“Acid spike” formation in the fast neutron radiolysis of supercritical water at 400 °C studied by Monte Carlo track chemistry simulations

Abstract

A reliable understanding of radiolysis processes in supercritical water (SCW) cooled reactors is required to ensure optimal water chemistry control. In this perspective, Monte Carlo track chemistry simulations of the radiolysis of pure, deaerated SCW at 400 °C by 2 MeV mono-energetic neutrons were carried out as a function of water density between 0.15 and 0.6 g/cm3. The yields of hydronium ions (H3O+) formed at early time were obtained based on the G values calculated for the first three generated recoil protons. Combining our calculated G(H3O+) values with a cylindrical track model allowed us to estimate the concentrations of H3O+ and the corresponding pH values. An abrupt, transient, and highly acidic pH response (“acid spikes”) was observed at early times around the “native” fast neutron and recoil proton trajectories. This intra-track acidity was found to be strongest at times of less than a few tens to a hundred of picoseconds, depending on the value of the density considered (pH ∼ 1). At longer times, the pH gradually increased for all densities, finally reaching a constant value corresponding to the non-radiolytic, pre-irradiation concentration of H3O+, due to the autoprotolysis of water. Interestingly, the lower the density of the water, the longer the time required to reach this constant value. Because many in-core processes in nuclear reactors critically depend on the pH, the present work raises the question whether such highly acidic pH fluctuations, though local and transitory, could promote or contribute to corrosion and degradation of materials under proposed SCW-cooled reactor operating conditions.

Generation of ultrafast, transient, highly acidic pH spikes in the radiolysis of water at very high dose rates: relevance for FLASH radiotherapy

“…35 We calculated the yields (or G-values) 36 and the concentrations of radiolytically formed H 3 O + and the corresponding pH values that prevailed in these track regions immediately after irradiation as a function of time for both low and high LET, 16,25 at ambient and elevated temperatures, 37 even under supercritical conditions. 38,39 In all cases studied, an abrupt, transient acidspike response was observed around the "native" radiation tracks. This response is a very sensitive function of radiation quality (LET) and temperature.…”

Characterizing the Early Acidic Response in Advanced Small Modular Reactors Cooled with High-Temperature, High-Pressure Water

“…This example provides a clear illustration of how water's pH might vary in the early stages of radiolysis under varying dose rate conditions, specifically at a nominal pressure of 25 MPa. Our prior studies [16,32,33] have shown that the initial acidic response in water exposed to high-LET radiation is markedly more significant than that observed with low-LET radiation. Conversely, using 300 MeV protons as a representative model to evaluate acidic spikes in SCWRs and SMRs can be seen as a cautious approach, particularly in terms of the actual magnitude of the effect.…”