Simulated high-level liquid waste was irradiated by 60Co gamma radiation, and changes in the gas-phase concentrations of the products H2, O2, and NOx that accumulated in the absence of sweeping air were measured. The H2 concentration reached a steady-state value of much less than 4% in line with the value predicted from the previously derived mathematical expression. The simulated dissolver solution was also irradiated, and another steady-state H2 concentration of much less than 4% was obtained in accordance with the corresponding predicted value. These experimental results lend strong support for the applicability of a mathematical expression in predicting the H2 concentration in a tank in the case of a sweeping-air function loss.