PNNL

Abstract

During geologic disposal of spent nuclear fuel(SNF) in an engineered nuclear waste repository, once all other barriers have degraded, oxidizing may occur at the solid-water interface owing to a self-generated radiolytic field. The repository design includes large quantities of iron (Fe), that is anticipated to corrode under an anoxic environment, and generate hydrogen (H2) gas [1,2]. This H2 gas is thought to be able to suppres the dissolution of SNF through a catalytic reaction with noble metal particles (NMP) that are pre-existing in the SNF. This interactions leads to the decomposition of the major oxidant, hydrogen peroxide (H2O2) [3]. These processes are edescribed in the Fuel Matrix Degradation (FMD) model that is being used to predict SNF degradation rates. The NMP, therefore,plays a important role within the FMD model.. Buck and co-workers characterized NMP in SNF using transmission electron microscopy (TEM). With TEM characterization, NMPs were well-crystallized with a composition ofMolybdenum (Mo), Technetium (Tc), Ruthenium (Ru), and Palladium (Pd)[4]. These samples were left untouched for 8 years, and we then re-characterized the samples using scanning transmission electron microscopy (STEM). Surprisingly, very few noble metal particles remained in the sample. From the elemental mapping using EDS, Mo was no longer detected, and the Tc signal was very weak. Therefore, we hypothesized that NMPs in solution are unstable under a radiolytic environment. To support our hypothesis, we performed an in-situ liquid cell STEM experiments on Pd-Ru NMPs. We synthesized Pd-Ru NMPs by following the literature with modification [5]. Poly(N-vinyl-2-pyrrolidone) (PVP) was dissolved in triethyleneglycol (TEG), and the solution was heated with stirring up to 200 °C. Noble metal precursors (RuCl3•nH2O and PdCl2) were dissolved in deionized water. The precursor solution was slowly added to TEG solution for 40 minutes. After synthesis, Ru-Pd nanoparticles were suspended in water. To mimic the actual environment, we applied in situ liquid cell experiments. We prepared the liquid cell holder capped with Pd-Ru nanoparticles in water solution. In STEM, we utilized the electron beam as the source of both observation and the radiation. We filmed Pd-Ru nanoparticles in water using STEM. The particles in water were tracked while the electron beam was irradiated. We observed the particle size decreased with the electron beam irradiation. After 22 minutes of the electron beam irradiation, the particle size had been reduced from around 167.1494 nm2 to near 0 nm2. Furthermore, smaller particles have been dissolved completely, and we could no longer observe them in the images. This result tells us that NMPs dissolve in water with electron beam irradiation. Although more statistics are necessary, this provides some insight into how fast the particles dissolve and the mechanism of NMP dissolution. With elemental mapping using STEM- Electron Energy-Loss Spectroscopy (EELS), the Pd signals became weaker while we continuously acquired spectra. Therefore, we could confirm that Pd-Ru nanoparticles, and not other particles in the system, were dissolving. More data on the behavior of both Pd and Ru is needed to make comparisons on the relative rates of dissolution of the two elements. This will allow us measure the dissolution kinetic difference of the various noble metal elements. In this study, NMP were irradiated by the electron beam in water and their behavior was observed by STEM imaging and EELS. From images, we observed Pd-Ru nanoparticles dissolution. These results suggest that the NMP may be unstable in water exposed to a radiolytic field.