Microstructural Processes Leading To Fracture In Nuclear Graphites.

Neighbour, Gareth Bryan (1993). Microstructural Processes Leading To Fracture In Nuclear Graphites. PhD thesis University of Bath.

URL: https://purehost.bath.ac.uk/ws/portalfiles/portal/...

Abstract

Thermal nuclear reactors, such as Magnox and AGR, contain graphite moderator blocks in which irradiation-induced internal stresses are generated. Measurement of the internal stresses is required by the nuclear power industry and acoustic emission (AE) monitoring has been identified as a possible method. There is some evidence that a Kaiser effect occurs in graphites, i.e., on stressing a graphite that has been subject to a prior stress, the onset of AE occurs at the previous peak stress. The work reported in this thesis examines the suitability of AE for this purpose. The microstructures of three nuclear graphites (PGA, IM1-24 and VNEC) were characterised qualitatively and quantitatively using optical and electron microscopy and AE was monitored during both monotonic and cyclic loading to failure in tensile, compressive and flexural test modes. For unirradiated graphites, the Kaiser effect was not found in cyclic loading, but a Felicity effect was observed, i.e., the onset of AE occurred below the previously applied peak stress. The Felicity effect was attributed to time-dependent relaxation and recovery processes and was characterised using a new parameter, the Recovery ratio. It was shown that AE can be used to monitor creep strain and creep recovery in graphites at zero load. The AE-time responses from these experiments were fitted to equations similar to those used for creep strain-time at elevated temperatures. The number of AE counts from irradiated graphites were greater than those from unirradiated graphites, subject to similar stresses, due to increases in porosity caused by radiolytic oxidatioa A Felicity effect was also observed on cyclic loading of irradiated graphites, but no evidence for a Kaiser effect was found for irradiated graphites loaded monotonically to failure. Thus internal stresses in irradiated graphites could not be measured using AE. This was 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.

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