The Behavior of ThO₂-Based Fuel Rods During Normal Operation and Transient Events in LWRs

The thermal, mechanical, and chemical behavior of both thorium and uranium dioxide (ThO₂-UO₂) and thorium and plutonium dioxide (ThO₂-PuO₂)-based fuels 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 PuO₂ as well as being a more stable waste form. The behavior is described for three different designs of ThO₂-based fuels: a homogeneous mixture of ThO₂-UO₂, a microheterogeneous arrangement of the ThO₂ and UO₂, and a homogeneous mixture of ThO₂-PuO₂. The behavior was calculated with widely known LWR analysis tools extended for ThO₂-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) HEATING7 for 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 ThO₂-based fuels relative to 100% UO₂ fuel: (a) higher thermal conductivity, lower density and volumetric heat capacity, less thermal expansion, and higher melting point; (b) higher fission-gas production for ²³³U fission than ²³⁵U fission, but a lower gas diffusion coefficient in the ThO₂ than in the UO₂; (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 ThO₂ 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 ThO₂-UO₂-based fuels with respect to safety is generally equal to or better than that of 100% UO₂ fuel. The in-service and transient temperatures in the most promising neutronic design of microheterogeneous ThO₂-UO₂-based fuel are greater than the temperatures in 100% UO₂ fuel but are still within normal LWR safety limits. The reactor kinetics parameters for ThO₂-PuO₂-based fuel cause a higher transient reactor power for some postulated accidents, but in general, the margin of safety for ThO₂-PuO₂ fuels is equal to or greater than that in 100% UO₂ fuels.