THE BEHAVIOR OF ThO_2-BASED FUEL RODS DURING NORMAL OPERATION AND TRANSIENT EVENTS IN LWRs

摘要

The thermal, mechanical, and chemical behavior of both thorium and uranium dioxide (ThO_2-UO_2) and thorium andplutonium dioxide (ThO_2-PuO_2)-basedfuels during in-service and hypothetical accident conditions in light water reactors (LWRs) is described. These fuels offer the possibility for increased proliferation resistance and a reduction in the stockpile of weapons-grade and reactor-grade PuO_2 as well as being a more stable waste form. The behavior is described for three different designs of ThO_2-based fuels: a homogeneous mixture of ThO_2-UO_2, a microheterogeneous arrangement of the ThO_2 and UO_2, and a homogeneous mixture of ThO_2-PuO_2. The behavior was calculated with widely known LWR analysis tools extended for ThO_2-based fuels: (a) MATPRO for calculating material properties, (b) FRAPCON-3 for calculating in-service fuel temperature and fission-gas release, (c) VIPRE-01 for calculating the possibility for departure from nucleate boiling, (d) HEATING7for calculating in-service two-dimensional temperature distributions in microheterogeneous fuel, (e) SCDAP/RELAP5-3D for calculating the transient reactor system behavior and fuel behavior during loss-of-coolant accidents, and (f) FRAP-T6 for calculating the vulnerability of the cladding to cracking due to swelling of the fuel during hypothetical reactivity-initiated accidents. The analytical tools accounted for the following differences in ThO_2-based fuels relative to 100% UO_2 fuel: (a) higher thermal conductivity, lower density and volumetric heat capacity, less thermal expansion, and higher melting point; (b) higher fission-gas production for ~(233)U fission than ~(235)U fission, but a lower gas diffusion coefficient in the ThO_2 than in the UO_2; (c) less plutonium accumulation at the rim of the fuel pellets; (d) greater decay heat; (e) microheterogeneous arrangement of fuel; and (f) more-negative moderator temperature and Doppler coefficients and a smaller delayed-neutron fraction. The newly developed models for ThO_2 were checked against data from the light water breeder reactor program. Calculations by these analytical tools indicate that the in-service and transient performance of homogeneous ThO_2-UO_2-based fuels with respect to safety is generally equal to or better than that of 100% UO_2 fuel. The in-service and transient temperatures in the most promising neutronic design of microheterogeneous ThO_2-UO_2-based fuel are greater than the temperatures in 100% UO_2 fuel but are still within normal LWR safety limits. The reactor kinetics parameters for ThO_2-PuO_2 -based fuel cause a higher transient reactor power for some postulated accidents, but in general, the margin of safety for ThO_2-PuO_2 fuels is equal to or greater than that in 100% UO_2 fuels.