Effects of Source and Geometry Modeling on the Shielding Calculations for a Spent Nuclear Fuel Dry Storage Cask

Surface dose rate distribution over a spent nuclear fuel dry storage cask was realistically evaluated using the MONACO with Automated Variance Reduction using Importance Calculations (MAVRIC) computational sequence in the SCALE6 code system, with special emphasis on the effects of detailed modeling on the source term and cask geometry. The first storage cask in Taiwan has been fabricated and will be ready for loading of the designated spent fuels from Taiwan Power Company's first nuclear power plant. A test run is scheduled for 2013.

Neutron and gamma-ray source terms of the first batch of 56 spent fuels were determined one by one according to their specifications, burnup histories, and cooling times. The geometry of the cask was modeled in detail including the prescribed loading pattern of 56 spent fuels in the canister. MAVRIC was modified to allow specification of the source intensity and the axial distribution for each fuel bundle, and this resulted in a factor of 3 difference in the calculated surface dose rates from fuel gammas. The main purpose for such comprehensive and detailed modeling was to compare the results with a simplified model and to predict a dose rate distribution as realistically as possible in preparation for making a high-quality comparison with field measurements. In addition to checking assumptions adopted in the safety analysis report, the results of this study can provide useful guidance for the preparation of a health physics program during the test run and, more importantly, pave the way for establishing a valuable benchmark problem.