Large radiation back-flux from Monte Carlo simulations of fusion neutron-material interactions

TL;DR

This study quantifies the magnitude and implications of radiation back-fluxes, including neutrons, gamma rays, and electrons, from neutron-material interactions in fusion reactors using Monte Carlo simulations, highlighting their impact on plasma dynamics and disruption mitigation.

Contribution

It provides the first detailed quantification of prompt and delayed radiation back-fluxes across different materials and wall thicknesses in fusion reactors, emphasizing their significance.

Findings

Neutron and gamma back-fluxes are comparable to incident flux.

Electron back-fluxes are lower but can cause sheath reversal.

Delayed gamma and electron back-fluxes can reach 7% of prompt fluxes.

Abstract

Radiation back-fluxes, generated from neutron-material interactions in fusion power reactors, can dramatically impact the plasma dynamics, e.g., by seeding runaway electrons during disruptions via Compton scattering of background electrons by wall-emitted gamma radiation. Here, we quantify these back-fluxes, including neutrons, gamma rays, and electrons, using Monte Carlo calculations for a range of structural material candidates and first wall thicknesses. The radiation back-flux magnitudes are remarkably large, with neutron and gamma radiation back-fluxes on the same order of magnitude as the incident fusion neutron flux. Electron back-fluxes are two orders of magnitudes lower, but are emitted at sufficiently high energies to provide a relatively large back-current through the sheath which may cause sheath reversal. Material configuration plays a key role in determining back-flux magnitudes. The structural material chiefly determines the neutron back-flux magnitude, while the first wall thickness principally attenuates the gamma ray and electron back-fluxes. In addition to prompt back-fluxes, which are emitted immediately after fusion neutrons impact the surface, significant delayed gamma ray and electron back-fluxes arise from nuclear decay processes in the activated materials. These delayed back-flux magnitudes range from 2%--7% of the prompt back-fluxes, and remain present during transients when fusion no longer occurs. During disruptions, build-up of delayed gamma radiation back-flux represents potential runaway electron seeding mechanisms, posing additional challenges for disruption mitigation in a power reactor compared with non-nuclear plasma operations. This work highlights the impact of these radiation back-fluxes plasma performance and demonstrates the importance of considering back-flux generation in materials selection for fusion power reactors.