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Kinetics and mechanisms of the reaction of air with nuclear grade graphites: IG-110
Abstract
The work presented in this report is part of an ongoing effort in the microgravimetric evaluation of the intrinsic reaction parameters for air reactions with graphite over the temperature range of 450 to 750°C. Earlier work in this laboratory addressed the oxidation/etching of H-451 graphite by oxygen and steam. This report addresses the air oxidation of the Japanese formulated material, IG-110. Fractal analysis showed that each cylinder was remarkably smooth, with an average value, D, the fractal dimension of 0.895. The activation energy, Ea, was determined to be 187.89 kJ/mol indicative of reactions occurring in the zone II kinetic regime and as a result of the porous nature of the cylinders. IG-110 is a microporous solid. The low initial reaction rate of 9.8 × 10-5 at 0% burn-off and the high value (764.9) of Φ, the structural parameter confirm this. The maximum rate, 1.35 × 10-3 g/m2s, was measured at 34% burn-off. Reactions appeared to proceed in three stages and transition between them was smooth over the temperature range investigated. Both Ea and 1n A did not vary with burn-off. The value of ΔS, the entropy of activation, was - 41.4 eu, suggesting oxygen adsorption through an immobile transition state complex. Additional work is recommended to validate the predictions that will be made in relation to accident scenarios for reactors such as the modular high temperature gas-cooled reactor where fine grained graphites such as IG-110 could be used in structural applications.
... According to the first assumption, the ASA will be gradually decreased by chemical reaction since the grains shrink, while the second assumption leads to a contrary trend. The experiment of graphite-air oxidation using IG-110 by Fuller and Okoh (1997) supports the second model, in which the graphite oxidation rate reached a peak value at mass loss fraction of 38% [26]. In addition to the expansion of micro pores, chemical reaction also opens those isolated pores. ...
... According to the first assumption, the ASA will be gradually decreased by chemical reaction since the grains shrink, while the second assumption leads to a contrary trend. The experiment of graphite-air oxidation using IG-110 by Fuller and Okoh (1997) supports the second model, in which the graphite oxidation rate reached a peak value at mass loss fraction of 38% [26]. In addition to the expansion of micro pores, chemical reaction also opens those isolated pores. ...
... The complex micro structure of nuclear graphite leads to various values in literature. For example, a value of 765 [26], 268 [61], and 80 [41]- [43] have been used for nuclear graphite IG-110. As a result, the value of Fm also differs significantly. ...
Nuclear graphite is proposed for use in High-temperature Gas-cooled Reactor (HTGR) designs as the neutron moderator, reflector, and core structural material. During the normal operation of an HTGR, a small amount of moisture can exist in the primary helium circuit even with a helium purification system included to remove any impurities present in the primary coolant. In addition, a large amount of moisture can quickly enter the primary side during a steam ingress accident for those HTGRs that feature a steam Rankine cycle as the power conversion unit. The moisture can react with nuclear graphite in high-temperature environments, which degrades mechanical strength of graphite. Therefore, it is necessary to investigate the graphite-steam oxidation phenomena in detail to facilitate future HTGR licensing and deployment. In this research, the oxidation behavior of nuclear graphite IG-110 by steam was investigated under various temperature, moisture concentration, and hydrogen partial pressure conditions. A graphite-steam oxidation test facility was constructed to obtain high-resolution experimental data. The reaction environment was jointly controlled by a tube furnace, a peristaltic pump, and gas mass controllers. The concentrations of production gases CO and CO2 were measured online by a gas chromatography, which were then used to derive the oxidation rates. A total of 141qualified data points of the kinetic oxidation rates were collected at temperatures 850 to 1100 °C with steam partial pressure up to 20 kPa and hydrogen partial pressure varied from 0 to 3 kPa. Boltzmann-enhanced Langmuir-Hinshelwood (BLH) reaction rate equation was obtained through multivariable optimization. The overall mean relative difference between the predicted oxidation rate and the experimental data is 24%, with the maximum difference being 55%. In addition, experiments were performed to investigate the effect of mass loss on graphite oxidation rate. It was believed the graphite-moisture reaction expands the existing micro pores in graphite and opens those originally isolated pores, both resulting in an increase of active surface area. In the experiment, the graphite mass loss fraction was found to have a more prominent effect on increasing the oxidation rate at lower temperatures. Furthermore, a multiphysics model was developed for graphite-steam oxidation. The numerical model couples all important physical processes, including the kinetic chemical reaction, multi-species transport, free and porous flow, heat transfer, and microporous structure evolution. The multiphysics model was validated against our experimental data. Our comparisons show that the numerical model can well simulate the apparent oxidation rate and accurately predict the post-oxidation density distribution. The validated model was then applied to the prototypic MHTGR design for normal operating conditions. The chronic graphite-moisture oxidation during a full MHTGR service period of 36-months was simulated. The simulation indicates that at the end of the 36-month operation, the maximum local graphite mass loss can reach to about 85%. However, the oxidation is well confined within a thin layer of about 0.5 mm thickness into the graphite surface. Therefore, chronic graphite-moisture oxidation will not significantly decrease the mechanical strength of graphite, nor jeopardize the integrity of graphite fuel blocks in MHTGR during its normal operation.
... Bulk densities vary by grade but typically range from 1.7-1.9 g/cm 3 , versus a theoretical density of 2.26 g/cm 3 , indicating a range of 15-25% porosity. Oxidation can occur internally at the pore walls, leading to growth of E-mail address: ryanmpaul@gmail.com the pore network as material is consumed, sometimes without any major change to sample dimensions [3][4][5] . ...
... g/cm 3 , versus a theoretical density of 2.26 g/cm 3 , indicating a range of 15-25% porosity. Oxidation can occur internally at the pore walls, leading to growth of E-mail address: ryanmpaul@gmail.com the pore network as material is consumed, sometimes without any major change to sample dimensions [3][4][5] . The resulting increase in pore volume has been shown to profoundly decrease mechanical strength [ 1 , 6-8 ], which has profound performance and safety implications. ...
... As an example, the top left of Fig. 1 shows an optical micrograph of G-90 graphite, which has a bulk density of 1.90 g/cm 3 (approximately 15% porosity) [ 4 , 5 ]. Maximum grain size is 500 μm and impurities are less than 50 ppm. ...
Thermal oxidation mass loss of synthetic graphite can be life-limiting for high-temperature applications. The complex microstructure of synthetic graphite has made it difficult to predict oxidation-induced changes.Specifically, there are no accepted models for predicting oxidation pore growth, which affects properties and performance.
This paper presents the application of a three-dimensional microstructure-based model for synthetic graphite oxidation in the kinetic regime. Pore structure is described with randomly-placed spheres, whose growth rate is dependent on the penetration depth of oxidant. Pore structure, intrinsic reactivity, sample size, geometry, and initially closed porosity are included in the model. Oxidation mass loss occurs faster for finer pore size, lower bulk density, higher reactivity, and/or smaller samples.
For model validation, 30 datasets representing ten (mostly nuclear) high-purity graphite grades were selected, covering air oxidation temperatures from 450 to 750 °C and sample masses spanning four orders of magnitude. Agreements between model and experiment were judged reasonable, particularly the effects of microstructure and sample mass. For the temperature effect, aggregated reactivity ratios showed an activation energy of 187 kJ/mol, within expected kinetic range. To the author's knowledge, this model is the first three-dimensional microstructure-based approach for graphite air oxidation in the kinetic regime.
... According to Eto and Growcock [4]'s research , the mechanical strength of nuclear graphite is significantly changed by the variation of graphite density, and as a result, approximately 10 percent density change may cause around 50 percent mechanical strength reduction depending on the materials. According to other graphite oxidation research (Fuller and Okho [5], Hino and Hishida [6], Hinsen et al. [7], Katcher and Moorman [8], Kim and NO [9], Moorman [10]), it takes around 7 or 8 days for all the graphite to be transformed into CO or CO 2 gas under 700 C oxidation condition, compared to 2 ~ 3 days over 900 o C. In reality, the graphite temperature can be increased up to 1500 o C in the accident conditions, and it means that the mechanical strength of the graphite can be seriously weakened causing the core collapse. ...
... In previous research (Fuller and Okho [5], Mills [11]), the mechanism of the time-dependent graphite oxidation is qualitatively well analyzed and explained. According to their results, the graphite oxidation occurs both on the external surface and in the internal pores. ...
... In this case, at the beginning, the oxidation increases the internal pore size, which increases the active surface area. However, as the oxidation proceeds, the enlarged pores are collapsed and the total active surface area starts to decrease at around 30~40 % burn-off (Fuller et al. [5]), Kim [12]) as shown in Figure 1. At high temperature (more than 1000 o C), the oxidation reaction simply reduces the size of the graphite, which continuously increases or decreases the active surface area. ...
An air-ingress accident in a VHTR is anticipated to cause severe changes of graphite density and mechanical strength by oxidation process resulting in many side effects. However, quantitative estimations have not been performed yet. In this study, the focus has been on the prediction of graphite density change and mechanical strength using a thermal hydraulic system analysis code. For analysis of the graphite density change, a simple graphite burn-off model was developed based on the similarity concept between parallel electrical circuit and graphite oxidation considering the overall changes of the graphite geometry and density. The developed model was implemented in the VHTR system analysis code, GAMMA, along with other comprehensive graphite oxidation models. GT-MHR 600 MWt reactor was selected as a reference reactor. From the calculation, it was observed that the main oxidation process was derived 5.5 days after the accident following natural convection. The core maximum temperature reached up to 1400 o C. However it never exceeded the maximum temperature criteria, 1600 o C. According to the calculation results, most of the oxidation occurs in the bottom reflector, so the exothermic heat generated by oxidation did not affect the core heat up. However, the oxidation process highly decreased the density of the bottom reflector making it vulnerable to mechanical stress. In fact, since the bottom reflector sustains the reactor core, the stress is highly concentrated on this part. The calculations were made for up to 11 days after the accident and 4.5% of density decrease was estimated resulting in 25% mechanical strength reduction.
... The instant oxidation rates vary from 2.5 to 1312.8 g/m 2 /h with the treatment temperature increase from 900 to 1400 C. The rates at the short oxidation times fluctuate but approach a constant value after long time oxidation treatment. This difference can be attributed to microstructure changes at the early stage of the oxidation or effects of impurities on the sample surfaces [24,27]. When graphite samples were exposed to oxidants, pores became enlarged with opening of closed pores, which brought new surface areas for reaction [24,25,27,28]. ...
... This difference can be attributed to microstructure changes at the early stage of the oxidation or effects of impurities on the sample surfaces [24,27]. When graphite samples were exposed to oxidants, pores became enlarged with opening of closed pores, which brought new surface areas for reaction [24,25,27,28]. For example, the instant oxidation rate increased with time and the maximum was observed at 38% burnoff for the IG-110 graphite oxidation in air, where the pore opening rate became the same as the pore merging rate [27]. ...
... When graphite samples were exposed to oxidants, pores became enlarged with opening of closed pores, which brought new surface areas for reaction [24,25,27,28]. For example, the instant oxidation rate increased with time and the maximum was observed at 38% burnoff for the IG-110 graphite oxidation in air, where the pore opening rate became the same as the pore merging rate [27]. Also, impurities on the graphite surface can act as catalysts for oxidation [25,28]. ...
Water leakage in accidental conditions of high temperature gas-cooled reactors is one of the most critical problems that can compromise the integrity of different nuclear components. In this study, oxidation behaviors of nuclear graphite IG-110 in water ingress accidental conditions were investigated. Mass loss and oxidation rates were evaluated after oxidation tests at temperatures up to 1400 °C in an Ar-20 vol% H2O mixed atmosphere. The activation energy decreased from 318.6 to 148.9 kJ/mol with temperature, indicating two different oxidation regimes. The cross-sections of the oxidized samples were systematically characterized. The corresponding logarithmic porosity profiles showed a temperature dependency. Pore formation moved toward near-surface regions with increasing temperature and preferential binder oxidation, with filler particle degradation. Furthermore, oxidant concentration profiles and oxidation depths were estimated using a theoretical model and compared with the experimental results. This work provides important benchmark data and safety analysis guidance for the accident scenario in high temperature gas-cooled reactors.
... [7c]) [29]. It should also be noted for comparison purposes that the effective activation energy in the temperature range of 550°C to 700°C matches relatively well with a number of nuclear graphite oxidation studies where effective activation energies range from 170-210 kJ/mol [48][49][50][51][52][53][54][55][56][57][58][59]. ...
... An initial increase in surface area up to approximately 40% oxidation is observed, after which the surface area begins to decline. This phenomenon has been observed experimentally by Fuller and Okoh using nuclear-grade graphite [55]. Assuming a constant ratio of ASA to TSA, the oxidation rate per unit volume would locally mirror the change in TSA. ...
... A C C E P T E D ACCEPTED MANUSCRIPT 55 model is suggested to properly account for all three considerations. The model uses an intrinsic oxidation rate normalized to the reactive surface area, which permits the same rate equation to be used for all nuclear-grade graphites. ...
For the next generation of nuclear reactors, HTGRs specifically, an unlikely air ingress warrants inclusion in the license applications of many international regulators. Much research on oxidation rates of various graphite grades under a number of conditions has been undertaken to address such an event. However, consequences to the reactor result from the microstructural changes to the graphite rather than directly from oxidation. The microstructure is inherent to a graphite's properties and ultimately degradation to the graphite's performance must be determined to establish the safety of reactor design. To understand the oxidation induced microstructural change and its corresponding impact on performance, a thorough understanding of the reaction system is needed. This article provides a thorough review of the graphite-molecular oxygen reaction in terms of kinetics, mass and energy transport, and structural evolution: all three play a significant role in the observed rate of graphite oxidation. These provide the foundations of a microstructurally informed model for the graphite-molecular oxygen reaction system, a model kinetically independent of graphite grade, and capable of describing both the observed and local oxidation rates under a wide range of conditions applicable to air-ingress.
... Below 400 o C, graphite oxidation is negligible. The prevailing oxidation modes are categorized based on temperature as follows: [23][24][25][26] Mode ( ...
... A 10% decrease in graphite density could reduce its structural strength by up to 50%, depending on the type of graphite. 25,27,28 Owing to the low rates of reaction, oxidants concentration in this mode is uniform throughout the pores (Figs. 4a and 5a). ...
... The pores eventually break through, decreasing the internal surface area (Fig. 5), and reducing the corrosion rate. 25,26 This reduction in reactivity may be moderated by an increase in surface area through access to originally closed pores. ...
A massive air or steam ingress in High Temperature Reactors (HTRs) nominally operating at 600-950 o C is a design-basis accident requiring the development and validation of graphite oxidation and erosion models to examine the impact on the potential fission products release and the integrity of the graphite core and reflector blocks. Nuclear graphite is of many types with similarities but also differences in the microstructure, volume porosity, impurities, type and size of filler coke particles, graphitization and heat treatment temperatures, and the thermal and physical properties. These as well as the temperature, types and partial pressures of oxidants affects the prevailing oxidation mode and kinetics of the oxidation processes of graphite in HTRs. This paper reviews the graphite crystalline structure, the fabrication procedures, characteristics, chemical kinetics and modes of oxidation of nuclear graphite for future model developments.
... According to Eto and Growcock [4]'s research , the mechanical strength of nuclear graphite is significantly changed by the variation of graphite density, and as a result, approximately 10 percent density change may cause around 50 percent mechanical strength reduction depending on the materials. According to other graphite oxidation research (Fuller and Okho [5], Hino and Hishida [6], Hinsen et al. [7], Katcher and Moorman [8], Kim and NO [9], Moorman [10]), it takes around 7 or 8 days for all the graphite to be transformed into CO or CO 2 gas under 700 C oxidation condition, compared to 2 ~ 3 days over 900 o C. In reality, the graphite temperature can be increased up to 1500 o C in the accident conditions, and it means that the mechanical strength of the graphite can be seriously weakened causing the core collapse. ...
... In previous research (Fuller and Okho [5], Mills [11]), the mechanism of the time-dependent graphite oxidation is qualitatively well analyzed and explained. According to their results, the graphite oxidation occurs both on the external surface and in the internal pores. ...
... In this case, at the beginning, the oxidation increases the internal pore size, which increases the active surface area. However, as the oxidation proceeds, the enlarged pores are collapsed and the total active surface area starts to decrease at around 30~40 % burn-off (Fuller et al. [5]), Kim [12]) as shown in Figure 1. At high temperature (more than 1000 o C), the oxidation reaction simply reduces the size of the graphite, which continuously increases or decreases the active surface area. ...
... In prismatic core HTGR and VHTR, a partial weight loss due to in-pore gasification of the massive graphite support columns in the lower plenum could compromise their structural strength, suggesting a plausible collapse of the reactor core. A weight loss of 10% could reduce the mechanical strength of the nuclear graphite structure by about 50% (Fuller and Okoh, 1997;Kim et al., 2008). Furthermore, the air ingress into the helium coolant channels and subsequent weight loss of graphite in the reactor core region could expose the Tristructural-Isotropic (TRISO) coated-fuel particles and release the trapped radioactive fission products within the prismatic hexagonal fuel elements. ...
... Numerous experiments have been carried out with relatively small specimens of different grades of nuclear graphite to measure the total gasification rate and the transient weight loss at different flow conditions, compositions of the inlet gas, and specimen temperatures (Fuller and Okoh, 1997;Xiaowei et al., 2004;Chi and Kim, 2008;Hinssen et al., 2008;Kane et al., 2011;Contescu et al., 2012). Experimental results have shown that the orientation of the cutting plane of the relatively small specimens affects the oxidation reactivity (Xiaowei et al., 2005). ...
... Although easy to implement, the empirical approach is limited to the range of experimental measurements used to determine the apparent activation energies and the rate coefficients in the Arrhenius expressions of the total gasification rate. The applicability of these expressions to reactor safety analysis involving significantly massive graphite structures needs future verification (Figure 1) (Fuller and Okoh, 1997;Xiaowei et al., 2004;El-Genk and Tournier, 2011;El-Genk and Tournier, 2012a;El-Genk and Tournier, 2012b). Furthermore, the empirical approach exhibits discontinuities in the rate predictions at the transitions between the primary modes of gasification. ...
This paper provides chemical kinetics parameters for the gasification of nuclear graphite grades of IG-110, IG-430, NBG-18 and NBG-25 and presents empirical correlations for their surface areas of free active sites as a function of mass. The kinetics parameters for the four elementary chemical reactions of gasification of these grades of nuclear graphite include the values and Gaussian distributions of the specific activation energies and the values of the pre-exponential rate coefficients for the adsorption of oxygen and desorption of CO and CO 2 gases. The values of these parameters and the surface area of free active sites for IG-110 and NB-25, with fine and medium petroleum coke filler particles, are nearly the same, but slightly different from those of NBG-18 and IG-430, with medium and fine coal tar pitch coke filler particles. Recommended parameters are applicable to future safety analysis of high and very high temperature gas cooled reactors in the unlikely event of a massive air ingress accident.
... A conventional empirical approach has been adapted by many investigators for predicting the graphite gasification rate using Arrhenius relationships. The apparent activation energy and rate coefficient in these relationships are determined empirically from a linear best fit of the gasification rate measurements at low and intermediate temperatures [4][5][6][7][8]. At high temperatures, graphite gasification becomes progressively diffusion limited. ...
... This paper examines the effectiveness of a recently developed phenomenological oxidation kinetics model [15] to calculating the gasification rates of different types of nuclear graphite for wide ranges of temperatures and weight loss fractions. The model results are compared with the reported measurements by Chi and Kim [5] and Fuller and Okoh [4] for IG-110, IG-430 and NBG-25 graphite cylinders in atmospheric dry air. The Gaussian-like distributions of the specific activation energies for the adsorption of oxygen and desorption of CO, the initial surface area of free active sites and the effective kinetics rate coefficients for the primary oxidation reactions are obtained using a multi-parameter optimization algorithm from the reported transient measurements of the gasification rates and weight loss at different temperatures in the experiments [4,5]. ...
... The model results are compared with the reported measurements by Chi and Kim [5] and Fuller and Okoh [4] for IG-110, IG-430 and NBG-25 graphite cylinders in atmospheric dry air. The Gaussian-like distributions of the specific activation energies for the adsorption of oxygen and desorption of CO, the initial surface area of free active sites and the effective kinetics rate coefficients for the primary oxidation reactions are obtained using a multi-parameter optimization algorithm from the reported transient measurements of the gasification rates and weight loss at different temperatures in the experiments [4,5]. At high temperatures, when graphite gasification is diffusion limited, the model calculates the effective diffusion velocity of oxygen in the boundary layer using a semi-empirical correlation developed for air flows at Reynolds numbers ranging from 0.001 to 100. ...
A phenomenological oxidation kinetics model of graphite is presented and its results are compared with the reported experimental gasification data for nuclear graphite of IG-110, IG-430 and NBG-25. The model uses four elementary chemical kinetics reactions, employs Gaussian-like distributions of the specific activation energies for adsorption of oxygen and desorption of CO gas, and accounts for the changes in the effective surface areas of free active sites and stable oxide complexes with weight loss. The distributions of the specific activation energies for adsorption and desorption, the values of the pre-exponential rate
coefficients for the four elementary chemical reactions and the surface area of free active sites are obtained from the reported measurements using a multi-parameter optimization algorithm. At high temperatures, when gasification is diffusion limited, the model calculates the diffusion velocity of oxygen in the boundary layer using a semi-empirical correlation developed for air flows at Reynolds numbers ranging from 0.001 to 100. The model also accounts for the changes in the external surface area, the oxygen pressure in the bulk gas mixture and the effective diffusion coefficient in the boundary layer with weight loss. The model results of the total gasification rate and weight loss with time in the experiments agree well with the reported measurements for the three types of nuclear graphite investigated, at
temperatures from 723 to 1226 K and weight loss fractions up to ˜0.86.
... However, the time it takes to reach a specific weight loss depends on temperature, and hence the mode of oxidation ( Fig. 1), the oxygen partial pressure, and the type of graphite. Below 400 • C, graphite gasification is negligible, but at higher temperatures ( Fig. 1), it progressively proceeds through 3 basic modes (Walker et al., 1959;Fuller and Okoh, 1997;Xiaowei et al., 2004;El-Genk and Tournier, 2012), which depend on temperature: ...
... The resulting decrease in the solid density by gasification in this mode could eventually compromise the structural integrity of graphite structural components. A 10% decrease in graphite density could reduce the structural strength of nuclear graphite by up to 50% (Fuller and Okoh, 1997;Kim et al., 2008). In practice, since the oxygen partial pressure and weight loss in the volume pores decreases almost exponentially with distance into the graphite structure (Growcock et al., 1980;Hinssen et al., 2008), Mode (a) does not exist by itself, but in combination with the in-pores diffusion-limited gasification, Mode (b), which dominates at intermediate temperature. ...
... Note that the approach of Su and Perlmutter (1985) does not account for the opening of initially closed volume pores, and would not be applicable to volume porosities >60% and/or weight loss >40%, when graphite fragmentation likely occurs. Results of the NBG-18 nuclear graphite gasification experiments of Hinssen et al. (2008) and those of Fuller and Okoh (1997) for nuclear graphite grades of K018, K022 and IG-110 confirmed that the maximum ASA typically occurs at a weight loss of ∼35%. In the next section, the present chemical-reaction kinetics model, with the input parameters determined by the multi-parameter optimization algorithm, is applied to the gasification of the NBG-18 nuclear graphite specimens in the experiments of Chi and Kim (2008) and Hinssen et al. (2008) (Tables 1 and 2). ...
This paper introduces a chemical kinetics model and compares its calculations with reported measurements of weight loss and total gasification rate for different NBG-18 nuclear graphite specimens in experiments performed at 876–1226 K. Results show that the gasification rate is chemical-kinetics limited at low and intermediate temperatures and diffusion-limited at high temperatures. At high temperatures, the model calculates the diffusion velocity of oxygen through the boundary layer using a developed correlation for Reynolds numbers of 0.006–1000. The agreement of the calculations with reported measurements of the total gasification rate and transient weight loss confirms the soundness of the chemical kinetics approach and validates the developed model and the multi-parameter optimization algorithm for determining the chemical kinetics parameters, based on reported measurements. These parameters are the values and Gaussian-like distributions of the specific activation energies for oxygen adsorption and desorption of CO, the specific activation energy for desorption of CO2, the initial surface area of free active sites and the rate constants for the four elementary chemical reactions in the model. The performed parametric analyses for NBG-18 nuclear graphite specimens investigated the effects of temperature and oxygen partial pressure on total gasification rate and production rates of CO and CO2 gases, for wide ranges of temperatures and oxygen partial pressures.
... The F b factors of IG-110 and H451 were measured by the experimental setup and for NBG-10, NBG-18, and V483T, the data published in Fuller and Okoh (1997). Figures 8 and 9 show the plots of F b or surface area density versus burn-off. ...
... Figure 8-(a) shows the results for IG-110 graphite. This graph includes three datasets: Fuller and Okoh (1997), Kim et al. (2006), and (3) the experimental results obtained in this work. According to the figure, the data from Kim (2006) and this work show very good agreement, but the data from Fuller and Okoh (1997) shows some discrepancies from other data. ...
... This graph includes three datasets: Fuller and Okoh (1997), Kim et al. (2006), and (3) the experimental results obtained in this work. According to the figure, the data from Kim (2006) and this work show very good agreement, but the data from Fuller and Okoh (1997) shows some discrepancies from other data. The reason is not yet identified. ...
Graphite oxidation in an air-ingress accident is presently a very important issue for the reactor safety of the very high temperature gas cooled-reactor (VHTR), the concept of the next generation nuclear plant (NGNP) because of its potential problems such as mechanical degradation of the supporting graphite in the lower plenum of the VHTR might lead to core collapse if the countermeasure is taken carefully. The oxidation process of graphite has known to be affected by various factors, including temperature, pressure, oxygen concentration, types of graphite, graphite shape and size, flow distribution, etc. However, our recent study reveals that the internal pore characteristics play very important roles in the overall graphite oxidation rate. One of the main issues regarding graphite oxidation is the potential core collapse problem that may occur following the degradation of graphite mechanical strength. In analyzing this phenomenon, it is very important to understand the relationship between the degree of oxidization and strength degradation. In addition, the change of oxidation rate by graphite oxidation degree characterization by burn-off (ratio of the oxidized graphite density to the original density) should be quantified because graphite strength degradation is followed by graphite density decrease, which highly affects oxidation rates and patterns. Because the density change is proportional to the internal pore surface area, they should be quantified in advance. In order to understand the above issues, the following experiments were performed: (1)Experiment on the fracture of the oxidized graphite and validation of the previous correlations, (2) Experiment on the change of oxidation rate using graphite density and data collection, (3) Measure the BET surface area of the graphite. The experiments were performed using H451 (Great Lakes Carbon Corporation) and IG-110 (Toyo Tanso Co., Ltd) graphite. The reason for the use of those graphite materials is because their chemical and mechanical characteristics are well identified by the previous investigations, and therefore it was convenient for us to access the published data, and to apply and validate our new methodologies. This paper presents preliminary results of compressive strength vs. burn-off and surface area density vs. burn-off, which can be used for the nuclear graphite selection for the NGNP.
... The oxidation process of graphite is affected by various factors, including temperature, pressure, oxygen concentration, types of graphite, graphite shape and size, flow distribution, etc. The effects of these factors have been documented by a number of previous investigations , Fuller et al. 1997, Moorman 1984, and good models have been developed for estimating the graphite oxidation process in an air-ingress accident. ...
... In this report, the F b factor has been experimentally obtained as a function of burn-off for various forms of graphite: IG-110, H451, NBG-10, NBG-18, and V483T. The F b factors of IG-110 and H451 were measured by the experimental setup used in Section 4.2, and for NBG-10, NBG-18, and V483T, the data published in Fuller and Okoh (1997), Moorman et al. (1999), andHinssen et al. (2008) have been used. Figure 4-16 (a) shows the results for IG-110 graphite. ...
... Figure 4-16 (a) shows the results for IG-110 graphite. This graph includes three datasets: Fuller and Okoh (1997), Kim et al. (2006), and (3) the experimental results obtained in this work. According to the figure, the data from Kim et al. (2006) and this work show very good agreement, but the data from Fuller and Okoh (1997) shows some discrepancies from other data. ...
The US Department of Energy is performing research and development (R&D) that focuses on key phenomena that are important during challenging scenarios that may occur in the Next Generation Nuclear Plant (NGNP) Program / GEN-IV Very High Temperature Reactor (VHTR). Phenomena identification and ranking studies (PIRT) to date have identified the air ingress event, following on the heels of a VHTR depressurization, as very important (Schultz et al., 2006). Consequently, the development of advanced air ingress-related models and verification and validation (V&V) are very high priority for the NGNP program. Following a loss of coolant and system depressurization, air will enter the core through the break. Air ingress leads to oxidation of the in-core graphite structure and fuel. The oxidation will accelerate heat-up of the bottom reflector and the reactor core and will cause the release of fission products eventually. The potential collapse of the bottom reflector because of burn-off and the release of CO lead to serious safety problems. For estimation of the proper safety margin we need experimental data and tools, including accurate multi-dimensional thermal-hydraulic and reactor physics models, a burn-off model, and a fracture model. We also need to develop effective strategies to mitigate the effects of oxidation. The results from this research will provide crucial inputs to the INL NGNP/VHTR Methods R&D project. This project is focused on (a) analytical and experimental study of air ingress caused by density-driven, stratified, countercurrent flow, (b) advanced graphite oxidation experiments, (c) experimental study of burn-off in the bottom reflector, (d) structural tests of the burnt-off bottom reflector, (e) implementation of advanced models developed during the previous tasks into the GAMMA code, (f) full air ingress and oxidation mitigation analyses, (g) development of core neutronic models, (h) coupling of the core neutronic and thermal hydraulic models, and (i) verification and validation of the coupled models.
... This is consistent with results from the literature on the oxidation of graphite in air that revealed three oxidation regimes. 13,20,21,22 At low temperatures (Regime I), the oxidation rate is controlled by chemical reaction as oxygen diffuses into the bulk (uniform reaction throughout the bulk). At high temperatures (Regime III), the oxidation rate is controlled by diffusion of reactants and reaction products to and from the graphite surface since chemical reaction is very rapid (surface reaction dominant). ...
... Regime II corresponds to the intermediate temperature range where both mechanisms make significant contributions to the oxidation rate and the transition from one to the other occurs. Various studies have found the transition occurring anywhere from 500 to 900°C 10,20,21 ; however, the oxidant gas flow rate has also been suggested as a contributing factor governing the transition from reaction-controlled to diffusion-controlled oxidation. 13 Due to differing, nonstandard test conditions from one study to another, care should be taken when comparing this work to other studies done on oxidation rates of matrix graphite; however, recognizing the oxidation regimes in this work gives insight into the practical differences between RDKRS and ARB-B1 matrix graphite. ...
The oxidation behavior of matrix‐grade graphite in air‐ or steam‐ingress accident scenarios is of great interest for high‐temperature gas reactors (HTGRs). In this study, the microstructures of two variants of matrix‐grade graphite based on the German A3‐3 and A3‐27 formulations were characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy, and correlated to oxidation behavior observed through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Through TEM imaging and selected area electron diffraction (SAED), a higher volume fraction of partially graphitized carbon was identified in the A3‐3 type graphite than in the A3‐27 type. This structure is believed to have contributed to the accelerated oxidation exhibited by A3‐3 in the chemical reaction‐controlled oxidation regime.
... Author E a (kJ/mol) Chi and Kim [2] 159 Fuller and Okoh [11] 188 Contescu et al. [7] 201 Wang et al. [3] 205 Kim et al. [12] 218 Table 5 Mass loss rates, area-normalized and weight-normalized oxidation rates for IG-110 and NBG-18, average of two trials. Magnification was around 5000X. ...
... The oxidation rate of IG-110 in dry medical-grade air was also compared to recently reported oxidation data from Fuller and Okoh [11] and Chi and Kim [2]. Fig. 3 shows that the oxidation rates obtained in this study are in very good agreement with the available data for IG-110 for a large range of temperatures. ...
... In such an event, gasification within the volume pores of the support columns could compromise their mechanical strength and possibly result in a collapse of the reactor core. A weight loss of as little as 10% could reduce the mechanical strength of nuclear graphite by about 50% (Fuller and Okoh, 1997;Kim et al., 2008). The ingress of air into the reactor core flow channels and subsequent weight loss or erosion of nuclear graphite could expose coated fuel particles and release the fission products trapped in the graphite surrounding the particles. ...
... Below 673 K, graphite gasification is negligible, but, the rate increases exponentially with increased temperature. The three primary modes of graphite gasification with increasing temperature are shown in Fig. 1 (Fuller and Okoh, 1997;Xiaowei et al., 2004;El-Genk and Tournier, 2012;Ogawa, 1987) and discussed briefly next. ...
Gasification of nuclear graphite in the unlikely event of massive air ingress in High-Temperature and Very-High Temperature gas-cooled Reactors is a safety concern, requiring accurate and reliable predictions of the erosion rate of the external surface and within volume pores. At low temperature, gasification occurs within the open pores gradually degrading the mechanical strength of graphite components. Gasification shifts gradually to the external surface with increasing temperature. At high temperatures, although the rates of chemical reactions increase exponentially with temperature, they are limited by the oxygen diffusion to the external surface. A semi-empirical Sh correlation is developed to calculate the oxygen diffusion velocity. It is based on an extensive database of reported measurements of the convective heat transfer coefficient for heated wires and cylinders in air, water and paraffin oil flows at 0.006 ≤ Re ≤ 2.42 × 105 and 0.068 ≤ Pr ≤ 35.2 and the mass transfer coefficient at 4.8 ≤ Re ≤ 104 and Sc = 0.609 and 1300–2000. The database also includes reported values of the averaged Sh for gasification of a cylinder of V483T nuclear grade graphite (300 mm long and 200 mm in dia.) at 1141–1393 K in ascending cross-flow of nitrogen gas containing 5 vol.% oxygen at 533 ≤ Re ≤ 1660. The Sh correlation is within ±8% of the compiled 807 data points and applicable to both internal and external parallel and cross-flow conditions. When implemented in a chemical-reaction kinetics model, the calculated gasification rates are consistent with reported measurements for different size specimens of nuclear graphite grades NBG-18, NBG-25, IG-11, IG-110 and IG-430 at intermediate and high temperatures in atmospheric air (0.08 ≤ Re ≤ 30).
... Hinnsen et al. [20] 773-1173 Fuller and Okoh [22] 723-973 Blanchard [23] 873-1173 Luo et al. [9] 873-1073 Contescu et al. [24] 923-1023 ...
... Fig. 3 suggests that when NBG-18 has lost 25-55% of its initial mass with no gas switching, it experiences nearly identical mass loss rates at 1473 K and 1873 K, which can be viewed as the maximum rate obtainab le without the use of gas switching. Fuller and Okoh reported the maximum oxidation rate to occur at about 40% burnup [9,22] while Su and Perlmute r reported the maximum rate to occur at about 20-30% burnup [9,32]. The use of ''gas switching'' has ensured that the samples reach the target temperature gradually and do not lose any mass as the system is heated. ...
One of the most severe accident scenarios anticipated for VHTRs is an air ingress accident caused by a pipe break. Graphite oxidation could be severe under these conditions. In this work, the oxidation rate of NBG-18 nuclear-grade graphite was studied thermogravimetrically for different oxygen concentrations and with temperatures from 873 to 1873 K. A semi-empirical Arrhenius rate equation was developed for the temperature range of 873–1023 K. The activation energy of NBG-18 was 187 kJ/mol and the order of reaction was 1.25. The penetration depth of oxidant was about 3–4 mm for NBG-18 oxidized at 973 K. Increased porosity and changes in external geometry became more prominent at higher temperatures from about 1173 to 1873 K. The surface of oxidized NBG-18 was characterized by SEM, EDS, FTIR and XPS.
... The temperature boundaries of the two oxidation regimes are not well defined, and many other factors can affect the results. Discerning between effects of particular test conditions (sample geometry and size, air flow distribution, local temperature gradients, etc) and those related to true material properties (crystallite size and morphology, impurity content, etc) is not straightforward (Fuller, 1997;Jiang, 2000;Bhattacharya, 2003, Hahn, 2005. However, the influence of shape-and geometry-related factors that control diffusion can be better understood using data obtained from measurements on large size samples of graphite (Kim, 2006). ...
... The gasification rate of graphite increases varies with the burn-off degree, to a maximum at about 40-50 % burn-off, and then decreases; this was explained as an effect of development of additional porosity (Fuller, 1997) or of heterogeneity of oxidation sites (Hurt, 2005). Experience has shown that the most linear part of the weight vs. time curve is at weight losses ranging between 5-10 % of the original weight; this particular range was selected in the proposed ASTM for calculation and comparing of oxidation rates. ...
Oxidation behavior of graphite is of practical interest because of extended use of graphite materials in nuclear reactors. High temperature gas-cooled reactors are expected to become the nuclear reactors of the next generation. The most critical factor in their safe operation is an air-ingress accident, in which case the graphite materials in the moderator and reflector would come in contact with oxygen at a high temperature. Many results on graphite oxidation have been obtained from TGA measurements using commercial instruments, with sample sizes of a few hundred milligrams. They have demonstrated that graphite oxidation is in kinetic control regime at low temperatures, but becomes diffusion-limited at high temperatures. These effects are better understood from measurement results with large size samples, on which the shape and structural factors that control diffusion can be more clearly evidenced. An ASTM test for characterization of oxidation resistance of machined carbon and graphite materials is being developed with ORNL participation. The test recommends the use of large machined samples (~ 20 grams) in a dry air flow system. We will report on recent results and progress in this direction.
... The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. [22] 193-225 kJ/mol Zhou et al. [63] 220 kJ/mol Fuller et al. [64] 187.89 kJ/mol 2.30E + 10 1/s I.M. De Cachinho Cordeiro et al. ...
The oxidation of graphene-based material (i.e. graphite, graphene) is a reaction of immense importance owing to its extensive industrial application (i.e. nanocomposites, flame retardants, energy storage). Although immense experimental works were carried out for identifying the thermal degradation and oxidation process of graphene, they generally lack atomistic-level observation of the surface reactions, thermal formation pathways from solid to product volatiles and structural evolutions during oxidation. To analyse the favourable properties of graphene from its carbon-chain molecular structure viewpoint, it is essential to investigate graphene-based materials at an atomic level. This study bridges the missing knowledge by performing quantitative reactive forcefield coupled molecular dynamics simulation (MD-ReaxFF) to determine the oxidation kinetics of graphite under computational characterisation schemes with temperatures ranging from 4000 K to 6000 K. The kinetics parameters (i.e. activation energy) were extracted through proposed numerical characterisation methods and demonstrated good agreement with the thermogravimetric analysis experiments and other literature. Activation energy at 193.84 kJ/mol and 224.26 kJ/mol were extracted under the isothermal scheme by two distinct characterisation methods, achieving an average relative error of 11.3 % and 2.5 % compared to the experiment data, which is 218.60 kJ/mol. In comparison, the non-isothermal simulations yielded 214.53 kJ/mol, with a significant improvement on the average relative error of 1.86 %.
... Another possible approach for graphite could be large scale gasification; under optimum conditions, graphite is known to readily oxidise to the gas phase i.e. gasification, the topic of which has provided a wealth of literature [10][11][12]. These previous authors have monitored the gasification process by measuring the oxidation-induced mass loss of samples in situ, through thermal gravimetric analysis (TGA); however this method offers little insight into the gaseous species being evolved as a result of oxidation. ...
This study has investigated the laboratory scale thermal oxidation of nuclear graphite, as a proof-of-concept for the treatment and decommissioning of reactor cores on a larger industrial scale. If showed to be effective, this technology could have promising international significance with a considerable impact on the nuclear waste management problem currently facing many countries worldwide. The use of thermal treatment of such graphite waste is seen as advantageous since it will decouple the need for an operational Geological Disposal Facility (GDF). Particulate samples of Magnox Reactor Pile Grade-A (PGA) graphite, were oxidised in both air and 60% O2, over the temperature range 400–1200°C. Oxidation rates were found to increase with temperature, with a particular rise between 700–800°C, suggesting a change in oxidation mechanism. A second increase in oxidation rate was observed between 1000–1200°C and was found to correspond to a large increase in the CO/CO2 ratio, as confirmed through gas analysis. Increasing the oxidant flow rate gave a linear increase in oxidation rate, up to a certain point, and maximum rates of 23.3 and 69.6 mg / min for air and 60% O2 respectively were achieved at a flow of 250 ml / min and temperature of 1000°C. These promising results show that large-scale thermal treatment could be a potential option for the decommissioning of graphite cores, although the design of the plant would need careful consideration in order to achieve optimum efficiency and throughput.
... The kinetics of the thermal oxidation of carbon/carbon composite materials in an oxidizing atmosphere have been extensively studied [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] because the thermal stability/ resistibility of carbon materials is one of the most important factors in determining the potential applications of materials that effectively utilize their physical characteristics such as lightness, strength, and conductivities. Thermal oxidation of carbon is a complex solid-gas reaction because of the intrinsic heterogeneity of the reaction, as has been studied in detail for the oxidation reaction of graphite [18][19][20][21][22][23][24][25][26][27][28] and in many solid-state and solid-gas reactions [29][30][31][32]. In carbon/carbon composites, the situation is more complex because of the additional heterogeneity introduced by the compositional and structural factors of the composites [4,[6][7][8]. ...
Thermal oxidation of carbon/carbon composites in an oxidizing atmosphere is a multistep process regulated by the intrinsic heterogeneity of the solid–gas reaction, the additional heterogeneity of the compositional and structural characteristics of the composite, and how these two properties change as the reaction progresses. By focusing on the overlapping features of the component reaction steps, the kinetic characterization of the multistep kinetic process was studied to reveal the correlation between the thermal oxidation behavior and the compositional and structural characteristics of carbon/carbon composites. Using commercially available mechanical pencil leads as a typical model system for a carbon/carbon composite, the thermal behaviors of two different leads manufactured by different companies were investigated comparatively via thermoanalytical techniques and morphological observations. On the basis of a reaction model considering the different reactivities of the main (graphite) and secondary (carbonized polymer) carbon components, the kinetic features of two partially overlapping reaction steps were revealed via a kinetic deconvolution analysis of the thermoanalytical data for the thermal oxidation process. The kinetic results were correlated with the compositional and structural characteristics of carbon/carbon composites using morphological observations of the partially reacted samples. Herein, the practical usefulness of the kinetic analysis in characterizing carbon/carbon composites is discussed.
... For this a thorough understanding of the oxidation behaviour of the graphite is required, and this has previously been studied on virgin PGA graphite [25]. This study highlighted that PGA graphite exhibits the three regimes of thermal oxidation that have been observed on various other types of nuclear graphite [26][27][28]. The one of most relevance for this work was the low temperature (<600°C) chemical rate regime. ...
Pile Grade A graphite was used as a moderator and reflector material in the first generation of UK Magnox nuclear power reactors. As all of these reactors are now shut down there is a need to examine the concentration and distribution of long lived radioisotopes, such as ¹⁴C, to aid in understanding their behaviour in a geological disposal facility. A selection of irradiated graphite samples from Oldbury reactor one were examined where it was observed that Raman spectroscopy can distinguish between underlying graphite and a surface deposit found on exposed channel wall surfaces. The concentration of ¹⁴C in this deposit was examined by sequentially oxidising the graphite samples in air at low temperatures (450°C and 600°C) to remove the deposit and then the underlying graphite. The gases produced were captured in a series of bubbler solutions that were analysed using liquid scintillation counting. It was observed that the surface deposit was relatively enriched with ¹⁴C, with samples originating lower in the reactor exhibiting a higher concentration of ¹⁴C. Oxidation at 600°C showed that the remaining graphite material consisted of two fractions of ¹⁴C, a surface associated fraction and a graphite lattice associated fraction. The results presented correlate well with previous studies on irradiated graphite that suggest there are up to three fractions of ¹⁴C; a readily releasable fraction (corresponding to that removed by oxidation at 450°C in this study), a slowly releasable fraction (removed early at 600°C in this study), and an unreleasable fraction (removed later at 600°C in this study).
... Below 400 o C, graphite oxidation is negligible, but at higher temperatures, the rate increases exponentially with temperature. The three primary modes of graphite gasification with increasing temperature are ( Fig. 1): 1,3,7 Mode (a) (~ 400 -800 K) in which gasification is solely controlled by the kinetics of the elementary chemical reactions (Fig. 1), and its rate increases exponentially with temperature. In this mode, gasification occurs mostly within the open volume pores, thus does not change the outside dimensions. ...
The safety analysis of High-Temperature and Very High Temperature gas-cooled Reactors requires reliable estimates of nuclear graphite gasification as a function of temperature, among other parameters, in the unlikely event of an air ingress accident. Although the rates of prevailing chemical reactions increase exponentially with temperature, graphite gasification at high temperatures is limited by oxygen diffusion through the boundary layer. The effective diffusion velocity depends on the total flow rate and pressure of the bulk air-gas mixture. This paper developed a semi-empirical Sherwood number correlation for calculating the oxygen diffusion velocity. The correlation is based on a compiled database of the results of convective heat transfer experiments with wires and cylinders of different diameters in air, water and paraffin oil at 0.006 < Re < 1,604 and 0.068 < Sc < 35.2, and of mass transfer experiments at 4.8 < Re < 77 and 1,300 < Sc < 2,000. The developed correlation is within + 8% of the compiled database of 567 data points and consistent with reported gasification rate measurements at higher temperatures in experiments using different size specimens of nuclear graphite grades of NBG-18 and NB-25, IG-11, IG-110 and IG-430 in atmospheric air at 0.08 < Re < 30. Unlike the Graetz solution that gives a constant Sh of 3.66 at Re < 1.0, in the present correlation Sh decreases monotonically to much lower values with decreasing Re.
... It is therefore a good emitter and absorber of thermal radiation [6]. Although thermal emissivity is an intrinsic physical property of materials, it is affected significantly by the surface conditions [7][8][9][10]. However Systematic and quantitative data on the relationship between the thermal emissivity and surface morphology and roughness have not yet been reported in the literature [8,11,12]. ...
We study the relationships between the thermal emissivity of nuclear graphites (IG-110, PCEA, IG-430 and NBG-18) and their surface structural change by oxidation using scanning electron microscope and X-ray diffraction (XRD). The nonoxidized (0% weight loss) specimen had the surface covered with glassy materials and the 5% and 10% oxidized specimens, however, showed high roughness of the surface without glassy materials. During oxidation the binder materials were oxidized first and then graphitic filler particles were subsequently oxidized. The 002 interlayer spacings of the non-oxidized and the oxidized specimens were about . There was a slight change in crystallite size after oxidation compared to the nonoxidized specimens. It was difficult to find a relationship between the thermal emissivity and the structural parameters obtained from the XRD analysis.
... The experimental results for the gasification of nuclear graphite grades of IG-110, IG-430, NBG-18 and NBG-35 in atmospheric airflow, covering a wide range of temperatures and weight losses, successfully validated the determined values of the chemical kinetics parameters. When used in a developed chemical-reactions kinetics gasification model, the model results compared favorably IG-110, air [11] IG-110, air [12] NBG-25, air [12] IG-110, air [13] IG-110, He-O 2 (5.09 mol%) [14] IG-110, He-O 2 (20 mol%) [15] PGX, air [11] IG-430, air [12] NBG-18, air [12] NBG-10, air [16] PGX, air [16] AG13-01, TGA test, air [16] AG13-01, Vertical Furnace, air [16] PCEA, air [17] PGX, He-O 2 (1.37 mol%) [14] 9 10 11 12 13 14 15 16 Fuel matrix, air [16] A3-27 Binder, air [18] A3-27 Filler, air [18] A3-27 Fuel Matrix [19] (c) Fuel matrix and filler with the experimental measurements of the total gasification rate and transient weight loss [24][25][26]. The determined values of the chemical kinetics parameters depend on the graphite grade (e.g., origin of binder and filler particles; petroleum or coal pitch) and the filler particles size (super-fine, fine or medium)) [27]. ...
... High temperature oxidation of graphite and C/C has been subject to an extensive amount of experimental and theoretical investigations due to its practical importance in aerospace industry [14][15][16][17][18][19][20][21][22], nuclear energy industry [23,24] as well as in coal/char combustion industry [25]. Usually, the stagnation gas flow configuration is employed in the investigation. ...
The effects of surface temperature (1400–2000 °C), flow velocity gradient (130–600 s−1) and ambient pressure (5–101 kPa) on the oxidation behavior of graphite were studied in O2. The weight loss rate between 1400 and 1800 °C is independent of the surface temperature but slightly decreases at 2000 °C. Besides, it displays quasi-parabolic increases with increasing the flow velocity gradient and the ambient pressure but levels off above the ambient pressure of 20 kPa. According to the theoretical analysis, the enhancement of the gas-phase CO–O2 reaction has a joint reduction effect on the weight loss rate.
... The rapid development of high temperature gas-cooled reactor (HTGR) boosts the study of isotropic graphite. Due to its low neutron absorption cross-section, corrosion resistance, and good mechanical properties at high temperature, graphite is used as structural material and moderator to thermalize fast neutrons from the fission process [1,2]. But the environment of neutron irradiation in reactors during operation inevitably induces changes in dimension and physical properties to graphite, promoting stress and even cracks [2][3][4]. ...
As structural material and moderator in high temperature gas-cooled reactor (HTGR), nuclear graphite endures large flux of irradiation in its service time. The microstructure of nuclear graphite is a topical issue studied to predict the irradiation property of graphite and improve manufacturing process. In our present work, the pores in graphite are focused, and the relationship between pore and irradiation behavior is discussed. Three kinds of nuclear graphite (IG-11, NBG-18, and HSM-SC) are concerned, and their porosity, pore size, and morphology before and after irradiation are studied, respectively. A comparison between the three graphites shows that dense small pores which are uniformly distributed in graphite bring better irradiation property because the pores can accommodate some of the internal stress caused by irradiation expansion. Coke particles of small size and a thorough mixture between coke and binder are suggested to obtain such pores in nuclear graphite and thus improve irradiation property.
... Graphite is an important material of construction for the reactor core and the fuel pebbles. Knowledge of the high temperature oxidative behaviour of the graphite materials utilized in such reactors is important for design and accident modelling purposes [1,2]. The nuclear fuel is embedded in a graphite matrix to form the pebbles. ...
The oxidation, in a neat oxygen atmosphere, of high-purity and highly crystalline natural graphite and synthetic Kish graphite was investigated. The physico-geometric model function of the kinetic rate equation was experimentally determined by isothermal thermogravimetric analysis at 650 °C. Analytic solutions for basic flake shapes indicate that this function strictly decreases with conversion. However, for both samples the experimental data trend was a rapid initial increase followed by the expected decrease to zero. High resolution field emission scanning electron microscopy, of partially oxidized flakes, provided plausible explanations for this discrepancy. Rapid development of macroscopic surface roughness during the initial stages of oxidation was evident and could be attributed to the presence of catalytic impurities. Large fissures along the planes of the natural graphite and the initiation, growth and coalescence of internal cavities in the Kish graphite were observed. Flake models incorporating the latter two features are difficult to analyse analytically. However, a facile probabilistic approach showed that reasonably good agreement with experimental data was possible.
... Gas-solid reactions are the net result of two concurrent processes whose rates have a very different variation with temperature: The rate of chemical oxidation increases much faster with temperature than the effective diffusivity of the oxidant in the porous structure of graphite. This fact causes transition between regime 1 (kinetic control) and regime 2 (inpore diffusion control) of oxidation, usually reported as a change of slope in the Arrhenius representation of oxidation rates [3]. Interestingly, the Arrhenius plot of oxidation rates for the examples analyzed here was perfectly linear (Fig. 4) although the oxidation profile was clearly not uniform (Fig. 3). ...
... It is generally accepted that the dominant mechanism of graphite oxidation varies with the temperature [8]. Oxidation is controlled by the kinetics of the chemical process at low temperatures, but becomes diffusion-limited at high temperatures, and is strictly limited to mass transfer in the boundary layer at very high temperatures [9]. ...
This document reports on initial activities at ORNL aimed at quantitative characterization of porosity development in oxidized graphite specimens using automated image analysis (AIA) techniques. A series of cylindrical shape specimens were machined from nuclear-grade graphite (type PCEA, from GrafTech International). The specimens were oxidized in air to various levels of weight loss (between 5 and 20 %) and at three oxidation temperatures (between 600 and 750 oC). The procedure used for specimen preparation and oxidation was based on ASTM D-7542-09. Oxidized specimens were sectioned, resin-mounted and polished for optical microscopy examination. Mosaic pictures of rectangular stripes (25 mm x 0.4 mm) along a diameter of sectioned specimens were recorded. A commercial software (ImagePro) was evaluated for automated analysis of images. Because oxidized zones in graphite are less reflective in visible light than the pristine, unoxidized material, the microstructural changes induced by oxidation can easily be identified and analyzed. Oxidation at low temperatures contributes to development of numerous fine pores (< 100 m2) distributed more or less uniformly over a certain depth (5-6 mm) from the surface of graphite specimens, while causing no apparent external damage to the specimens. In contrast, oxidation at high temperatures causes dimensional changes and substantial surface damage within a narrow band (< 1 mm) near the exposed graphite surface, but leaves the interior of specimens with little or no changes in the pore structure. Based on these results it appears that weakening and degradation of mechanical properties of graphite materials produced by uniform oxidation at low temperatures is related to the massive development of fine pores in the oxidized zone. It was demonstrated that optical microscopy enhanced by AIA techniques allows accurate determination of oxidant penetration depth and of distribution of porosity in oxidized graphite materials.
This work compared the 3D Random Pore Model (3D-RPM) with experimental and characterization data to systematically study the effect of nuclear graphite microstructure and air oxidation temperature. It is well known that oxidation-induced weight loss at elevated temperatures degrades graphite structure and properties; however, a fundamental understanding of the role of graphite microstructure is still unclear. In this work, three diverse grades of nuclear graphite—IG-110, NBG-18, and PCEA—were examined and tested at air oxidation temperatures of 600, 650, 700, and 750 °C following ASTM D7542. The 3D-RPM reproduced the microstructure and temperature dependence of mass loss curves for these grades. For the first time, measurements and modeling were combined to show how bulk density and the amount and spatial distribution of open and closed porosity affects oxidation. IG-110 was less dense and had an open pore network that is finer and more uniformly spread, and showed fastest oxidation. PCEA porosity was less uniform and showed less oxidation, while NBG-18 was more dense, had the least fine and uniform porosity, and showed the slowest oxidation. In general, oxidation proceeds faster if open porosity is more uniformly distributed and can incrementally access closed pore surface area with more ease.
The oxidation of carbon-based material (i.e. graphite, graphene) is a reaction of immense importance owing to its extensive industrial application (i.e. nuclear reactors, aerospace composites). Carbon oxidation is also one of the most fundamentally essential heterogeneous reactions. Although comprehensive experimental studies on the oxidation of graphite have been conducted, one of the major limiting factors is the lack of atomistic-level observation on thermal formation pathways from solid-state to gas volatiles and material morphological study during oxidation. To bridge the knowledge gap, this study proposed a scale-bridging molecular dynamics (MD) simulation based on a reactive-force-field (ReaxFF) potential to elucidate the pyrolysis kinetics of graphite basal plane structure under hyperthermal oxygen conditions in a wide temperature range (i.e. 4000K-6000K). The pyrolysis kinetics parameters (i.e. activation energy) were extracted through numerical characterization and demonstrated good agreement with the thermogravimetric analysis experiments and pioneer literature. The species breakdown analysis from the ReaxFF simulation has also further indicated the feasibility of MD as an applicable approach to analyze the pyrolysis and chemical reaction mechanisms of graphite.
In a High-Temperature Gas-cooled Reactor (HTGR), radiation is the dominant form of heat transfer due to the high temperature environment. Therefore, the emissivity of the core materials (mainly nuclear grade graphite) is important for reactor safety assessment. In this paper, the emissivity of nuclear grade graphite IG-110 was measured in the temperature range from 500 °C to 1000 °C by using an infrared thermometer. Besides, the impact of the graphite oxidation, which may take place in a postulated air ingress accident, was also evaluated. As a result, it was found that the emissivity of IG-110 grade graphite decreases slightly as the temperature increase. Moreover, a relatively high emissivity was detected in the pre-oxidized specimen. Based on the measurement data, two experimental correlations were suggested for the engineering applications. It could also be concluded that the commonly used value of the graphite emissivity (0.8), is conservative for engineering judgment.
Gas chromatography and numerical methods were used to investigate the oxidation behavior of SNG742 nuclear grade graphite at 500–1100°C with different oxidizing gas flow rates. A four-step overall reaction mechanism was used to describe the graphite oxidation reaction process, which reasonably explains the peak of CO mole fraction at around 700°C with lower flow rates. The kinetic parameters of apparent oxidation reaction and four-step reaction mechanism of SNG742 graphite were obtained through curve fitting of the measured data. The apparent activation energy of SNG742 nuclear graphite is 200.7 ± 14.2 kJ/mol. The effects of temperature and gas velocity on the oxidation rate were analyzed. The graphite oxidation process was effectively simulated using CFD method with porous media model and the four-step reaction mechanism. The simulation results show that increasing the gas flow rate can reduce the thickness of the diffusion boundary layer and influence the diffusion control regime of graphite oxidation. The four-step overall reaction mechanism can better describe the graphite oxidation process than the one-step overall reaction. The porous medium model can effectively simulate the mass diffusion process and surface chemical reaction in the interior porous region of the graphite.
The thermal behaviors of carbon/carbon (C/C) composites in flowing air were investigated on the basis of mechanical pencil leads with different hardness values and diameter sizes as a model system. Two separated mass-loss processes were observed during heating the mechanical pencil leads in air, which are attributed to the evaporation/decomposition of an impregnation agent and the subsequent thermal oxidation of the residual C/C composite. The thermal behaviors were invariant among the mechanical pencil leads with different diameter sizes, but they systematically changed with hardness. Variations in the thermal behaviors can be quantified by the mass-loss value during the evaporation/decomposition of the impregnation agent, in addition to the kinetic deconvolution analysis that was applied to the multistep thermal oxidation process of carbon components with different reactivities. These results correlate the thermal behavior with the compositional and structural characteristics of C/C composites, which can be useful for characterization and product control.
Graphite is widely used as moderator, reflector and structural materials in the high temperature gas-cooled reactor pebble-bed modular (HTR-PM). In normal operating conditions or water/air ingress accident, the nuclear graphite in the reactor may be oxidized by air or steam. Oxidation behavior of nuclear graphite IG-110 which is used as the structural materials and reflector of HTR-PM is mainly researched in this paper. To investigate the penetration depth of oxygen in IG-110, this paper developed the one dimensional spherical oxidation model. In the oxidation model, the equations considered graphite porosity variation with the graphite weight loss. The effect of weight loss on the effective diffusion coefficient and the oxidation rate was also considered in this model. Based on this theoretical model, this paper obtained the relative concentration and local weight loss ratio profile in graphite. In addition, the local effective diffusion coefficient and oxidation rate in the graphite were also investigated.
The effects of temperature on the oxidation behavior of the A3-3 matrix graphite (MG) in the temperature range 798-973 K in air with a flow rate of 100 ml/min to burn-offs of 10-15 wt%, were investigated by a home-made thermo-gravimetric experimental setup. The oxidation rate (OR) increases significantly with the temperature. The OR at 973 K is over 70 times faster than at 798 K. The oxidation kinetics of A3-3 MG in air at temperatures up to 973 K is in the reaction control regime, where the activation energy is 176 kJ/mol and the Arrhenius equation could be described as: OR=2.9673×108·exp(-21124.8/T) wt%/min. The relatively lower activation energy of MG than that of structural nuclear graphite indicates that MG is more easily oxidized.
Pile grade A (PGA) graphite was used as a material for moderating and reflecting neutrons in the UK's first generation Magnox nuclear power reactors. As all but one of these reactors are now shut down there is a need to understand the residual state of the material prior to decommissioning of the cores, in particular the location and concentration of key radio-contaminants such as 14C. The oxidation behaviour of unirradiated PGA graphite was studied, in the temperature range 600-1050°C, in air and nitrogen using thermogravimetric analysis, scanning electron microscopy and X-ray tomography to investigate the possibility of using thermal degradation techniques to examine 14C distribution within irradiated material. The thermal decomposition of PGA graphite was observed to follow the three oxidation regimes historically identified by previous workers with limited, uniform oxidation at temperatures below 600°C and substantial, external oxidation at higher temperatures. This work demonstrates that the different oxidation regimes of PGA graphite could be developed into a methodology to characterise the distribution and concentration of 14C in irradiated graphite by thermal treatment.
This study consists of three main parts. The first part characterizes IG- and NBG-grade nuclear graphites (IG-110, IG-430, NBG-18, and NBG-17) in terms of the size and shape of filler particles and how the forming method affects the pore distribution. The second part presents an experimental investigation of nuclear graphite oxidation at temperatures ranging from 700 to 1100 degrees C in air and correlates this with the theory of active sites on graphite. Mercury porosimetry is used to quantify the pore structure development at various temperatures. X-ray diffraction analysis of selected graphites is conducted to determine the crystallographic parameters. Results of mercury porosimetry and scanning electron microscopy images are correlated with the theory of active sites on graphite in order to demonstrate the relationship between pore distribution and active sites. The third part of the study presents two experiments. The first experiment considers the effects of size of samples with the same aspect ratio and the other considers actual-sized fuel pellets and graphite sleeves to evaluate the degradation of graphite components in an air-ingress scenario.
In the investigative study of the Isotropic Graphite IG-110 of Toyo Tanso, the shore hardness test, compressive test, bending test and fracture toughness test were conducted. The compressive test, the bending test and the fracture toughness test were performed by MTS-810. Test velocity of the compressive and the bending test were 0.5 mm/min and the fracture toughness test were 0.1 mm/min. The results were compared with manufacturer data of Toyo Tanso. Through irradiation test using HANARO research reactor, hardness and strength of nuclear graphite IG-110 was examined. Strength and hardness of irradiated steel is higher than non-irradiated, but nuclear graphite IG-110 declined unlike steel. To search for characteristic change of nuclear graphite IG-110 under the amounts of neutron, a repeat of the experiment was conducted.
In the high temperature gas-cooled reactor, the oxidation of graphite is inevitable as a result of impurities in helium coolant. As an air or water ingress accident would cause graphite components to oxidize more seriously, thereby it would affect the reactor normal operation and safety. Oxidation velocity and oxidation product of a selected graphite (excess material from 10MW High Temperature Gas-cooled Reactor) were studied, samples are oxidized between 400°C to 1200 °C with gas flow rates ranging from 125 to 500 ml/min. Relationship between oxidation conditions and surface properties of oxidized graphite is also elaborated by means of gas chromatography and scanning electron microscopy for scanning of graphite surface.
Pyrolytic carbon (PyC) coating is deposited on IG-110 nuclear graphite to protect it against the impregnation of molten Flinak salt. Raman spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and X-ray photoelectron spectroscopy (XPS) are used to investigate the effect of defects induced by 12C+ ion irradiation on the fluorination of PyC coating in Flinak salt. Results show evidence for the formation of CF bond. And the defects induced by ion irradiation facilitate the fluorination of PyC coating in Flinak salt.
Graphite is suitable materials as a moderator, reflector, and supporter of a nuclear reactor because of high tolerance to the high temperature and neutron irradiations. Because graphite is so weak to the oxidation, its oxidation study is essentially demanded for the operation and design of the nuclear reactor. This work focuses on the effect of the surface oxidation of graphite according to the surface treatment. With thermogravimeter (TG), oxidation characteristics of the isotropic graphite are measured at the three temperature areas, and oxidation ratio and amounts are estimated as changing the surface roughness. Furthermore, the polished graphite surface produced fom the surface treatment is investigated with the Raman spectroscopic study. Oxidation behaviors of the surface are also evaluated as elimination the polished layer by washing with strong sonication.
Graphite has hexagonal closed packing structure with two bonding characteristics of van der Waals bonding between the carbon layers at c axis, and covalent bonding in the carbon layer at a and b axis. Graphite has high tolerant to the extreme conditions of high temperature and neutron irradiations rather than any other materials of metals and ceramics. However, carbon elements easily react with oxygen at as low as 400C. Considering the increasing production of today of hydrogen and electricity with a nuclear reactor, study of oxidation characteristics of graphite is very important, and essential for the life evaluation and design of the nuclear reactor. Since the oxidation behaviors of graphite are dependent on the shapes of testing specimen, critical care is required for evaluation of nuclear reactor graphite materials. In this work, oxidation rate and amounts of the isotropic graphite (IG-110, Toyo Carbon), currently being used for the Koran nuclear reactor, are investigated at various temperature. Oxidation process or principle of graphite was figured out by measuring the oxidation rate, and relation between oxidation rate and sample shape are understood. In the oxidation process, shape effect of volume, surface area, and surface to volume ratio are investigated at , based on the sample of ASTM C 1179-91.
Graphite is suitable for high temperature structural materials because of chemical stability as well as unique crystal structure. Especially, graphite can be used as a part of a nuclear reactor due to high tolerance at the extreme conditions of high temperature and neutron irradiations. Although study of oxidation properties or behaviors of graphite are very important and essential for the life and stability of the nuclear reactor, most of studies treat this theme lightly. This work focuses on the oxidation characteristics of several grade isotropic graphite of the nuclear reactor.
This paper presents a transient multicomponent mixture analysis tool developed to analyze the molecular diffusion, natural convection, and chemical reactions related to air ingress phenomena that occur during a primary-pipe rupture of a high temperature gas-cooled reactor (HIGR). The present analysis tool solves the one-dimensional basic equations for continuity, momentum, energy of the gas mixture, and the mass of each gas species. In order to obtain numerically stable and fast computations, the implicit continuous Eulerian scheme is adopted to solve the governing equations in a strongly coupled manner. Two types of benchmark calculations were performed with the data of prerious Japanese inverse U-tube experiments. The analysis program, based on the ICE technique, runs about 36 times faster than the FLUENT6 for the simulation of the two experiments. The calculation results are within a 10% deviation from the experimental data regarding the concentrations of the gas species and the onset times of natural convection.
The CO2 corrosion behavior of IG-110 nuclear graphite has been investigated using the gas chromatography method which allows the continuous analysis of the CO2/CO gas mixture at the outlet of the corrosion chamber. The effects of temperature and initial CO2 concentration are studied based on the Arrhenius-type reaction model. From 745 to 995 °C, the Arrhenius curve shows a linear behavior. For higher temperatures, a non-linear behavior is observed. The activation energy is calculated as 210 kJ/mole and is independent of the initial CO2 inlet concentrations of 10%, 14% and 17%. The corrosion behavior at 1145 °C, in the diffusion-controlled regime, has also been investigated. At this temperature, the interior of IG-110 graphite is severely attacked by CO2, and the material's surface morphology is changed drastically. A measurement of the corrosion rate against corrosion time shows that the corrosion rate initially increases to a maximum value at a weight loss degree of 30%–35%, after which it begins to decline.
Pyrolytic carbon (PyC) coatings were deposited on IG-110 nuclear
graphite by thermal decomposition of methane at ˜1830 °C. The
PyC coatings are anisotropic and airtight enough to protect IG-110
nuclear graphite against the permeation of molten fluoride salts and the
diffusion of gases. The investigations indicate that the sealing nuclear
graphite with PyC coating is a promising method for its application in
Molten Salt Reactor (MSR).
The chemical compatibility aspects of CVD β-SiC and SiCf/SiC composites with a VHTR specific helium coolant were examined. The specimens were exposed to helium gas containing 20 Pa H2, 5 Pa CO, 2 Pa CH4, and 0.02–0.1 Pa H2O, which is an expected VHTR coolant chemistry. Oxidation tests were carried out at 900 and 950 °C for up to 250 h. β-SiC and SiCf/SiC composites had an excellent compatibility with the expected VHTR helium coolant environment. The oxidation of β-SiC as a matrix material of the SiCf/SiC composite reacted in a passive oxidation regime owing to the presence of water vapor. A condensed version of the oxide SiO2 formed at an early stage of oxidation and the growth of this oxide layer was very limited as the oxidation time increased up to 250 h. The recession of the pyrolytic carbon interphase of SiCf/SiC composite could not be observed in the test range.
An intrinsic mathematical model is developed for the investigation of the gas–solid reaction kinetics of high-purity graphite and oxygen. This model is based upon the oxygen transfer mechanism and uses physically meaningful parameters that are directly comparable to the experimental and theoretical literature of the carbon–oxygen reaction system. The model was used to extract reaction parameters for NBG-18 polycrystalline graphite for oxygen/nitrogen mixtures with a total pressure of 100 kPa. Experimental temperatures ranged from 500 to 850 °C for oxygen partial pressures of 1, 5, 10, 20, and 40 kPa. The optimized model parameters are in good agreement with previously reported literature values.
A number of experiments was carried out to investigate the effect of moisture, always present in environmental air, on the graphite oxidation rate. A porous metal with 10 µ m pores was used to enhance the humidification at the outlet of the vertical column that is full of water and is designed to increase the moisture on the helium gas when it is passed through the porous media located at the bottom of the water column. The relative humidity of mixture was controlled between 0 % and 70 % by a humidity sensor. The experiment was performed at temperatures ranging from 873K through 1573 K, mole fractions of oxygen from 0.09 to 0.17, and relative humidity between 0 % through 70 % at the normal condition. Assuming that the effect of moisture only affects the mass transfer, we derived a theoretical model for mass transfer that included the fast homogeneous CO combustion reaction. The present model showed that the mass transfer rate of humid air is a half of the mass transfer rate for dry air. The predictions by the model agree with experimental data within 17%.
We have talked about the large variability of coal as a precursor for carbons as we go from anthracite to bituminous to lignite. We can produce carbons of large variability in properties, like microporosity, by pretreating the coal and changing the nature of the precursor with which we start. We can produce a wide variety of carbons which, today, have important commercial applications. For tomorrow, there is great promise of producing new carbons of still greater commercial applications.
The catalytic activity of zinc in the gasification of single crystals of graphite has been studied by a controlled atmosphere electron microscopy technique. Continuous observation showed that catalyst particles are only active when in contact with steps on the cleavage surface of the graphite crystal and cut channels into the steps. Quantitative estimates of channel propagation rates as a function of oxygen pressure and temperature have been obtained. It is suggested that the catalyst composition is close to ZnO.
The fuel elements for the MHTGR are manufactured from nucleargrade graphite and have core-residence times of about 1000 days that are determined from considerations based on nuclear-design criteria. To ensure structural integrity and minimize the risk to plant investment, a potential limitation of fuel-element life may result from graphite corrosion in the high-temperature regions of the core (T ≅ 1100 °C). This corrosion is caused primarily by reactions of graphite with the low concentrations of steam (0.01–0.1 ppm) that are normally present in the helium coolant Economical operation of the reactor requires that the steam concentrations be maintained at such low levels that corrosion rates will not impact the normal fuel cycle. In this paper, we develop a graphite-corrosion model and address this important practical problem.
A description of an apparatus for studying heterogeneous gas-solid reactions in the one to one hundred μ pressure and 600 to 1300° temperature range is presented. The data for the graphite-oxygen reaction from 600 to 800° are presented. The surface reaction is zero order with an 80 kcal. per mole activation energy. On samples thicker than 0.1 mm., the diffusion of oxygen into the pores in the graphite results in an observed one-half order reaction with a 42 kcal. per mole activation energy. Carbon monoxide is shown to be the primary reaction product.
The effects of catalysts on the kinetics of the thermal reaction of graphite with air are described. Catalysts are impurities initially present in natural or artificial graphites, which accumulate on the surface during burn off, and impurities deliberately introduced to obtain positive (K, Na) or negative (P2O5) catalysis. Catalysts affect the reaction rate, the ratio, the activation energy and the pre-exponential factor of the Arrhenius equation, and correlations have been found between these kinetic parameters which, on the other hand, have practically the same values for the purest graphites. The results are explained by the coexistence of two independent reactions, uncatalyzed and catalyzed, occuring respectively on the clean and the contaminated fractions of the surface.
Experiments on the oxidation of short tubes of graphite in air (1–4) have been extended to long channels of graphite where the oxygen is depleted as it flows through the channel. The theory of reaction in porous solids which was developed for the earlier work has been extended to cover this more general case. It is shown that a useful approximation to the general theory can be obtained by considering the channel as a series of short sections and introducing the axial concentration effects via the problem boundary conditions. The theory was tested by experiments carried out in a vertical channel of graphite 14 ft long, 2 in. i.d. and 8 in. o.d. constructed from six blocks of British nuclear grade graphite. The inner graphite surface was exposed to mixtures which flowed through the bore of the tube. Evidence of the radial diffusional effect was obtained from gas concentration measurements at the outer surface. The oxygen was depleted by chemical reaction as it flowed through the channel bore giving rise to an oxygen concentration profile which was experimentally measured. No corresponding axial concentration gradient at the exterior surface was observed. The derived theory allowed the diffusional effects to be quantitatively estimated. Good agreement between the present results and previously reported values for similar graphites was obtained.
Graphon, a highly graphitized carbon black, was first oxidized to eight levels of burn-off between 0 and 35% to introduce varying amounts of active surface area. Following the cleaning of the activated samples by heating at 950°C in a vacuum of 10−5 torr, chemisorption of oxygen between 300–625°C was studied. At a low O2 pressure, i.e. 0.5 torr, the saturation amount of oxygen adsorbed sharply increased at temperatures above 400°C, suggesting the presence of at least two types of active sites. The kinetics of the reactivity of O2 with the Graphon samples was studied between 450–675°C. From the kinetics, the number of the more active sites could be determined. For the Graphon sample of maximum burn-off, the area occupied by these sites is 3.5 m2/g or about 3% of the total surface. This value agrees closely with that obtained from low pressure O2 chemisorption results at 300°C. The rate of oxygen interaction on the more active sites (I) is given by ko2(I)po2, TAS(I)[1-θ (I)]2, where ko2(I) = 1.6 × 10−5 exp (−29,000/RT) cm sec−1.
The German graphite development programme for High Temperature Reactors has been based on the assumption that reactor graphite for core components with lifetime fluences of up to 4 × 1022 neutrons per cm2 (EDN) at 400°C can be manufactured from regular pitch coke. The use of secondary coke and vibrational moulding techniques have allowed production of materials with very small anisotropy, high strength, and high purity which are the most important properties of reactor graphite. A variety of graphite grades has been tested in fast neutron irradiation experiments. The results show that suitable graphites for modern High Temperature Reactors with spherical fuel elements are available.