Investigation of plutonium diffusion profiles in Opalinus Clay rock via TOF-SIMS and rL-SNMS

Abstract

The capability of the combined approach of time-of-flight secondary ion mass spectrometry (TOF-SIMS) and resonant laser secondary neutral mass spectrometry (rL-SNMS) for the analysis of diffusion samples of

in Opalinus Clay (OPA) under aerobic conditions was investigated at the micrometer scale. The speciation of Pu in the diffusion reservoir with OPA pore water (pH 7.6) was determined as ${\text{PuO}}_{\text{2}}^{\text{+}}$ using capillary electrophoresis coupled to inductively coupled plasma mass spectrometry (CE-ICP-MS). Using modern 3D printing techniques, a simple and easily scalable experimental setup was developed and adapted to the requirements of TOF-SIMS and rL-SNMS. Together, these techniques allowed for the observation of the pristine diffusion profile of Pu while retaining information about the heterogeneous clay rock. For the experiment with 35 days of in-diffusion, the modeling of an averaged diffusion profile of approximately 300 µm length resulted in $D_a$ = 3.2(4) × 10^-15 m² s^-1. TOF-SIMS and rL-SNMS mappings showed heterogeneous distributions of Pu inside the clay rock and correlations with the matrix elements Fe and Ca, pointing to pyrite and a cementing calcite phase as reactive mineral phases.