An investigation of acoustic emission from an irradiated nuclear graphite

Abstract

Acoustic emission (AE) responses from unirradiated nuclear graphites subject to cyclic loading have been shown to exhibit the Felicity effect, i.e., AE is detected at stresses below the previous peak stress. This has been attributed to time dependent recovery processes which occur upon unloading and at zero stress [1]. This paper describes work comparing similar unirradiated and irradiated nuclear sleeve graphites. The number of AE event counts from irradiated graphite was shown to be greater than that from unirradiated graphite, subjected to similar stresses. This effect was attributed to the increase in porosity caused by radiolytic oxidation. A Felicity effect was observed on cyclic loading of irradiated graphite, but no evidence for a Kaiser effect was found for irradiated graphite loaded monotonically to failure. The absence of a Kaiser effect can be attributed to relaxation and recovery processes that occur in the considerable time interval between removing the irradiated graphite from the reactor and post-irradiation examination. Therefore, it was concluded that AE monitoring is not a suitable technique for measuring internal stresses in irradiated graphites.