Photoneutron production in heavy water reactor fuel lattice from accelerator-driven bremsstrahlung

Abstract

The coupling of low-energy electron linear accelerators (eLINACs) to a large heavy water reactor is proposed to create an accelerator-driven photoneutron source (ADS). Photoneutron yields of 1012 pn/s per kW of beam power can be achieved in the ADS-CANDU concept where the wide fuel channel spacing of heavy water reactors represents a near-optimal geometry for conversion of accelerator-driven bremsstrahlung into photoneutrons in the heavy water moderator with minimal parasitic attenuation of photons in the fuel channels. Twenty MeV electron beam energy is most efficient at producing photoneutrons despite having the largest fuel channel shielding effect (28% attenuation compared to an infinite heavy water medium). The majority of photoneutrons are produced during source-photon first flights, so the spatial distribution and emission spectra of the ADS in the secondary (γ,n) converter are correlated to the doubly differential angle and energy distribution of the bremsstrahlung emitted from the (e-,γ) converter. Compton-scattered photons and tertiary bremsstrahlung originating from the electron–positron pair and recoil electron secondary particles are important contributors to the photoneutron yield of higher energy eLINACs systems. Photonuclear data for the deuterium photoneutron reaction cross section and secondary electron transport and tertiary bremsstrahlung production physics implemented in Monte-Carlo radiation transport codes can dominate ADS simulation results and are the same magnitude as the physical phenomena such as the fuel channel shielding effect.