Cesium Chemistry in the LWR severe accident and towards the decommissioning of Fukushima Daiichi Nuclear Power Station

Researches on the fission product chemistry made after the severe accident of the Fukushima Daiichi Nuclear Power Station were reviewed with focus on the Cs chemistry in terms of two regimes, namely, the accidental source term and the long-term source term via aqueous phase toward the decommissioning. For the accidental source term, Cs chemical interaction with Mo, B, and Si under steam atmosphere greatly affect the behavior of release from fuel and revaporization from the deposit on the structural material surface in the reactor coolant system. Regarding the unique issue of long-term source term via aqueous phase, Cs penetration into concrete and fuel debris leaching were mentioned as the main sources of FPs. Efforts on the preparation of thermodynamic data for the Cs complex oxides were described. All these Cs chemical behaviors should be modelled and validated/verified through the analysis and evaluation of the actual samples including fuel debris that would be taken from the Fukushima Daiichi Nuclear Power Station in near future.