well as for the TiO2 agglomerates used in ARTIST Phase I. In Phébus tests, which produced more prototypical aerosols, SEM micrographs showed fairly compact, sintered agglomerate structures unlike the ARTIST TiO2 particles. Consequently, intra-particle forces can be expected to be higher in reality than in these TiO2 agglomerates. Nevertheless, the issue is of some importance since the generation of submicron particles from supramicron ones leads to less-efficient retention of the aerosols concerned. In the first instance, accident sequences other than SGTR inducing highly-turbulent flows need to be identified; comparison of plant calculations without and with (assumed) break-up for SGTR and the other pertinent sequences would then allow evaluation of the risk relevance of the break-up phenomenon. If the risk impact is significant then, in the absence of relevant data for severe accident aerosols, data for prototypical particles are needed in highly-turbulent accident-relevant conditions.
Impact of Chemistry: The fundamental importance of chemistry to prediction of source terms arising from potential severe accidents is well established. Nuclear safety codes model chemical reactions using the thermodynamic equilibrium approach, which relies on having thermodynamic data for each chemical species covered. Unfortunately, the uncertainties associated with some of these data are often large and divergence between data series originating from different sources can be considerable. It is recommended (as is being done for the ASTEC code) that the thermo-chemical data used by codes be thoroughly verified and that key sources of uncertainty be identified. Moreover, while the thermodynamic approach is pragmatic, it is of limited value when addressing accident scenarios in which slow chemical kinetics precludes achievement of thermodynamic equilibrium (e.g. secondary-side, cold-leg and containment conditions). In this context it is recommended to:
• Assess the value of the simplistic approach (taken in ASTEC/SOPHAEROS and VICTORIA codes) of assigning user-defined cut-off temperatures below which chemical reactions in the RCS do not occur. Use of PHÉBUS FP results with respect to iodine in the RCS should prove helpful;
• Follow closely experimental programmes that are investigating reaction rates for iodine species (e.g., the French CHIP series);
• Assess whether FP species other than iodine, which may reach the containment in the vapour phase, require kinetic modelling (especially important for hot-leg and high-pressure sequences where vapour fractions at the RCS breach will be significant).
• Lastly, the effects of radiolysis in the RCS are unknown. Radiolysis may have little impact in the core region (temperatures being so high that only simple atomic and radical species exist), but may be important in cooler regions of the RCS involving significant deposits (high local dose rate) such as in a cold-leg sequence. In terms of direct consequences for aerosols, one effect will be reduction of the threshold super-saturation at which vapours nucleate since a high density of electrically-charged condensation nuclei will form. In terms of consequences for the source term, the meagre state of knowledge renders even qualitative evaluation difficult.
Revaporisation of deposits: Although significant progress has been made regarding studies on revaporisation from several projects (Phébus FP, the EC 4th Framework Programme projects RVP and REVAP-ASSESS and the EXSI project), experimental work on revaporisation remains scarce; further theoretical and experimental developments are necessary to understand the revaporization process. In addition, the safety relevance ought to be further demonstrated by assessing, conceivably, the impact of a weak source of fission products from the reactor coolant system for some hours after the main release-from-core phase.
Containment Issues
Charge effects: Small deposits of aerosols on the outer walls of the 10 m3 containment vessel in the PHÉBUS test FPT0 could not be explained by electrophoretic effects. However, even if charge effects on aerosol deposition are not seen to be significant in test facilities (with or without a radiation field) there is no firm evidence that this effect would be negligible in an accident. At present there is no consensus among experts on whether further investigations of charging effects are necessary or not.
Mixed aerosols in condensing atmospheric conditions: Although there has been considerable progress in modelling aerosol deposition as a function of relative humidity, a comparison of the adequacy of code results from ISP 37 and ISP 44 indicate that there is still some work to be done to ensure satisfactory coupling between thermal hydraulic and aerosol models so that these capture correctly aerosol behaviour in most environments. An additional uncertainty in modelling aerosol behaviour in the containment in humid conditions arises from determining the hygroscopicity associated with a mixture of aerosols of different compositions. Finally, there is some uncertainty regarding the density of multi-component aerosols, and whether this parameter is important for accident conditions with a wide variety of aerosol components. Both the PHÉBUS tests and the KAEVER experiments suggest that an average aerosol particle composition and size might be attained in containment. The KAEVER experiments further suggest that this average particle would behave like the most hygroscopic of its individual components. Confirmation of these findings, in a large- or intermediate-scale experiment performed under saturated conditions might significantly simplify modelling aerosol behaviour in wet conditions.
Mechanical Resuspension: Compared with deposits in the primary system, deposits in containment will have significantly lower particle loadings, and be distributed over larger areas. Resuspension of such deposits is possible as a consequence of a breach of containment and/or a hydrogen burn. It is recommended that experimental studies be performed on resuspension of real or simulated deposits at containment-typical loadings, particularly during flow disturbances, as a function of the deposit composition and history (dry, wet, wet then dried in situ etc.) and as the result of hydrogen deflagrations.
In addition, probabilistic studies should be undertaken to evaluate the safety significance of containment resuspension.
Re-entrainment in the Containment: Re-entrainment of particulate fission products will occur at several water and core melt pools during a severe accident. The release rates of radiological materials are relatively small but the sources are persistent. A significant contribution to the source term by re-entrainment is possible in the late accident phase. However, reliable analytical investigations including risk relevant aspects do not exist yet. With a few exceptions, there are only a few codes able to simulate the FP release from boiling or flashing sumps.
Up to now all re-entrainment tests including ThAI have been made on ideal systems with appropriate concentrations of soluble and insoluble materials but without surfactants or impurities. Additional experiments are required to evaluate entrainment under realistic conditions. After improvement and validation of re-entrainment models for realistic conditions is achieved reliable accident calculations to quantify the effect of re-entrainment from boiling pools on the source term will be possible. In the light of these results it may be desirable to investigate measures reducing the release of fission products by re-entrainment.
Pool scrubbing: Some BWR and PWR severe accident scenarios involve transport of radioactive aerosols through pools of water where particles can be retained. This phenomenon, known as pool scrubbing, has the potential to reduce the source term. Results provided by both stand-alone and integral code models indicate satisfactory agreement with simple experiments for integral retention. However, a systematic experimental database is required for validation purposes. Particular attention should be given to removal of aerosols during formation and subsequent disintegration and coalescence of bubbles, and the effects of submerged structures and contaminants (surfactants).
Removal by sprays: This issue has been extensively investigated by the French organisations CEA and IRSN using specific apparatuses and the CARAIDAS, MISTRA and TOSQAN test facilities. The data should be made accessible to the nuclear community, at least the OECD partners. Validated modelling based on these experimental investigations has been implemented in the codes ASTEC and TONUS. The ASTEC model can be found in the open literature. Further work on containment sprays is low priority for countries that have access to this data but in other countries and for certain advanced designs it remains important to establish effective removal by spray systems and both experimental and analytical efforts continue.
Influence of recombiners: Phenomenological experiments have demonstrated that there is a potential for the operation of Passive Autocatalytic Recombiners (PARs) to generate volatile molecular iodine by thermal decomposition of metal-iodide containment aerosols. Scoping calculations indicate that this conversion process might be a significant contribution to the molecular iodine in the containment atmosphere. It has also to be recognised that iodine may not be the only fission product concerned, e.g., some formation of the highly volatile species ruthenium tetroxide might be possible in the conditions expected within PARs though no investigation of this has been performed. Further experimental investigations in realistic conditions (mixed-aerosol and mixed-atmosphere composition) are necessary.
Hydrogen-burn effects on suspended aerosols: Heat release by hydrogen burns may have a strong effect on aerosol characteristics and could possibly liberate volatile forms of iodine. Experiments in the ThAI facility demonstrate that there is a clear effect of hydrogen-combustion-induced flows on resuspension of already deposited CsI aerosol. These findings have to be expanded to consider other types of relevant aerosol species and other plant applications. However, chemical effects on iodine-containing aerosols have not been studied; information from the ongoing investigation of the impact of recombiners on aerosols may be relevant.
Release from MCCI pool: The behaviour of aerosols formed after the release from a molten corium concrete pool was investigated in the 1990s in the ACE phase C and BETA tests and more recently in the OECD-MCCI and EC Framework Programmes (LPP and MP projects). However, efforts to further aerosol code development and validation from these experiments has been limited. As there is currently little known activity in this area it is recommended that the following steps be initiated in the near future:
● Recalculation of the old test results to evaluate the thermodynamic basis of the present models by comparing concrete erosion processes and aerosol nucleation due to vaporization from the free upper surface and into gas bubbles in the corium concrete pool,
● Obtain information on the characterization of concrete-based aerosols; conventional (non-nuclear) knowledge about concrete aerosols may be a good starting point;
● Evaluate information from the EC projects MP and LPP from the specific view of fission products release due to MCCI;
Penetration of aerosols through leak paths: From a review of available data and models including recent research in SARNET on dry aerosol transport in cracks, the recommendation can be made that additional separate effects and integral tests are required:
● Separate effect tests should provide a detailed characterization of the scenarios and the phenomena necessary for model development and validation of individual depletion mechanisms;
● Integral tests should focus on measuring overall process variables such as mass retained to provide a data base for checking the overall model performance.
The boundary conditions (hydraulic diameter, curvature of the path, fluid composition pressure drop, wall temperature, and aerosol size) for these experiments should, obviously, be as close as possible to those postulated under accident conditions.
For wet aerosols, it is recommended that additional experiments be performed to characterise aerosol transport through sequential expansion and contraction regions representative of the leak path from containment to the outside atmosphere and to evaluate the extent of leak-path plugging.
Fire aerosols: Little is known about the properties or amounts of aerosols produced from fires, and how mixing of flaky fire aerosol with nuclear aerosol particles would impact on the aerosol depletion rate in containment. There is a need for further experimental investigation of fire aerosols especially from cable fires. This could be done in a future step of the OECD PRISME and/or the Sandia CAROLFIRE projects. The aerosol production rate, the particle size distribution and the shape factors are of main concern. It is recommended that existing multi-component aerosol models be extended in order to simulate the interaction of fire aerosols with a pre-existing nuclear aerosol.
A new major version of the European severe accident integral code ASTEC, developed by IRSN with some GRS support, was delivered in November 2015 to the ASTEC worldwide community.Main modelling features of this V2.1 version are summarised in this paper. In particular, the in-vessel coupling technique between the reactor coolant system thermal-hydraulics module and the core degradation module has been strongly re-engineered to remove some well-known weaknesses of the former V2.0 series. The V2.1 version also includes new core degradation models specifically addressing BWR and PHWR reactor types, as well as several other physical modelling improvements, notably on reflooding of severely damaged cores, Zircaloy oxidation under air atmosphere, corium coolability during corium concrete interaction and source term evaluation.Moreover, this V2.1 version constitutes the back-bone of the CESAM FP7 project, which final objective is to further improve ASTEC for use in Severe Accident Management analysis of the Gen.II-III nuclear power plants presently under operation or foreseen in near future in Europe. As part of this European project, IRSN efforts to continuously improve both code numerical robustness and computing performances at plant scale as well as users' tools are being intensified.Besides, ASTEC will continue capitalising the whole knowledge on severe accidents phenomenology by progressively keeping physical models at the state of the art through a regular feed-back from the interpretation of the current and future experimental programs performed in the international frame.
The effective capture and storage of radiological iodine ((129)I) remains a strong concern for safe nuclear waste storage and safe nuclear energy. Silver-containing mordenite (MOR) is a longstanding benchmark for iodine capture; however, the molecular level understanding of this process needed to develop more effective iodine getters has remained elusive. Here we probe the structure and distribution of iodine sorbed by silver-containing MOR using differential pair distribution function analysis. While iodine is distributed between gamma-AgI nanoparticles on the zeolite surface and subnanometer alpha-AgI clusters within the pores for reduced silver MOR, in the case of unreduced silver-exchanged MOR, iodine is exclusively confined to the pores as subnanometer alpha-AgI. Consequently, unreduced silver-containing zeolites may offer a more secure route for radioactive iodine capture, with the potential to more effectively trap the iodine for long-term storage.
An efficient venturi scrubber system making use of heterogeneous nucleation and condensational growth of particles was designed and tested to remove fine particles from the exhaust of a local scrubber where residual SiH4 gas was abated and lots of fine SiO2 particles were generated. In front of the venturi scrubber, normal-temperature fine-water mist mixes with high-temperature exhaust gas to cool it to the saturation temperature, allowing submicron particles to grow into micron sizes. The grown particles are then scrubbed efficiently in the venturi scrubber. Test results show that the present venturi scrubber system is effective for removing submicron particles. For SiO2 particles greater than 0.1microm, the removal efficiency is greater than 80-90%, depending on particle concentration. The corresponding pressure drop is relatively low. For example, the pressure drop of the venturi scrubber is approximately 15.4 +/- 2.4 cm H2O when the liquid-to-gas ratio is 1.50 L/m3. A theoretical calculation has been conducted to simulate particle growth process and the removal efficiency of the venturi scrubber. The theoretical results agree with the experimental data reasonably well when SiO2 particle diameter is greater than 0.1 microm.
A new major version of the European severe accident integral code ASTEC, developed by IRSN with some GRS support, was delivered in November 2015 to the ASTEC worldwide community. Main modelling features of this V2.1 version are summarised in this paper. In particular, the in-vessel coupling technique between the reactor coolant system thermal-hydraulics module and the core degradation module has been strongly re-engineered to remove some well-known weaknesses of the former V2.0 series. The V2.1 version also includes new core degradation models specifically addressing BWR and PHWR reactor types, as well as several other physical modelling improvements, notably on reflood-ing of severely damaged cores, Zircaloy oxidation under air atmosphere, corium coolability during corium concrete interaction and source term evaluation. Moreover, this V2.1 version constitutes the backbone of the CESAM FP7 project, which final objective is to further improve ASTEC for use in Severe Accident Management analysis of the Gen.II–III nuclear power plants presently under operation or foreseen in near future in Europe. As part of this European project , IRSN efforts to continuously improve both code numerical robustness and computing performances at plant scale as well as users' tools are being intensified. Besides, ASTEC will continue capitalising the whole knowledge on severe accidents phenomenology by progressively keeping physical models at the state of the art through a regular feedback from the interpretation of the current and future experimental programs performed in the international frame.
The Fukushima-1 accident highlighted the relevance of having a passive wet trap for fission products before being released from the BWR-Mark-I containments. Even though pool scrubbing was heavily investigated in the 80's and 90?s of last century, a peer review of data obtained and codes developed at that time, has led to conclude that there are significance knowledge gaps for nuclear safety that should be investigated. This paper summarizes the major insights gained from the literature survey conducted, the anticipated conditions under which experimental research should be focused on and the available facilities in which experimentation will take place within the framework of the EU-PASSAM project. Despite the large body of information available, a lack of systematic analysis of major influencing variables on pool scrubbing, particularly those of hydrodynamic nature, has been found. Among them, particular emphasis will be placed in upcoming EU-PASSAM campaigns on: high injection velocities of the carrier gas (i.e., jet injection regime), gas rise hydrodynamics under both bubbly regime (low gas velocity) and churn-turbulent flow (high gas velocity) and the effect of submerged structures. The resulting database is expected to allow a comprehensive validation of existing hydrodynamic models in integral safety codes and to serve as the basis for new model development, if necessary. The long-term behavior of trapped fission products will be also experimentally explored. Additionally, the paper will briefly outline the EU-PASSAM framework.
Iodine is one of the most radiotoxic fission product released from fuel during a severe nuclear power plant accident. Within the containment building, iodine compounds can react e.g. on the painted surfaces and form gaseous organic iodides.
In this study, it was found out that gaseous methyl iodide (CH 3 I) is oxidised when exposed to beta radiation in an oxygen containing atmosphere. As a result, nucleation of aerosol particles takes place and the formation of iodine oxide particles is suggested. These particles are highly hygroscopic. They take up water from the air humidity and iodine oxides dissolve within the droplets. In order to mitigate the possible source term, it is of interest to understand the effect of beta radiation on the speciation of iodine.
An extensive evaluation was carried out to determine the optimal silver ion-exchanged level for the removal of methyl iodide at high temperatures up to 400°C. Based on the degree of silver utilization, the optimal silver loading and temperature were about 10 wt% and 175°C, respectively. The physical and chemical properties of silver ion-exchanged zeolite were characterized by instrumental analysis such as BET, TG/DTA and SEM-EDS. Adsorption dynamics was also studied at different temperatures, and methyl iodide concentrations. A simple dynamic model was formulated by employing the linear driving force (LDF) approximation inside adsorbent particles, and the nonisothermal Langmuir-Freundlich equation. The model equations were solved numerically by an orthogonal collocation method. The proposed dynamic model satisfactorily simulated the experimental breakthrough results.
Wet electrostatic precipitators (ESPs) are good options for effective control of sulfuric acid aerosol emission. However, various problems caused by materials and non-uniform distribution of water film limited the applicability of typical wet ESPs. Research on ESP technology has tried to find more suitable and anti-corrosive methods to solve these imperfections. This research was inspired by the requirement to replace rigid collection electrode by single terylene or polypropylene fabrics. A patented system was designed, and the capillary difference between terylene and polypropylene fabrics was illustrated. Contrastive V–I curves of different collection electrodes were investigated under same conditions. The effects of several important parameters on the removal of sulfuric acid aerosol were analyzed. The results demonstrated that the variations of absorbed mass were significantly influenced by physical properties of the liquids and the structure of fabrics. The behavior of the new ESP was consistent with the typical ESP using a thimbleful of water penetrating terylene or polypropylene collection electrode via capillary flow. The collection efficiencies by terylene and polypropylene fabrics were higher than those by fiberglass reinforced plastics (FRP) under certain conditions. The collection efficiency had linear relationship with specific surface area (SCA) and mass concentration. The collection efficiency increased with increasing electric field strength, average diameter of particles and with decreasing gas temperature. As long as there was any water on the collector surface, any particle would exhibit similar collection efficiencies, whether of high resistivity or not. Experimental and theoretical investigations indicated that single terylene or polypropylene collection electrode had significant advancement which could improve wet ESP applications, such as superior performance and continuous operation ability compared with typical materials.
The feasibility of the use of high intensity acoustic waves to combat airborne spreading of accidentally released radioactive or toxic material is investigated. The potential areas of application range from small-scale laboratory spills and fires to major releases of hazardous material into the atmosphere. Results are presented on the acoustic agglomeration of a submicron combustion aerosol produced by burning rubber in a 4.5 m3 chamber. Of particular interest in these experiments is the rate of decrease of the mass loading and how this scales with acoustic power at a source frequency of 21 kHz. Finally some extrapolations of the results will be made with regard to the problem of ‘open-air’ scavenging together with a brief discussion of some of the difficulties to be expected.
Spray systems in nuclear reactor containments are described. The scrubbing of aerosols from containment atmospheres by spray droplets is discussed. Uncertainties are identified in the prediction of spray performance when the sprays are used as a means for decontaminating containment atmospheres. A mechanistic model based on current knowledge of the physical phenomena involved in spray performance is developed. With this model, a quantitative uncertainty analysis of spray performance is conducted using a Monte Carlo method to sample 20 uncertain quantities related to phenomena of spray droplet behavior as well as the initial and boundary conditions expected to be associated with severe reactor accidents. Results of the uncertainty analysis are used to construct simplified expressions for spray decontamination coefficients. Two variables that affect aerosol capture by water droplets are not treated as uncertain; they are (1) [open quote]Q[close quote], spray water flux into the containment, and (2) [open quote]H[close quote], the total fall distance of spray droplets. The choice of values of these variables is left to the user since they are plant and accident specific. Also, they can usually be ascertained with some degree of certainty. The spray decontamination coefficients are found to be sufficiently dependent on the extent of decontamination that the fraction of the initial aerosol remaining in the atmosphere, m[sub f], is explicitly treated in the simplified expressions. The simplified expressions for the spray decontamination coefficient are given. Parametric values for these expressions are found for median, 10 percentile, and 90 percentile values in the uncertainty distribution for the spray decontamination coefficient. Examples are given to illustrate the utility of the simplified expressions to predict spray decontamination of an aerosol-laden atmosphere.
Nuclear Engineering and Design j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / n u c e n g d e s a b s t r a c t A steam generator tube rupture in a pressurized water reactor may cause accidental release of radioac-tive particles into the environment. Its specific significance is in its potential to bypass the containment thereby providing a direct pathway of the radioactivity from the primary circuit to the environment. Under certain severe accident scenarios, the steam generator bundle may be flooded with water. In addi-tion, some severe accident management procedures are designed to minimize the release of radioactivity into the environment by flooding the defective steam generator secondary side with water when the steam generator has dried out. To extend our understanding of the particle retention phenomena in the flooded steam generator bundle, tests were conducted in the ARTIST and ARTIST II programs to determine the effect of different parameters on particle retention. The effects of particle type (spherical or agglomerate), particle size, gas mass flow rate, and the break submergence on particle retention were investigated. Results can be summarized as follows: increasing particle inertia was found to increase retention in the flooded bundle. Particle shape, i.e., agglomerate or spherical structure, did not affect retention signif-icantly. Even with a very low submergence, 0.3 m above the tube break, significant aerosol retention took place underlining the importance of the jet–bundle interactions close to the tube break. Droplets were entrained from the water surface with high gas flow rates carrying aerosol particles with them. However, compared to particle retention in the water close to the tube break, the effect of droplet entrainment on particle transport was small.
Proceedings of the International Congress on Advances in Nuclear Power Plants 2010, ICAPP 2010. San Diego, CA, USA, 13 - 17 June 2010 Vol.2, 1084 - 1091 In case of a hypothetical severe accident it is very likely that iodine at least partly deposits on painted walls of a reactor containment building. Iodine may react with painted surfaces to form organic iodine species. These organic species are a possible source of volatile iodine, which may increase the fraction of releasable iodine. Therefore, it is important to study the transport of organic iodine in containment conditions. Another question is, in which form are the organic iodides transported as gaseous molecules or as aerosol particles resulting from organic iodides reacting with radiolysis products. To answer this last question methyl iodide was fed into the EXSI facility in an air mixture. In some experiments the flow contained also humidity. The reactions took place in a quartz tube heated either to 50°C, 90°C or 120°C. UV-light was used as a source of radiation to produce ozone from oxygen. A separate generator was also applied to reach higher ozone concentrations. Nucleated aerosol particles were collected on plane filters and gaseous iodine species were trapped in trapping bottles. Aerosol mass flow rate and size distribution as well as speciation of gaseous reaction products were measured with several on-line instruments. Collected aerosol particles were analysed with SEM. It was found that the formation of aerosol particles was very fast when ozone and methyl iodide were present in the facility. Even a very low concentration of ozone produced high number concentration of particles. The measured aerosol mass concentration increased with increasing temperature and ozone concentration. Because the particle diameter was quite small (<180 nm), their settling velocity is low. Therefore, iodine containing aerosols may exist in containment atmosphere for a long period of time. Part of methyl iodide was always transported through the facility regardless of experimental conditions. All ozone was consumed in the reactions when only UV-light was used as its source. With a separate generator, ozone concentration was sufficiently high to be detected at the outlet of the facility and thus enabling the study of the reaction kinetics. The transported fractions for both methyl iodide and ozone decreased with increasing reaction temperature. The main gaseous reaction products were methanol and formaldehyde. Especially at elevated temperature other reaction products, such as formic acid and methyl formate, became important as well.
Removing very fine particles in the 0.01-1 micro m range generated in diesel combustion is important for air pollution abatement because of the impact such particles have on the environment. By forming larger particles, acoustic agglomeration of submicron particles is presented as a promising process for enhancing the efficiency of the current filtration systems for particle removal. Nevertheless, some authors have pointed out that acoustic agglomeration is much more efficient for larger particles than for smaller particles. This paper studies the effect of humidity on the acoustic agglomeration of diesel exhausts particles in the nanometer size range at 21 kHz. For the agglomeration tests, the experimental facility basically consists of a pilot scale plant with a diesel engine, an ultrasonic agglomeration chamber a dilution system, a nozzle atomizer, and an aerosol sampling and measuring station. The effect of the ultrasonic treatment, generated by a linear array of four high-power stepped-plate transducers on fumes at flow rates of 900 Nm(3)/h, was a small reduction in the number concentration of particles at the outlet of the chamber. However, the presence of humidity raised the agglomeration rate by decreasing the number particle concentration by up to 56%. A numerical study of the agglomeration process as a linear combination of the orthokinetic and hydrodynamic agglomeration coefficients resulting from mutual radiation pressure also found that acoustic agglomeration was enhanced by humidity. Both results confirm the benefit of using high-power ultrasound together with humidity to enhance the agglomeration of particles much smaller than 1 micro m.
Sonochemistry is generally associated with the use of power ultrasound in liquid media. Under such circumstances acoustic cavitation can drive a range of reactions and processes. The use of airborne ultrasound in processing is less familiar because of the difficulties that relate to the use of ultrasound in gaseous systems. Firstly there is a greater attenuation (power loss) in the transmission of sound through air compared with that through liquid. Secondly the transfer of acoustic energy generated in air into a liquid or solid material is inefficient due to the mismatch between acoustic impedances of gases and solids or liquids. Despite this, applications do exist for airborne ultrasound but the source must be very powerful and efficient. In this way one can obtain levels of intensities at which it is possible to use ultrasound for specific applications such as to agglomerate fine dusts and to break down foams.
Two-phase flow hydrodynamics characterization for understanding the aerosol retention in liquid pools
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PASSAM “State-of-the-art report” - Technical Bases for Experimentation on Source Term Mitigation Systems
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Status Report on Filtered Containment Venting
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Experimental study on iodine chemistry (EXSI) - Containment experiments with elemental iodine
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