The Mechanism of Proton Irradiation-Induced Creep in Ultra-Fine Grain Graphite.

[摘要] The objective of this work was to determine the mechanism of proton irradiation-induced creep of ultra-fine grain graphite.Graphite is currently used as a structural material in nuclear reactors in the United Kingdom and is planned for future use in the Very High Temperature Reactor.Temperature and dose gradients within these structural components result in build-up of stresses that would surpass the fracture stress, but irradiation-induced creep allows for relaxation of some of these stresses.Thus it is critical to understand the mechanism that controls irradiation-induced creep in graphite to be able to predict the integrity of these current and future structural components.This work used POCO Graphite Inc. grade ZXF-5Q, which is an ultra-fine grain graphite.The proton irradiation-induced creep experiments were performed with a range of experimental conditions to investigate the effects of applied tensile stress, dose rate, irradiation temperature, and total accumulated dose.These experiments showed a linear dependence of creep rate on applied stress and dose rate, an Arrhenius dependence of creep compliance on temperature, and no effect of accumulated dose out to 1dpa.Using the experimental dependencies, it was found via process of elimination that the most probable mechanism controlling irradiation-induced creep Stress-Induced Preferential Absorption (SIPA) of defects at dislocations.This mechanism was further supported with the investigation of the post-irradiation Young’s modulus.The Young’s modulus was not affected by total dose thus agreeing with a steady-state creep regime, higher applied stress reduced Young’s modulus thus agreeing with either traditional SIPA or anisotropic diffusion SIPA; and finally, Young’s modulus increased with increasing temperature thus agreeing with the calculated interstitial concentrations being higher for the higher temperature experiments.Microstructural analysis investigated the effect of experimental conditions on the lattice spacing, uniformity of lattice spacing, and crystallite sizes.The results of these measurements agreed with the dependencies expected for the newly-proposed mechanism of radiation damage in graphite, which occurs when vacancy lines and loops disassociate into two dislocations, instead of the traditional theory that dimensional changes were due to the formation and growth of interstitial loops.

[发布机构] University of Michigan

[效力级别] Materials Science and Engineering

[关键词] Radiation Effects in Graphite;Materials Science and Engineering;Nuclear Engineering and Radiological Sciences;Engineering;Nuclear Engineering & Radiological Sciences