Assessment of post closure radioactive safety for the Korean reference disposal system: development of scenarios and quantitative assessment

The total system performance assessment (TSPA) on the Korean reference disposal system has been performed for different types of scenarios. Firstly two reference scenarios, the natural discharge and well ones are developed assessed. The natural discharge scenario assumes that a radionuclide is released from a waste container with an average lifetime of 1,000 years by intruding groundwater to a biosphere through a bentonite buffer and a natural barrier composed of a fractured porous rock and a major water conducting feature (MWCF). The well scenario describes that a radionuclide passing through a buffer enters a fractured rock which is intersected with a drinking well. Two scenarios are named as R1 and R2 respectively. The third scenario is for the initial waste container failure case. A waste container is apt to have initial defects during manufacturing and transportation to a deposition hole. The probability function of the ratio of waste container failure is assumed based on the engineering sense. The rest of waste containers are assumed to have full function of isolation of hazardous nuclides during the lifetime. This initial container failure scenario (ICF) has two different variations: one with a lifetime of 1,000 years ana the other with 10,000 years. Two variations are assessed for two different biosphere, natural discharge and well. The forth one is to assess the impact of excavation disturbed zones. Deposition tunnels are excavated by tunnel boring machine (TBM) or controlled blast (CB). The disturbed zone in assumed to be 30 cm and 1 meter for TBN and CB respectively. Six cases are developed for the EDZ scenarios considering all possible combination of changes in permeability a fracture aperture, and a porosity of a fractured rock. The fifth scenario stipulates the change of long term climate (LTC). The ice age assumed to be prevailed again after a few tens of thousand years. The advent of the ice age alters groundwater composition, pathways, and most importantly the nature of human life. Two cases are developed for the ice age scenario. The first one is the case that the groundwater pathway is shortened. This is feasible when a new shorter pathway is evolved between a repository and surface. Also it is possible when a future resident drills a deeper borehole to extract drinking water into a fractured rock adjacent to a repository. Even though groundwater is released into a ground surface, the pattern in usage of water will be significantly changed. To accommodate it, the dose conversion factors in the current boreal climate are used to assess the annual individual dose. The next scenario is still under development. It aims to deal with the case with a fault reactivation (FR) near a repository. Reactivation is assumed to reactivate and eventually widen an existing joint and its properties so that the groundwater flow speed and/or pathways will be altered. The TSPA 2006 by KAERI is to assess these two reference and four alternative scenarios with many sub-cases and parameter uncertainties for given Korean geological data set