Electrochemical behavior of matrix graphite in nitric acid by cyclic voltammetry

Abstract

Post-irradiation examination of irradiated spherical fuel elements in high-temperature gas-cooled reactors (HTGRs) requires the separation of integrated tri-structural isotropic (TRISO) particles from matrix graphite (MG). The electrochemical method is a promising approach for separating intact TRISO particles and exploring the fission product source terms in MG. In this work, the electrochemical behavior of MG and its three compositions, i.e., artificial graphite (AG), natural graphite (NG), and binder carbon (BC) were investigated experimentally by cyclic voltammetry using nitric acid as an electrolyte, and these three compositions showed very different electrochemical properties. Compared with BC, AG and NG exhibited relatively higher electrochemical reactivity controlled by diffusion, while BC performed almost identical adsorbing-controlled electrochemical behavior with MG composites, which probably resulted from encased AG and NG flakes by bulk BC. According to cyclic voltammetric and characterizing results of functional groups on the surface of the specimens, the deconsolidation reacting procedures of MG could be further interpreted into a two-step process. The first step was the electrochemical oxidation of BC by unsaturated , defective, and sp2 carbon atoms. In the second step, the NG and AG flakes were intercalated, oxidized and exfoliated by combined reactions of hydrolysis, intercalation, and oxygen evolution. The proposed electrochemical procedures obtained in this study provide the theoretical foundation to PIE investigations of HTGR spent fuels.