Study on the specifications of the basic core configurations of the modified STACY

Abstract

Since the compositions and properties of the fuel debris are uncertain, critical experiments are required to validate calculation codes and nuclear data used for the safety evaluation. For this purpose, the Japan Atomic Energy Agency (JAEA) has been modifying a critical assembly called “STACY.” The first criticality of the modified STACY is scheduled for spring 2024. This paper reports the consideration results of the core configurations of the modified STACY for the first criticality and inspections. The specifications of these core configurations were determined in advance, we tried to make them with a simplified computational model that considers the reactivity effect around the core. At the first criticality, two types of grid plates with different neutron moderation conditions (their hole spacings are 1.50  cm and 1.27  cm) were prepared. On the other hand, there is a limitation on the number of available UO₂ fuel rods. The core configurations for the first criticality satisfying these experimental constraints were designed by computational analysis. A cylindrical core configuration with a 1.50  cm pitch grid plate close to the optimum moderation condition needs 253 fuel rods to reach criticality. As to the 1.27 cm grid plate, we considered core configurations with 2.54  cm intervals by using doubled pitches of the grid plate. It will need 213 fuel rods for the criticality to be reached. In addition, the experimental core configuration was considered with steel/concrete simulant rods to simulate fuel debris conditions. This paper shows six core configurations with different conditions, and all of them satisfy the regulatory requests.