Evaluation method of hydrogen generation rate by water radiolysis in fuel debris canisters considering the radiation energy absorption rate of water dependence on particle size

An evaluation method of hydrogen generation rate due to water radiolysis caused by the radiation from fuel debris stored in fuel debris canisters was developed to ensure safe transport and storage of the fuel debris resulting from the core damage accidents in the Fukushima Daiichi Nuclear Power Station. Assuming that the radiation energy absorption rate of water (F value) is 1, that is all radiation energy is deposited in water mixed with the debris, the hydrogen generation rate will be overestimated. Therefore, to consider the shielding effect of the fuel debris itself, the F value dependence on the fuel debris particle size was examined. First, the hydrogen generation rate was obtained by the hydrogen generation test using particles (maximum size of 0.355 mm) of actual crushed spent nuclear fuel pellets. Then, the hydrogen generation rates obtained by this test were compared with those estimated using the F value calculated with the particle transport calculation code, PHITS. The F value was calculated assuming a system in which a spherical fuel debris particle was located at the center of a cube containing water. The PHITS code evaluated the hydrogen generation rate conservatively using the F value calculated for a fine particle (spherical fuel debris particle diameter of 0.0001 mm).