High Burnup Fuels for Advanced Nuclear Reactors

Abstract

The goal of this work is to select the best candidate fuel materials to deliver high burnup in advanced light water reactors. Uranium and thorium based fuels are considered. These fuel materials must be able to withstand nearly double the burnup of current LWRs in high irradiation fields. Reactor economics, safety, proliferation resistance, fuel reprocessing and spent fuel disposal are the most important factors to be addressed. High burnup will provide the opportunity for uninterrupted operation over long periods of time, reduction of spent fuel volume and improvement of proliferation resistance. Thus, effective power cycle maintenance and fuel management and reduced fuel storage needs will lead to more economic operation. Several uranium and thorium fuel forms are analyzed to predict their capability to withstand high burnups. Their fuel cycle cost is also considered. To compare the fuel options, simple indices characterizing the behavior of the materials at high burnup are defined. Indices for the thermal stress capability, stored energy and margin for melting are derived from non-dimensional analyses. To evaluate the fuel pin lifetime, a simplified fuel performance analysis code, FUELSIM (FUEL SIMulation code) was developed. The code utilizes the VENSIM simulation system, which allows for great flexibility in the change of governing relations, permits sensitivity analysis, and facilitates graphical outputs. Based on the sensitivity analysis by FUELSIM, dominant parameters are identified and a simplified expression is developed for predicting the increase in the pin internal pressure with burnup. For each material, we obtain a maximum attainable burnup at a given smear density. Cladding strain, internal pressure and fuel melting (or phase-change) temperature are the limiting factors used to obtain these burnups. From neutronic reactivity considerations, the needed [superscript 235]U enrichment can be specified. Thus, the fuel cycle cost for each material and smear density can be estimated. Metals, oxides, carbides and nitrides of uranium and thorium were examined. Although the results show that UN provides the highest potential for attaining high burnup and economic application in once-through cycles, it has limited compatibility with water. UO[subscript 2], at 90- 95% smear density, continues being the most feasible option as a nuclear material. Also, ThO[subscript 2]/UO[subscript 2] seems to offer as good or better potential performance and economics as UO[subscript 2]. However, more reliable data on the irradiation behavior of the different materials is needed before a definitive conclusion can be drawn. Also important in the evaluation of thorium/uranium cycles are attributes that were not considered here. These include the reduction in spent fuel volume, the improvement in proliferation resistance, possible power uprates and allowance for a higher peaking factor that may be possible by taking advantage of the increased margin that results from using fuels with lower stored energies.