Chemical-equilibrium-based Method for Calculating Multicomponent Dissolved Corrosion Product Distributions in Bulk-flow Regions of PWR Core Sub-channels

Oxidized corrosion products in pressurized water reactors (PWRs) deposit on fuel rods to form an oxide corrosion-product deposit layer, which threatens the safety and economic performance of nuclear power plants. In order to clarify the deposition mechanism of corrosion products and the growth law of the deposit layer, it is particularly critical to elucidate the distribution characteristics of multicomponent dissolved corrosion products in the coolant channels of the core. At present, a systematic calculation method for the distribution of dissolved corrosion products in the bulk-flow region of the core has not yet been established. In this paper, a chemical-equilibrium-based method for calculating the distribution characteristics of multicomponent dissolved corrosion products was proposed, an equilibrium-constant calculation model based on the HKF formulation and a multicomponent chemical-equilibrium model based on the law of mass action (LMA) were established, and the above models with a CFD solver through a Fluent UDF (user defined function) to calculate the concentration distributions and supersaturation characteristics of multicomponent dissolved corrosion products in the bulk-flow region of core subchannel coolant were integrated. Typical PWR subchannel geometric parameters and operating parameters for simulation were selected, and the three-dimensional distributions of temperature, pH, and the concentrations of each corrosion-product component in the coolant bulk-flow region under single-phase and multiphase conditions were obtained. By analyzing the simulation results, it identifies Fe(OH)₂ and Ni(OH)₂ as the main components of dissolved corrosion products; It verifies that pH and multicomponent concentrations are strictly affected by temperature, that their variation characteristics are closely related to temperature variation, and that when temperature increases, pH increases accordingly; It finds that the velocity distribution and temperature distribution exhibit azimuthal non-uniformity, leading to azimuthal non-uniformity in the distributions of pH and the concentrations of Fe-bearing and Ni-bearing species, which in turn causes non-uniform supersaturation distributions of dissolved corrosion products in solution and affects the nucleation and growth of particulate corrosion products as well as the thickness distribution of the corrosion-product deposit layer. The influence of subcooled nucleate boiling on the distribution of multicomponent solutes was also analyzed. The results show that as the coolant temperature increases to approach the saturation temperature, the local void fraction rises rapidly, forming a gas-liquid two-phase coexistence region that significantly enhances the local turbulent kinetic energy intensity and causes abrupt changes in heat and mass transfer behavior of the flow field, resulting in step changes in axial temperature, pH, and the concentrations of each corrosion-product component. This study couples and integrates multiple modules, saving the time cost of numerical calculations, and provides more detailed chemical boundary conditions and important theoretical basis for CRUD numerical calculations and practical tests. In addition, the azimuthal non-uniform distribution of CRUD can lead to local heat-transfer deterioration and hot-spot formation, increasing the risk of departure from nucleate boiling; The internal temperature of excessively thick CRUD rises sharply, stress corrosion correspondingly intensifies, and the probability of fuel cladding failure increases. Therefore, in reactor design, the above special phenomena need to be taken into consideration, for example by enhancing the effectiveness of mixing structures, adjusting fuel rod arrangement, or adopting cladding materials with anisotropic thermal conductivity to reduce azimuthal non-uniformity. This study has positive significance for reactor design and safe operation.