Radioactive Waste Disposal System Research Articles

고준위 폐기물 처분용기 내진 해석 모델 개발

고준위 방사성 폐기물 처분시스템은 지하 500 m 심도에서 암반에 터널을 뚫어 고준위폐기물 처분용기를 넣고 주위를 완충재로 매우는 형태이다. 많은 통계 자료에 의하면 한반도에서 매년 지진이 증가하는 추세이며, 지진이 발생할 경우 지하에서 발생된 전단력에 의해 처분용기가 손상될 수 있다. 더 나아가 방사성 유해물질이 유출되어 큰 환경 문제가 유발될 수 있다. 이에 본 논문에서는 지진에 대해 안전하게 보호할 수 있는 방법으로 내진형 완충재를 개발하였다. 내진 성능에 영향을 미치는 주요인자를 분석하여 내진형 완충재를 설계하였고, ABAQUS를 이용하여 전단해석모델을 개발하여 내진형 완충재의 성능을 평가하였다. In the underground 500 m depth, the high level radioactive waste disposal system is made by boring the tunnel in the base rock and putting the high level waste disposal canister that is the surrounding form with the buffer material. According to the many statistics, it is the tendency that the earthquake increases in the Korean peninsula every year. In case that the earthquake is generated, the disposal canister in the rock mass can be broken due to the shearing force in the underground. Furthermore, a major environmental problems can be caused by the radioactive harmful substances. In this study, the earthquake-proof type buffer material was developed with the protection method safely on the earthquake. The main parameter having an effect on the earthquake-resistant performance was analyzed and the earthquake-proof type buffer material was designed. The shear analysis model was developed and the performance of the earthquake-proof type buffer material was evaluated by using ABAQUS.

Multi‐species diffusion analysis in porous media under various dry density and temperature conditions: molecular dynamics simulation, homogenization analysis, and finite element method

SUMMARYIn porous media, chemical species that dissolve in pore water can be transported via diffusion mechanisms or advective fluxes, close to or far away from where precipitation occurs. In the case of a high‐level radioactive waste disposal system, compacted bentonite is used in a buffer material in an engineering barrier system to minimize the amount of specific nuclides that breach into the surrounding host rock. To minimize breaching, it is very important to understand the transport mechanism of multiple chemical species in porous media. In the following research, we introduced FEM analysis methods using the results of the molecular dynamics simulation and homogenization analysis (MD/HA) method. First, the diffusion coefficients of ions (Cl−, I−, and Na+) in different water layers of Na‐beidellite were calculated using the MD/HA procedure under various dry density (1.2, 1.6, and 2.0 Mg/m3) and temperature (293, 323, and 363 K) conditions. Next, using FEM analysis that used the MD/HA results as input parameters, the diffusion behaviors of ions in porous media were calculated. The results indicate that the diffusion coefficients of the interlayer water in Na‐beidellite are different from the diffusion coefficients under dry density conditions. Further, the concentration profiles (Ct/C0) of iodine and chloride are proportional to temperature but inversely proportional to dry density. Copyright © 2014 John Wiley & Sons, Ltd.

상대습도를 이용한 벤토나이트 완충재의 불포화 수리전도도 평가방안

상대습도 데이터를 이용하여 벤토나이트 완충재 블록의 불포화 수리전도도 변화를 평가하였다. 불포화 매질에서의 물의 흐름을 나타내는 일반적인 분석해를 통해 상대습도를 통한 불포화 수리전도도 계산방안을 도출하였고, 이를 실제 수행한 실내 물 유입 실험 결과에 적용하여 포화가 진행됨에 따라 변화하는 완충재 불포화 수리전도도 양상을 확인하였다. 일반적인 포화 상태와는 확연히 다르게 수두 구배와 물의 유출량이 시간에 따라 불규칙하게 변화하는 결과를 나타냈으며, 벤토나이트 완충재의 불포화 수리전도도는 시간에 따라 증가하는 경향을 보였다. 수분 흡수로 인한 벤토나이트 입자 팽창 때문으로 인한 매질 내 공극의 부피 및 크기 확대가 불포화 수리전도도값의 증가를 야기하는 것으로 판단되었고, 이러한 결과는 완충재 블록의 팽창 정도와 수리전도도의 상관성에 관한 추후 연구의 필요성을 제시하였다. 본 연구에서 수행된 불포화 수리전도도 평가 방안은 방사성폐기물 처분 시 완충재의 장기적인 수리학적 성능평가에 유용한 기술로 사용될 수 있을 것이다.

An optimal model of the representative habit data for complex environmental exposure systems

The concept of the representative person recommended by the International Commission on Radiological Protection (ICRP) is difficult to apply to dose assessment to the public because the representative habit data could not be determined in advance for general radiation exposure situations. So, a stylized method for constructing habit data of the representative person is needed to ensure consistency and validity of the dose estimation. This paper presents a generalized model of the representative habit data based on optimization by a linear programming technique to handle complex environmental exposure situations such as radioactive waste disposal. The model is devised in such a way that it can fully embody the ICRP's criteria for the representative person by ensuring the upper 5% exposure in the population and it is applicable to general exposure systems. The model was illustrated for Korea's low- and intermediate-level radioactive waste disposal system to show its applicability to complex exposure systems with multiple scenarios and pathways.

The long-term cement studies project: the UK contribution to model development and testing

AbstractThe international long-term cement studies (LCS) project aims to increase the understanding of the behaviour of cement within a radioactive waste disposal system and how hyper-alkaline leachates may interact with host rock. Such an understanding enables confident, robust and safety-relevant statements to be made concerning future system behaviour, irrespective of host rock, engineered barrier system, or waste type. The LCS project involves laboratory experiments, in situ tests and numerical modelling to address these issues. The agencies participating are Nagra (Switzerland), JAEA (Japan), the Nuclear Decommissioning Authority, Radioactive Waste Mangement Directorate (UK), Posiva (Finland) and SKB (Sweden).Project activities have included: the development of conceptual and theoretical models of cement–rock interaction; testing of numerical models against data from laboratory experiments and industrial and natural analogues of cement–rock reaction; and the synthesis and incorporation of performance assessment (PA) relevant data from analogue studies. Key threads running through these studies include an analysis of issues relating to upscaling of processes and data to the greater temporal and spatial scales relevant to PA, and investigations of modelling the changes in physical properties that accompany geochemical reaction. Here we present examples of the results from model test cases, highlighting the important issues that have arisen.

Molecular dynamics simulation of the diffusion of uranium species in clay pores

Molecular dynamics simulations were carried out to investigate the diffusive behavior of aqueous uranium species in montmorillonite pores. Three uranium species (UO22+, UO2CO3, UO2(CO3)22−) were confirmed in both the adsorbed and diffuse layers. UO2(CO3)34− was neglected in the subsequent analysis due to its scare occurrence. The species-based diffusion coefficients in montmorillonite pores were then calculated, and compared with the water mobility and their diffusivity in aqueous solution/feldspar nanosized fractures. Three factors were considered that affected the diffusive behavior of the uranium species: the mobility of water, the self-diffusion coefficient of the aqueous species, and the electrostatic forces between the negatively charged surface and charged molecules. The mobility of U species in the adsorbed layer decreased in the following sequence: UO22+>UO2CO3>UO2(CO3)22−. In the diffuse layer, we obtained the highest diffusion coefficient for UO2(CO3)22− with the value of 5.48×10−10m2s−1, which was faster than UO22+. For these two charged species, the influence of electrostatic forces on the diffusion of solutes in the diffuse layer is overwhelming, whereas the influence of self-diffusion and water mobility is minor. Our study demonstrated that the negatively charged uranyl carbonate complex must be addressed in the safety assessment of potential radioactive waste disposal systems.

Time-dependent strength degradation of granite

Understanding the time-dependent strength degradation and associated creep behavior is essential to the safety evaluation of a radioactive waste disposal system in rock. In this study, a series of constant loading tests under different confining pressures and temperatures with acoustic emission monitoring have been performed to study evolution of dilatancy (damage) and strength degradation. Source location analysis and moment tensor analysis were carried out to reproduce the progressive damage process during constant load helping to understand the failure mechanism from a microscopic point of view. In this paper, we report on the findings of the experimental results.

Development of scenario analysis and database for quantitative analysis of microbial effects on the repository performance

AbstractA scenario analysis was performed for microbial effect on the performance assessment (PA) of the high-level radioactive waste disposal system. Based on review of recent literature on PA and some results of our experiment, possible scenario of microbial effects on the Japanese high-level radioactive waste disposal system were analyzed. The result of the analysis shows that the microbial effect on groundwater evolution, which is caused by the microbial redox reaction, is one of the most important issues in assessing PA. The evolution not only affects radionuclide migration but also the life-time of metal overpack corrosion.To assess the microbial effect on groundwater evolution, a numerical estimation code is required, with reliable parameters for analysis of microbial activities used in the code. A biological parameter database has been developed to give the growth and metabolism of the six typical microbial groups to be used in the estimation code. For this purpose, about 573 data from 95 papers were examined to obtain effective data for disposal. The database is composed of significant data like specific growth rate, maximum growth rate, constant decay rate, and experimental condition for model assessment.

Double-layered buffer to enhance the thermal performance in a high-level radioactive waste disposal system

A thermal performance is one of the most important factors in the design of a geological disposal system for high-level radioactive wastes. According to the conceptual design of the Korean Reference disposal System, the maximum temperature of its buffer with a domestic Ca-bentonite is close to the thermal criterion, 100 °C. In order to improve the thermal conductivity of its buffer, several kinds of additives are compared. Among the additives, graphite shows the best result in that the thermal conductivity of the bentonite block is more than 2.0 W/m °C. We introduced the concept of a double-layered buffer instead of a traditional bentonite block in order to use the applied additive more effectively. The thermal analysis, based upon the three-dimensional finite element method, shows that a double-layered buffer could reduce the maximum temperature on a canister's surface by 7 °C under identical conditions when compared with a single-layered buffer. An analytical solution was derived to efficiently analyze the effects of a double-layered buffer. The illustrative cases show that the temperature differences due to a double-layered buffer depend on the thickness of the buffer.

DEVELOPMENT OF LOW-ALKALINE CEMENT USING POZZOLANS FOR GEOLOGICAL DISPOSAL OF LONG-LIVED RADIOACTIVE WASTE

To reduce uncertainties in the safety assessment of the disposal system for long-lived radioactive waste, cement was developed which generates leachates with a lower pH than that of ordinary cement paste. This cement is termed “low-alkaline cement”. Large amounts of pozzolans were used to produce the low-alkaline cement with ordinary Portland cement. Silica fume was found to be an effective pozzolans to reduce pH, but the needed large amount of silica fume reduced the workability of fresh concrete. Therefore, the authors also used fly ash with silica fume, to develop more workable low-alkaline cement, termed high-volume fly ash silica fume cement (HFSC). Two types of HFSC developed showed high compressive strength, smaller drying shrinkage and lower temperature rise than that of ordinary Portland cement. It was confirmed that HFSC could be used as self-compacting concrete. Therefore they can be applied as either structural or backfilling concrete in the disposal system.

Geochemical optimisation of a disposal system for high-level radioactive waste

Geochemical exploration commonly utilises observations from nature to help find, and explain the genesis of, anomalies such as ore bodies. Radioactive waste disposal involves the inverse process of using such expertise to design a geochemical anomaly (the repository) which will persist for geological timescales. The particular challenge is developing a disposal concept which is “robust” — based on materials and processes which can be relied upon to function in a predictable manner. With regard to long-term performance, experience has indicated that geochemical barriers generally tend to be more robust in this sense than mechanical or hydrological barriers. They are also often easier to support with natural analogue arguments. With the aim of developing a practical design of engineered barriers which will provide convincing long-term safety in a wide range of geological environments, optimisation of the materials under consideration is examined from a geochemical perspective.

Improbability of Igneous Intrusion Promoting a Critical Event in Spent Nuclear Fuel Disposed in Unsaturated Tuff

In their regulations, the U.S. Environmental Protection Agency and the U.S. Nuclear Regulatory Commission permit the omission of features, events, or processes with probabilities of <10(-4) in 10(4) yr (e.g., a constant frequency of <10(-8) per yr) in assessments of the performance of radioactive waste disposal systems. Igneous intrusion (or "volcanism") of a geologic repository at Yucca Mountain for radioactive waste is one disruptive event that has a probability with a range of uncertainty that straddles this regulatory criterion and is considered directly in performance assessment calculations. A self-sustained nuclear chain reaction (or "criticality") is another potentially disruptive event to consider, although it was never found to be important when evaluating the efficacy of radioactive waste disposal since the early 1970s. The thesis of this article is that the consideration of the joint event--volcanism and criticality--occurring in any 10,000-year period following closure can be eliminated from performance calculations at Yucca Mountain. The probability of the joint event must be less than the fairly well-accepted but low probability of volcanism. Furthermore, volcanism does not "remove" or "fail" existing hydrologic or geochemical constraints at Yucca Mountain that tend to prevent concentration of fissile material. Prior to general corrosion failure of waste packages, the mean release of fissile mass caused by a low-probability, igneous intrusive event is so small that the probability of a critical event is remote, even for highly enriched spent nuclear fuel owned by the U.S. Department of Energy. After widespread failure of packages occurs, the probability of the joint event is less than the probability of criticality because of the very small influence of volcanism on the mean fissile mass release. Hence, volcanism plays an insignificant role in inducing criticality over any 10(4)-yr period. We also argue that the Oklo reactors serve as a natural analogue and provide a rough bound on probability of criticality given favorable hydrologic or geochemical conditions on the scale of the repository that is less than 0.10. Because the product of this bound with the probability of volcanism represents the probability of the joint event and the product is less than 10(-4) in 10(4) yr, consideration of the joint event can be eliminated from performance calculations.

Modeling of transport and reaction in an engineered barrier for radioactive waste confinement

Bentonites have been proposed as buffer materials for barriers in geological disposal systems for radioactive waste. In these conditions the bentonite barriers may be submitted to changes of humidity, temperature variation, fluid interaction, mass transport, etc. This could modify the physico-chemical performance of the barrier, mainly on the interface with the steel container and with the geological barrier. The engineered barrier development necessitates thus the study of the physico-chemical stability of its mineral constituents as a function of time under the conditions of the repository in the long term. The aim of this paper is to simulate the chemical transformations of the engineered barrier and chemical-elements diffusion impact resulting from the temperature increase (disintegration reactions of wastes), the presence of fluids coming from the geological barrier, and from the liberation of iron (degradation of the containers). The current study was based on the laboratory experiments and mineral characterization achieved by LEM and GREGU research laboratories. The simulations to predict the chemical transformations of argillaceous barrier (MX80 bentonite) and chemical-elements diffusion impact (water-saturated medium) were carried out by using a thermo-kinetic hydrochemical code (KIRMAT: Kinetic Reactions and Mass Transport). This code considers a 1D multisolute mass transport system. Here, all simulations were made in reducing initial conditions ( P O 2 ≅0; E h =−200 mV) and the reaction temperature was considered as constant at 100 °C during 1000 years. The results show that the more significant chemicals transformations were (1) the Na/Ca–montmorillonite-to-Ca–montmorillonite conversion, (2) the montmorillonite-to-chlorite conversion and (3) the dissolution/precipitation of accessory minerals. The first chemical transformation represents about 22% in the worst case, i.e., near of the engineered barrier interfaces while the second chemical transformation only represents about 3% (near the container), i.e., a high concentration of iron favours the chlorite–FeAl precipitation. Concerning the dissolution/precipitation of accessory minerals, in general, it was observed that the quartz, microcline and albite were re-precipitated in the system; the calcite and biotite were partially dissolved, and the pyrite was kept inactive.

Kinetics of long-term illitization of montmorillonite—a natural analogue of thermal alteration of bentonite in the radioactive waste disposal system

A tertiary argillaceous rock formation and a quarternary intrusive rock which intruded into the former, distributed in the Nishikubiki district of Niigata prefecture in central Japan, has been investigated as a natural analogue of bentonite alteration in a repository for high-level radioactive waste. Lateral variation of clay mineral species in the aureole of intrusive rock has been examined and thermal analyses for the argillaceous rock has been carried out based on the cooling history of intrusive rock. Predominant clay mineral varies from montmorillonite to illite through illite/montmorillonite interlayers as the distance to the intrusive rock decreased. The thermal analyses indicate that temperature descended during the past 7.5×10 5 years from 543 to 288 K at a locality of argillaceous rock containing 75% illite in the interlayers. The apparent activation energy is evaluated to be about 107 kJ/mol for the reaction on the assumption that montmorillonite–illite conversion is governed by a first-order reaction. This value suggests little possibility of illitization of bentonite in the geological disposal system in the term of 10 5 years as long as the temperature of the repository environment is approximately 373 K or less.

Combining the radial basis function eulerian and Lagrangian schemes with geostatistics for modeling of radionuclide migration through the geosphere

To assess the long-term safety of a radioactive waste disposal system, mathematical models are used to describe groundwater flow, chemistry, and potential radionuclide migration through geological formations. A number of processes need to be considered, when predicting the movement of radionuclides through the geosphere. The most important input data are obtained from field measurements, which are not available for all regions of interest. For example, the hydraulic conductivity as an input parameter varies from place to place. In such cases, geostatistical science offers a variety of spatial estimation procedures. Methods for solving the solute transport equation can also be classified as Eulerian, Lagrangian and mixed. The numerical solution of partial differential equations (PDE) has usually been obtained by finite-difference methods (FDM), finite-element methods (FEM), or finite-volume methods (FVM). Kansa introduced the concept of solving partial differential equations using radial basis functions (RBF) for hyperbolic, parabolic, and elliptic PDEs. The aim of this study was to present a relatively new approach to the modeling of radionuclide migration through the geosphere using radial basis function methods in Eulerian and Lagrangian coordinates. In this study, we determine the average and standard deviation of radionuclide concentration with regard to variable hydraulic conductivity, which was modelled by a geostatistical approach. Radionuclide concentrations will also be calculated in heterogeneous and partly heterogeneous 2D porous media.

Modelling of radionuclide migration through the geosphere with radial basis function method and geostatistics

The modelling of radionuclide transport through the geosphere is necessary in the safety assessment of repositories for radioactive waste. A number of key geosphere processes need to be considered when predicting the movement of radionuclides through the geosphere. The most important input data are obtained from field measurements, which are not available for all regions of interest. For example, the hydraulic conductivity, as input parameter, varies from place to place. In such cases geostatistical science offers a variety of spatial estimation procedures. To assess the long term safety of a radioactive waste disposal system, mathematical models are used to describe the complicated groundwater flow, chemistry and potential radionuclide migration through geological formations. The numerical solution of partial differential equations (PDEs) has usually been obtained by finite difference methods (FDM), finite element methods (FEM), or finite volume methods (FVM). Kansa introduced the concept of solving PDEs using radial basis functions (RBFs) for hyperbolic, parabolic and elliptic PDEs. The aim of this study was to present a relatively new approach to the modelling of radionuclide migration through the geosphere using radial basis functions methods and to determine the average and sample variance of radionuclide concentration with regard to spatial variability of hydraulic conductivity modelled by a geostatistical approach. We will also explore residual errors and their influence on optimal shape parameters.

The OECD Nuclear Energy Agency Thermochemical Database Project

Abstract The OECD Nuclear Energy Agency (NEA) Thermochemical Database Project aims at making available a comprehensive, internally consistent and quality assured chemical thermodynamic database of selected chemical elements. This database should meet the specialized modeling requirements for safety assessments of radioactive waste disposal systems. This paper describes the evolution of the project from its inception in 1984 to the present organization as a semi-autonomous project under the aegis of the OECD NEA. The scientific principles behind the project organization have not been altered and the effort has resulted in the publication of five major reviews on the chemical thermodynamics of inorganic species and compounds of U, Np, Pu, Am and Tc. Currently, a second series of reviews is being prepared updating the existing, CODATA compatible, database and extending it to include selected data on inorganic species of compounds of Ni, Se and Zr as well as organic complexes with citrate, oxalate, EDTA and iso-saccharinic acid with U, Np, Pu, Am, Tc, Ni, Se, Zr and some other competing cations.

Micromechanics-based prediction of creep failure of hard rock for long-term safety of high-level radioactive waste disposal system

A high-level radioactive waste disposal project is under way in Japan. Isolation of the radioactive waste in a rock formation at great depth is considered as one of the promising disposal methods. Before proceeding with the disposal project, however, assessment of the long-term stability of rock mass surrounding waste packages is necessary. This paper presents a micromechanics-based model for predicting the creep failure of hard rock under compression. The subcritical crack growth due to stress corrosion cracking and interaction effects between cracks are considered as the main mechanisms of creep failure. An evolution problem of two interacting cracks is formulated and creep failure following tertiary creep is represented as unstable extension of the interacting cracks in the present model. The time to failure is calculated for different values of axial stress, confining pressure, and environmental conditions such as temperature and presence of water. The effects of water, temperature, and stress states on the long-term behavior are also included in the proposed model.

Performance Assessment of Low-Level Waste Disposal Facilities Using Coupled Unsaturated Flow and Reactive Transport Simulators

ABSTRACTRecent advances in the development of reactive chemical transport simulators have made it possible to use these tools in performance assessments (PAs) for nuclear waste disposal. Reactive transport codes were used to evaluate the impacts of design modifications on the performance of two shallow subsurface disposal systems for low-level radioactive waste. The first disposal system, located at the Hanford site in Richland, Washington, is for disposal of lowlevel waste glass. Glass waste blocks will be disposed in subsurface trenches, surrounded by backfill material. Using different waste package sizes and layering had a small impact on technetium release rates to the vadose zone. The second disposal system involves a hypothetical repository for low-level waste in Italy. A model of uranium release from a grout waste form was developed using the STORM reactive transport code. Uranium is predicted to be relatively insoluble for several hundred years under the high-pH environment of the cement pore water. The effect of using different filler materials between the waste packages on uranium flux to the vadose zone proved to have a negligible impact on release rates.

Degradation of phosphatic waste forms incorporating long-lived radioactive isotopes

AbstractThis paper provides a brief perspective on synthetic, phosphate-based waste forms for high level radioactive waste (HLW). Evidence in support of their long-term stability is then discussed by reference to the degradation of natural monazites with emphasis on the fate of released uranium, thorium and the rare earths (REE). It is apparent that the REE can be mobilized and fractionated at temperatures anticipated in a HLW repository (∼200°C). This provides an indication of the likely fate of the trivalent actinides (Am(III), Cm(III)) if incorporated in similar matrices. Thorium, though released on alteration of monazite, tends to re-concentrate locally in secondary, microcrystalline phases. In relative terms, U is readily removed from monazites. Although it can be re-concentrated in alteration products, the potential exists for substantial loss of U to groundwater. The findings of this research have important implications for the performance of radioactive waste disposal systems where there is a clear need for improved chemical data to describe the precipitation-dissolution of phosphate phases. It is concluded that monazite-like ceramics designed for the containment of HLW will retain tetravalent actinides but may release uranium in response to natural degradative processes.