Analysis of Plutonium-Fuelled Light Water Reactors

Analysis of physics problem areas and some economic aspects associated with plutonium utilization in water reactors are presented. From a physics standpoint, the major questions to be answered are: (1) Are there adequate data and design analysis methods for accurately predicting the nuclear performance of plutonium-fuelled reactors; and (2) What are the consequences on nuclear performance of plutonium fuelling! Cross-section uncertainties are reflected in uncertainties in the predicted multiplication constant, keff, of the reactor. An evaluation of these uncertainties due to the plutonium isotopes and the resulting uncertainties in fuel lifetime and fuel costs are presented. It is concluded that the largest uncertainty, in keff due to the cross-section uncertainties is of the order of 1.0% Δk/k(10 mk). An uncertainty of 10 mk in keff results in an uncertainty in fuel lifetime of approximately 5% or ∼1000 MWd/tonne. and fuel cycle cost uncertainty of the order of 15%. Further evaluation of the cross-sections in conjunction with theoretical methods have been made by comparing the calculated multiplication constant with results of light water-moderated critical experiments containing eight different plutonium fuels. Three of the fuels were plutonium-aluminium alloys having plutonium concentrations ranging from 1.8 to 5 wt.% and isotopic compositions ranging from 6 to 16 at.% 240Pu. The others were PuO2-UO2 fuels having PuO2 wt.% tanging from 1.5 to 6.6, and 240Pu at.% of 8, 16, and 24. Reasonably good agreement (∼ ±1% in keff) is obtained for the lower enrichment PuO2-UO2 fuels but poor agreement (0-8%) is obtained on the high buckling Pu-Al and PuO2-UO2 fuels. The major difficulty is suspected to be in properly accounting for leakage, which is under investigation. These analysis methods have been used to calculate the nuclear characteristics of plutonium-fuelled light water reactors. Plutonium compositions approximating those from water reactors and fast reactors blankets were assumed. It is shown that the reactivity coefficients are influenced by the low energy variation of 239Pu cross-sections and the additional resonance absorption in 240Pu, but no limiting characteristics are found. Similar fuel compositions were assumed in an evaluation of optimized utilization of plutonium in water reactors. It is found that the plutonium value is up to 28% more in an optimzed lattice (softer spectrum) than in a uranium lattice. It is also found that a substantial economic penalty ($2-4/g depending on assumed fabrication costs) may be incurred if the plutonium is recycled as partial enrichment in conjunction with enriched uranium in all fuel elements. This can be avoided by fully enriching with plutonium in natural or depleted uranium and employing this element in a limited number of reactors or a limited number of fuel elements within a reactor. (author)