Computational modeling of accelerating corrosion in primary coolants of a typical PWR

Abstract

High temperature and pressure environment along with large neutron and gamma-ray flux lead to significantly large corrosion in the stainless steel used in the primary coolant circuit of pressurized water reactors (PWRs). The corrosion products become activated in the reactor core upon circulation. Corrosion rate depends on reactor power and it can be a linear as well as a nonlinear function of time. Computer simulation offers a novel approach to study the effect of corrosion acceleration on coolant activity under transient conditions. We present two related mathematical models for corrosion acceleration under high temperature and flow rate transient conditions. In the first part of the work, we have formulated a mathematical model based on a system of coupled first order ordinary differential equations governing the isotopic concentration. In the second part, flow rate transients have been forced through non-homogeneous conditions. Numerical simulations show that for Na-24, Mn-56, Fe-59, Co-60 and Mo-99, the values of specific activities approach the equilibrium value fairly rapidly under normal operating conditions. Spikes are observed in the specific activity values of the primary coolant when corrosion acceleration and flow rate transients are employed in the model. Both the height and time of these spikes has been found to depend on the value of acceleration in corrosion.