Irradiation Induced Defect Evolution in Nuclear Graphite

Graphite will be used in the next-generation very-high-temperature gas-cooled reactor (VHTR) as a structural material and moderator. Under irradiation, graphite single crystals undergo lattice expansion along the c-axis and shrinkage parallel to the basal planes which have been attributed to the irradiation induced ballistic displacement of carbon atoms which accumulate as interstitial clusters between the basal planes, forcing the planes apart; however, there has been no experimental evidence for such interstitial clusters causing the dimensional change and hence there exists a fundamental disconnect between the accepted graphite irradiation behavioral model and the experimental studies on irradiated graphite. One of the reasons for this gap is that the studies on irradiation damage in graphite have generally been ex-situ analyses of neutron-irradiated specimens, but the defect evolution during irradiation in graphite is a complex, dynamic process in which multiple processes compete to determine the final outcome. The applicants propose to perform detailed in-situ electron-irradiation experiments using a transmission electron microscope (TEM) on nuclear graphite as well as highly oriented pyrolytic graphite (HOPG) specimens. This will enable capture of the damage processes live at the atomic level and identify the formation of different defect structures (vacancy and interstitial clusters, basal and prismatic dislocations, buckling, etc.) and the interactions between them. To ensure that the TEM specimens are defect-free, they intend to develop an oxidation-based technique in which mechanically thinned specimens will be oxidized in a controlled environment to create electron-transparent specimens. In-situ TEM analyses will be conducted at Boise State University. A 200kV TEM equipped with electron energy loss spectrometer (EELS) will be used for this purpose. In-situ irradiation measurements will be carried out using a double-tilt heating holder capable of reaching 1000 oC. Changes in the graphitic lattice and the bonding environment of carbon will be captured at different dose rates and temperatures. In addition to atomic-level imaging, we also intend to use EELS to study the changes in the bonding environment associated with the breakage of basal planes and formation of new defect structures. The Idaho National Laboratory (INL) will help perform neutron irradiation of these graphite samples through Advanced Graphite Creep (AGC) and SAM-II experiments. TEM studies on neutron-irradiated samples will be carried out and the irradiation-induced differences will be determined. In-situ TEM heating experiments will also be performed on neutron-irradiated samples to study defect annealing processes. Analysis of the irradiation-induced evolution of defect structures using atomic level in-situ TEM will help advance the understanding of irradiation induced deformation mechanisms.