Fuel cycle optimization of thorium and uranium fueled PWR systems

Abstract

The burnup neutronics of uniform PWR lattices are examined with respect to reduction of uranium ore requirements with an emphasis on variation of the fuel-to-moderator ratio (lattice pitch at constant fuel pin diameter) and the use of thorium. Fuel cycles using all combinations of the major fissile (U-235, U-233, Pu) and fertile (U-238, Th) species are examined. The LEOPARD code and prescriptions developed from a linear reactivity model are used to determine initial core and annual makeup fissile requirements for input into an in-house, simple, systems model, MASFLO-2, which calculates ore (and separative work) requirements per GWeyr for growing, declining, or finite-life nuclear electric systems. For low growth scenarios drier lattices are favored, and the thorium fuel cycle requires as much as 23% less ore than a comparably optimized uranium cycle with full recycle. For unmodified lattices, the thorium fuel cycle with full recycle exhibits long term uranium ore savings of 17% over the comparable uranium cycle with full recycle. For rapidly growing systems, drier lattices, and those using thorium, are less attractive because of their high startup inventories. Thus the introduction of thorium may increase ore and separative work requirements in the short term but will more than repay the ore investment in the very long term. Very little improvement was achieved by varying fuel pin diameter at a given fuel-to-moderator ratio, but itwas found to be slightly advantageous to recycle plutonium (or U-233) into dedicated reactors having individually optimized lattices: a strategy which may also be attractive for safeguards purposes.