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Radiation resistance of elastomeric O-rings in mixed neutron and gamma fields: Testing methodology and experimental results
Review of Scientific Instruments
Abstract
Materials and components employed in the presence of intense neutron and gamma fields are expected to absorb high dose levels that may induce deep modifications of their physical and mechanical properties, possibly causing loss of their function. A protocol for irradiating elastomeric materials in reactor mixed neutron and gamma fields and for testing the evolution of their main mechanical and physical properties with absorbed dose has been developed. Four elastomeric compounds used for vacuum O-rings, one fluoroelastomer polymer (FPM) based and three ethylene propylene diene monomer rubber (EPDM) based, presently available on the market have been selected for the test. One EPDM is rated as radiation resistant in gamma fields, while the other elastomers are general purpose products. Particular care has been devoted to dosimetry calculations, since absorbed dose in neutron fields, unlike pure gamma fields, is strongly dependent on the material composition and, in particular, on the hydrogen content. The products have been tested up to about 2 MGy absorbed dose. The FPM based elastomer, in spite of its lower dose absorption in fast neutron fields, features the largest variations of properties, with a dramatic increase in stiffness and brittleness. Out of the three EPDM based compounds, one shows large and rapid changes in the main mechanical properties, whereas the other two feature more stable behaviors. The performance of the EPDM rated as radiation resistant in pure gamma fields does not appear significantly better than that of the standard product. The predictive capability of the accelerated irradiation tests performed as well as the applicable concepts of threshold of radiation damage is discussed in view of the use of the examined products in the selective production of exotic species facility, now under construction at the Legnaro National Laboratories of the Italian Istituto Nazionale di Fisica Nucleare. It results that a careful account of dose rate effects and oxygen penetration in the material, both during test irradiations and in operating conditions, is needed to obtain reliable predictions.
... Elastomeric O-rings are widely used at CERN in a variety of applications, including but not limited to high radiation areas. EPDM-based elastomers are generally known to be more radiation resistant than other elastomeric materials [5], however large differences in the radiation tolerance of different commercial EPDM-based materials were observed [17]. Accordingly, it is not possible to assume that all EPDM-based materials exhibit the same resistance to radiation, and specific commercial products should be irradiated and tested. ...
... Both James Walker and Angst+Pfister EPDMs tested in this study largely exceed these end-points at the minimum investigated gamma dose of 2 MGy, suggesting that the most important degradation phenomena occur at lower doses. Figure 8 shows a comparison between three commercial EPDM O-rings including James Walker Shield-seal663®(referred to as EPDM 3 in the Graph), irradiated in the frame of a previous study performed in neutron and gamma mixed field radiation [17]. In this study, the neutron component of the dose corresponded to about 65% of the total, the rest being deposited by gamma radiation, at a very high total dose rate of approximately 0.7 MGy/h. ...
... In the graph, EPDM 3 corresponds to James Walker Shieldseal663®. Data extracted and elaborated from[17],[18]. ...
Irradiation tests are continuously needed at the European Organization for Nuclear Research (CERN), to experimentally assess the radiation tolerance of commercial non-metallic materials for use in high-radiation areas. The ’Radiation to Materials’ R2M activity organizes such tests and provides expertise in the interpretation of results, for the design and upgrade of devices absorbing critical doses during their lifetime. The various activities covered by the R2M are presented. Up to ten irradiation tests per year are organized in external
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Co facilities, in which samples absorb up to several MGy of gamma dose at dose rates in the kGy/h range. Cables, O-rings, lubricants, glues, optical materials, and insulators are examples of possible tested materials. In parallel, selected samples are irradiated in a mixed neutron and gamma irradiation station built close to the Neutron Time-Of-Flight facility. Dedicated research studies, mainly focused on oil, greases and elastomeric O-rings, are being performed to deepen the understanding of radiation effects in these critical materials. As an example, the mechanical and structural characterization of two elastomeric EPDM-based commercial materials irradiated in gamma radiation up to 3 MGy of dose is presented.
... Four commercially available elastomeric materials were selected for irradiation and testing. Three of them are ethylene-propylene-diene monomer (EPDM)-based O-rings, while one of them is a fluoropolymer (FPM)-based one [4]. Nine lubricating greases with different chemical compositions and one lubricating oil were selected [5,6]. ...
... For this reason, the light element composition of each product was measured using CHN (carbon, hydrogen, nitrogen) analysis. The measured composition was used in the simulations to calculate the neutron and gamma dose rate for each material [4,5]. ...
... Doses were delivered with very high dose rates corresponding to about 700 kGy/h and 340 kGy/h for EPDMs and FPM, respectively. Differences are due to the different hydrogen amount, about 8% for EPDMs and about 1% for FPM in mass [4]. All the irradiation were performed in air at atmospheric pressure in containers allowing a partial recirculation of air. ...
Lubricants and O-rings are necessarily used for the construction of many accelerator-driven facilities as spallation sources or facilities for the production of radioactive isotopes. During operation, such component will absorb high doses of mixed neutron and gamma radiation, that can degrade their mechanical and structural properties. Experimental radiation damage tests of these components are mandatory for the construction of the facility. Methodologies for irradiation in nuclear reactor mixed fields and post-irradiation examination of lubricating oils, greases and O-rings were developed and are here presented. Samples were characterized with standard mechanical and physical-chemical tests. Parametric studies on the dose rate effects have been performed on O-rings. A case studies for a specific O-ring application in a gate valve has been developed. Some of the tested samples showed a dramatic change of their properties with dose, while others remain stable. Results were collected on nine commercial greases, on one oil and on four commercial elastomeric O-rings. The most radiation resistant among the selected products are now considered for application in facilities under construction. The main mechanisms of neutron and gamma radiation damage on these polymers were investigated at the mechanical and structural level.
... In order to gather information on the resistance of polymeric Orings in such extreme conditions, in a previous work, two rubbers, namely ethylene propylene diene monomer (EPDM) rubber and a fluoroelastomer (FPM), were irradiated in reactor mixed neutron and gamma fields to investigate the effect of the adsorbed dose on their tensile properties and compression set values [11]. EPDM O-rings were then studied to assess their sealing performance both during the facility service time and the postservice storage phase [12]. ...
... Finally, elemental analyses were carried out to determine the content of the various elements, which was necessary to perform dosimetry calculations [11].The measured elemental composition of EPDM and FPM is reported in Table 1. ...
... Table 2 reports the dose rates of EPDM and FPM in the CT facility for the reactor operating at the nominal power (250 kW). Dose components and dose rates were calculated taking into account the elemental composition of materials and the irradiation conditions, as previously described in Ref. 11. Using the same methodology, dose rates for the two materials were calculated in the LS facility. ...
Elastomers are widely used in radioactive environments, where ionizing radiations induce a deterioration of their properties due to degradative phenomena occurring in the polymer structure. Their radiation resistance is usually assessed using γ‐rays and relatively low dose rates, but in actual applications, they are often exposed to mixed radiation fields and higher dose rates. Ethylene propylene diene monomer (EPDM) is known for its excellent resistance to γ‐rays but absorbs a larger dose by neutron interactions than fluoroelastomer (FPM). In this work, EPDM and FPM were irradiated in mixed neutron and gamma fields, using high dose rates (from 22 to 700 kGy h−1) and total absorbed doses between 0.2 and 3.5 MGy. The effects of irradiation were assessed by swelling tests, differential scanning calorimetry analysis and dynamic mechanical thermal analysis, and tensile tests. The results show that, even if irradiations were carried out in air, degradation took place under nonoxidative conditions owing to the high dose rates employed. Under such conditions, crosslinking is the dominant radiation‐induced reaction in both elastomers. Moreover, material degradation seems to be influenced mostly by the total absorbed dose and not by the type of radiation. Contrary to what observed at the lower dose rates employed with γ‐rays, major dose rate effects are not observed. POLYM. ENG. SCI., 2019.
... The remaining 15% is represented by gamma dose, mainly due to fission prompt gammas. More details about dosimetry can be found in [5]. The TIS life cycle includes a 15-day long post-service cooling time before removal from the facility to reduce the residual activity. ...
... Neutron damage studies on polymeric materials are presently very scarce in the literature. We developed protocols for irradiation and testing of elastomeric Orings [5] and lubricating greases [7]. They aim at the selection of radiation-resistant products to be used for the SPES construction. ...
... Two main components of the neutron energy spectrum are present: a fast component and a thermal component which originates from the moderation of fast neutrons due to the materials present in the reactor core. More details can be found in [5]. Fast neutrons are particularly effective in delivering dose to polymeric materials. ...
... This allows outer containers to be reused after decontamination, following a cooldown phase. (iii) Radiation tolerance: elastomeric seals should generally be avoided, as even the most radiation tolerant ones are expected to degrade at doses in the MGy range [19,45] (see Sec. V). A full-metal container body and graphite seals are, therefore, used to ensure leak tightness. ...
A new parasitic, mixed-field, neutron-dominated irradiation station has been recently commissioned at the European Laboratory for Particle Physics (CERN). The station is installed within the neutron time-of-flight (n_TOF) facility, taking advantage of the secondary radiation produced by the neutron spallation target, with neutrons ranging from 0.025 eV to several hundreds of MeV. The new station allows radiation damage studies to be performed in irradiation conditions that are closer to the ones encountered during the operation of particle accelerators; the irradiation tests carried out in the station will be complementary to the standard tests on materials, usually performed with gamma sources. Samples will be exposed to neutron-dominated doses in the MGy range per year, with minimal impact on the n_TOF facility operation. The station has 24 irradiation positions, each hosting up to 100 cm3 of sample material. In view of its proximity to the n_TOF target, inside protective shielding, the irradiation station and its operating procedures have been carefully developed taking into account the safety of personnel and to avoid any unwanted impact on the operation of the n_TOF facility and experiments. Due to the residual radioactivity of the whole area around the n_TOF target and of the irradiated samples, access to the irradiation station is forbidden to human operators even when the n_TOF facility is not in operation. Robots are used for the remote installation and retrieval of the samples, and other optimizations of the handling procedures were developed in compliance with radiation protection regulations and the aim of minimizing doses to personnel. The sample containers were designed to be radiation tolerant, compatible with remote handling, and subject to detailed risk analysis and testing during their development. The whole life cycle of the irradiated materials, including their post-irradiation examinations and final disposal, was considered and optimized.
... • Radiation tolerance; elastomeric seals should generally be avoided, as even the most radiation tolerant ones are expected to degrade at doses in the MGy range [20,42] (see Section 5). A full-metal container body and graphite seals are therefore used to ensure leak-tightness. ...
A new parasitic, mixed-field, neutron-dominated irradiation station has been recently commissioned at the European Laboratory for Particle Physics (CERN). The station is installed in the Neutron Time-Of-Flight (n\_TOF) facility, taking advantage of the secondary radiation produced by the neutron spallation target. The new station allows radiation damage studies to be performed in irradiation conditions that are closer to the ones encountered during the operation of particle accelerators; the irradiation tests carried out in the station will be complementary to the standard tests on materials, usually performed with gamma sources. Samples will be exposed to neutron doses in the MGy range per year, with minimal impact on the n TOF facility operation. The station has twenty-four irradiation positions, each hosting up to 100 cm3 of sample material. In view of its proximity to the n\_TOF target, inside protective shielding, the irradiation station and its operating procedures have been carefully developed taking into account the safety of personnel and to avoid any unwanted impact on the operation of the n\_TOF facility and experiments. Due to the residual radioactivity of the whole area around the n\_TOF target and of the irradiated samples, access to the irradiation station is forbidden to human operators even when the n\_TOF facility is not in operation. Robots are used for the remote installation and retrieval of the samples, and other optimizations of the handling procedures were developed in compliance with radiation protection regulations and the aim of minimizing doses to personnel.
... This is a special grade EPDM-based material designed to be radiation tolerant and declared by the producer as resistant up to a gamma dose of 1.6 MGy [45]. Additionally, this material was irradiated in a mixed neutron and gamma radiation field in a previous study [46], reporting moderate damage at 0.7 MGy and only severe damage at 1.5 MGy [47], which are well above the 0.12 MGy expected during Run 3. ...
Two 6 t beam dumps, made of a graphite core encapsulated in a stainless steel vessel, are used to absorb the energy of the two Large Hadron Collider (LHC) intense proton beams during operation of the accelerator. Operational issues started to appear in 2015 during LHC Run 2 (2014–2018) as a consequence of the progressive increase of the LHC beam kinetic energy, necessitating technical interventions in the highly radioactive areas around the dumps. Nitrogen gas leaks appeared after highly energetic beam impacts and instrumentation measurements indicated an initially unforeseen movement of the dumps. A computer modelling analysis campaign was launched to understand the origin of these issues, including both Monte Carlo simulations to model the proton beam interaction as well as advanced thermo-mechanical analyses. The main findings were that the amount of instantaneous energy deposited in the dump vessel leads to a strong dynamic response of the whole dump and high accelerations (above 200 g). Based on these findings, an upgraded design, including a new support system and beam windows, was implemented to ensure the dumps' compatibility with the more intense beams foreseen during LHC Run 3 (2022–2025) of 539 MJ per beam. In this paper an integral overview of the operational behaviour of the dumps and the upgraded configurations are discussed.
... For these reasons, experimental radiation resistance tests of candidate products for use in high-power environment where neutron radiation dominates are necessary. Irradiation and testing methodologies have been developed in the last years, dedicated to the selection of elastomeric O-rings to be used in the SPES project [4,5]. The objective of this study is to examine nine commercial greases for selection for applying to use in ESS and SPES facilities [6]. ...
... In particular, mixed radiation data are necessary for the design and construction of new facilities producing intense neutron fields. We dedicated a previous study to the development of a methodology for testing elastomeric O-rings irradiated in neutron and gamma mixed fields [21]. A following study was dedicated to the experimental determination of the end-of-life conditions of a specific EPDM O-ring to be installed in a gate valve of the SPES facility. ...
Many of the moving components in accelerator and target environments require lubrication. Lubricants in such environments are exposed to high fluxes of secondary radiation, which originates from beam interactions with the target and from beam losses. The secondary radiation is a mix of components, which can include significant fractions of neutrons. Lubricants are radiation-sensitive polymeric materials. The radiation-induced modifications of their structure reduce their service lifetime and impose additional facility maintenance, which is complicated by the environmental radioactivity. The study of the lubricants radiation resistance is therefore necessary for the construction of new generation accelerators and target systems. Nevertheless, data collected in mixed radiation fields are scarce. Nine commercial greases were irradiated at a TRIGA Mark II Research Reactor to serve for the construction of new accelerator projects like the European Spallation Source (ESS) at Lund (Sweden) and Selective Production of Exotic Species (SPES) at Legnaro, (Italy). Mixed neutron and gamma doses ranging from 0.1 MGy to 9.0 MGy were delivered to the greases. For an experimental quantification of their degradation, consistency was measured. Two of the greases remained stable, while the others became fluid. Post-irradiation examinations evidence the cleavage of the polymeric structure as the dominant radiation effect. Dose and fluence limits for the use of each product are presented. Apart from the scientific significance, the results represent an original and useful reference in selecting radiation resistant greases for accelerator and target applications. Keywords: Materials science, Aerospace engineering, Industrial engineering, Nuclear engineering, Materials chemistry, Mechanical engineering, Nuclear reactor irradiation, Polymer degradation, Neutron damage, Lubricating grease, Radiation effect, Mixed radiation field
... 10,11 A comparative study of fluoro-elastomers and EPDM-based elastomers confirmed that damage due to neutron and g radiation fields were minimal for EPDM elastomers. 12 Abdel-Aziz et al. 13 studied the influence of co-agent and fumed silica on thermal and electrical resistivity in EPDM/modified MMT nanocomposites. They also reported on thermal stability and activation energy for thermal degradation of g-irradiated specimens. ...
In this work, blends of ethylene propylene diene monomer rubber and chlorobutyl rubber were reinforced with organo-modified layered silicate (nanoclay) to enhance their performance in radiation as well as hydrocarbons environments. The mechanical properties of the nanocomposites increased (up to 57%) and solvent transport coefficients decreased (by 30%) with increasing nanoclay content. The enhancement in properties was attributed to the dispersion of nanoclay platelets in the ethylene propylene diene monomer–chlorobutyl rubber blends and the chemical interaction between nanoclay and the polymer which were confirmed by morphological and spectroscopic analysis, respectively. The effect of nanofiller content on the mechanical properties, solvent uptake and thermal degradation of blends exposed to gamma radiation was investigated by irradiating the nanocomposites with gamma rays for cumulative doses of 0.5, 1 and 2 MGy. The ethylene propylene diene monomer–chlorobutyl rubber nanocomposites with 5 phr nanoclay had the best retention of mechanical properties and solvent sorption coefficients on exposure to radiation. Depending on the dose of cumulative radiation exposure, chain scission and/or crosslinking occurred in the nanocomposites, resulting in varying degrees of changes in properties.
One of the key priorities in modern mechanical engineering is to ensure the safe, long-lasting operation of systems while maximizing energy efficiency and withstanding extreme and damaging conditions. This is especially relevant in environments subject to radiation, in which materials, depending on their chemical and elemental composition, exhibit specific limitations. In this review, we summarize the current state of the art in research and development regarding the performance of lubricants in radiation fields and the effectiveness of radiation-tolerant lubricated systems. We discuss the current understanding of radiation-damage mechanisms in dry-film lubricants, oils, and greases, and we summarize the experimental results of the irradiation studies performed to date. We compile and compare the established performance of various material types and chemistries in intense ionizing radiation fields. Finally, we provide an overview of future research directions, highlighting the challenges faced in irradiation studies, considerations relating to the selection of materials, new facilities and experiments aimed at advancing the field, and other issues related to the safe application and use of lubricant materials in radiation environments.
The article discusses the main aspects of the use of rubber products based on triple ethylene propylene rubbers of domestic and foreign production, filled with an anti-radiation additive based on a composition of oxides of rare earth elements, as radiation-resistant elastomeric materials. The use of elastomers in the nuclear industry makes it possible to solve many urgent problems and ensure the operation of many critical products and mechanisms, the functioning of which is not possible without the use of elastic materials. This paper presents the results of the study and comparison of the physico-mechanical and operational properties of rubbers based on various ethylene propylene rubbers of domestic and foreign production with the addition of anti-radiation additive VKR-5M for their use as radiation-resistant elastomeric materials. The basic physical, mechanical and operational characteristics of rubber mixtures and rubbers based on ethylene propylene rubbers of domestic and foreign production have been studied. The main mechanisms and properties of radiation aging of elastomers, as well as ways to increase their resistance to ionizing radiation are considered. The paper presents the results of studies of frost resistance, heat resistance, radiation and thermal radiation resistance of rubbers based on ethylene propylene rubbers, which contain an anti-radiation additive based on the composition of rare earth element oxides WRC-5M, the advantages and disadvantages of various brands of domestic and foreign ethylene propylene rubbers in various operating conditions, and conclusions are drawn about the effectiveness of the introduction of anti-radiation additives WRC-5M, which increases the radiation resistance of rubbers. Based on the analysis of the data obtained during the work, the most radiation-resistant elastomeric base for rubbers used in conditions of increased radiation exposure was determined.
EPDM O-rings of gate valves employed for the construction of a second-generation accelerator for the production of neutron-rich Radioactive Ion Beams were studied in order to assess their sealing performance both during the facility service time and the post-service storage phase. Several O-ring specimens were at first exposed to different dose levels of mixed neutron and gamma radiations. Correspondent modifications of physical and mechanical properties of the material were investigated by means of uniaxial tensile tests, dynamic mechanical analyses, aging, compression set and vacuum leak tests. A hyperelastic strain energy function was adopted to fit the mechanical response of the material as a function of the absorbed dose. The minimum squeeze degree that guarantees O-ring sealing efficiency at different irradiation levels was determined by varying the interference between O-rings and grooves. A finite element model of the vacuum leak test was then set up to assess the contact pressure level required to ensure sealing. Numerical simulations of the gate valve main O-ring were subsequently carried out. By comparison of the predicted contact pressure and strain levels with experimental results, a life prediction map, as function of the service time, the storage time and the O-ring squeeze degree, was proposed.
The effect of mixed gamma and neutron radiation on the properties of O-rings made of elastomeric materials differing for composition and declared radiation resistance, a fluoroelastomer and an ethylene-propylene diene rubber, was investigated experimentally. The O-rings were subject to mixed gamma and neutron irradiation for various hours, leading to significant changes in the mechanical behaviour. These variations were further explored with tests that allowed highlighting the changes in the macromolecular architecture: solvent swelling tests, to evaluate the occurrence of two competitive phenomena of crosslinking and cleavage, and dynamic mechanical thermal analysis, to better interpret the mechanical response as due to the evolution of the crosslinked structure and of the temperature dependence.
The Selective Production of Exotic Species (SPES) facility, now under construction at Legnaro National Laboratories of INFN, is a second-generation accelerator for the production of neutron-rich ion beams. The radioactive nuclear species are produced by fission of a 238U target, on which a 200μA primary proton beam of 40MeV energy impinges. Materials and components constituting the Target and Ion Source assembly and the Front-End supporting structure are subjected to serious radioactive damage due to intense neutron and photon fields present under operating conditions. In the framework of the SPES project, experimental campaigns aimed at testing the radiation hardness of critical materials and components of potential use in the construction were started. Irradiations were conducted in a reactor mixed field of neutrons and photons in order to reproduce, as close as possible, the actual environmental service conditions. Results obtained for different types of elastomeric materials used for construction of vacuum O-rings, as well as preliminary results obtained for lubricating oils and greases, are presented. Materials under consideration are both conventional ones as well as materials specifically developed for applications in the presence of ionizing radiation. The latter materials were previously tested mainly in gamma radiation fields.
Facilities making use of the Isotope Separator On-Line (ISOL) method for the production of Radioactive Ion Beams (RIB) attract interest because they can be used for nuclear structure and reaction studies, astrophysics research and interdisciplinary applications. The ISOL technique is based on the fast release of the nuclear reaction products from the chosen target material together with their ionization into short-lived nuclei beams. Within this context, the SPES (Selective Production of Exotic Species) facility is now under construction in Italy at INFN-LNL (Istituto Nazionale di Fisica Nucleare — Laboratori Nazionali di Legnaro). The SPES facility will produce RIBs mainly from n-rich isotopes obtained by a 40 MeV cyclotron proton beam (200 μA) directly impinging on a uranium carbide multi-foil fission target. The aim of this work is to describe and update, from a comprehensive point of view, the most important results obtained by the analysis of the on-line behavior of the SPES production target assembly. In particular an improved target configuration has been studied by comparing different codes and physics models: the thermal analyses and the isotope production are re-evaluated. Then some consequent radioprotection aspects, which are essential for the installation and operation of the facility, are presented.
The SPES Radioactive Ion Beam (RIB) facility at INFN-LNL is in the construction phase. It is based on the ISOL method with an UCx Direct Target able to sustain a power of 10 kW. The primary proton beam is delivered by a high current Cyclotron accelerator, with energy 35-70 MeV and a beam current of 0.2-0.5 mA. Neutron-rich radioactive ions will be produced by proton induced Uranium fission in the UCx target at an expected fission rate in the order of 10 13 fissions per second. The exotic isotopes will be re-accelerated by the ALPI superconducting LINAC at energies of 10A MeV and higher, for masses in the region A=130 amu at expected rate on the secondary target of 10 7 – 10 9 pps. The SPES project has the aim to provide high intensity and high-quality beams of neutron-rich nuclei as well as to develop an interdisciplinary research center based on the cyclotron proton beam.
DCNS provides the French Navy with ships and services, including
through-life support (TLS). For that activity, the lifetime of seals has
to be studied, either because periods between maintenance stops are
longer, or because some seals cannot be replaced in short time and
without removing big equipments.Among different kinds of seals we
studied, we focus on EPDM seals. First of all, we studied their thermal
ageing in service, through accelerated ageing procedures and Arrhenius
law. Their mechanical properties after 19 and 32 years were estimated.
They showed a reasonable evolution.Then, the effects of gamma-radiations
were studied at different doses: 10, 20 and 400 kGy combining
temperature and radiation and 18, 33 and 250 kGy (only radiation). In
both cases, seals were irradiated after thermal ageing.Mechanical
properties (compression, tensile, and hardness) were measured at the
surface and in the middle of the seals (10×10 mm2
section and 68 mm diameter). In addition, physicochemical tests were
performed (TGA and DSC).The results showed that in normal conditions,
the properties of the EPDM seals vary a little till 32 years in service
(<20% in compression, and <30% in elongation at break in
tension).On the other hand, with doses of 250 and 400 kGy, the evolution
of the seals is important (+130% compression force).These results are in
good concordance with other studies on this kind of polymer, where the
dose at which there is a significant modification is about 250 kGy
(Madani, 2004; Zaharescu and Podina, 2001).
We have previously presented a quantitative model that combines diffusion and kinetic expressions with chemical/mechanical property relationships to predict the time development of diffusion-limited oxidation profiles in polymers. This model was shown to be applicable to thermal aging environments and should be generally applicable to environmental stresses for which total damage to the material is directly proportion to total exposure time. In this work, we use a fluorocarbon elastomer to demonstrate applicability of the model to ionizing-radiation environments.
An EPDM seal was qualified for use in nuclear power plants (NPPs). The primary function of the seal is to prevent leakage of unwanted fluids and gases during long-term normal operation, as well as during postulated accidents which may occur at the end of the planned service lifetime. Tested samples were compressed between two massive metal plates (the same compression as under regular service conditions) and exposed to an accelerated ageing procedure to simulate up to 10 years of operation, including accident simulation. Mechanical properties, compression set and a gas leakage test were carried out. The minimum necessary compression to keep the required tightness under all conditions during normal service and during accident conditions was found and the service lifetime was assessed for two seal thicknesses.
Various data has indicated that some elastomers have much higher radiation resistance than Viton. Nine samples of elastomers were irradiated with gamma rays. Two Ethylene Propylene Diene compounds, EPDM's, were found to exhibit acceptable properties for o-rings after radiation levels of 5x10Y rads, while Viton failed at 1x10X rads. Vacuum tests also were favorable so EPDM o-rings were chosen as seals in the Energy Saver cryostat vacuum system.
Over the past decade a range of facilities have been built which produce accelerated beams of short-lived radioactive nuclei. The advent of these beams has revolutionised nuclear physics research by removing the straightjacket that Nature had held us in when only the few stable nuclei were available to be used to initiate nuclear reactions. We can now probe further, and in more detail, towards the limits of nuclear stability, uncovering new structural phenomena and measuring the key nuclear reactions that control the energy output and element production in explosive astrophysical sites. A new generation of facilities is now under construction or planning that will have the capability to extend our reach ever further. In this talk we will survey the capabilities and time lines of these facilities, as well as looking at the science areas that each will address.
In the presence of oxygen, polymeric materials may undergo diffusion-limited heterogeneous degradation with significant oxidation occurring only near the surfaces. It is important in studying polymer degradation (and in designing accelerated aging experiments) to be able to determine under what conditions heterogeneous degradation occurs, and to estimate the extent of significant oxidative penetration under those conditions. We describe here the use of several techniques for identification of oxidative degradation gradients. A rapid determination of oxidation depth is accomplished by optical examination of metallographically polished cross-sectioned samples; oxidized and nonoxidized regions are distinguished by differences in surface reflectivity. A more detailed determination of the shapes of degradation gradients is accomplished by performing a series of sensitive determinations of relative hardness changes across the surface of cross-sectioned, polished samples. Typical oxidation depths for the commercial polymers examined are on the order of fractions of millimeters over a dose-rate range of 104–106 rad/h. Significant variations among different materials are found, as would be expected given differences in oxygen consumption and permeation rates. A detailed example is given of the tensile property behavior of a Viton material over a range of dose rates where the degradation is seen to change from strongly heterogeneous at high dose rates to homogeneous as the dose rate is lowered. Degradation differences in this material are very pronounced: at high dose rates the polymer undergoes primarily crosslinking to give a hard brittle material, whereas under lower dose rates, where oxygen permeation is complete, the polymer undergoes predominantly scission to yield a soft, easily stretchable material.
A rigorous test of theoretical treatments for diffusion-limited oxidation was completed by conducting an extensive series of radiation-initiated oxidation experiments on a commercial EPDM material. Oxidation profiles were monitored from density changes; profiles were obtained versus sample thickness, radiation dose rate and surrounding oxygen partial pressure. The resulting profile shapes and magnitudes could be quantitatively fit with a two-parameter theoretical treatment based on oxidation kinetics containing unimolecular termination reactions. The theoretical parameters derived from fitting allowed quantitative confirmation of a governing theoretical expression relating these parameters to independently measured values for the oxygen consumption and permeation rates.
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