Simulation of corrosion product activity in extended operating cycles of PWRs under flow rate transient and nonlinearly rising corrosion rates coupled with pH effects

摘要

Corrosion product activity (CPA) has been studied in a typical pressurized water reactor (PWR) under flow rate transients, having extended burn-up cycles with time dependent pH-level and boric acid variations. The simulations show CPA buildup towards saturation following the mixing of boron in primary coolant and a decreasing trend following the increase in pH value. In the presence of boric acid and pH control, the nonlinearly rising corrosion rates result in enhanced saturation levels when compared with the constant corrosion rate case. The behavior of CPA has been compared in multiple cycles for both no flow rate transient case and a transient with enriched boric acid (40%) in coolant. In first cycle when flow rate transient is applied the saturation value goes up and during second cycle, the peak value for the flow rate transient case becomes about 13% higher when compared with no flow rate transient scenario. The saturation CPA value in second cycle for transient case is also 14% higher when compared to saturation value for no transient case in the same cycle. However, in subsequent cycles the effect of flow rate transient from first cycle on CPA gets diminished. When only flow rate is allowed to change and power is kept constant a pronounced rise in saturation value of CPA occurs. However, in the case of power-following flow rate transient, only small rise in the saturation values is observed.