THE PLUTONIUM FEEDBACK APPROACH TO NUCLEAR POWER

Nuclear parameter variations are presented for sodium graphite reactors using Pu-spiked natural U as fuel. The fuel feed is assumed to be natural U, and the important variables are the initial amount of excess reactivity, lattice spacing, and alpha , the ratio of Pu/sup 239/ to U/sup 235/ in the feed material. The system is called "steady state" in that the ratios, N/sub 40/N/sub 49/ = sigma /sub c(49)// sigma /sub a(40)/ and N/sub 41//N/sub 4 9/ = sigma /sub c(49)/ / sigma /sub a(41)/, obtained from setting the build-up equations to zero are assumed for the feed concentrations, and the feed material to the reactor is always the same. During irradiation, the U/sup 235/ and U/sup 238/ concentrations steadily decline while the Pu isotope concentrations initially increase, then decline. To ensure sufficient plutonium for feed material, it is necessary to remove the fuel from the reactor before the Pu content drops below its initial value. Although the reactivity variations presented were calculated specifically for sodium graphite reactors, they may be applied to any thermal reactor using Pu-spiked natural U as fuel. The reactivity changes are determined primarily by the fuel characteristics and are only slightly dependent on the other material in the reactor core. An analysis which estimates the effect of Pu feedback opcration fuel costs is given. (auth) in terms of nuclear power cost reduction are discussed (auth