Measurement of Free Fission-Gas Pressure in Operating UO₂-Filled Fuel Rods

A differential-type pressure transducer suitable for use in a fast-neutron environment at elevated temperatures has been developed. The sensitivity of this device is approximately 0.25 lb/in.² with the ability to withstand an unbalance pressure of over 1500 lb/in.² from either direction. A number of stainless-steel-clad, UO₂-filled fuel rods equipped with these transducers have been irradiated in the Vallecitos Boiling Water Reactor to exposures up to roughly 4000 MWd/t peak. These experiments have shown that fission-gas release is negligible from UO₂ operated below the recrystallization temperature. For operation at higher temperatures, observed fission-gas pressures were in qualitative agreement with measured void volume and quantity of free gas found in post-irradiation examination. The decrease in void volume during operation has been calculated for a fuel rod and the calculated value used to estimate a mean fuel temperature.