| Summary: | 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.
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