Evaluation of hydrogen gas production at transient criticality in uranyl nitrate solution

Yoshida, Ryoichiro ; Yamane, Yuichi  ; Abe, Hitoshi 

At criticality accident on solution fuel, it is known that hydrogen gas is generated with nuclear fission. Because of the temporary decrease effect on power through density decrease and the possibility which causes secondary accident such as explosion, it is important on safety evaluation at criticality accident that to estimate relating the amount of hydrogen gas and the number of nuclear fissions. The time history data of concentration of hydrogen gas in vent gas when the transition critically used uranyl nitrate solution, which was obtained from Transient Experiment Critical Facility (TRACY), could not be explained by simple diffusion, because of peak about 500 seconds later from reactivity addition and after that long tail. Then, calculating the transition behavior of hydrogen gas in reactor core tank by combining multiple models such as Taylor dispersion, and evaluating concentration of hydrogen gas result reproducing the measured value well. The amount of hydrogen gas also evaluated 4 mol per 10 fissions.