Scenario Development Methodology for Performance Assessment of Near-surface Lilw Repository Based on Feps and Interaction Matrix Approach

Systematic procedure of developing radionuclide release scenarios was established based on FEP list and Interaction Matrix for the near-surface LILW repository. The relevant FEPs were screened by experts’ review in terms of domestic situation and combined into scenarios on the basis of Interaction Matrix analysis. A computer program named IMFEP_NS was developed to view and select project FEPs, to make its Interaction Matrix at user’s disposal, and to visualize the interaction between FEPs and Interaction Matrix. The concept of approach to generate scenarios for entire domain is to divide the whole system domain into three sections: Near-field, Far-field, and Biosphere. Possible sub-scenarios were generated within each sectional subscenario set composed by assembling relevant FEPS and Interaction Matrix in advance, and then scenarios for entire system were built up with sub-scenarios of each section. As an application of established scenario generation approach, sixteen design scenarios and two alternative scenarios for near-surface repository were evaluated. Finally, a reference scenario and other noteworthy scenarios were selected through experts’ scenario screening. INTRODUCTION The generation of scenarios and their associated justification methodology has become a very important aspect of confidence building for the post-closure safety assessment of radioactive waste repository. Although lots of scenarios were recognized from the past scenario development studies, it has been needed to establish a systematic framework and development procedure(1). To supplement this needs, the Rock Engineering System matrix method which utilizing Features, Events and Processes(FEP) and Interaction Matrix(IM) was examined and adopted(2,3). This study is focused on the development and application of a methodology, based on FEP and IM, to systematize the procedure for developing radionuclide release scenarios in a near-surface LILW repository in Korea. FEPs database was set up and underwent experts’ review to screen out those irrelevant to domestic situation in the first step of scenario development. And then, IM was created in connection with the qualified FEPs. A computer program named IMFEP_NS was developed for this purpose. It was possible to recognize and develop scenarios by combining FEPs on the basis of IM. The procedure for moving from a comprehensive FEPs database to a set of justified scenarios are often poorly developed and documented. In this study, an approach to generating scenarios WM’02 Conference, February 24-28, 2002, Tucson, AZ 2 from those screened FEPs and IM was developed on the basis of divide-and-combine concept. It consists of two stages of procedure, firstly dividing the repository system domain into three sections from near-field to biosphere and developing sub-scenarios for each section in advance, and then combining sub-scenarios of each section to buildup the scenarios for entire system. Scenario development methodology using the developed approach was applied for generating a complete set of scenarios to be used in the safety/performance assessment of near-surface disposal system in Korea. Scenario screening by expert judgment is also applied. After experts’ review on these developed scenarios, reference scenarios expected to have relatively high probability were selected. APPROACH TO SCENARIO DEVELOPMENT Systematic procedure of developing radionuclide release scenarios was established based on FEP list and IM for the near-surface LILW repository. The relevant FEPs were screened by experts’ review in terms of domestic situation and combined into scenarios on the basis of IM analysis. IMFEP_NS (Interaction Matrix and FEP Viewer for Scenario Development of Near Surface Repository) was developed to view and select project FEPs from an extensive FEP database, to make its IM at user’s disposal, and to visualize the interaction between FEPs and IM. FEPs can be mapped to the all matrix components thought to have interaction by using this tool. Figure 1 illustrates a screen clip on FEP database viewer within the IMFEP_NS. Fig. 1. An illustrative clip of IMFEP_NS The concept of approach to developing scenarios from those screened FEPs and IM for entire domain is shown in Figure 2. The whole system domain was divided into three sections such as Near-field, Far-field, and Biosphere. Possible sub-scenarios were generated within each WM’02 Conference, February 24-28, 2002, Tucson, AZ 3 sectional sub-scenario set labeled their own numbers in advance. Each sectional sub-scenario was composed by assembling relevant FEPs along the directions shown in IM. A number of subscenarios could be generated in each section. Though 2 scenarios are possible from combination of n FEPs theoretically, it is needed to identify a limited number of representative scenarios rather than comprehensively identify every possible scenario. The existing list of generic scenarios can serve as a guidance of what scenario to consider for a given geologic condition and disposal facility type. After it was done over three sections to pick out one subscenario from a sectional sub-scenario set, scenarios for the complete domain were created by combining these three sub-scenarios into one overall scenario. Applicable scenarios set could be established by iterating this work for all possible combinations. To identify the more important scenarios, the ranking system based on expert judgment was introduced to the scenario screening. Importance of consequence, probability, and uncertainty were used as screening criteria for possible scenarios. Fig. 2. Concept of approach to scenario development APPLICATION OF SCENARIO DEVELOPMENT METHODOLOGY Established scenario development methodology was applied to generate various design and alternative scenarios for performance assessment of a hypothetical engineered vault disposal facility in Korea. To limit the number of all possible scenarios under control, the highest-level assumptions were introduced to categorize the created scenarios into design scenario in this study. Here the term of Near field Far field Biosphere Near field Sub-Scenarios Far field Sub-Scenarios Scenarios Biosphere Sub-Scenarios Near-field Sub -scenario #1 Near-field Sub –scenario #2 Far-field Sub -scenario #1 Far-field Sub –scenario #2 Far-field Sub –scenario #3 Near-field Sub –scenario #i Near-field Sub –scenario #l Far-field Sub –scenario #j Far-field Sub –scenario #m Biosphere Sub -scenario #1 Biosphere Sub –scenario #2 Biosphere Sub –scenario #3 Biosphere Sub –scenario #k Biosphere Sub –scenario #n Scenario #1