Monte Carlo track-chemistry simulations of fast neutron radiolysis in supercritical water at 400–600 °C and 25 MPa

Abstract

Understanding the radiation chemistry and behavior of transient species in supercritical water-cooled reactors (SCWRs), including small modular variants (SCW-SMRs), is essential for evaluating corrosion risks. Operating beyond water’s critical point, these reactors encounter unique challenges as intense radiation alters coolant chemistry and threatens material integrity. Here, we employ Monte Carlo track-chemistry simulations to quantify the radiolytic yields (G values) of e⁻_aq, ^•OH, H^•, H₂, H₂O₂, H₃O⁺, and OH⁻ in SCWR and SCW-SMR coolants exposed to 2-MeV fast neutrons at 400–600 °C and 25 MPa. Calculations track the first three elastically scattered recoil protons, with initial energies of 1.264, 0.465, and 0.171 MeV, from ∼1 ps to 100 μs. The temporal profiles of our calculated yields resemble those reported for low-linear-energy-transfer (LET) radiation, such as ⁶⁰Co γ rays, fast electrons, or 300-MeV protons. However, under fast neutron irradiation, charge recombination between e⁻_aq and H₃O⁺ within spurs or tracks is markedly enhanced, reflecting the high-LET nature of neutrons. In the homogeneous chemical stage of radiolysis, simulations reveal a pronounced rise in G(^•OH) and G(H₂) alongside a reduction in G(H^•). This behavior is driven by H^• + H₂O → ^•OH + H₂, a key pathway for H₂ formation that may help mitigate net water radiolysis and reduce corrosion. Under supercritical conditions, the very low G(H₂O₂) indicates that H₂O₂ likely contributes little to material oxidation. Furthermore, in situ H₃O⁺ formation by recoil-proton irradiation transiently acidifies native track regions, suggesting that this localized acidity could promote corrosion. Overall, these results provide critical insights into the radiolytic processes in SCWRs and SCW-SMRs, informing strategies for optimized water-chemistry control and enhanced material protection.