Proton irradiation-induced creep of ultra-fine grain graphite

The objective of this work was to systematically study the effects of applied tensile stress, dose rate, temperature, and accumulated dose on the in situ proton irradiation-induced creep behavior of graphite. Experiments were performed on ultra-fine grain POCO Graphite Inc. ZXF-5Q. The irradiation creep rate exhibited a linear dependence on applied tensile stress, dose rate, and temperature and no dependence on dose to a total accumulated value of 1.0 dpa. These dependencies showed reasonably good agreement with in-reactor creep data. Analysis of the crystal structure and crystallite size parameters following irradiation under load revealed that the most-probable source of structural change is formation of interstitial clusters between basal planes and uncollapsed vacancy lines, in agreement with changes observed for graphite irradiated in-reactor. The parametric dependencies and the change of the lattice parameters, and crystallite dimensions, were compared with dependencies from several potential mechanisms of irradiation-induced creep and revealed, via process of elimination, that Stress-Induced Preferential Absorption (SIPA) is the most likely mechanism to explain irradiation-induced creep in this material.