Analysis of backfill performance of engineered barrier from view point of nuclide migration

This report describes barrier performance of geological disposal of high level radioactive waste. Three different simulation codes, SWIFT, FEMWASTE, and ourown code RASACAL, were used to analyse migration behavior in both disposal pit and horizontal tunnel. Neptunium 237 was chosen as a dominant nuclide in hazardous impaction due to water intrusion accident in disposal site. Some topical results are as follows; 1. In the case of compacted bentonite, Np-237 migtates in vertical direction of pit because of smaller adsorption coefficient than surrounding rock medium. 2. Migration in horizontal tunnel is mainly depends on adsorption characteristics of backfilling materials. The effect of groundwater velocity in horizontal tunnel may be minor when the velocity range is small. 3. Although the mass flux from disposal pit is normally a function of water velocity, a maximum mass flux value was found. (author)