Irradiation Effect on Noble Metal Particles in Water Using In Situ Liquid Cell Transmission Electron Microscopy

Noble metal particles (NMP) in spent nuclear fuel (SNF) play a key role as catalysts to suppress the oxidative dissolution of SNF in the presence of hydrogen gas generated from iron corrosion. Therefore, to predict SNF degradation rate which is described in Fuel Matrix Degradation (FMD) model, the stability of NMP in a radiation environment is important. Herein, we analyzed NMP extracted from SNF using transmission electron microscopy (TEM) and found that NMPs became significantly degraded after nearly decade of intact storage. Furthermore, we investigated the stability of NMPs under irradiation environment using in-situ liquid cell TEM experiments. We synthesized Pd NMPs and Pd-Ru NMPs to study the irradiation effect on NMPs in water for in-situ liquid cell TEM experiments. Dissolution of both types of particles in water was observed owing to the formation of radiolytic species in water. In this regard, we confirmed that higher electron dose rate accelerates the dissolution rate. Also, we have checked the irradiation effect on NMPs without water environment. Instead of dissolution, Pd-PdO core-shell structure was formed with higher dose rate in the absence of water in the system. In addition, after the system reaches super-saturation of Pd²⁺ ions in water due to Pd NMPs dissolution and evaporation of water, Pd dendrite formation was observed by reducing Pd²⁺ ions with electron beam. This confirms that Pd NMPs dissolves and Pd²⁺ ions still exist in water solution. From these in-situ observations, the stability of NMPs under irradiation environments is a potential concern, which, in turn, may impact the ability of these particles to limit SNF degradation.