DEVELOPMENT OF CRITICALITY RISK EVALUATION METHOD FOR FUEL DEBRIS IN FUKUSHIMA DAI-ICHI NPS

摘要

After the accident of Fukushima Dai-ichi Nuclear Power Station, there has been still damaged nuclear fuel in the reactor buildings, which is called "fuel debris" and contains an amount of fissile materials more than the minimum criticality amount. Measured data of Xe gas show that it is currently subcritical, which must be kept until finishing retrieval work. In the latest plan, retrieving fuel debris is to start in 2025. The International Research Institute for Nuclear Decommissioning, IRID, has been developing the technology for decontamination in reactor building. A developed method will be reviewed and granted by the Nuclear Regulatory Authority in Japan, NRA, before the start of the retrieving. Criticality risk information will be needed for decision making on following two subcriticality issues, -Criticality control of fuel debris to keep it subcritical until the end of retrieval work. -Safe operation for retrieving fuel debris to keep it subcritical. Because fuel debris has been out of criticality control and the key condition for its current subcriticality is unknown, it is important to obtain criticality risk information as soon as possible. Applying conventional Probabilistic Safety Assessment, PSA, to fuel debris is difficult, because it is based on the probability of becoming out of criticality control due to troubles such as equipment failure, operation mistake and so on. A new method, which has been developed by Japan Atomic Energy Agency to technically support the NRA, is designed to estimate criticality risk based on a probability distribution of k_(eff) due to uncertainty of composition and geometry of fuel debris, which method will provide risk information for the two subcriticality issues as well as for the tolerance level of criticality risk if it is unavoidable. Developed method contains four steps: two steps of analysis of criticality risk and two steps of quantification of the risk. A procedure of risk quantification has been defined, in which systematic analysis of criticality risk can be done by making "status matrix." A trial analysis by using the procedure shows that some events which can initiate criticality are able to be supposed to happen during maintenance period until the beginning of retrieving fuel debris, and that analysis also indicates countermeasures should be considered as soon as possible in order to reduce criticality risk for public and workers.