Stepwise Release of Fission Gas from UO₂ Fuel

Measurements of the fission-product gas pressure in UO₂ fuel elements during irradiation have shown that a significant amount of gas is released during power transients. The gas appears to he released as the element power is decreased to zero during reactor shutdown; little release occurs as the power is raised at startup. It is postulated that gas trapped in bubbles or in a central void is released by cracking or stress-induced movement during the power transient.