Spent fuel corrosion and the impact of iron corrosion – The effects of hydrogen generation and formation of iron corrosion products

Canister designs for deep geological disposal of spent nuclear fuel typically involve large amounts of iron. Canister failure will likely involve corrosion of the canister at the disposal depth several hundred meters underground, where conditions are typically anoxic. Spent nuclear fuel corrosion and dissolution will therefore likely proceed together with anoxic iron corrosion. The iron corrosion can suppress spent fuel corrosion by creation of strongly reducing conditions from Fe(II) formation and generation of large quantities of hydrogen. In an attempt to separate the effect of hydrogen from that of Fe(II) and iron corrosion products, two experiments were performed. In both cases, spent nuclear fuel powders were used in autoclaves filled with simplified granitic groundwater initially pressurized with argon. In the first experiment, the autoclave contained iron powder. Following the initial radionuclide release at start-up, the concentration of redox sensitive radionuclides decreased with uranium reaching UO2 (am) solubility levels within months. Release of non-redox sensitive elements such as Cs ceased after 223 days. Continuous hydrogen generation was observed. XRD and Raman spectroscopy of the iron corrosion products identified magnetite. The second experiment was therefore performed starting with magnetite powder. In this case, the autoclave argon atmosphere remained constant. Radionuclide release was, however, continuous with uranium reaching >2 × 10−5 M after 275 days. The magnetite powder of the second experiment was analyzed and found to be composed of mainly magnetite with a small fraction of hematite. In the iron corrosion experiment, Mo, Tc and U where found to have high iron corrosion product affinities. However, in the magnetite experiment, U, Tc and Mo displayed weak affinity for the corrosion products, indicating presence in the higher oxidation state. The results thus point to hydrogen evolution being the main contributor to the apparent inhibition of radionuclide release in the iron corrosion experiment.