Debris deposits at the bottom of the containment vessel of the Fukushima Daiichi Unit 1 reactor on March 17 (International Nuclear Decommissioning Research and Development Organization, Hitachi GE Nuclear Energy, Inc. (Courtesy of International Nuclear Decommissioning Research and Development Institute, Hitachi GE Nuclear Energy)

May 26, 2022
On May 26, TEPCO announced that it had detected high levels of neutron radiation, which is emitted when uranium and plutonium contained in nuclear fuel undergo nuclear fission, in sediment found at the bottom of the containment vessel at the Fukushima Daiichi Nuclear Power Plant’s Unit 1 reactor. TEPCO announced that it had detected high levels of neutron radiation, which is emitted when uranium and plutonium contained in nuclear fuel fuse. A TEPCO representative said, “It is presumed to be derived from molten nuclear fuel (debris). It is natural to assume that the debris is contained in the sediment.
　TEPCO will focus on examining the thickness of the deposits near where the neutron rays were detected and the types of radioactive materials contained in the deposits.
　According to TEPCO, on March 20 and 21, underwater robots were used to survey four locations at the bottom of the containment vessel, and neutron rays were detected in all of them. The values at three locations near the openings in the base of the pressure vessel were particularly high.
https://www.tokyo-np.co.jp/article/179742?fbclid=IwAR0oljxEJF2Q5XzUE859cfr1DfSmtvct8xkg6FcU2uIEpIKvtlrh3qm9aag