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  NextGen MURR Working Group established in Missouri
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Measurements of the total pressure from irradiated (U,Pu)-mixed oxide were analyzed 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_IF = max(p_AG + p_FP, p_sat). The total pressure over irradiated fuel p_IF is equal to the pressure sum from ambient gas p_AG and released fission products in the gaseous state p_FP when this sum is greater than the saturation vapor pressure of fresh (U,Pu)-oxide p_sat. In this regime, fuel boiling is suppressed. At sufficiently high temperatures when p_sat > p_AG + p_FP, the oxide begins to boil and the total pressure p_IF reaches the fresh fuel saturation vapor pressure p_sat. The switch-over in the controlling mechanism occurred at ∼5200 K.