Unexpected large radiation back-flux from neutron irradiation of materials in fusion power reactors

Neutron-material interactions in fusion power reactors lead to an intense radiation back-fluxes to the plasma core. These back-fluxes can impact the plasma boundary conditions via electron emission, in turn affecting the core plasma performance, or can seed runaway electrons via Compton scattering by gamma rays. We have quantified these back-fluxes, including prompt contributions from neutron collisions and delayed contributions from nuclear decay processes, for various material configurations using coupled radiation transport simulations and nuclear decay calculations. The neutron back-flux, which is entirely prompt, has the same magnitude as the incident flux. Gamma ray back-fluxes range from 14% to 52% of the incident flux, and electron back-fluxes are two orders of magnitude smaller. The first wall weakly attenuates the neutron back-flux, but strongly attenuates gamma ray and electron back-fluxes. As a result, the gamma ray and electron back-fluxes are emitted with large ~MeV energies. Delayed back-fluxes range from 2--6% of the prompt back-fluxes, which is significant enough to influence post-thermal-quench disruption dynamics. This work provides a route to optimize materials selection in fusion reactors based on plasma performance impact from neutron irradiation.