Thermal diffusivity of tungsten irradiated by protons in spallation environment up to 26.5 dpa

Tungsten is chosen as the target material of European Spallation Source (ESS), where it will be irradiated by a high energy (2 GeV) and high power (5 MW) pulsed proton beam to produce neutrons to be used by neutron scattering intruments. For designing a target with a high availability, it is important to determine the evolution of the temperature and thermal stresses in tungsten during operation. The thermal stress in the target is mainly determined by temperature gradients which depend on thermal diffusivity of tungsten. Reduced thermal diffusivity results in increased beam-induced thermomechanical stress in the target, which affects the survivability and lifetime of the target. As irradiation alters material properties, knowledge on the impact of irradiation on thermal diffusivity is crucial to the ESS target design and operation. In this work, the Laser Flash Analysis (LFA) technique was used to investigate the thermal diffusivity of three highly irradiated samples with displacement damage doses 9.5, 25.1 and 26.5 dpa. Due to the high radioactivity of the specimens, no surface polishing or blackening could be applied on the samples prior to the LFA measurements. Therefore, an attempt was made to study the effect of measured surface roughness on LFA data and the obtained results for irradiated samples were calibrated accordingly. The data shows a drastic reduction of the thermal diffusivity for all three samples by up to 50 %. The thermal diffusivites of the three specimens are overlapping within measurement uncertainty ranges, and are independent of the damage dose. Further, the recovery of the thermal diffusivity of irradiated tungsten was also observed after annealing at 1000 °C for an hour. The results showed the annealing effect was less pronounced compared to specimens irradiated at lower dose and lower temperature. These phenomena are speculated to be owing to the formation of larger dislocation loops and defect clusters in the high dose samples during radiation, resulting in a smaller contribution to the recovery achieved by post irradiation annealing.