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Nuclear Fuel Cycle (NFC) Technology and Policy Program

Design and Performance Assessment of a PWR Whole-Assembly Seed and Blanket Thorium Based Fuel Cycle

D.Wang, M.J. Driscoll, and M.S. Kazimi

MIT-NFC-TR-026 (October 2000)

Abstract

A novel once-through thorium fuel cycle for existing PWRs was investigated based on a heterogeneous core concept. The concept, called the Whole-Assembly Seed and Blanket design (WASB), differs from the Radkowsky Thorium Fuel (RTF) design in which the seed and blanket units are in the same assembly. In the WASB arrangement the seed and blanket units each occupy one full-size PWR assembly and the assemblies are arranged in a checkerboard array.

The computer code used in this work is CASMO-4. Benchmark comparisons between CASMO-4 colorset calculations and the more general MOCUP (MCNP+ORIGEN) demonstrated good agreement. The WASB design uses annular UO2 seed pins and 90% ThO2-10% UO2 blanket fuel pins. Er2O3 is used in the central voids of the seed pins as burnable poison to hold down excess reactivity, flatten the power sharing, and control power peaking. The seed fuel is 19.5% enriched UO2 and follows a 4.5 cycle length while the blanket follows a 13.5 year cycle. Economic considerations show that the WASB design is comparable to the RTF design, and markedly better than conventional PWRs in natural uranium utilization, but worse in separative work utilization. Reactivity coefficients of moderator and fuel were calculated and found favorable. Control rod worth was also investigated and compared to conventional PWRs. All appear satisfactory for retrofitting into current PWRs.

Analyses show that the proliferation resistance attributes of the Whole-Assembly Seed and Blanket design are significantly better than that of the current all UO2 fuel cycle of operating PWRs because of reduced plutonium production (by a factor of approximately 4) and degraded plutonium isotope mix in the discharged fuel. However, care should be taken to denature U-233 by the addition of small amounts of U-238 to the thorium fuel in the blanket. Because of the high burnup achievable, fewer spent fuel assemblies (about 1/3 less) have to be stored and disposed of than today's PWRs.