Experiments on Concrete-loaded core in the modified STACY

Araki, Shohei  ; Arakaki, Yu; Maekawa, Tomoyuki; Kamikawa, Yutaka; Hasegawa, Kenta; Yoshikawa, Tomoki ; Tada, Yuta; Sumiya, Masato; Seki, Masakazu; Aizawa, Eiju; Ishii, Junichi; Izawa, Kazuhiko ; Iwahashi, Daiki*; Shiba, Shigeki*; Gunji, Satoshi  

In order to investigate the effect on criticality characteristics of fuel debris containing concrete composition in the Fukushima Daiichi Nuclear Power Plant, critical experiments loaded concrete rods were conducted in the modified STACY. The experimental cores of the modified STACY were composed of light water, fuel rods, and concrete rods with grid plates with 15.0-mm lattice intervals in the core tank. Each fuel rod consisted of a zirconium alloy clad tube (9.5-mm outer diameter) and UO pellets (8.2-mm diameter) with 4.98wt.% 235U enrichment. Each concrete rod was composed of concrete simulant pellets (7.0-mm diameter) and an aluminum alloy clad tube (9.5-mm outer diameter). The concrete simulant pellets were fabricated using an aggregate-free mortar in order to ensure a uniform composition of the pellets, and the composition was evaluated by chemical analysis. Three experimental cores were configurated with 9, 25 and 69 concrete rods. The critical water level data were obtained. An effect of composition uncertainty of the concrete simulant pellet was estimated by sensitivity analysis using MCNP6.2 with JENLD-4.0. The sensitivity analysis estimated the effect to be less than 3 pcm. The calculation results performed using MCNP6.2 with ENDF/B-VII.1, JENDL-4.0, and JENDL-5 libraries showed a tendency to overestimate the multiplication factor according to increasing the loading amount of the concrete rods.