Closure Of Repository Research Articles

Assessment of the evolution of the redox conditions in a low and intermediate level nuclear waste repository (SFR1, Sweden)

The evaluation of the redox conditions in an intermediate and low level radioactive waste repository such as SFR1 (Sweden) is of high relevance in the assessment of its future performance.The SFR1 repository contains heterogeneous types of wastes, of different activity levels and with very different materials, both in the waste itself and as immobilisation matrices and packaging. The level of complexity also applies to the different reactivity of the materials, so that an assessment of the uncertainties in the study of how the redox conditions would evolve must consider different processes, materials and parameters.This paper provides an assessment of the evolution of the redox conditions in the SFR1. The approach followed is based on the evaluation of the evolution of the redox conditions and the reducing capacity in 15 individual waste package types, selected as being representative of most of the different waste package types present or planned to be deposited in the SFR1. The model considers different geochemical processes of redox relevance in the system. The assessment of the redox evolution of the different vaults of the repository is obtained by combining the results of the modelled individual waste package types.According to the model results, corrosion of the steel-based material present in the repository keeps the system under reducing conditions for long time periods. The simulations have considered both the presence and the absence of microbial activity. In the initial step after the repository closure, the microbial mediated oxidation of organic matter rapidly causes the depletion of oxygen in the system. The system is afterwards kept under reducing conditions, and hydrogen is generated due to the anoxic corrosion of steel. The times for exhaustion of the steel contained in the vaults vary from 5ky to more than 60ky in the different vaults, depending on the amount and the surface area of steel. After the complete corrosion of steel, the system still keeps a high reducing capacity, due to the magnetite formed as steel corrosion product.The redox potential in the vaults is calculated to evolve from oxidising at very short times, due the initial oxygen content, to very reducing at times shorter than 5years after repository closure. The redox potential imposed by the anoxic corrosion of steel and hydrogen production is on the order of −0.75V at pH 12.5. In case of assuming that the system responds to the Fe(III)/magnetite system, and considering the uncertainty in the pH due to the degradation of the concrete barriers, the redox potential would be in the range −0.7 to −0.01V.A Monte-Carlo probabilistic analysis on the rate of corrosion of steel shows that the reducing capacity of the system provided by magnetite is not exhausted at the end of the assessment period, even assuming the highest corrosion rates for steel.Simulations assuming presence of oxic water due to glacial melting, intruding the system 60ky after repository closure, indicate that magnetite is progressively oxidised, forming Fe(III) oxides. The time at which magnetite is completely oxidised varies depending on the amount of steel initially present in the waste package.The behaviour of Np, Pu, Tc and Se under the conditions foreseen for this repository is discussed.

Biogeochemical changes at early stage after the closure of radioactive waste geological repository in South Korea

Permanent disposal of low- and intermediate-level radioactive wastes in the subterranean environment has been the preferred method of many countries, including Korea. A safety issue after the closure of a geological repository is that biodegradation of organic materials due to microbial activities generates gases that lead to overpressure of the waste containers in the repository and its disintegration with the release of radionuclides. As part of an ongoing large-scale in situ experiment using organic wastes and groundwater to simulate geological radioactive waste repository conditions, we investigated the geochemical alteration and microbial activities at an early stage (∼63days) intended to be representative of the initial period after repository closure. The increased numbers of both aerobes and facultative anaerobes in waste effluents indicate that oxygen content could be the most significant parameter to control biogeochemical conditions at very early periods of reaction (<35days). Accordingly, the values of dissolved oxygen and redox potential were decreased. The activation of anaerobes after 35days was supported by the increased concentration to ∼50mgL−1 of ethanol. These results suggest that the biogeochemical conditions were rapidly altered to more reducing and anaerobic conditions within the initial 2months after repository closure. Although no gases were detected during the study, activated anaerobic microbes will play more important role in gas generation over the long term.

EBS behaviour immediately after repository closure in a clay host rock: HE-E experiment (Mont Terri URL)

Abstract The evolution of the clay-based engineered barrier system (EBS) of geological repositories for radioactive waste has been the subject of many research programmes during the last decade. The early post-closure thermal behaviour is elucidated by the HE-E experiment, a 1:2 scale heating experiment (at the Mont Terri Rock Laboratory), which was implemented in the first semester of 2011, with the initiation of the heating phase in June 2011. A maximum temperature of 140 °C was reached in June 2012. After 15 months of heating, the temperature evolution in the EBS and the Opalinus Clay reflects the design calculations, and thermally induced porewater overpressures are being measured at a few metres’ distance in the Opalinus Clay. Seismic methods proved to be a sensitive tool for the continuous characterization of changes of EBS and Opalinus Clay properties. Design modelling and predictive modelling based on the as-built parameter dataset with established coupled codes (TOUGH, CODE_BRIGHT; using various geometries) are described. The results indicate that the models are generally in agreement with the observations and capable of capturing the evolution of the experiment.

Enhanced Sealing Project (ESP): evolution of a full-sized bentonite and concrete shaft seal

Abstract A full-scale shaft seal was designed and installed in the 5 m-diameter access shaft at Atomic Energy of Canada Limited's (AECL's) Underground Research Laboratory at the point where the shaft intersects an ancient water-bearing, low-angle thrust fault at a depth of c. 275 m in granitic rock. The seal consists of a 6 m-thick bentonite-based component sandwiched between 3 m-thick, keyed upper and lower concrete components. This design was adopted in order to limit the mixing of saline groundwater from the deeper regime with the fresher, near-surface groundwater regime. Construction of the shaft seal was done as part of Canada's Nuclear Legacy Liabilities Program. A jointly funded monitoring project, called the Enhanced Sealing Project (ESP), was developed by AECL (Canada) and jointly funded by NWMO (Canada), SKB (Sweden), Posiva Oy (Finland), and ANDRA (France), and since mid 2009 the thermal, hydraulic and mechanical evolution of the seal has been constantly monitored. The evolution of the type of seal being monitored in the ESP is of relevance to repository closure planning by demonstrating the functionality of shaft seals. Although constructed in a crystalline rock medium, the results of the ESP are relevant to the performance of seals in a variety of host rock types.

사용후핵연료 심층처분을 위한 지하연구시설(URL)의 필요성 및 접근 방안

This study gives a conceptual and basic direction to develop a URL (underground research laboratory) program for establishing the performance and safety of a deep geological disposal system in Korea. The concept of deep geological disposal is one of the preferred methodologies for the final disposal of spent nuclear fuel (SNF). Advanced countries with radioactive waste disposal have developed their own disposal concepts reasonable to their social and environmental conditions and applied to their com-mercial projects. Deep geological disposal system is a multi-barrier system generally consisting of an engineered barrier and natural barrier. A disposal facility and its host environment can be relied on a necessary containment and isolation over timescales envisaged as several to tens of thousands of years. A disposal system is not allowed in the commercial stage of the disposal program without a validation and demonstration of the performance and safety of the system. All issues confirming performance and safety of a disposal system include investigation, analysis, assessment, design, construction, operation and closure from planning to closure of the deep geological repository. Advanced countries perform RD&D (research, development & demonstration) programs to validate the performance and safety of a disposal system using a URL facility located at the preferred rock area within their own territories. The results and processes from the URL program contribute to construct technical criteria and guidelines for site selection as well as suitability and safety assessment of the final disposal site. Furthermore, the URL program also plays a decisive role in promoting scientific understanding of the deep geological disposal system for stakeholders, such as the public, regulator, and experts.

Reducing Uncertainties Affecting the Assessment of the Long-Term Corrosion Behavior of Spent Nuclear Fuel

Reducing the uncertainties associated with extrapolation to very long term of corrosion data obtainable from laboratory tests on a relatively young spent nuclear fuel is a formidable challenge. In a geologic repository, spent nuclear fuel may come in contact with water tens or hundreds of thousands of years after repository closure. The corrosion behavior will depend on the fuel properties and on the conditions characterizing the near field surrounding the spent fuel at the time of water contact. This paper summarizes the main conclusions drawn from multiyear experimental campaigns performed at JRC-ITU to study corrosion behavior and radionuclide release from spent light water reactor fuel. The radionuclide release from the central region of a fuel pellet is higher than that from the radial periphery, in spite of the higher burnup and the corresponding structural modifications occurring at the pellet rim during irradiation. Studies on the extent and time boundaries of the radiolytic enhancement of the spent fuel corrosion rate indicate that after tens or hundreds of thousands of years have elapsed, very small or no contribution to the enhanced corrosion rate has to be expected from α radiolysis. A beneficial effect inhibiting spent fuel corrosion due to the hydrogen overpressure generated in the near field by iron corrosion is confirmed. The results obtained so far point toward a benign picture describing spent fuel corrosion in a deep geologic repository. More work is ongoing to further reduce uncertainties and to obtain a full description of the expected corrosion behavior of spent fuel.

Assessment of the evolution of the redox conditions in the SKB ILW-LLW SFR-1 repository (Sweden)

ABSTRACTThe evaluation of the redox conditions in the Swedish ILW-LLW repository, SFR-1, is of high relevance in the performance assessment. The SFR-1 repository contains heterogeneous types of wastes, of different activity levels and with different materials in the waste and in the matrices and packaging. Steel and concrete-based materials are ubiquitous in the repository. The assessment presented in this work is based on the evaluation of the redox conditions and of the reducing capacity in 15 individual and representative waste package types in SFR-1. A combination of the individual models is used to determine the redox evolution of the different vaults in the repository. The results of the model indicate that in the initial time after repository closure, O2 is consumed through degradation of organic matter and metal corrosion during the initial time after repository closure. Afterwards, the system is kept under reducing conditions for long time periods, and H2(g) is generated due to the anoxic corrosion of steel forming magnetite as main corrosion product. The time at which steel is depleted varies with the amount and characteristics of steel and ranges from 5,000 to over 60,000 years. After complete steel corrosion, the reducing capacity of the system is mainly given by magnetite. The calculated redox potential under the chemical conditions imposed by the massive amounts of cements in the repository is in the order of -0.75 V (at pH 12.5). In case of assuming that the Eh of the system is controlled by the interaction between Fe(III)/Magnetite as a result of groundwater/magnetite interactions, redox potentials in the range -0.7 to -0.01V are calculated, considering the uncertainty in the pH prevalent in the system If the absence of oxic disturbances the Eh of the repository system would be kept reducing. In the event of oxidising and diluted glacial meltwater intrusion, magnetite would gradually convert into Fe(III) oxides, buffering the redox potential of the system and preventing it from oxidation for long time periods.

Np(V/VI) redox chemistry in cementitious systems: XAFS investigations on the speciation under anoxic and oxidizing conditions

Abstract The use of cementitious materials is foreseen to immobilize long-lived intermediate level wastes that may contain significant amounts of 237Np. Predicting the release of Np from a cement-based repository requires an adequate understanding of its interaction with the main sorbing components of cement. Although Np(IV) will prevail under repository conditions after depletion of O2, Np(V) is expected to control the chemistry of Np in the early stage after repository closure as well as in the presence of oxidizing waste forms (i.e. high content of NO 3 - ). Moreover, little is known of the stability of Np(VI) under hyperalkaline conditions, resulting in a rather ill-defined Np(V/VI) redox chemistry in cementitious environments. XANES studies on Np(V)-doped calcium silicate hydrates (C S H phases) and hardened cement paste (HCP) samples prepared under anoxic and under oxidizing conditions (provided by 5 × 10−3 M NaClO) clearly showed the predominance of Np(V) and Np(VI), respectively. The presence of two different neptunyl moieties was confirmed by EXAFS, revealing significantly shorter Np Oax and Np Oeq distances obtained for the samples with NaClO (1.79–1.85 A and 2.24–2.25 A) compared to those prepared under anoxic conditions (1.88–1.89 A and 2.39 A). This indicates oxidation of Np(V) to Np(VI) in the presence of NaClO. The short Np–Oeq distances along with structural properties of the Si-shells (number of neighboring Si atoms, bond distances) suggest that an incorporation mechanism is responsible for Np(V/VI) uptake by C S H phases. The coordinative environments of both Np redox states in cement paste were found to be similar to those in C S H phases, indicating C S H as the uptake-controlling phase. In contrast to Np(V), the molecular environment of Np(VI) in C S H phases depends not only on the Ca:Si ratio of these phases but also on pH, presumably reflecting differences between the hydrolysis scheme of the two Np redox states under these pH conditions. EXAFS data further confirmed the predominance of aqueous Np(VI) species (as Np VI O 2 ( OH ) 4 2 - ) in hyperalkaline and oxidizing tetramethylammonium hydroxide (TMA-OH) solutions. Under anoxic conditions and in the presence of CO 3 2 - (as an impurity of TMA-OH), predominance of a mixed Np(V) OH CO3 species was indicated by EXAFS. These Np(V/VI) aqueous species are not considered in the current NEA thermodynamic selection and, therefore, deserve further attention.

Migration Behaviour of Lanthanides in Compacted Bentonite with Iron Corrosion Product Using Electrochemical Method

ABSTRACTAfter the closure of a high-level waste repository, corrosion of the carbon steel overpack will occur. The corrosion products can then migrate into bentonite and affect the migration behavior of radionuclides in bentonite. Therefore, electrochemical experiments, with Fe2+ supplied by anodic corrosion of carbon steel, were carried out to study trivalent lanthanides in compacted bentonite. The interface between a carbon steel coupon and bentonite (dry density, 1.5 Mg/m3) was spiked with a tracer solution containing Nd(NO3)3, Eu(NO3)3, Dy(NO3)3, and Er(NO3)3. The carbon steel coupon was connected as the working electrode to a potentiostat and held at a constant potential between -550 and 0 mV (vs. Ag/AgCl reference electrode) for 7 days. A model using dispersion and electromigration could explain the measured profiles in the bentonite specimens. The best-fit electromigration velocity was related to the applied electric potential and was 1.0–3.8 nm/s for Nd, Eu, Dy, and Er ions. For these lanthanides, the best-fit dispersion coefficient was also related to the applied potential and was 0.8–1.6 μm2/s, and the dispersion length was calculated as 0.2 mm from the linear relationship between the dispersion coefficient and electromigration velocity. Finally, the apparent diffusion coefficient for these lanthanides was estimated as 0.6–0.9 μm2/s.

Evaluation of nuclide release scenarios for a hypothetical LILW repository

A program for the safety assessment and performance evaluation of a low- and intermediate-level radioactive waste (LILW) repository system has been developed. Utilizing GoldSim (2006), the program evaluates nuclide release and transport into the geosphere and biosphere under various disruptive natural and manmade events and scenarios that can occur after a waste package failure. We envisaged and illustrated these events and scenarios as occurring after the closure of a hypothetical LILW repository, and they included the degradation of various manmade barriers, pumping well drilling, and natural disruptions such as the sudden formation of a preferential flow pathway in the far-field area of the repository. Possible enhancement of nuclide transport facilitated by colloids or chelating agents is also dealt with. We used the newly-developed GoldSim template program, which is capable of various nuclide release scenarios and is greatly suited for simulating a potential repository given the geological circumstances in Korea, to create the detailed source-term and near-field release scheme, various nuclide transport modes in the far-field geosphere area, and the biosphere transfer. Even though all parameter values applied to the hypothetical repository were assumed, the illustrative results, particularly the probabilistic calculations and sensitivity studies, may be informative under various scenarios. ( GoldSim, LILW, Nuclide transport, Safety assessment, Scenario).

Bentonite Alteration Due to Thermal-Hydro-Chemical Processes During the Early Thermal Period in a Nuclear Waste Repository

After closure of an underground nuclear waste repository, the decay of radionuclides will raise the temperature in the repository, and the bentonite buffer will resaturate by water inflow from the surrounding host rock. The perturbations from these thermal and hydrological processes are expected to dissipate within hundreds to a few thousand years. Here, we investigate coupled thermal-hydro-chemical processes and their effects on the short-term performance of a potential nuclear waste repository located in a clay formation. Using a simplified geometric configuration and abstracted hydraulic parameters of the clayey formation, we examine geochemical processes, coupled with thermohydrologic phenomena, and potential changes in porosity near the waste container during the early thermal period. The developed models were used for evaluating the mineral alterations and potential changes in porosity of the buffer, which can affect the repository performance. The results indicate that mineral alteration and associated changes in porosity induced by early thermal and hydrological processes are relatively small and are expected to not significantly affect flow and transport properties. Chlorite precipitation was obtained in all simulation cases. A maximum of one percent volume fraction of chlorite could be formed, whose process may reduce swelling and sorption capacity of bentonite clay, affecting the performance of the repository. The illitization process was not obtained from the present simulations.

Biogeochemical processes in a clay formation in situ experiment: Part G – Key interpretations and conclusions. Implications for repository safety

Abstract The in situ porewater chemistry (PC) experiment carried out in the Opalinus Clay formation at the Mont Terri Rock Laboratory, Switzerland for a period of 5 a allowed the identification and quantification of the biogeochemical processes resulting from and affected by an anaerobic microbial disturbance. The unintentional release of degradable organic compounds (mainly glycerol) induced microbially-mediated SO4 reduction in the borehole with concomitant significant geochemical changes in the circulating water and the adjacent porewater. These changes included a decrease in SO 4 2 - concentration and pH and an increase in pCO2 and alkalinity relative to the non-affected formation water. However, the cation composition of the water and the mineralogy of the clay close to the borehole wall showed very little change. This is explained by (1) the strong chemical buffering processes in the clay and (2) by the diffusion-limited flux of solutes. With the aid of a reactive transport model with a minimum set of kinetic parameters for the hypothesised degradation reactions, the evolution of solutes in the borehole could be modelled adequately. The model was also applied to the prediction of restoration times upon depletion of the C source and results indicated restoration times to undisturbed conditions of about 15 a, but also highlighted the rather large uncertainties inherent in the geochemical model. Nevertheless, the simulations provided additional evidence of the high pH buffer capacity of the Opalinus Clay. The results from the microbiological investigations do not allow unambiguous identification of the origin of the microbial population in the borehole. Possible sources were the drilling procedure, the artificial porewater, and perhaps some revival of indigenous dormant strains. Regardless of the origin of the microbes, the results from the PC experiment underlined the importance of anaerobic microbial activity in the “disturbed” Opalinus Clay, facilitated by the introduction of space, water and organic material, in rapidly establishing very reducing conditions. The PC experiment also yielded valuable insight with regard to the safety of a high-level radioactive waste repository emplaced in Opalinus Clay. Anaerobic microbial perturbations in the clay host rock may occur from the construction and excavation procedures and emplaced organic by-products. The resulting effects on porewater chemistry, i.e., especially on pH and Eh, may affect the mobility of radionuclides eventually released from the waste. However, the overall results of the PC experiment suggest that such effects are temporary and spatially limited because of the large buffering capacity and diffusive properties of the clay formation. Nevertheless, the results also indicate that the amounts of organic materials in a high-level waste repository should be kept small in order to achieve background conditions within a short time period after repository closure. A further conclusion from the PC experiment is that commonly used equipment materials may not display commonly assumed inert behaviour. This particularly holds for the gel-type “robust” reference electrodes, which may release substantial amounts of glycerol.

Migration Behavior of Alkali Earth Ions in Compacted Bentonite With Iron Corrosion Product Using Electrochemical Method

AbstractCarbon steel overpack will corrode by consuming oxygen introduced during repository construction after closure of repository, that will keep the environment in the vicinity of repository reducing. The iron corrosion products can migrate in bentonite as ferrous cations (Fe2+) through the interlayer of montmorillonite replacing the exchangeable sodium ions in the interlayer. This replacement of sodium may affect the migration behavior in the altered bentonite not only for redox-sensitive elements but also the other ions. Therefore we have carried out electrochemical analysis, of calcium, strontium or barium with the ferrous ion supplied by anodic corrosion of iron coupons in compacted bentonite. Fifteen micro liters of tracer solution containing 8.6 M of CaCl2 or 3.0 M of SrCl2 or 1.5 M BaCl2 were sspiked on the interface between the iron coupon and bentonite, for which the dry density was in the range of 1.4 to 1.5 Mg/m3, before assembling. The iron coupons were connected as working electrodes to the potentiostat and held at a constant supplied potential between - 500 to +300 mV (vs. Ag/AgCl reference electrode) for up to 7 days. Calcium and strontium could migrate faster and deeper into the bentonite than iron in each condition, while barium could migrate slower than iron. A model using dispersion and electromigration can explain the measured profiles in the bentonite specimens. The fitted value of electromigration velocity was a function of applied electrical potential and 10 to 23 nm/s for calcium, 11 to 19 for strontium, around 4 nm/s for barium and 5 to 15 nm/s for iron, respectively. Alternatively, the fitted value of the dispersion coefficient was not a function of applied potential, and the values were 3 - 8 × 10-12 m2/s for calcium, 2 - 4 × 10-12 m2/s for strontium, 5 - 10 × 10-12m2/s for barium and 3 - 9 × 10-12 m2/s for iron, respectively.

閉鎖時の意思決定における地層処分モニタリングのあり方に関する検討

現在, 我が国では原子燃料サイクルの過程で発生する高レベル放射性廃棄物を, 数万年以上といった長期にわたり人間の生活環境から隔離するため, 地層処分が計画されている. この地層処分施設は, 操業安全性と閉鎖後安全性の両方を確保するように設計される必要があり, 閉鎖後安全性は施設が閉鎖された後に, モニタリングまたは制度的管理に依存することなく, 人工バリアおよび天然バリアによって提供されることとなる. しかしながら, 最近では閉鎖措置の一環で地下水モニタリング等も検討対象となり, さらに社会科学の観点からも閉鎖後のモニタリングに関する必要性が議論されつつある. このような状況を踏まえると, 高レベル放射性廃棄物処分の長期に亘る安全性の確保の観点から, モニタリングのあり方を検討することが重要となる. そこで, 本論では処分事業において重要と考えられる閉鎖時の意思決定の段階に着目し, まず, 閉鎖時の意思決定の考え方を整理したうえで, 閉鎖時の意思決定におけるモニタリングのあり方 (モニタリングの役割, モニタリング実施時の制約条件, および具体的なモニタリング計画の検討方法) について考察を試みた. このあり方は, 今後サイト調査から最終閉鎖後の各段階を対象に一貫性を持ったモニタリング計画の検討と, 社会との合意形成に向けたモニタリングに関する議論の起点になるものであると考えている.

Evaluation of mineral precipitation potential in a spent nuclear fuel repository

In this study, the potential for mineral precipitation reactions to occur during the excavation, disposal, backfilling and closure of a deep geological repository for the final disposal of spent nuclear fuel was evaluated with the assistance of hydrogeochemical modelling. Four modelling exercises, corresponding to the main expectable geochemical scenarios in the Excavation Damaged Zone (EDZ) throughout repository evolution, were carried out and the times for sealing of the discontinuities owing to mineral precipitation in each of them were evaluated and discussed. The simulations show that mineral precipitation reactions are thermodynamically feasible in most of the studied cases. The main mineral phases potentially responsible for the hydraulic sealing of the EDZ are calcite and ferric oxyhydroxides, being the estimated volumes occupied by the precipitation of calcite between one and three orders of magnitude larger than the volume of precipitating ferric phases. The estimated times for complete sealing of the EDZ may vary between several hundreds to more than 1 million years. The shortest sealing times (less than 3,000 years) are obtained for the mixture of groundwaters from the repository depth with dilute infiltration waters.

Migration Behavior of Potassium and Rubidium in Compacted Bentonite Under Reducing Condition With Iron Corrosion Product

AbstractCarbon steel overpack will corrode by consuming oxygen introduced by repository construction after closure of repository and then will keep the reducing environment in the vicinity of repository. The iron corrosion products can migrate in bentonite as ferrous ion through the interlayer of montmorillonite replacing exchangeable sodium ions in the interlayer. This replacement of sodium with ferrous ion may affect the migration behavior in the altered bentonite not only for redox-sensitive elements but also the other ions. Therefore the authors have carried out electromigration experiments of potassium or rubidium with source of iron ions supplied by anode corrosion of iron coupon in compacted bentonite. Five to fifteen micro liter of tracer solution containing 3.3 M of KCl or 2.2 M of RbCl was spiked on the interface between an iron coupon and bentonite, which dry density was around 1.4 Mg/m3, before assembling. The iron coupon was connected as the working electrode to the potentiostat and was held at a constant supplied potential between - 600 and 300 mV vs. Ag/AgCl reference electrode for up to 8 days. Potassium could migrate faster and deeper in bentonite specimen than iron in each condition. On the other hand rubidium could migrate slower than iron. Migration velocity was a function of applied electrical potential and 8 to 14 nm/s for potassium, 5 to 10 nm/s for iron and 3 to 5 for rubidium, respectively. Dispersion coefficient was also a function of applied potential and 10 to 14 × 10−12 m2/s for potassium, 4 to 8 overv 10−12 m2/s for rubidium and 2 to 4 overv 10−12 m2/s for iron, respectively. Diffusion experiments were also carried out for comparison. Potassium and rubidium might migrate slightly slower in the altered bentonite by iron corrosion than in ordinary compacted bentonite.

Implementation of Localized Corrosion in the Performance Assessment Model for Yucca Mountain

A total system performance assessment (TSPA) model has been developed to analyze the ability of the natural and engineered barriers of the Yucca Mountain repository to isolate nuclear waste over the period following repository closure. The principal features of the engineered barrier system are emplacement tunnels (or “drifts”) containing a two-layer waste package (WP) for waste containment and a titanium drip shield to protect the WP from seeping water and falling rock. The 25-mm-thick outer shell of the WP is composed of Alloy 22, a highly corrosion-resistant nickel-based alloy. There are five nominal degradation modes of the Alloy 22: general corrosion, microbially influenced corrosion, stress corrosion cracking, early failure due to manufacturing defects, and localized corrosion (LC). This paper specifically examines the incorporation of the Alloy 22 LC model into the Yucca Mountain TSPA model, particularly the abstraction and modeling methodology, as well as issues dealing with scaling, spatial variability, uncertainty, and coupling to other submodels that are part of the total system model, such as the submodel for seepage water chemistry.

Redox front formation in an uplifting sedimentary rock sequence: An analogue for redox-controlling processes in the geosphere around deep geological repositories for radioactive waste

Subsurface redox fronts control the mobilization and fixation of many trace elements, including potential pollutants such as certain radionuclides. Any safety assessment for a deep geological repository for radioactive wastes needs to take into account adequately the long-term redox processes in the geosphere surrounding the repository. To build confidence in understanding these processes, a redox front in a reduced siliceous sedimentary rock distributed in an uplifting area in Japan has been studied in detail. Geochemical analyses show increased concentrations of Fe and trace elements, including rare earth elements (REEs), at the redox front, even though concentrations of reduced rock matrix constituents show little change. Detailed SEM observations revealed that fossilized microorganisms composed of amorphous granules made exclusively of Fe and Si occur in the rock’s pore space. Microbial 16S rDNA analysis suggests that there is presently a zonation of different bacterial groups within the redox band, and bacterial zonation played an important role in the concentration of Fe-oxyhydroxides at the redox front. These water–rock–microbe interactions can be considered analogous to the processes occurring in the redox fronts that would develop around geological repositories for radioactive waste. Once formed, the Fe-oxyhydroxides within such a front would be preserved even after reducing conditions resume following repository closure.

SIGNIFICANCE OF 14C AND 228Ra IN TERMS OF THE PROPOSED YUCCA MOUNTAIN HIGH-LEVEL RADIOACTIVE WASTE REPOSITORY

C and Ra are two of the radionuclides that have either been identified as being potentially significant in terms of releases from the proposed Yucca Mountain high-level radioactive waste repository, or are specifically cited for consideration and evaluation in the regulations promulgated by the U.S. Nuclear Regulatory Commission. The purpose of this study was to estimate the concentrations and associated doses for these two radionuclides, if released under conditions of a scenario assumed to apply to a repository containing some of the features of the one proposed at Yucca Mountain, NV, and to compare these estimates to the regulatory limits for that facility. For C, the postulated condition was that an annual fractional release of 10 of its total remaining inventory occurs beginning at 10,000 y after repository closure. For Ra, the same fractional release rate was assumed, but in this case it was presumed to occur when the Ra inventory was projected to reach a maximum at more than 10 y after repository closure. The estimated concentrations and doses were, in turn, compared to the concentration limit, specified in the Ground Water Protection Standards (GWPSs) in the case of Ra, or derived, in the case of C, on the basis of the regulatory dose rate limit. Due to the small inventory of C in the waste, and its short half-life relative to the performance period evaluated, its estimated concentration in the ground water would be slightly more than 4% of the derived GWPS. Due to the relatively small initial inventory of Th, the precursor of Ra, and the correspondingly small quantities of higher atomic number actinides that could, through decay, produce additional quantities of Th, its estimated concentration in the ground water would be less than 3% of the GWPS, leaving the remaining portion of the limit for potential contributions from Ra. At the same time, however, it must be recognized that, in this case, the regulations require that any contributions of naturally occurring Ra and Ra already present in the ground water must be included in the determination of compliance. If this is done, the total concentration of Ra, combined with the naturally occurring concentration of Ra, would be about 10.5% of the limit. In a similar manner, the committed doses due to the annual consumption of each of these two radionuclides in ground water and food, produced in the local biosphere, were evaluated in terms of the Individual Protection Standard (IPS). Based on these analyses, the estimated effective dose for C, using the coefficients in Federal Guidance Report (FGR) No. 13, was 4.15 muSv y, less than 3% of the IPS. For Ra, the comparable estimate at the time of maximum inventory, excluding in this case the contributions from naturally occurring Ra and Ra, was 7.39 muSv y, representing about 5% of the IPS. Based on the value assumed for the fractional release rate (10 y), it was concluded that neither C nor Ra will be significant in terms of either the applicable GWPS or the IPS. While it was recognized that, due to the time spans involved, these analyses were primarily an academic exercise, it is believed that the perspectives and accompanying insights are useful.

Migration Behavior of Plutonium in Compacted Bentonite Under Reducing Condition Using Electromigration

AbstractCarbon steel is one of the candidate overpack materials for high-level waste disposal and is expected to assure complete containment of vitrified waste glass during an initial period of 1000 years in Japan. Carbon steel overpack will be corroded by consuming oxygen introduced by repository construction after closure of repository and then will keep the reducing environment in the vicinity of repository. The migration of iron corrosion products through the buffer material will affect migration of redox-sensitive radionuclides. Therefore the authors have carried out electromigration experiments with source of iron ions supplied by anode corrosion of iron coupons attached to compacted bentonite. Authors tried to use plutonium in this experimental configuration to obtain the knowledge of migration behavior of actinides. Authors also used cesium as reference. The concentrations of iron and sodium showed nearly complementary distributions. It is expected that iron ion could migrate as ferrous ion through the interlayer of montmorillonite replacing exchangeable sodium ions in the interlayer. Concentration profiles of plutonium in bentonite grew as time supplying electric potential as long as 168 h. Plutonium migrated from the iron anode toward cathode as deeper than 1 mm of the interior of bentonite even in 48 h, though plutonium could not diffuse 1 mm for 2 years. On the other hand, profiles of cesium seemed to be controlled by ordinary diffusion because of large diffusion coefficient of cesium in bentonite as large as 10$^{-12}$ m$^{2}$/s. Drift of the cesium profile by electric potential gradient could be observed clearly after 240 h at individual experiment for cesium. Apparent dispersion coefficients of plutonium were calculated from the profiles and were as large as 10$^{-13}$ m$^{2}$/s. Since plutonium migration was accelerated by electric potential, plutonium chemical species would have positive charge and were estimated as PuOH$^{2+}$ or PuCl$^{2+}$ by the thermodynamic calculation. Thus this experiment can provide a diffusion field for cations under a reducing condition with ferrous ions in water-saturated bentonite.