Studies of nuclear criticality safety of fuel debris using the modified STACY critical assembly

Gunji, Satoshi  ; Araki, Shohei   ; Yoshikawa, Tomoki ; Arakaki, Yu; Aizawa, Eiju; Seki, Masakazu; Ishii, Junichi; Izawa, Kazuhiko ; Shiba, Shigeki*; Iwahashi, Daiki*

The details of the fuel debris generated in the TEPCO's Fukushima Dai-ichi Nuclear Power Station accident are still not fully understood, and its critical properties are being evaluated using nuclear calculations with various parameters. On the other hand, criticality experiments are required to validate these computations because the fuel debris contains materials such as concrete for which nuclear data is not well evaluated and has heterogeneous and non-uniform compositions. For this purpose, the critical assembly STACY was modified from a solution fuel system to a light water-moderated heterogeneous system. This modification was completed at the end of 2023, and the operation restarted in the spring of 2024. To simulate the criticality characteristics of the fuel debris, 70 rod-shaped samples of concrete composition and stainless steel with the same diameter as the UO fuel rods were prepared, and equipment was also installed to prepare pellet-shaped samples and load them into the experimental core. The results of these experiments simulating fuel debris characteristics were analyzed using the MCNP6 code and the JENDL nuclear data libraries, and it was shown that these nuclear calculations are valid for evaluating the criticality characteristics of fuel debris.