Dose Equivalent Rate Benchmark Calculations of a Dry Storage Cask for Spent Fuel by 3D Monte Carlo code

In Japan, 9 PWR plants have restarted in recent three years. Consequently storing spent fuel assemblies will be a critical issue to be solved in the near future because spent fuel pools in each plant will be filled up with their spent fuel assemblies generated by plant operation. One candidate solution for storing spent fuel assemblies is to store them in dry casks instead of spent fuel pools, because dry casks provide high sealing integrity of radioactive material and structural toughness. Many designers have therefore been trying to apply their new dry cask designs with an advanced shielding calculation method instead of conventional 2D calculation codes which have been used in licensing examination by a regulatory agency. The 3D Monte Carlo calculation codes can evaluate dose equivalent rate of cask with complex structure more sophisticatedly and accurately than 2D calculation codes, and it has become more common to be used in detailed calculation for designing. To apply 3D Monte Carlo calculations to design of dry casks, its Verification & Validation procedure has to be performed. However, only a few ‘detailed benchmark problems’ are available to verify 3D Monte Carlo codes for dry cask application. Thus, MHI conducted an experiment with a trial small dry cask which contains a spent fuel and has shielding structures similar to actual dry casks, and measured dose equivalent rates at its surfaces and surroundings. MHI then performed 3D Monte Carlo calculations by MCNP code with detailed analysis model with spent fuel assembly, cask structure including shielding, chemical compositions of materials and so on. The calculated dose equivalent rates agreed well with the measured values within 20% deviations. Therefore, it can be concluded that MCNP code can calculate dose equivalent rates of dry cask for licensing.