Radiolytic gas voids are generated by the motion of fission fragments during prompt criticality of fissile solutions. The generation of radiolytic gas voids decreases with the power degradation due to the reactivity feedback. Meanwhile, the expansion of voids against the inertia of the fissile solution leads to an increase in pressure, which has a potential impact on the integrity of the pressure boundary structures. The volume of radiolytic gas voids must be determined for the evaluation of negative reactivity feedback, maximum pressure and release rate of fission products.