Neutron Capture on Stable Fission Product~139La Contribution to Delayed-Photoneutrons in Heavy Water Reactors

摘要

Delayed-photoneutrons (pns) produced from hard gamma rays incident on deuterium contribute ~5% to the delayed neutron fraction (β) of heavy water reactors. The gamma-emitting fission fragments can have much longer half-lives than the conventional 6-groups of neutron decay precursors, so pns play an important role in controllability of heavy water reactors. The ~(1-40)Ba~(1-40)La decay chain with parent ~140Ba (12.753 d l~½ & cumulative fission yield X_c = 6.2%) and daughter~140La (1.679 d T~½ & independent fission yield Xi = 5.3 x 10~(-3)%) populates the longest-lived fi-pn Groups 7&8. The ~140La decay scheme includes 2.52 MeV gamma ray with 3.46% yield and a few other lower-intensity gamma lines above the D(γ,n) threshold. The dclayed-pn data used in CANDU analysis for the past 60 years is derived from a patchwork of fission experiments (some unpublished) and zero power reactor measurements, and until recently no detailed isotopic analyses had been performed [1]. A cursory survey of the chart of the nuclides found many stable/long-lived isotopes (~139La, ~86Kr,~87Rb, ~137Cs,~82Se) whose radioactive activation products emit hard gammas. However, the thermal (n,γ) cross sections are small (< 1 b to mb), except for stable ~139La (9 b and 6.41% x_c). The study hypothesis is: fuel cycle-dependent neutron activation reactions on stable isotopes in the fuel contribute to delayed-pn fractions. This work quantifies the neutron capture on ~139La contribution to the ~140La delayed-pn group for the natural uranium and DUPIC (Direct Use of spent PWR fuel In CANDU) fuel cycles for CANDU reactors.