Multi-physics simulation of CRUD deposition and lithium meta-borates precipitation in pressurized water reactors

Abnormal power shifts in fuel assemblies lead to a decrease in the safety and economy of PWRs, especially during the long-term and high-power operation of the PWRs. To predict the power shift, it is necessary to predict both CRUD deposition on the surface of the fuel assemblies and boron hideout within the CRUD deposits. A model to simulate the chemistry of soluble boron and lithium was developed and coupled to the previous multi-physics model for heat transfer and growth of CRUD deposits to simulate the boron hideout within the growing CRUD deposits. The two simulation cases for the long-term operation of PWRs with two thermal power levels were simulated with various removal rates of the CRUD deposits during shutdown practices and the porosity of the CRUD deposits based on the literature survey. The model predicted significant increases in the CRUD deposition and boron hideout along with a power increase, and its behavior is well matched to that of axial offset anomaly. The minimum thickness of the CRUD deposits for the occurrence of boron hideout was investigated through simulation, and it showed that the minimum thickness is dependent on the power level of the PWRs.