Deep geological disposal is generally accepted to be the most practical approach to handling radioactive wastes. Bentonite has been considered as a buffer material in deep geological disposal repositories (DGR) for high-level radioactive wastes. Evaluating the effect of short-term bentonite alteration on EBS performance has limitations in safety assessment over thousands of years. Information on bentonite characteristics under various conditions obtained from natural systems can be used to evaluate long-term safety of bentonite buffer. The purpose of this study was to investigate mineralogical and physicochemical characteristics of bentonite in the Naah mine located in Yangnam-myeon, Gyeongju-si for a natural analogue of the bentonite barrier in DGR. A total of 15 samples were collected at regular intervals from the bentonite layer and andesitic lapilli tuff (i.e., parent rock) at the boundary with the bentonite layer. The bentonite layer is located at a depth of about 1 m below the ground surface. Each sample was separated into particles < < 75 μm and particles < 2 μm through grinding and sedimentation processes. The separated subsamples were characterized mineralogically and physiochemically using various analytic techniques. Bentonite samples have a similar SiO2/Al2O3 ratio to the parent rock and a lower (Na+K)/Si ratio than the parent rock, indicating depletion of alkali components during bentonitization. The parent rock and bentonite samples have similar mineral composition (i.e., quartz, feldspars, opal-cristobalite-tridymite and montmorillonite). Results of XRD analysis on the randomly distributed particles < 2 μm indicate that bentonite is mostly composed of Ca-montmorillonite, which is a typical dioctahedral smectite. Results of FTIR and VNIR analysis indicate that montmorillonite contained in bentonite is Al-dioctahedral montmorillonite, and Al is substituted with Mg in some octahedron units. The mineralogical and physicochemical characteristics are similar regardless of sampling location. These results suggest that bentonite potentially exposed to weathering, located near the ground surface, has hardly altered.

In the design of a spent-fuel (SF) storage, the consideration of burnup credit brings the benefits in safety and economic views. According to it, various SF burnup measurement systems have been developed to estimate high fidelity burnup credit, such as FORK and SMOPY. Recently, there are a few attempts to localize the SF burnup measurement system in South Korea. For the localization of SF burnup measurement systems, it is very important to build the isotope inventory data base (DB) of various kinds of SFs. In this study, we performed DeCART2D/MASTER core follow calculations and McCARD single fuel assembly (FA) burnup analyses for Hanbit unit 3 and confirmed the characteristic of the isotope inventory over burnup. Firstly, the core follow calculations for Cycles 1~7 were performed using DeCART2D/MASTER code system. The core follow calculation is very realistic and practical because it considers the design conditions from its nuclear design report (NDR). Secondly, the Monte Carlo burnup analyses for single FAs were conducted by the McCARD Monte Carlo (MC) transport code. The McCARD code can utilize continuous energy cross section library and treat complex geometric information for particle transport simulation. Accordingly, the McCARD code can provide accurate solutions for burnup analyses without approximations, but it needs huge computing resources and time burden to perform whole-core follow calculations. Therefore, we will confirm the effectiveness of the single McCARD FA burnup analyses by comparing the DeCART2D/MASTER core follow results with the McCARD solution. From the results, the use of single FA burnup analyses for the establishment of the DBs will be justified. Various FAs, that have different 235U enrichments and loading pattern of fuel rods and burnable absorbers, were considered for the burnup analyses. In addition, the results of the sensitivity analyses for power density, initial enrichment, and cooling time will be presented.