The electrochemical deconsolidation mechanism of graphite matrix in HTGR spherical fuel elements

Abstract

In High-Temperature Gas-cooled Reactor (HTGR)s, coated fuel particles are dispersed in a graphite matrix, which forms both compact and spherical fuel elements. As for the post irradiation examination (PIE) of HTGR irradiated fuels, the electrochemical deconsolidation method in which graphite matrix is disintegrated into graphite powder is one of the feasible processing methods. The graphite matrix is composed of natural graphite, artificial graphite and glassy carbon from carbonized resin, which have different effects on electrochemical deconsolidation. In this work, synthesized graphite matrix specimens were produced by intentionally varying the resin binder content in the graphite matrix raw materials from 5% to 50%, in order to investigate the electrochemical deconsolidation mechanism of graphite matrix in HTGR fuel elements. The results showed that increasing the glassy carbon content increased the degree of chemical oxidation, with more pronounced production of graphite oxide. The driving force for graphite matrix deconsolidation is proposed to be a synergetic effect of the expansion stress around closed- and through-hole microstructure originating from oxidatively destroyed graphite layers. This work will not only help to more thoroughly understand the electrochemical deconsolidation procedure of HTGR fuels, but also contribute to the development of a feasible method to manage radioactive graphite waste.