The effects of fission vs fusion-like neutron spectra on REBCO activation and microstructure

REBCO-based superconducting magnets are considered crucial to the long-awaited realization of commercially viable controlled thermonuclear fusion via compact reactor architectures. To expand the current understanding of how the inherently reduced shielding of these proposed designs affects the radiation-sensitive high temperature superconductors, the activation spectra and microstructure of REBCO tapes irradiated more traditionally with fission-reactor neutrons were compared to those irradiated with the higher-fidelity fusion-like spectrum produced by thick target deuteron breakup in a first-of-its-kind irradiation experiment on these coated conductors. For the fusion-like case, several peaks in the activation spectrum confirmed high-threshold nuclear reactions, and SEM revealed superficial alterations to the most intensely irradiated area. TEM revealed that although both sets of microstructures appeared strained, the 14 MeV-irradiated damage manifested as milder patches of subcascades and large voids, with no large amorphous clusters classically associated with REBCO fission irradiations, as confirmed by our own fission samples. Computational analysis on the visually indistinguishable respective lattice strains revealed comparable damage metrics of the fusion-like samples to fission samples, despite the much lower total fluences on the former. The nuclear physics origins of these findings and their implications for the superconducting properties are also discussed. These results present an important preliminary step in the rigorous technical readiness qualifications required by this pivotal technology in the burgeoning REBCO-based compact fusion industry.