Applicability Evaluation of Candidate Technologies for Nuclear Material Quantification in Fuel Debris at Fukushima Daiichi Nuclear Power Station - Active Neutron Technique – (Interim Report)

The severe loss-of-coolant accidents of Fukushima Daiichi Nuclear Power Station (1F) produced fuel debris in the reactor cores of Units 1-3. Under the collaborative program with United States Department of Energy (DOE), Japan Atomic Energy Agency (JAEA) and Central Research Institute of Electric Power Industry (CRIEPI) have surveyed technologies for nuclear material quantification of fuel debris at 1F since 2012. Four research groups in JAEA and CRIEPI have evaluated independently the applicability for four technologies, passive neutron technique, passive gamma technique, active neutron technique and active gamma technique, by simulation code and small scale measurement tests. All parties recognized the importance of the characterization study on each candidate technology for establishment of the concept of integrated measurement system that combines several measurement technologies for accurate quantification. For the characterization study, standard fuel debris and canister models were developed. We, Nuclear Science and Engineering Center of JAEA, are working on the development of a non-destructive assay (NDA) measurement system using the Fast Neutron Direct Interrogation (FNDI) method. The FNDI method is a kind of active neutron technique and can measure the total amounts of fissile materials (U-235, Pu-239, Pu-241). We have already carried out design analyses of an NDA measurement system for measuring the debris assuming use of the Three Mile Island (TMI) canister model. The result was presented at the Institute of Nuclear Materials Management (INMM) 56th Annual Meeting. Since then, we have modified the design of the NDA measurement system for the aforementioned fuel debris and canister models. In this paper, we provide the calculation and evaluation results using the modified NDA measurement system. Moreover, we provide analytical investigations of the influence of fuel debris including high fissile material content on measurements.