Density Dependence of the Radiolysis Yields of Primary Species From Fast Neutron-Irradiated Supercritical Water at 400°C

Abstract

A reliable understanding of radiolysis processes in a supercritical water (SCW)-cooled reactor is crucial in order to develop chemistry control strategies that minimize the corrosion and degradation of materials. However, direct measurement of the chemistry in reactor cores is difficult due to the extreme conditions of high temperatures and pressures and mixed neutron and γ-radiation fields that are not compatible with normal chemical instrumentation. Thus, chemical models and computer simulations are an important route of investigation for predicting the detailed radiation chemistry of the coolant in a SCW reactor and the consequences for materials. Surprisingly, there is only limited information on the fast neutron radiolysis of water at high temperatures and no experimental data are yet available on the radiolysis yields for fast neutron irradiation of SCW. In this work, Monte Carlo simulations were used to calculate the G-values for the primary species e^-_aq, H•, H₂, •OH, and H₂O₂ formed from the radiolysis of pure, deaerated SCW (H₂O) by 2 MeV monoenergetic neutrons at 400 °C as a function of water density in the range of ~0.15-0.6 g/cm³. The 2 MeV neutrons were considered representative of a fission-neutron flux in a reactor. For light water, the moderation of these neutrons after knock-on collisions with water molecules generated mostly recoil protons of 1.264, 0.465, 0.171, and 0.063 MeV whose linear energy transfer (LET) values varied from ~3 to 46 keV/micrometer for densities over the range of ~0.15 to 0.6 g/cm³. Neglecting oxygen ion recoils and assuming that the most significant contribution to the radiolysis came from these first four recoil protons, the fast neutron yields were estimated as the sum of the G-values for these protons after appropriate weightings were applied according to their energy. The simulation results were compared with available experimental data for low-LET (⁶⁰Co γ-rays or fast electrons) SCW radiolysis. Most interestingly, the reaction of H• atoms with water was found to play a critical role in the formation yields of H₂ and •OH at 400 °C. Recent work has recognized the potential importance of this reaction above 200 °C, but its rate constant is still controversial.

Keywords: Generation IV supercritical water (SCW) reactor, radiolysis, fast neutrons, recoil protons, gamma-irradiation, linear energy transfer (LET), primary species, yields (G-values).