Development of Basic NDA Technologies for Nuclear Material Accountancy of Debris of Melted Fuel Formed in Severe Accidents

In severe nuclear reactor accidents such as Units 1 - 3 of Fukushima-Daiichi NPP (Nuclear Power Plant), melted fuel (MF) is formed. In the case of Fukushima-Daiichi NPP, it is being planned that the damaged reactors are going to be decommissioned completely within 30-40 years, with the removal of MF from the reactors beginning around 2021. The forming of MF in a severe accident of a nuclear reactor may change the category of the nuclear reactor from “item facility” (i.e., “a facility where all nuclear material (NM) is kept in item form and the integrity of the item remains unaltered during its residence at the facility” [2]) to “non-item facility.” When operators take the MF out from the reactors, chunks of MF are going to be broken 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. Taking this removal process into account, the damaged reactor could be categorized as “a bulk-handling facility” (i.e., “a facility where NM is held, processed or used in bulk form” The NM accountancy is essential for the safeguards of a bulk-handling facility. This paper presents a categorization of debris of MF and a selection of possible Non-Destructive Assay (NDA) technologies which could be applied to the NM accountancy of MF debris. The paper introduces two NDA technologies for NM accountancy of debris: Neutron Resonance Densitometry (NRD) for particle-like (or grain-like) debris, and NRF (Nuclear Resonance Fluorescence) NDA using LCS (Laser Compton Scattered) gamma-rays (mono-energetic gamma-rays) for cut or small rock-like debris. The paper also describes about the recent development of these two technologies and near future plan.