Analysis of Measured Total Pressures from Irradiated Solid and Liquid (U,Pu)-Mixed Oxide

Abstract

This paper analyzes measurements of the total pressure from irradiated (U, Pu)-mixed oxide with respect to the fission product release kinetics and availability for pressure generation in Bethe-Tait excursions. Two pressure sources acting on a millisecond time scale were identified: release of grain boundary fission products (gases and volatiles such as cesium) triggered by grain boundary separation and release of formerly intragranular fission products due to fuel boiling. The former process can provide pressures on a megapascal scale early, and the latter process, late in the accident progression. No fission product release was observed from nonboiling liquid fuel. Based on the experimental data, a model was formulated for the total pressure over irradiated (u, Pu)-oxide. Fuel vapor and gases interact by a suppression mechanism: P{sub IF} = max (P{sub ag} + P{sub FP}, P{sub sat}). The total pressure over irradiated fuel P{sub IF} is equal to the pressure sum from ambient gas P{sub AG} and released fission products in the gaseous state P{sub FP} when this sum is greater than the saturation vapor pressure of fresh (U, Pu)-oxide P{sub sat}. In this regime, fuel boiling is suppressed. AT sufficiently high temperatures when P{sub sat} {gt} P{sub AG} + P{sub FP},more » the oxide begins to boil and the total pressure P{sub IF} reaches the fresh fuel saturation vapor pressure P{sub sat}. The switch-over in the controlling mechanism occurred at {approximately}5200 K.« less