Publication Date

8-2013

Type of Culminating Activity

Dissertation

Degree Title

Doctor of Philosophy in Materials Science & Engineering

Department

Materials Science and Engineering

Supervisory Committee Chair

Darryl P. Butt, Ph.D.

Abstract

The very high temperature reactor (VHTR) is the latest generation of high-temperature gas-cooled reactors (HTGR). The VHTR has higher outlet temperatures than a traditional HTGR with outlet temperatures up to 1000 °C. This high outlet temperature permits emissions-free process heat in the form of high-quality steam for high temperature industrial applications. Moreover, the high temperatures of the reactor could potentially be used for hydrogen production from water or high efficiency electrical power production (~50% efficiency of thermal to electrical power conversion).

The VHTR is designed to employ helium as its coolant and uses graphite for its neutron moderator and as a key structural component for the core. Graphite is used because of it excellent structural stability at high temperatures, high thermal inertia, and the relatively low cost of its production. Since graphite is a key component to the VHTR, the integrity of the graphite is critically linked to the operable lifetime of the reactor. Thorough characterization of the graphite material used, as well as a complete understanding of the mechanisms behind graphite’s life-limiting phenomena is critical to understanding the limits of safe operation for the VHTR. Graphite is not a new high temperature nuclear material; in fact, it has been used in nuclear reactor designs since the very first reactor went critical in December of 1942. Due to its continued use in reactors, a major focus has been placed on understanding graphite’s long-term degradation in a radiation environment, and thus many of the phenomena responsible for degradation are well known.

Unfortunately, graphite is a complicated/complex material in that the properties are highly dependent upon the initial source of carbon, as well as variations in the coke type, size and relative quantities of filler and binder, and the manufacturing process used. Thus, each graphite used for nuclear applications is in some sense a new material with unique properties that must be thoroughly characterized before use in a reactor.

This dissertation research is focused on the pre-irradiation characterization of IG-110, PGX, NBG-18, and PCEA commercial graphites, the atomic level defects involved in irradiation induced shrinkage and swelling of these graphite materials, and finally the development of a unified reaction model for the oxidation of all high-purity nuclear graphites with oxygen. While similar characterizations and mechanistic studies have been made, many of the techniques used in this study such as electron energy loss spectroscopy (EELS), image processing and analysis, and filtering of high resolution lattice images were either impractical or unavailable in the past. This dissertation seeks to build on past studies of classical reactor grade graphites and use modern experimental techniques to further our understanding of the specific graphites examined and the underlying mechanism that contribute to graphite degradation.

In Chapter Two, microstructural characterization of the filler and binder materials is performed. All grades examined were well graphitized in both the binder and filler, although the spatial domains of crystallites were significantly smaller in the binder. Turbostratic graphite, indicated by an elliptical diffraction pattern, was present in all grades. The microcracks, which are known to contribute to the bulk materials shrinkage and later swelling, were found to vary significantly in size, shape, and quantity with graphite grade.

Chapter Three examines the atomic scale defects responsible for irradiation induced swelling and microcrack closure via transmission electron microscopy under electron beam irradiation. Utilization of noise-filtering in the frequency domain of lattice images and videos allowed analysis of the formation of vacancy loops, interstitial loops, and resulting dislocations with unprecedented clarity. The dislocations were observed to undergo positive climb resulting in the formation of extra basal planes. This in addition with the reduction in atomic density evidenced by electron energy loss spectroscopy is believed to be responsible for the graphite swelling in the c-direction and microcrack closure.

Using optical microscopy, the macro-scale features of the filler particles and macro-porosity were characterized in Chapter Four. The average size and shape of the two-dimensional cross-sections of the filler particles for each grade was determined. A qualitative trend was found between the aspect ratio of the particles and the degree of alignment of the particle crystallites. To characterize the porosity, image analysis was performed using code written in matlab. Probability densities were determined for the size and shape of the macroporosity. Furthermore, a preferred orientation was observed for all grades characterized. The code for two-dimensional analysis used for the corresponding publication is currently being modified to analyze three-dimensional input data from µX-ray CT scans and will be published in a future journal article.

In Chapter Five, the oxidation of NBG-18 nuclear graphite was studied. A reaction model was developed based upon the actual oxygen transfer mechanism for the graphite-oxygen reaction system. The parameters are therefore physically meaningful and directly related to individual elementary reaction rates within the mechanism. The Arrhenius parameters are in excellent agreement with experimental and theoretical measurements of the same elementary reactions. Given the wide variety of high-purity graphite sources used in this literature and excellent agreement between measured and predicted values, the developed intrinsic model should be applicable to all nuclear-grade graphites. Moreover, the model can be extrapolated outside the experimental temperature and pressure range with much larger degrees of certainty due to the relationship of the fitted parameters to the physical reaction mechanism.

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