Melting Heat Rating of UO₂-25 wt% PuO₂ Fuels

For UO₂-25 wt% PuO₂, the quantity kdT was established at 49 ± 5 W/cm, where T_s = 700°C, k is the fuel thermal conductivity, and T_s and T_m refer to fuel surface and melt temperatures, respectively. This result is obtained from measurements on six individual fuel pins. Each pin was surrounded by a calorimeter consisting of a heavy-walled aluminum sleeve with thermocouples at two radial positions. Heat generation rates measured calorimetrically were supplemented by postirradiation bumup analyses for ¹⁴⁰ Ba, ¹⁴¹ Ce, and ⁹⁵Zr. Fuel pins were irradiated for 10 h at steady-state peak heat ratings after which they were rapidly quenched to preserve structures representative of peak power operation. Fractional fuel melting was correlated with heat rating and the correlation extrapolated to zero melting to yield melting heat rating. Flux depression corrections are required to apply these measurements, which were made in a thermal flux, to fast flux environments. These were obtained by measuring radial burnup profiles in several irradiated cross sections and numerically integrating the results.