Micro-heterogeneous thorium based fuel concepts for pressurized water reactions

Abstract

The purpose of this work is to evaluate the potential for enhancing the performance of the once-through PWR fuel cycle by appropriate design of thorium containing fuel with concurrent consideration of neutronics, thermal-hydraulics, and proliferation resistance. CASMO-4 is employed to model a fuel assembly consisting of a homogeneously fueled ThO2-UO2 lattice of conventional design. It is found that homogeneously mixed ThO2-UO2 fuel in general has better physics behavior than all-UO2 fuel, e.g. more negative MTC and less reactivity swing. The H/HM ratio can be optimized to extend the burnup of both ThO2-UO2 and all-UO2 fuel about 10% and 5% respectively. However, it is found that the all-UO2 fuel provides higher potential energy generation (or achievable burnup) than the homogeneously mixed ThO2-UO2 fuel. MOCUP (MCNP+ORIGEN) is used to model unit cells of the micro-heterogeneous fuel. The key findings for the best micro-heterogeneous configurations are as follows: (1) For a given U-235 inventory, a 20% to 30% increase in fuel cycle achievable bumup above the homogeneous case is possible, therefore 10% to 15% above the all-UO2 fuel; (2) For certain axially heterogeneous configurations, a "burnable poison" reactivity suppression effect appears at the beginning of irradiation. Analysis of the possible mechanisms behind these effects shows that they are due to a combination of changes in self-shielding, local fissile worth, and conversion ratio.

(cont.) Economic evaluations show that the homogeneous ThO2-UO2 fuel is 20%-30% more expensive than the equivalent all-U fuel but micro-heterogeneous fuel can be comparable to or even cheaper than the all-UO2 fuel. In terms of the plutonium content, the micro-heterogeneous Th/U fuel is more proliferation-resistant than the homogeneous Th/U fuel, and the homogeneous Th/U fuel is more proliferation-resistant than the all-UO2 fuel. However, the uranium produced in the ThO2 zone of any undenatured micro-heterogeneous design should be considered of proliferative concern. Adding natural or depleted uranium to the ThO2 zone can make the material not weapon-usable at the expense of degrading the neutronic performance. Placing some enriched uranium within the thorium zone may be needed to reduce local thermal peaking to a manageable level.