Defect Evolution in High-Temperature Irradiated Nuclear Graphite
Authors: Johns Steve 
Published: January 2021

Graphite has historically been used as a moderator material in nuclear reactor designs dating back to the first man-made nuclear reactor to achieve criticality (Chicago Pile 1) in 1942. Additionally, graphite is a candidate material for use in the future envisioned next-generation nuclear reactors (Gen IV); specifically, the molten-salt-cooled (MSR) and very-high-temperature reactor (VHTR) concepts. Gen IV reactor concepts will introduce material challenges as temperature regimes and reactor lifetimes are anticipated to far exceed those of earlier 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. The macroscopic response of graphite subjected to displacing irradiation is often modeled semi-empirically based on irradiation data of specific graphite grades (some of which are obsolete). The lack of an analytical description of the response of nuclear graphite subjected to irradiation is due in part to the complex microstructure of synthetic semi-isotropic graphites.

Chapter One provides a general overview of the application, processing, and irradiation-induced property changes of nuclear graphite. The key properties affected by displacing irradiation include, but are not limited to, coefficient of thermal expansion (CTE), irradiation creep, and irradiation-induced dimensional change. Additionally, historical models of radiation damage in nuclear graphite, including their inadequacies in accurately describing property changes, are discussed. It should be noted that a comprehensive explanation for all irradiation-induced property change is beyond the scope of this work, which is focused on the evolution of novel atomic-level defects in high-temperature irradiated nuclear graphite and the implications of these defects for the current understanding of irradiation-induced dimensional change.

Chapter Two is focused on the development of a novel oxidation-based transmission electron microscopy (TEM) sample-preparation technique for nuclear-grade graphite. Conventionally, TEM specimens are prepared via ion-milling or a focused ion beam (FIB); however, these techniques require the use of displacing radiation and may result in localized areas of irradiation damage. As a result, distinguishing defect structures created as artifacts during sample preparation from those created by electron- or neutron-irradiation can be challenging. Bulk nuclear graphite grades IG-110, NBG-18, and highly oriented pyrolytic graphite (HOPG) were oxidized using a new jet-polishing-like setup where oxygen is used as an etchant. This technique is shown to produce self-supporting electron-transparent TEM specimens free of irradiation-induced artifacts; thus, these specimens can be used as a baseline for in situ irradiation experiments as they have no irradiation-induced damage.

Chapter Three examines the dynamic evolution of defect structures in nuclear graphite IG-110 subjected to electron-irradiation. As use of fast neutrons for irradiation experiments is dangerous, expensive, and time consuming, electron-irradiation is arguably a useful surrogate; however, comparisons between the two irradiating particles is also discussed. In situ video recordings of specimens undergoing simultaneous heating and electron-irradiation 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 to crystallites.

Neutron-irradiated nuclear graphite IG-110 was supplied by Idaho National Laboratory as part of the Advanced Graphite Creep capsule experiments (AGC-3). Chapter Four reports the preliminary characterization of IG-110 neutron-irradiated at 817°C to a dose of 3.56 displacements per atom (dpa). Shown is experimental evidence of a ‘ruck and tuck’ defect occurring in high-temperature neutron-irradiated nuclear graphite. The ‘ruck and tuck’ defect arises due to irradiation-induced defects. The interaction of these defects results in the buckling of atomic planes and the formation of a structure composed of two partial carbon nanotubes. The “buckle, ruck and tuck” model was first theoretically predicted via computational modeling in 2011 as a plausible defect structure/mechanism occurring in high-temperature neutron-irradiated graphite by Prof. Malcolm Heggie et al. Chapter Four shows the first direct experimental results to support the “buckle, ruck and tuck” model.

Chapter Five further characterizes nuclear graphite IG-110 neutron-irradiated at high temperature (≥800 °C) at doses of 1.73 and 3.56 dpa. Results show further evidence to support the “buckle, ruck and tuck” model and additionally show the presence of larger concentric shelled fullerene-like defects. Fullerene-like defects were found to occur in disordered regions of the microstructure including within nanocracks (Mrozowski cracks). These results agree with high-temperature electron-irradiation studies which showed the formation of fullerene-like defects in-situ and give additional validity to the use of high-flux electron-irradiation as a useful approximation to neutron-irradiation. Furthermore, Chapter Five gives 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.