Measurements of the Total Pressure from Irradiated (U,Pu)-Mixed Oxide

Irradiated (U.Pu)-mixed oxide with 5% burnup was heated in the pulsed Annular Core Research Reactor at Sandia National Laboratories. The tests were typical of prompt Bethe-Tait excursions in terms of heating method (nuclear fission), heating period (milliseconds), and temperatures attained (up to 7700 K). Fission products provided high pressures at temperatures at which fresh fuel shows only a negligible vapor pressure. Fission product release became measurable as soon as the temperature exceeded the steady-state irradiation temperature of the fuel sample. The fission product pressures reached 1.3 to 2.5 MPa at 3000 K over solid fuel, and 2.5 to 5 MPa at 4000 K over liquid fuel. The total amount of fission product released corresponded to ∼30 to 75% of the fission gas inventory. The amount of fission product released increased with the fuel heating rate. Under rapid heating, the total pressure over irradiated (U,Pu) oxide is controlled by a suppression mechanism. At any given temperature, the gaseous components (xenon, cesium, and ambient gas) suppress fuel boiling if their pressure p_gas is higher than the fresh fuel saturation vapor pressure p_sat of unirradiated fuel. If p_sat exceeds p_gas, the total pressure is, to a first approximation, equal to P_sat. Under millisecond heating, the total pressure from irradiated fuel may be taken as p_tot = max(P_gas,P_sat).