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

Yun Long, Larry J. Siefken, Pavel Hejzlar, Eric P. Loewen, Judith K. Hohorst, Philip E. MacDonald, Mujid S. Kazimi

Nuclear Technology / Volume 147 / Number 1 / July 2004 / Pages 120-139

Technical Paper / Thoria-Urania NERI

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.

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