Effect of a 0.3-eV Resonance Cross Section for Plutonium on the Coolant Void Reactivity in a Heavy Water Lattice

Plutonium fuel could be utilized in the entire core of a heavy water-moderated, boiling light water-cooled pressure-tube-type reactor (HWR). The coolant void reactivity, however, depends on the various parameters of the lattice. It is especially significant to clarify the effect of plutonium nuclides on the void reactivity.

The void reactivities in the infinite HWR lattices have been parametrically analyzed to clarify the effects of changes in the lattice parameters on the void reactivity using the WIMS-D4 code with the JENDL-3.1 nuclear data. At present, it is known that the behavior of the void reactivity can be clarified by separating the components of fuel nuclides, neutron cross sections, energy groups, and regions in the lattice cell from the global reactivity effect, using the important reaction rates.

If the fuels are the same in the macroscopic absorption cross section for the 2200 m/s neutron, it has been shown that the void reactivity shifts further to a negative direction in a narrower pitch lattice and in the plutonium-fueled lattice with a higher content of ²³⁹Pu rather than in the uranium one. The effect of reducing the void reactivity to the negative by fissile plutonium is caused mainly by the presence of the resonance cross section at ~0.3 eV of ²³⁹Pu. The higher the content of ²³⁹Pu, the less the recovery of dipped neutron flux within the resonance energy width due to a decrease in the thermal neutron scattering of hydrogen with an increase in coolant void fraction, so that the decreased resonance fission rate for ²³⁹Pu contributes to the more negative direction for the void reactivity.

On the other hand, resonance at ~0.3 eV for ²⁴¹Pu does not have an important role for the void reactivity because its resonance cross section is smaller than that of ²³⁹Pu.