The Influence of Hydrogen on the Radiolytic Oxidation of UO2

Licentiate thesis, 2020

Spent nuclear fuel from the nuclear fuel cycle contains radiotoxic nuclides which must be safely stored for over 100 000 years. The Swedish final repository concept, KBS-3, is based on engineered and geological barriers that prevent the nuclear fuel from coming in contact with groundwater, which is the most credible vector to transport the radionuclides into the biosphere. In the safety assessment of a repository, the water intrusion scenario must therefore be investigated.

The UO2 matrix contains the majority of the long-lived radiotoxic elements. As the U(IV) form is highly insoluble, the release of the radiotoxic nuclides is largely governed by oxidation of the UO2 matrix into the much more soluble U(VI) form. Oxidation can occur due to the formation of radiolytic oxidants through the ionization or excitation of water molecules in contact with fuel.

Oxidation of UO2 pellets using external Am-241 sources was studied under conditions where the UO2 surface and the source were separated by 30 µm water. H2 was shown to suppress the surface oxidation as well as dissolution. This was shown by direct measurement of the surface oxidation state using XPS, as well as through concentration measurements in solution using mass spectrometry (ICP-MS).

Oxidative dissolution of 10 and 24 wt% Pu-doped MOX pellets was also studied under Ar and D2 atmospheres. The D2 atmosphere suppressed the uranium dissolution. However, corrosion of the stainless-steel materials present in the autoclave system was also observed. A calculation model was also developed for calculating dose-rates from α-doped UO2 based material.