Applicability Evaluation of Candidate Technologies for Nuclear Material Quantification in Fuel Debris at Fukushima Daiichi Nuclear Power Station - Passive Gamma Technique -

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. JAEA’s Integrated Support Center for Nuclear Nonproliferation and Nuclear Security (ISCN) is in charge of development of the passive gamma technique in collaboration with Tokyo Tech. Some fission products (FPs) such as Ce and Eu seem to be very low volatile even in high temperature environment in the severe accident of nuclear reactors, and they are supposed to chemically coexist with fuel elements such as uranium and plutonium. In our passive gamma ray spectroscopy, first we measure the gamma rays from such FPs and estimate their amount and burnup. Then we multiply the mass ratio of the FPs and nuclear materials, and we obtain the mass of nuclear material of interest. According to the standard fuel debris and canister models mentioned above, we performed some simulation of leakage gamma rays from the canister of debris. For that the gamma ray source spectra were derived from the composition of the standard fuel debris and the photon transport calculation was carried out using MCNP. In addition, we conducted experiments of gamma ray measurement from intact spent fuels irradiated in the Experimental Fast Reactor Joyo in order to validate the prediction performance of the simulation by the comparison of the experiments and simulation.