The mechanisms of alteration of a homogeneous U0.73Pu0.27O2 MOx fuel under alpha radiolysis of water

Abstract

An homogeneous MOx fuel with 24 wt% of plutonium was leached in a carbonated water (10−2 M) under argon ([O2] < 1 vppm) for one year in order to assess the leaching behaviour of U–Pu oxide solid solutions and more specifically to simulate the behaviour of Pu enriched agglomerates characteristic of heterogeneous Mimas MOx fuel. The alpha activity of the pellets was 2.2 × 109 Bq. g−1. Two successive dissolution regimes were observed: an initial dissolution with a uranium release rate of 1.2 × 10−4 molU.m−2. d−1, and then, a long-term dissolution regime with a rate of 7.6 × 10−6 molU.m−2. d−1. The H2O2 concentration was under the detection limit of 1 × 10−7 mol L−1. Pu concentration in the homogeneous solution was constant around 10−9 mol L−1 throughout the duration of the experiment, in accordance with a thermodynamic equilibrium controlled by an amorphous Pu(OH)4 phase. SEM – WDS analysis confirmed a Pu-enriched layer at the surface of the pellets with Pu contents up to 39 wt%. This Pu-enriched layer becomes more resistant against leaching than the pristine surface. Despite this Pu enrichment, H2O2 concentration remained very low in the homogeneous solution. Different mechanisms of consumption can be considered, such as the oxidative dissolution of the pellet, the precipitation of U or Pu peroxides or the catalytic disproportionation. As the precipitation of peroxides at the surface was discarded by Raman spectroscopy and the oxidative dissolution was very limited, the low H2O2 concentration was likely due to a higher catalytic disproportionation of H2O2 by the Pu-enriched layer. Mass balance calculation showed that H2O2 disproportionation represented 99% of the H2O2 consumption for the homogeneous MOx, against 86% for UO2 pellets. These results shed new light on the Pu stabilizing mechanisms against the oxidative dissolution that can be applied to model the behaviour of different MOx fuels under long-term disposal conditions.