Figure: Scattering measurements on fine-grain nuclear graphite taken using neutrons and X-rays. results using USAXS (green) and WAXS (blue) are compared with data presented by Zhou et. al (orange). These measurements span vast length-scales covered by different measurement techniques shown in real space on the top x-axis.

Lead P.I. - Dr. David Sprouster and Dr. Lance Snead

Since the original designs of nuclear reactors in the 1940s, graphite has been an essential material for in-core components, it remains integral to the designs of the next-generation reactors. Graphite, made from crystallized carbon, has the unique property of efficiently scattering neutrons combined with a low absorption rate which makes it ideal for use as a neutron moderator or reflector. Nuclear graphite is a composite material formed by crystalline filler particles joined together with a disordered graphite binder which creates a complex structure with noticeable porosity. Each grade of nuclear graphite has a unique pore morphology that changes under irradiation by neutrons. When damaged by irradiation, nuclear graphite initially undergoes densification, and, upon further irradiation, the graphite reaches a so-called turnaround point in which it begins expanding. This expansion leads to cracking and material failure, which limits the material’s lifetime. The complex irradiation-induced changes of nuclear graphite need to be quantitatively understood to accurately predict the lifetime of graphite components. In this project, we aim to improve our understanding of nuclear graphite’s response to irradiation by investigating the pore structure using scattering and imaging techniques. We propose to combine ultra small angle x-ray scattering (USAXS) with X-ray computed tomography (XCT) to nondestructively examine the morphology of the pore structure using fractal mathematical models. Fractal models have a direct correspondence to the probability of brittle fracture resulting from microcrack growth, which is statistically represented using a Weibull distribution. We plan to develop and evaluate a new methodology to nondestructively determine the Weibull parameters for irradiated nuclear graphite, enabling the probability of failure to be predicted under various irradiation doses and temperatures.

The major tasks for this project will be as follows. First, prepare unirradiated and irradiated graphite samples for measurements. Second, reanalyze published datasets collected using destructive tests to directly compare them with our nondestructive techniques. Third, perform XCT on samples our group previously measured with USAXS to optimize data collection procedures. Fourth, conduct XCT measurements on the remaining samples. Fifth, measure all the samples with USAXS and investigate our methodology for determining the Weibull parameters using a fractal model.