Speaker
Description
The details of the fuel debris generated in the Tokyo Electric Power Company Holdings’
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$_2$ fuel
rods were prepared, and equipment was also installed to prepare pellet-shaped samples and
load them into the experimental core.
We will report the results of these experiments, plans for making benchmarks, and expected
contributions of the modified STACY to the Fukushima Dai-ichi decommissioning work.
Acknowledgments
The modification of the STACY critical assembly and their experimental activities were
performed under the auspices of the Secretariat of Nuclear Regulation Authority (S/NRA/R)
of Japan.
