Analysis of photoneutron flux distribution in water-shielded reactors

In a water-shielded reactor, the photonuclear reaction γ(d, p)n takes place between γ-rays emitted from reactor core and deuterons contained in the shielding water surrounding the core. Photoneutrons produced by the reaction are distributed in the water layer in addition to the neutrons originated from the core. In this report, it is shown that the distribution of the thermal photoneutron flux φ_i^γ in water not close to the core surface is approximately given by the simple formula: φ_i^γ = Σ_i Σ_γnⁱ/Σ₂/(1-μ_i²τ)(1-μ_i²L²) φ_γⁱ, (L² = D₂/Σ₂) where φ_γⁱ and μ_i represent the γ-ray flux in the i-th energy group and its linear absorption coefficient in water, Σ_γnⁱ is the macroscopic cross section for the photoreaction, D₂ and Σ₂ are the diffusion coefficient and the absorption cross section for thermal neutrons in water and √τ is the slowing-down length in water. Comparison of the calculated results to the experiments in BSR-1 shows that the above formula reproduces the photoneutron distribution reasonably well in a region where the neutron flux originated from the reactor core is negligibly small. In particular the above expression for photoneutrons together with the calculated neutron flux originated from the core can explain a bend in the observed neutron flux distribution occurring at about 1.8 m from the core surface of BSR-1. (author)