Critical experiment plans on the new STACY to clarify the criticality characteristics of the molten core-concrete interaction products

Gunji, Satoshi  ; Araki, Shohei   ; Izawa, Kazuhiko ; Suyama, Kenya

It is considered that a large amount of fuel debris was generated in the TEPCO's Fukushima Dai-ichi Nuclear Power Station accident. In particular, the criticality characteristics of the fuel debris, including concrete components, which are products of molten core-concrete interaction (MCCI), have not been well investigated. In this study, to plan physical simulation in critical experiments at the critical assembly using pseudo fuel debris samples including concrete, we evaluated the sensitivity to the effective multiplication factor of the Si and Ca cross sections in the concrete-simulant sample based on the results of elemental analysis of the prototype. These sensitivity calculations were carried out for each sample loading method and composition. We focused on the energy profile of the sensitivity of the Ca capture reaction and confirmed that the shape of the sensitivity energy profile changed depending on the sample compositions and neutron moderation conditions. We could know the characteristics of each experimental method by clarifying the trends of sensitivity obtained in different experimental cases. It was found that increasing the amount of concrete in the samples and changing the neutron moderation conditions in the experimental core configurations produced similar changes in the shape of the sensitivity energy profile. This result shows the possibility of reproducing the characteristics of MCCI products through practical critical experiments using concrete materials that do not contain fissile materials.