Examination of I and Cs releases during late phase of Fukushima Daiichi NPP accident by using I/Cs ratio of source terms evaluated reversely by WSPEEDI code with environmental monitoring data

Hidaka, Akihide   ; Yokoyama, Hiroya

To clarify what happened during the Fukushima accident, the phenomena within RPV and the discussion of ties with the environmental monitoring are very important. However, the previous study has not necessarily advanced until the present that passed almost six years from the accident. The present study investigated I and Cs release behaviors during the late phase of the accident based on I/Cs ratio of the source terms that were recently evaluated backward by WSPEEDI code based on environmental monitoring data. The I release from the contaminated water in the basement of 1F2 and 1F3 reactor buildings was evaluated to be about 10% of I source term. The increase in Cs release from March 21 to 23 and from March 30 to 31 could be explained by the release of CsBO which is formed as a result of chemical reactions of Cs with BC due to re-ascension of the core temperature caused by slight shortage of the core cooling water.