A Systematic Study of Moderation Effects On Neutronic Performance of UO[subscript 2] Fueled Lattices

Abstract

This report addresses the physics of reactor cores that can be operated for 10 to 15 years without refueling — inspired by the objective of enhanced nuclear fuel cycle performance with regard to economics and resistance to weapon proliferation. Proliferation resistance is a primary consideration in this design. The long life operation reduces the routine access to the internals of the reactor vessel, therefore reducing the possibility for clandestine production of nuclear weapons. Additionally, reduction of reactor shutdown time can result in improved safety and economics. As a first step, the most promising fuel lattice characteristics to achieve long life from a physics point of view are studied. These studies also define the design tradeoffs involved in conceptualizing such cores. Moderation effects on UO[subscript 2] fueled lattices are analyzed systematically using state-of-the-art computer codes (CASMO-4 and MOCUP). The standard 4-loop Westinghouse pressurized water reactor (PWR) is taken as our reference core and single unit cell analysis is employed. To change the moderator-to-fuel ratio, which is characterized by the hydrogen-to-heavy-metal (H/HM) atom number ratio, various methods are adapted including varying water density, fuel density, fuel rod diameter, and fuel rod pitch. Higher burnup potential as well as longer core endurance (burnup times heavy metal mass) would be desirable. For a given initial enrichment, the results show that higher reactivity-limited burnup is achievable by either a more wet lattice or much drier lattice than normal. However, epithermal lattices are distinctly inferior performers. In terms of longer endurance, current PWR lattice parameters are about the optimum. Higher burnup and endurance can be achieved with higher initial enrichment. Characteristics of the spent fuel from high burnup UO[subscript 2] fueled lattices have been examined. The variation of isotopic mix and quantity of plutonium with moderator-to-fuel ratio for UO[subscript 2] fueled lattices has been studied to clarify the impact on its proliferation resistance. And Np production as a function of H/HM has been computed as a measure of long-term radiological hazard for high level waste disposal. It is shown that Np is mildly affected by the H/HM ratio and the current PWR lattice is close to optimum configuration. However, high burnup is significantly beneficial as a way to make the plutonium isotopic mix less attractive as a weapon material.