Modelling of Radiolysis Enhanced Corrosion of PWR Primary Circuit Materials

This paper presents a comprehensive review and case-based application of modeling radiolysis-enhanced corrosion phenomena in the primary circuit of pressurized water reactors(PWRs), with particular emphasis on the theoretical development and engineering deployment of the LwrChem simulation tool recently introduced in China. Radiolysis of reactor coolant, driven by gamma and neutron fluxes in the reactor core, produces chemically active species that influence redox conditions and, in turn, affect the corrosion potential of structural materials. The LwrChem model couples detailed radiolysis kinetics, electrochemical equilibrium calculations, and spatially resolved three-dimensional core physics to simulate coolant chemistry evolution with high accuracy and strong spatial fidelity. In this study, a typical four-loop PWR was modeled using a 178-compartment spatial decomposition to capture coolant behavior across core and non-core regions. Two distinct operational scenarios were investigated: one under nitrogen-free conditions, and the other reflecting nitrogen-pressurized conditions representative of volumetric control tank(VCT) operation with (7-20)×10⁻⁶ N₂ concentrations. Simulation results demonstrated that, under standard hydrogen injection conditions(2.76×10⁻⁶), LwrChem successfully predicted steady-state hydrogen peroxide concentrations near 80×10⁻⁹ in high-irradiation zones and electrochemical corrosion potential(E_CP) values ranging from –740 mV to –650 mV vs. standard hydrogen electrode(SHE). These predictions were validated against experimental data with deviations within ±50 mV, confirming model accuracy. When nitrogen was introduced into the system, the model captured significant hydrogen consumption(e.g., 500×10⁻⁹ reduced to 346×10⁻⁹) and the formation of ammonia(NH₃) concentrations up to 3500×10⁻⁹, indicating a substantial shift in the redox environment. The appearance of oxidizing species such as NO₂ suggested a potential NH₃/NO₂ synergistic mechanism in accelerating corrosion, although NO₂ was not explicitly included in the electrochemical model. Moreover, spatial heterogeneity in E_CP values across fuel assemblies was revealed, with lower potentials observed at the top of high-power bundles, highlighting the importance of axial redox variation in understanding localized corrosion and crud-induced power shift(CIPS) mechanisms. The study further introduces the integration of LwrChem with the SIMULATE5 core simulation platform, enabling high-resolution, node-level corrosion risk assessment based on actual power and radiation field distributions. This coupling improves predictive capability for identifying oxidation hotspots and supports targeted water chemistry control strategies, including hydrogen injection optimization. Overall, this work represents the first in-depth engineering validation of LwrChem under Chinese and the first systematic modeling of nitrogen pressurization effects on engineering application of coolant chemistry and corrosion. The model demonstrates strong scalability, broad applicability, and high reliability under extreme conditions, including high temperature, high pressure, and mixed-radiation fields. These capabilities position LwrChem as a valuable computational tool for predicting radiolysis-affected corrosion behavior, supporting nuclear power plant life extension, improving coolant chemistry management, and contributing to the safe, efficient development of advanced reactor technologies.