Laser Ultrasonic Sensing of Oxidation-Induced Microstructural Changes in Nuclear Graphites

Serving both as a structural and as a functional material, nuclear graphites have a range of useful mechanical, chemical and radiation-related behaviors that are important for nuclear applications. The raw materials used as feedstock along with the manufacturing processes used for production of these materials results in a complicated microstructure composed of graphite filler, graphitized pitch binder, and voids/defects that include porosity and microcracks. These heavily influence the overall elastic moduli of nuclear graphites and must be taken into account when interpreting ultrasonic measurements. Indeed, the measured elastic moduli are generally close to the Reuss limits - the theoretical, lower bounds for the moduli - but no attempts have been made to explain these results within the broader context of elasticity in porous materials containing microcracks. In this work, we report ultrasonic measurements of elastic moduli for nuclear graphites as a function of the bulk porosity and interpret them using relatively simple, physics-based models that describe the effects of porosity and microcracking on modulus. Modifications to these models are explored to account for microstructure-related effects on modulus such as those that are associated with oxidation-induced porosity. These models can be used to interpret a broad range of ultrasonic measurements reported previously for nuclear graphites, and the insight gained into the elastic responses of these materials can be used to guide the use of ultrasonic methods to assess the structural integrity of nuclear graphites.