Investigation of Size Effects in Test Methods Used for Irradiated and Oxidized Graphite

Abstract

Postirradiation examination (PIE) of graphite samples trepanned from the UK reactor cores has been carried out for more than 50 years. Due to the nature of the material, there are sample size and geometry restrictions and no standard test methods to cover measurements on this material. Nevertheless, these measurements are used to support the continued operation of the UK reactor cores, and hence a large program of trials is carried out to provide confidence that each method is accurate and reproducible. These trials typically involve a study of size effects using virgin graphite and simulant materials for the irradiated and oxidized graphite, but a corresponding study with irradiated samples is usually not possible. This paper combines the work of two UK studies to investigate the size effect of the PIE test methods on irradiated graphite on the basis of the characterization of graphite used in the advanced gas-cooled reactors (AGRs) and Magnox reactors. The AGR study focused on the static Young's modulus, three-point bend strength of unnotched and notched beams, and the work of fracture. The Magnox study focused on the coefficient of thermal expansion, diametral compression, and flexural strength. The two studies used large irradiated graphite samples from “installed sets” (i.e., precharacterized graphite samples installed in the reactor cores before the start of operation for monitoring purposes). Large Magnox samples that were trepanned from the reactor core after shutdown were also tested. The purpose of these investigations was to relate the graphite measurements normally undertaken on small trepanned samples to property values obtained using standard test methods on irradiated material. The sample selection was such that it covered as wide a range of dose and radiolytic weight loss as possible. This paper outlines the methodology, results, and conclusions for each of the studies and provides some guidelines for similar studies on new graphites.