Evolution of the uranium concentration in dissolution experiments with Cr-(Pu) doped UO2 in reducing conditions at SCK CEN

Christelle Cachoir,Karel Lemmens,Thierry Mennecart

doi:10.1557/s43580-021-00027-y

Abstract

Cr-doped UO2-based model materials were prepared at SCK CEN, mimicking modern LWR fuels, to understand the influence of Cr doping on the spent fuel dissolution behaviour in geological repository conditions. Tests were carried out with four model materials: depleted UO2, Cr-doped depleted UO2, Pu-doped UO2 and Pu-Cr-doped UO2. Static dissolution experiments have been performed up to 4 months in autoclaves under 10 bar H2 pressure with a Pt/Pd catalyst in media at pH 13.5 and at pH 9. The Cr-doping appeared to reduce the U concentrations by a factor 6 at pH 13.5, but it had no or not much effect at pH 9.Graphic abstract

Highlights

In recent years, doped nuclear fuels have been developed with the objective of reducing the quantity of fuel used per energy output in operating i.e. Gd doping to control excess fuel reactivity without control rod, and more recently Cr (+ Al) doping to minimize fission gas release thanks to the resulting larger grain size
For the active experiments (Fig. 3a-green symbols) a high initial U release of 1.7 × 1 0–7 for Pu-doped UO2 (Pu-UO2) and 1.0 × 1 0–7 mol.L−1 for Pu-Cr-doped depleted U O2 (Cr-UO2) is observed with no clear influence of Cr up to 10 days
Afterwards, while the U concentration remained around 1.0 × 1 0–7 mol.L−1 for Pu-Cr-UO2, the U concentration for Pu-UO2 increased

Summary

Introduction

In recent years, doped nuclear fuels have been developed with the objective of reducing the quantity of fuel used per energy output in operating i.e. Gd doping to control excess fuel reactivity without control rod, and more recently Cr (+ Al) doping to minimize fission gas release thanks to the resulting larger grain size. The experiments by SCK CEN focused on Cr-doped materials, with a larger grain size than standard fuels. Static dissolution experiments were carried out with the four model materials, added as pellets: ‘inactive tests’ with UO2 and Cr-UO2 and ‘active tests’ with Pu-UO2 and PuCr-UO2. Each autoclave is filled with 30 ml of solution and one pellet of (doped) U O2 (Table 1), corresponding to an initial ratio of U O2 surface area to solution volume (SA/V) of 6 m−1 and 3 m−1 for the inactive and the active experiments, respectively. When the U concentration within the washing solution stayed in the range of the U(IV) solubility (10–8.5 ± 1 M) [13], the pellets were transferred from the PE bottle to the autoclaves and the leaching experiment was started for a period up to 4 months. We focus only on the evolution of the U concentrations

Results and discussion

Conclusions

Compliance with ethical standards