Study on the basic core analysis of the new STACY

Gunji, Satoshi  ; Yoshikawa, Tomoki ; Araki, Shohei   ; Izawa, Kazuhiko ; Suyama, Kenya

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, JAEA has been modifying a critical assembly called "STACY". The first criticality of the new STACY is scheduled for spring 2024. This paper reports the consideration results of the core configurations of the new STACY at the first criticality. We prepared two sets of gird plates with different neutron moderation conditions (their intervals are 1.50 cm and 1.27 cm). However, there is a limitation on the number of available UO fuel rods. In addition, we would like to set the critical water heights for the first criticality at around 95 cm. This is to avoid the reactive effect of the aluminum alloy middle grid plates (Approx. 98 cm high). The core configurations for the first criticality satisfying these conditions were constructed by computational analysis. A square core configuration with the 1.50 cm grid plate that is close to the optimum moderation condition needs 261 fuel rods to reach criticality. As to the 1.27 cm grid plate, we considered two core configurations with 1.80 cm intervals by using a checkerboard arrangement. One of them has two regions core configuration with 1.27 and 1.80 cm intervals, and the other has only 1.80 cm intervals. They need 341 and 201 fuel rods for the criticality, respectively. This paper shows these three core configurations and their calculation models.