Ten years after the NPP accident at Fukushima : review on fuel debris behavior in contact with water

Following the NPP accident, several hundred tons of heat-generating corium and fuel debris have been cooled permanently by millions of m(3) of flowing. Knowledge on the interaction with water is crucial for any decommissioning planning. Starting from knowledge on the evolutions of the accident in the three reactor cores and associated fuel debris formations and some additional isotopic and physio-chemical information on debris fragments collected in Fukushima soils, we review the temporal evolution of the chemistry and leached radionuclide contents of the cooling water, comparing measured concentration ratios of the actinides and fission products in the water to reported results of laboratory leaching studies with either spent nuclear fuel or simulated fuel debris. As for spent fuel leaching, the fractions of inventories of Cs-134,Cs-137 in the cooling water are orders of magnitude larger than that of the actinides. After more than 10 years of fuel debris/water contact, Cs-137 release rates have decreased by about a factor of 100. The total release of actinides from the fuel debris is orders of magnitude lower than that of Cs-134,Cs-137 or of Sr-90. This high stability makes direct disposal of fuel debris in suitable containers after decommissioning a viable option.