The fate of radiolytic oxidants during spent fuel leaching in the presence of dissolved near field hydrogen

Abstract

Summary In most European disposal concepts, one expects large amounts of dissolved hydrogen produced by the anoxic corrosion of iron inside a damaged container. At repository temperatures (<100 °C), dissolved hydrogen is quite inert and is not expected to contribute to the redox capacity of the deep groundwaters. In a previous work from this laboratory we observed a large impact of dissolved hydrogen on the dissolution of the spent fuel in stainless steel autoclaves. In this work we report data on spent fuel dissolution obtained using quartz lined autoclaves to ensure that there is no contact between the solution and the metallic parts; we also made a number of other experimental improvements. The autoclave was filled with a solution 10 mM NaCl, 2 mM NaHCO3 and with H2+0.03% CO2 at a pressure of 0.5 MPa in the remaining free volume. The leaching of PWR spent fuel powder, placed in a gold basket, was studied during more than one year by analysing solution samples taken at regular time intervals. Special care was devoted to the study of the initial phase of the dissolution that was not investigated previously. In spite of the ten times lower hydrogen concentration as compared to the study in stainless steel autoclaves, extremely low concentrations of uranium (less than 10-9 M) were again measured in the solution samples; this was the case also for most of the redox sensitive fuel components. The uranium levels in solution remained practically constant during the whole leaching period, indicating the absence of any oxidative dissolution of the spent fuel matrix. In order to follow the fate of radiolytic oxidants, gas phase analyses were also carried out. The radiolytic oxygen levels in the autoclave measured after one year leaching were below the detection limit. A discussion of the fate of the radiolytic oxidants in these experiments and the mechanism of the hydrogen activation is also presented. The main conclusion is that for concentrations of dissolved hydrogen above 4 mM, no measurable oxidative dissolution of the UO2 matrix seems to occur in the studied systems.