Effect of water molecules co-intercalation and hydrolysis on the electrochemical deconsolidation of matrix graphite in aqueous nitric acid

Abstract

Separation of complete TRISO-coated fuel particles from matrix graphite is challenging but important for the post-irradiation examination (PIE) of high-temperature gas-cooled reactor (HTGR) fuel elements. Electrochemical technology is considered one of the most promising and effective approaches in this regard. Herein, to explore the influence of HNO3 concentration on galvanostatic deconsolidation, 4%–68% HNO3 were taken as examples. The matrix graphite anodes gradually exfoliated to fragments in 34% HNO3, while expanding in other concentrations. The characterization results of deconsolidated fragments and galvanostatic curves identified the highest oxygen content was reached in 34% HNO3 due to the combined effect of intercalation, hydrolysis and oxygen evolution. In higher HNO3 concentrations like 68%, the intercalating agent predominates and therefore the intercalation and following hydrolysis reactions are the main causes of matrix graphite deconsolidation, with no oxygen evolution occurring. In contrast, in lower HNO3 concentrations like 4%, the violent oxygen evolution reaction hinders the intercalation and hydrolysis reactions. In short, only the combined interaction of intercalation, hydrolysis and oxygen evolution can deconsolidate the bulk matrix graphite into a more homogeneous powder. This work will provide more insights into the electrochemical deconsolidation mechanism and process of both HTGR fuel elements and other graphite materials.