Measurement of Coefficient of Thermal Expansion of Small Graphite Specimens

Abstract

Coefficient of thermal expansion (CTE) gives a key insight into the thermomechanical properties of a material, representing the dimensional expansion of a material for a given temperature change. It is of particular relevance to graphite components within reactors, providing insight into the thermal strains experienced at operating temperatures. In combination with other internal strains and external loads, these thermal strains can have a direct effect on component performance, as well as operational lifetimes. Hence, a good understanding of how CTE changes with irradiation is essential for any graphite reactor component in both existing and newly built reactors. This understanding relies on accurate measurements of small specimens with noncompliant geometries. The most commonly used method of measuring CTE is push-rod dilatometry, favored for its simplicity, and this technique is covered by ASTM E228. This and other CTE measurement standards place specific size requirements on samples, which can be difficult to achieve for irradiated graphite samples recovered from reactors or irradiated in material test reactors. The National Nuclear Laboratory (NNL) has used this technique to routinely measure the CTE of small samples trepanned from Magnox and advanced gas-cooled reactor (AGR) graphite over the past three decades as part of the graphite core monitoring campaigns. The samples measured at the NNL are not compliant with the geometry requirements mandated by ASTM E228. This paper examines two studies on nonstandard compliant geometry AGR samples. The first is a comparison between dilatometry and electronic speckle pattern interferometry, and the second is a dilatometry study conducted on samples of similar material but differing geometries. The results are used to discuss typical challenges and aspects of the measurements that require addressing to demonstrate confidence in dilatometry and optical interferometry CTE measurements on noncompliant geometries.