Effect of 0.3eV resonance cross section for plutonium on coolant void reactivity in heavy water lattice

Kowata, Yasuki; Fukumura, Nobuo

Plutonium fuel could be utilized in the entire core of heavy water moderated, boiling light water cooled pressure-tube-type reactor (HWR). The void reactivity, however, depends on the various parameters of the lattice. It is especially significant to clarify the influence of plutonium nuclides on the void reactivity. The void reactivities in the infinite HWR lattices have been parametrically analyzed to clarify the influences of changes in the lattice parameters on the void reactivity using the WIMS-D4 code with the JENDL-3.1 nuclear data. In this lattice calculation, it has been known that the behavior of the void reactivity can be made clear by separating the components for fuel nuclides, neutron cross sections, energy group and regions in lattice cell from the void reactivity using the important reaction rates. If the macroscopic 2200m/s neutron absorption cross section of fuel is identical each other, it has been shown that the void reactivity of the HWR lattice shifts further to the negative side in the narrower pitch lattice, and in the plutonium lattice than in the uranium lattice. The effect reducing the void reactivity to the negative by plutonium is caused mainly by the presence of the resonance cross section at around 0.3eV of Pu. Because the higher the content of Pu is, the less the recovery effect of neutron density within the resonance energy due to decrease in the thermal neutron scattering of hydrogen is with increase in coolant void fraction, so that the decreased resonance fission rate for Pu contributes to the more negative side for the void reactivity.