Development of basic NDA technologies of nuclear material accountancy of debris of melted fuel formed in severe accidents

Seya, Michio; Harada, Hideo   ; Kitatani, Fumito  ; Koizumi, Mitsuo ; Tsuchiya, Harufumi ; Iimura, Hideki  ; Kureta, Masatoshi   ; Takamine, Jun ; Hajima, Ryoichi; Hayakawa, Takehito; Shizuma, Toshiyuki; Angell, C.; Bolind, A.

The forming of MF in severe accidents of nuclear reactors such as Units 1 - 3 of Fukushima-Daiichi NPP (Nuclear Power Plant) inevitably change the category of the nuclear reactor from "item facility" to "bulk-handling facility". At removal of the MF it is necessary to break chunks of MF into relatively small blocks by using some tools, resulting in debris that consists of cut or small rock-like debris and particle (or grain)-like debris in bulk form. This paper presents a categorization of debris of MF and two possible non-destructive assay (NDA) technologies for precise measurement which could be applied to the NM accountancy of MF debris. One of them is Neutron Resonance Densitometry (NRD) for particle-like (or grain-like) debris, and another is NRF (Nuclear Resonance Fluorescence) NDA using LCS (Laser Compton Scattered) -rays (mono-energetic -rays) for cut or small rock-like debris. The paper also describes about the recent development of these two technologies.