Radiolysis of supercritical water at 400 °C: density dependence of the rate constant for the reaction of hydronium ions with hydrated electrons

Interaction of hydrogen with actinide dioxide (011) surfaces

The corrosion and oxidation of actinide metals, leading to the formation of metal-oxide surface layers with the catalytic evolution of hydrogen, impacts the management of nuclear materials. Here, the interaction of hydrogen with actinide dioxide (AnO₂, An = U, Np, or Pu) (011) surfaces by Hubbard corrected density functional theory (PBEsol+U) has been studied, including spin-orbit interactions and non-collinear 3k anti-ferromagnetic behavior. The actinide dioxides crystalize in the fluorite-type structure, and although the (111) surface dominates the crystal morphology, the (011) surface energetics may lead to more significant interaction with hydrogen. The dissociative adsorption of hydrogen on the UO₂ (0.44 eV), NpO₂ (-0.47 eV), and PuO₂ (-1.71 eV) (011) surfaces has been calculated. It is found that hydrogen dissociates on the PuO₂ (011) surface; however, UO₂ (011) and NpO₂ (011) surfaces are relatively inert. Recombination of hydrogen ions is likely to occur on the UO₂ (011) and NpO₂ (011) surfaces, whereas hydroxide formation is shown to occur on the PuO₂ (011) surface, which distorts the surface structure.

Formation of Local, Transient “Acid Spikes” in the Fast Neutron Radiolysis of Supercritical Water at 400 °C: A Potential Source of Corrosion in Supercritical Water-Cooled Reactors?

The use of supercritical water (SCW) in GEN IV reactors is a logical approach to the ongoing development of nuclear energy. A proper understanding of the radiation chemistry and reactivities of transients in a reactor core under SCW conditions is required to achieve optimal water chemistry control and safety. A Monte Carlo simulation study of the radiolysis of SCW at 400 °C by incident 2 MeV monoenergetic neutrons (taken as representative of a fast neutron flux in a reactor) was carried out as a function of water density between ∼150 and 600 kg/m3. The in situ formation of H3O+ by the generated recoil protons was shown to render the “native” track regions temporarily very acidic (pH ∼ 1). This acidity, though local and transitory (“acid spikes”), raises the question whether it may promote a corrosive environment under proposed SCW-cooled reactor operating conditions that would lead to progressive degradation of reactor components.

Yields of primary species in the low-linear energy transfer radiolysis of water in the temperature range of 25–700 °C

Monte Carlo track chemistry simulations were used to calculate the yields (G values) for the radical (e_aq⁻, H˙, ˙OH) and molecular (H₂, H₂O₂) species formed in low-LET water radiolysis from ∼1 ps to 1 ms between 25 and 700 °C, at 25 MPa pressure.