Total System Performance Assessment Research Articles

Comparison of Numerical Analysis Methods of APro for the Total System Performance Assessment of a Geological Disposal System

Comparison of Numerical Analysis Methods of APro for the Total System Performance Assessment of a Geological Disposal System

A modularized numerical framework for the process-based total system performance assessment of geological disposal systems

This study developed a safety assessment tool for geological disposal systems called APro, a systemically integrated modeling system based on modularizing and coupling the processes which need to be considered in a geological disposal system. Thermal, hydraulic, chemical, canister failure, radionuclide release and transport processes were considered in the current version of APro. Each of the unit processes in APro consists of a single Default Module, and several Alternative Modules which can increase the flexibility of the model. As an initial stage of developing the modularization concept and modeling interface, the Default Modules of each unit process were described, with one Alternative Module of chemical process. The computation part of APro is mainly a MATLAB workspace controlling COMSOL and PHREEQC, which are coupled by an operator splitting scheme. The APro model domain is a stylized geological disposal system employing the Swedish disposal concept (KBS-3 type), but the repository layout can be freely adjusted. In order to show the applicability of APro to the total system performance assessment of geological disposal system, some sample simulations were conducted. From the results, it was confirmed that coupling of the thermal and hydraulic processes and coupling of the canister failure and the radionuclide release processes were well reflected in APro. In addition, the technical connectivity between COMSOL and PHREEQC was also confirmed.

Sequential Bayesian Updating Module of Input Parameter Distributions for More Reliable Probabilistic Safety Assessment of HLW Radioactive Repository

A Bayesian approach was introduced to improve the belief of prior distributions of input parameters for the probabilistic safety assessment of radioactive waste repository. A GoldSim-based module was developed using the Markov chain Monte Carlo algorithm and implemented through GSTSPA (GoldSim Total System Performance Assessment), a GoldSim template for generic/site-specific safety assessment of the radioactive repository system. In this study, sequential Bayesian updating of prior distributions was comprehensively explained and used as a basis to conduct a reliable safety assessment of the repository. The prior distribution to three sequential posterior distributions for several selected parameters associated with nuclide transport in the fractured rock medium was updated with assumed likelihood functions. The process was demonstrated through a probabilistic safety assessment of the conceptual repository for illustrative purposes. Through this study, it was shown that insufficient observed data could enhance the belief of prior distributions for input parameter values commonly available, which are usually uncertain. This is particularly applicable for nuclide behavior in and around the repository system, which typically exhibited a long time span and wide modeling domain.

Development of User-friendly Modeling Interface for Process-based Total System Performance Assessment Framework (APro) for Geological Disposal System of High-level Radioactive Waste

A user-friendly modeling interface is developed for a process-based total system performance assessment framework (APro) specialized for a generic geological disposal system for high-level radioactive waste. The APro modeling interface is constructed using MATLAB, and the operator splitting scheme is used to combine COMSOL for simulation of multiphysics and PHREEQC for the calculation of geochemical reactions. As APro limits the modeling domain to the generic disposal system, the degree of freedom of the model is low. In contrast, the user-friendliness of the model is improved. Thermal, hydraulic, mechanical and chemical processes considered in the disposal system are modularized, and users can select one of multiple modules: “Default process” and multi “Alternative process”. APro mainly consists of an input data part and calculation execution part. The input data are prepared in a single EXCEL file with a given format, and the calculation part is coded using MATLAB. The final results of the calculation are created as an independent COMSOL file for further analysis.

Progress of the long-term safety assessment of a reference disposal system for high level wastes in Korea

KAERI developed a reference repository system for the disposal of radioactive wastes resulting from the pyroprocessing of PWR spent nuclear fuels (A-KRS; Advanced Korean Reference Disposal System). To check the design feasibility of this system, we developed a total system performance assessment (TSPA) tool using Goldsim program (GoldSim, 2006) and assessed the exposure dose rates for the reference scenario and three alternative scenarios such as earthquake, well intrusion, and initial defect of waste packages by using this tool. And then, we compared the exposure dose rates for each scenario with a draft supplementary safety goal, 10 mSv/yr, which was suggested by the regulatory body in Korea. In the original TSPA tool, we use a single source term model by assigning the same reference radionuclide inventory data for each waste package and the same shortest distance from the waste package to the MWCF (Major Waster Conducting Features) for conservatism. We developed a multi-lattice source term model by assigning various radionuclide inventory data and various distance from the waste package to the MWCF for more realistic safety assessment. Then, we found that the exposure dose rates for the case of multi-lattice source term model was one order of magnitude lower than those for the case of single source term model. According to the draft safety guideline, the primary safety goal is expressed in risk, that is, the total annual risk for the representative person resulting from the radiation exposure should not exceed 1.0 × 10−6/yr. Therefore, we developed a methodology for complex scenarios to perform the risk-based safety assessment of a HLW repository, and applied it to a reference repository system. And we found that the suggested methodology can be used to perform the risk-based long-term safety assessment of a radioactive waste repository. We can make various risk profiles by making various kinds of complex scenarios with this methodology, and they can be used to support the development of safety cases for acquiring public acceptance.

A-KRS 처분 시스템 확률론적 안전성 평가

A GoldSim template program for a safety assessment of a hybrid-typed repository system, called A-KRS, in which two kinds of pyro-processed radioactive wastes, low-level metal wastes and ceramic high-level wastes that arise from the pyro-processing of PWR nuclear spent fuels are disposed of, has been developed. This program is ready both for a deterministic and probabilistic total system performance assessment which is able to evaluate nuclide release from the repository and farther transport into the geosphere and biosphere under various normal, disruptive natural and manmade events, and scenarios. The A-KRS has been probabilistically assessed with 9 selected input parameters, each of which has its own statistical distribution for a normal release and transport scenario associated with nuclide release and transport in and around the repository. Probabilistic dose exposure rates to the farming exposure group have been evaluated. A sensitivity of 9 selected parameters to the result has also been investigated to see which parameter is more sensitive and important to the exposure rates.

우리나라의 고준위폐기물 처분을 위한 FEP과 시나리오 개발

The impacts influenced on the performance and safety of a repository are classified as units of Features, Events, and Processes (FEP), for the total system performance assessment (TSPA) related to the permanent disposal of HLW. The importance is evaluated in consideration of the frequency, consequence, regulation, suitability of a specific site, etc. and then these are grouped as a similar FEP. A scenario describing the migration of radionuclide from the repository to the biosphere is derived from understanding the interaction among these groups. KAERI has developed the KAERI FEP lists by review and collation of the foreign studies. The KAERI FEP list has been reviewed by several Korean experts. The five major scenarios describing possible future evolutions of the geological disposal system have been developed by RES and PID methods. Also the CYPRUS which is a KAERI integrated database management system for the total system performance assessment (TSPA) related to the permanent disposal of HLW has been developed and the results of the FEP and scenario development have been uploaded in this system.

A-KRS 처분 시스템 결정론적 안전성 평가

파이로 처리 방사성 폐기물 처분 시스템 A-KRS에 대한 결정론적, 확률론적 안전성 평가가 가능하도록 골드심 프로그램을 개발하였다. 처분장에서 핵종이 유출되어 다양한 처분 시스템 내 매질을 이동하는 것에 관련된 주요 시나리오를 도출하기 위하여 처분 시스템을 구성하는 각 요소 별로, 그리고 처분 시스템 내 핵종의 이동에 관여하는 FEP을 인지 선별하여 수학적 모델을 개발하고 이를 골드심 템플릿 프로그램으로 개발하였다. 이 프로그램을 이용하여 폐쇄후 방사선적 안전성 평가를 결정론적으로 수행하여 5개의 정상 및 비정상 시나리오를 평가하여 그 결과를 상호 비교하였다. A GoldSim template program for a safety assessment of a hybrid-typed repository system, called "A-KRS," in which two kinds of pyro-processed radioactive wastes, low-level metal wastes and ceramic high-level wastes that arise from the pyro-processing of PWR nuclear spent fuels are disposed of, has been developed. This program is ready both for a deterministic and probabilistic total system performance assessment which is able to evaluate nuclide release from the repository and farther transport into the geosphere and biosphere under various normal, disruptive natural and manmade events, and scenarios. The A-KRS has been deterministically assessed with 5 various normal and abnormal scenarios associated with nuclide release and transport in and around the repository. Dose exposure rates to the farming exposure group have been evaluated in accordance with all the scenarios and then compared among other.

HISTORICAL DEVELOPMENT AND EVOLUTION OF EPRI'S POST-CLOSURE DOSE ASSESSMENT OF POTENTIAL RELEASES TO THE BIOSPHERE FROM THE PROPOSED HLW REPOSITORY AT YUCCA MOUNTAIN

This paper describes the development and evolution of the Electric Power Research Institute's (EPRI) post-closure dose assessment for potential releases of radionuclides from the proposed High Level Waste repository at Yucca Mountain. The starting point for this work was the 1995 publication of Technical Bases for Yucca Mountain Standards by the Commission on Geosciences, Environment and Resources of the National Research Council. This report proposed the development and application of an individual risk-based standard for releases from the repository to replace the existing one, which was based on radionuclide release limits. This in turn implied the development and application of methods to assess radiation doses to humans. Accordingly, EPRI produced a methodology for such dose assessment as part of its Total System Performance Assessment program for the proposed Yucca Mountain repository site. The methodology initially addressed releases via groundwater and then releases associated with extrusive igneous events. The methodology was updated and applied over the following years to take account of regulatory developments, changes in estimates of the source term to the biosphere, peer review through international model comparison exercises, new site generic data, and new data concerning conditions at the point of compliance in Amargosa Valley. The main outputs were Biosphere Dose Conversion Factors, which relate radionuclide levels in environmental media to the annual individual doses to a member of a hypothetical critical group and to the regulator-defined Reasonably Maximally Exposed Individual. Most recently, consideration has been given to uncertainty in the dose estimates based on a probabilistic analysis. The paper provides a perspective on the evolution of the dose assessments in response to the developments listed above.

THE USE OF NUMERICAL MODELS IN SUPPORT OF SITE CHARACTERIZATION AND PERFORMANCE ASSESSMENT STUDIES FOR GEOLOGICAL REPOSITORIES

The paper is describing work being developed in the frame of a 5-year IAEA Coordinated Research Programme (CRP) started in late 2005. Participants gained knowledge of modelling methodologies and experience in the development and use of rather sophisticated simulation tools in support of site characterization and performance assessment calculations. These goals were achieved by a coordinated effort, in which the advantages and limitations of numerical models are examined and demonstrated through a comparative analysis of simplified, illustrative test cases. This knowledge and experience should help them address these issues in their own country’s nuclear waste program. Coordination efforts during the first three years of the project aimed at enabling this transfer of expertise and maximizing the learning experience of the participants as a group. This was accomplished by identifying common interests of the participants (i.e., Process Modelling and Total System Performance Assessment methodology), and by defining complementary tasks that are solved by the members. Synthesis of all available results by comparative assessments is planned in the coming months. The project will be completed end of 2010. This paper is summarizing activities up to November 2009.

Current Status of Radioactive Waste Management in Korea

Korean energy security is dependent on the peaceful and safe utilization of nuclear energy. The Korean government’s policy on nuclear energy can be summarized with two principles: promoting the peaceful uses of nuclear energy and ensuring its safe use and storage. Since April 1978 Korea has strongly relied on nuclear energy for its electricity generation. As of today, 20 nuclear power plants, 16 pressurized water reactors PWRs and 4 Canada Deuterium Uranium CANDU reactors are in operation, and 4 PWRs are under construction. The installed nuclear capacity is 17.7 GWe, representing 27% of the nation’s total installed capacity. The nuclear share in electricity remains at around 37.8%, reaching a level of 146.8 TWh. New power reactors in Korea Standard Nuclear Power Plants are fully designed by domestic technologies. A more advanced reactor named AP1400 will be commissioned in 2020. The Achilles’ heel of nuclear energy is radioactive waste. Even though proper scientific management of radioactive waste has been found and proved safe for lowand intermediate-level waste LILW disposal and an interim storage of spent nuclear fuel and even the introduction of permanent disposal of high-level radioactive waste HLW is envisaged to be feasible, the public perception on these issues is not supportive. The public’s everlasting “not in my backyard” refrain has hindered the implementation of Korea’s radioactive waste management program for the last two decades. Overcoming the safety issues involved in radioactive waste management is not a simple scientific problem. It requires attention to both state-of-the-art science and technology and the psychology of the general and local public. In fact, it is a transscientific issue, a fusion of science and socioeconomic studies. The Korean Atomic Energy Commission provided clear milestones for constructing two key facilities: the LILW repository by 2008 and the centralized storage for spent fuel by 2016. With the full support of the Korean government, institutes, and general public, Korea Hydro Nuclear Power KHNP secured a site for the LILW repository in 2005. An underground repository with the capacity for 100,000 drums is under construction, and will be in commission by 2012. Long-term safe management of spent nuclear fuel is necessary not only to store spent fuel over a long period of time but also to potentially recycle it under full observance of the nuclear nonproliferation duties. The permanent disposal of HLW is the research and development R&D topic for a future generation and involves protecting the environment and minimizing the economic burden. Since 1997 Korea Atomic Energy Research Institute KAERI has been developing a permanent disposal facility for HLW and a total system performance assessment TSPA . Its current R&D activities are focused on the preliminary conceptual design of the Korean Reference Disposal System KRS , development of the

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.

Analysis of the Treatment, by the U.S. Department of Energy, of the FEP Hydrothermal Activity in the Yucca Mountain Performance Assessment

The U.S. Code of Federal Regulations, 10 CFR Part 63, stipulates that the expected performance of the geological high-level nuclear waste repository must be demonstrated through a total system performance assessment (TSPA). The TSPA represents an analysis that identifies all features, events, and processes (FEPs) that might affect the disposal system and examines the effects of the identified FEPs upon the performance of the system. Secondary minerals from the thick unsaturated zone of Yucca Mountain were deposited from waters with temperatures up to 70-90 degrees C. U-Pb dating constrained the ages of the elevated temperatures to the period between 10 and 5-6 million years ago. Relatively youthful circulation of thermal waters (hydrothermal activity) would be of concern for the safety of the disposal facility. A phenomenological model was advanced by the U.S. Department of Energy (DOE), which proposed that the minerals were deposited by infiltrating meteoric waters that were heated upon contact with the bedrock; it was hypothesized that the latter was conductively heated by a shallow silicic magma body. The model rendered processes responsible for elevated water temperatures inconsequential for the safety of the proposed nuclear waste facility. However, attempts by DOE at validating the model by means of numeric thermal simulations and analogue system observations were unsuccessful. Regulations specify two criteria for exclusion of a FEP from consideration in the TSPA: low probability and low consequence. The lack of a plausible phenomenological model makes it impossible to apply either of these two criteria to the FEP Hydrothermal Activity. Despite the lack of a valid criterion for exclusion, it was excluded from the TSPA. Both the development of DOE's thermal model and the formal FEP analysis were associated with deviations from DOE's quality assurance regulations.

Development of the KAERI FEP, scenario, and assessment method database for permanent disposal of HLW in Korea

To develop the scenarios of radionuclide release after the closure of a deep geological repository in Korea, the Features, Events and Processes (FEPs) are collected. Based on the screening procedures with expert opinions, important FEPs are identified. FEPs are then classified into base and probabilistic ones. Base FEPs create the base scenarios and probabilistic FEPs create the alternative scenarios. Combining these FEPs into integrated FEPs (IFEPs) turns out to easily construct the RES matrix. After an RES matrix is developed, a scenario is developed for each corresponding RES matrix. Then assessment context and a flow chart are developed to be used for the total system performance assessment.

Approach to Assessing the Potential Effects of Colloidal Radionuclide Transport on Nuclear Waste Repository Performance

The U.S. Nuclear Regulatory Commission (NRC) is independently evaluating technical issues such as colloid-facilitated radionuclide transport in preparation for reviewing an anticipated license application from the U.S. Department of Energy (DOE) for a potential high-level nuclear waste repository at Yucca Mountain, Nevada. For performance assessment computer simulations of evolving conditions many years into the future, the influence of colloids in enhancing radionuclide transport is difficult to estimate and highly uncertain. NRC staff is conducting a multipronged approach to assessing whether or not these uncertainties are sufficiently represented by performance assessment models. Preliminary simplified calculations providing a conservative estimate of calculated dose from colloidal Pu suggest that an effect on dose is plausible. A more sophisticated effort involves analytical modeling of colloidal Pu transport that uses laboratory and field data to represent more accurately processes such as kinetic controls on sorption (attachment) and desorption (detachment) of radionuclides at colloid surfaces. This modeling effort shows that slow desorption of radionuclides from colloids is a factor that could enhance radionuclide migration. Finally, an abstraction of colloidal transport is being implemented in the NRC total-system performance assessment model in order to integrate potential colloidal effects at the system level. This implementation is flexible enough that a variety of sensitivity studies can be conducted that will aid identification of the model parameters most significant to transport.

Risk assessment of uniform and localized corrosion of alloy 22

The risk associated with the performance of alloy 22 waste package (WP) in the potential repository for high-level nuclear waste at Yucca Mountain (YM) was assessed using the Nuclear Regulatory Commission’s Total-System Performance Assessment (TPA) Code. The high-temperature (above 100 °C) deliquescence relative humidity (RH) from mixed salt deposits on the WP surface was evaluated by lowering the critical RH (RHcritical) for aqueous corrosion to 35 to 60 pct. For the base case values of the critical potential for initiation of localized corrosion, the estimated dose increased from 0.05 to 1 mrem/year in 10,000 years by altering RHcritical. For the modified case, by adding more data and refining the fitting, the estimated dose increased to 3.8 mrem/year at 10,000 years without lowering RHcritical. Using the new repassivation potential with nitrate as an inhibitor without changes in RHcritical, the estimated dose decreased to 0.03 mrem/year at 10,000 years. Taking credit for the remaining surface area of the WP after failure by localized corrosion reduces the estimated dose from 4 to 0.4 mrem/year. Anodic sulfur segregation at the interface of metal and passive film and subsequent spalling of passive film may enhance uniform corrosion. The cyclic process of fast active corrosion upon sulfur segregation followed by slow passive corrosion upon repassivation is unlikely to significantly reduce the WP lifetime.

“Reference Biospheres” for solid radioactive waste disposal: the BIOMASS Methodology

The BIOMASS Theme 1 project has developed a methodology for the logical and defensible construction of ‘assessment biospheres’: mathematical representations of biospheres used in the total system performance assessment of radioactive waste disposal. The BIOMASS Methodology provides a systematic approach to decision making, including decisions on how to address biosphere change. The BIOMASS Methodology was developed through consultation and collaboration with many relevant organisations, including regulators, operators and a variety of independent experts. It has been developed to be practical and to be consistent with recommendations from ICRP and IAEA on radiation protection in the context of the disposal of long-lived solid radioactive wastes. The five main steps in the methodology are described in this paper. The importance of a clear assessment context, to clarify intentions and to support a coherent biosphere assessment process within an overall repository performance assessment, is strongly emphasised. A well described assessment context is an important tool for ensuring consistency across the performance assessment as a whole. The use of interaction matrices has been found to be helpful in clarifying the interactions between different habitats within the biosphere system and the significant radionuclide transfer pathways within the biosphere system. Matrices also provide a useful means of checking for consistency.

The evaluation of corrosion processes for engineered barrier systems

The U.S. Nuclear Regulatory Commission (NRC) established a corrosion program to evaluate the predicted performance of the U.S. Department of Energy’s proposed engineered barrier system for the potential high-level waste geologic repository at Yucca Mountain, Nevada. The risk-informed, performance-based regulation established by the NRC for the potential licensing of the proposed repository is introduced in this article with a discussion of resolution methods for various technical issues. These resolution methods include probability or consequence screening, deterministic confirmatory research investigations, and the use of the Total-System Performance Assessment Code to determine the risk significance of specific materials degradation processes on the total system performance. An example of the deterministic analysis method is the evaluation of the susceptibility of mill-annealed and gas-tungsten-arc welded alloy 22 to lead-assisted stress corrosion cracking in deaerated PbCl2 and Pb(NO3)2 solutions at 95°C. Also in this article is an evaluation of the risk significance of high-temperature deliquescence-induced localized corrosion of the alloy 22 waste package outer barrier on the total system performance.

Application of the TSPA Glass Degradation Model to Non-Conforming Waste Forms

AbstractAn approach is presented for determining if the models used to calculate the release of radionuclides from defense high-level radioactive waste (HLW) glass for total system performance assessment (TSPA) calculations can be used to account for the release of radionuclides from waste forms other than standard borosilicate glasses. The fractional release rates of radionuclides due to waste form degradation, the available surface area, and the radionuclide inventory in an alternative waste form can be compared with the corresponding models used in TSPA for HLW glasses to determine if those models adequately represent the waste form. This approach is demonstrated for the ceramic and metallic waste forms developed for electrometallurgically treated spent sodium-bonded nuclear fuel. Depending on the waste form, comparisons made with aspects of the HLW glass model may be based on similarities in degradation mechanisms or purely empirical.

Risk-Based Evaluation of Long-Term Safety for a Yucca Mountain Repository Using the IMARC TSPA Code

AbstractThe Electric Power Research Institute (EPRI) has conducted independent total system performance assessments (TSPAs) of the proposed Yucca Mountain spent fuel and high-level radioactive waste repository for 15 years. EPRI uses its TSPA code, IMARC, currently in its eighth version. The major results of the IMARC-8 analyses are presented in this paper. In all of the situations evaluated by EPRI using IMARC-8, the results for the reference repository concept for Yucca Mountain are well within the regulatory criteria established in the applicable regulations, 10 CFR 63 and 40 CFR 197. Further analyses indicate that at least two of the repository barriers must fail to function as anticipated for dose risks to rise as much as one millirem per year (1/15th of the regulatory limit).