Effect of Thermal Neutron Shielding on the Irradiation Hardening and Recrystallization of Tungsten

Several tungsten grades were irradiated at nominal 400°C, 800°C, and 1100°C to a dose of 0.84 dpa using an artificially tailored neutron spectrum in the BR2 reactor. The irradiated materials includes ITER specification grades, one advanced pure tungsten grade, a W0.5ZrC particle-reinforced grade, and two potassium-doped grades. A 0.5 mm gadolinium shield was employed to suppress the thermal neutron component of the BR2 spectrum, producing a less lethargic neutron spectrum that reduces the tungsten transmutation rate to a nominal value of 0.5 at.% Re/dpa, significantly lower than in unshielded BR2 spectrum where it was over 2 at.% Re/dpa.The objective of this work is to investigate the influence of the reduced thermal neutron fraction on post-irradiation mechanical properties, assessed via room-temperature Vickers hardness and elevated-temperature tensile testing. The lower transmutation rate of tungsten into rhenium and osmium leads to reduced irradiation hardening compared to previous BR2 and HFIR campaigns conducted without thermal neutron shielding. One of the pure tungsten grades, demonstrates superior post-irradiation mechanical properties, probably because of its specific manufacturing process. A warm rolled potassium-doped grade performs better than all the other materials of this study, as a result of strong crystallographic texture and high initial dislocation density, which seems to promote property retention under neutron irradiation for the preferred orientation. Furthermore, fractographic analysis of tensile specimens reveals irradiation-enhanced recrystallization, a consequence of the reduced transmutation rate under this spectrum.These findings underscore the critical role of neutron spectrum tailoring for assessing and benchmarking the radiation response of tungsten-based materials in the context of fusion applications.