Criticality characteristics of fuel debris mixed by fuels with different burnups based on fuel loading pattern

燃料装荷パターンに基づいて燃焼度の異なる燃料が混合した燃料デブリの臨界特性

渡邉 友章  ; 大久保 清志*; 荒木 祥平   ; 外池 幸太郎 

Watanabe, Tomoaki; Okubo, Kiyoshi*; Araki, Shohei; Tonoike, Kotaro

The fuel debris produced by the accident of the Fukushima Daiichi Nuclear Power Station (1F) is probably in a state of mixture of burned fuels with different burnups each other. In such a case, the mixing ratio of burned fuels in fuel debris would affect its criticality. This report shows computation results of criticality characteristics of fuel-debris compositions prepared by mixing nuclide compositions of burned fuels in various patterns based on a fuel loading pattern. The results indicate that fuel debris is potentially subcritical when 1-cycle fuels, whose average burnup is several GWd/t, are included homogeneously in fuel debris because remaining Gd and Gd in 1-cycle fuels works to reduce neutron multiplication. The results also indicate that Gd/U ratio well characterize criticality of fuel debris.