CONFIRMATORY EXAMINATION OF NEUTRON DETECTION EVENTS IN THE EARLY PHASE OF FDNP ACCIDENT

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At the main entrance of Fukushima Dai-ichi Nuclear Power Plant (FDNP), a monitoring car detected neutrons during two periods, in the early morning of 13th March 2011 and from the night of 14th to the early hours of 15th March 2011 (TEPCO, 2022). This neutron detection at the main entrance has been an unresolved item that is essential to evaluate the accident progression in the early phase of FDNP accident (specifically, the early phase of core degradation). In the present study, a confirmatory examination was conducted by heating the slices of spent fuel pin in a hot cell to examine the release of actinide nuclides. The results indicated that, in the early phase of accidents, actinide nuclides were released in the temperature range below 1773 K and neutrons were emitted by its spontaneous fission.