STACY Critical Experiments to Simulate Shape Changes of Fuel Debris

The Japan Atomic Energy Agency is conducting critical experiments to clarify the criticality characteristics of fuel debris using the Static Experiment Critical Facility (STACY). The geometric shape of fuel debris may change, such as joining or separation, due to the unintentional falling of fuel debris or processing during retrieval work. To confirm the validity of simulating these conditions by computations, a series of critical experiments was carried out with two core configurations close to each other. The geometrical change in the fuel debris was simulated by changing the relative distance between the two cores in a core configuration where a small core consisting of 3 × 5 or 5 × 3 fuel rods was close to a larger core with a pit shape to receive it.An experimental analysis was carried out using the Monte Carlo calculation codes MCNP and MVP and the Japanese evaluated nuclear data library JENDL. It was shown that the effective multiplication factor can be evaluated using a simple calculation model, with the calculation results showing that the values of the effective multiplication factor of MCNP were smaller than those of MVP and that the values of the effective multiplication factor of JENDL-4.0 were smaller than those of JENDL-5. In particular, in JENDL-5, it was shown that it is effective to correctly calibrate the temperature point of the thermal scattering law (TSL) used.