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Graduate Defense: Steve Johns

October 23 @ 1:00 pm - 3:00 pm MDT

Dissertation Information

Title: Defect Evolution in High-Temperature Irradiated Nuclear Graphite

Program: Doctor of Philosophy in Materials Science and Engineering

Advisor: Dr. Rick Ubic, Materials Science and Engineering, Co-Chair, and Dr. Karthik Chinnathambi, Materials Science and Engineering, Co-Chair

Committee Members: Dr. Peter Müllner, Materials Science and Engineering, and Dr. Will Windes, Materials Science and Engineering


Graphite has historically been used as a moderator material in many nuclear reactor designs and remains a candidate material for use in the future envisioned next-generation nuclear reactors (Gen IV). Gen IV reactor concepts will introduce new material challenges as temperature regimes and reactor lifetimes are anticipated to far exceed those of earlier generation reactors. Irradiation-induced defect evolution is a fundamental response in nuclear graphite subjected to irradiation. These defects directly influence the many property changes of nuclear graphite subjected to displacing radiation; however, a comprehensive explanation for irradiation-induced dimensional change remains elusive. This dissertation is focused on the characterization of high-temperature irradiation-induced defect evolution in nuclear graphite via transmission electron microscopy (TEM). Conventional TEM sample-preparation techniques require the use of displacing irradiation and may result in localized areas of irradiation damage. Novel oxidative TEM specimen preparation, which does not require displacing irradiation, was developed to produce baseline specimens. As use of fast neutrons for irradiation experiments is dangerous, expensive, and time-consuming, electron-irradiation is arguably a useful surrogate. In situ video recordings of specimens undergoing electron-irradiation and heating were used to analyze the dynamic atomic level defect evolution in real-time. Novel fullerene-like defect structures are shown to evolve as a direct result of high-temperature electron-irradiation and cause significant dimensional change. In situ electron-irradiation results were compared to those caused by actual neutron-irradiation at comparable doses and temperatures via ex situ characterization. Ex situ analysis confirms fullerene-like defects as a dominant defect type. Additionally, results show the first direct experimental evidence of defect structures previously hypothesized via computational modeling. These results provide valuable insight to unresolved quantitative anomalies of historical models of graphite expansion and may improve the understanding of current empirical and theoretical models of irradiation-induced property changes in nuclear graphite.