High-Resolution Multiphysics Analysis of CRUD Effects in Reactor Assemblies

This study develops a high-fidelity multiphysics coupling framework to systematically investigate the impact of Chalk River unidentified deposits (CRUD) on pressurized-water-reactor (PWR) assemblies, integrating neutronics, thermal hydraulics, and chemical kinetics modeling. CRUD, formed via subcooled nucleate boiling (SNB) and corrosion-product deposition, alters local boron concentrations, thereby inducing a power shift (CIPS) due to CRUD and generating asymmetric axial power distributions. The proposed framework adopts an internally coupled approach, utilizing Picard iteration and a background-mesh technique to resolve multiscale computational challenges while maintaining numerical accuracy and efficiency. Numerical simulations of a <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mn>17</a:mn><a:mo>×</a:mo><a:mn>17</a:mn></a:math> PWR fuel assembly reveal that CRUD deposition reduces the eigenvalue by 293 percent mille (pcm) and shifts the axial power peak downward by 21 cm, corresponding to a 6.27% axial offset. Radially, the effect of CRUD on power distribution remains minimal (approximately 0.04%). A critical observation is the dominant influence of surface heat flux on CRUD growth, with thicker deposits and elevated boron concentrations observed in high-power regions. CRUD-induced perturbations also exhibit a “neighboring phenomenon,” wherein CRUD-free fuel rods experience comparable axial power shifts due to neutron-flux disturbances. These findings underscore the intricate coupling between power distribution, SNB, and CRUD chemistry, providing valuable insights for enhancing reactor performance and safety.