Multicycle Study of Effects of Coolant Chemistry on Corrosion Product Activity in Pwrs for Extanded Cycles under flow rate and Power Transients

Abstract

In nuclear power plants, corrosion and deposition of activated corrosion products on the surfaces in contact with primary coolant lead to safety hazard and financial losses. Corrosion and radiation-field buildup can be restricted through coolant chemistry control. Plant designers and operators need computing methods to predict the behavior of corrosion products in the primary coolant circuit for evaluating water chemistry specifications and plant activity level. In this work, a detailed methodology has been developed for modeling and simulation of corrosion product activity in primary coolant circuit of a pressurized water reactor and its dependence on coolant chemistry under various plant operating conditions. Set of differential equations governing the physical processes for activation and transport of corrosion products in a primary coolant circuit of a typical PWR have been used to frame the model. Based upon this mathematical model, computer program CPAIR-P/PH has been written in FORTRAN-77. This computer code has been used to study the change in activity for a typical pressurized water reactor with linearly as well as nonlinearly accelerating corrosion rates, various pH change rates and boron enrichment levels for extended operating cycles. Effects of flow rate transients and power perturbations have also been compared for extended cycles. Results for 60Co, 59Fe, 56Mn, and 99Mo show that the specific activity in primary coolant circuit reaches rapidly to equilibrium under normal plant conditions. For an extended 24-month cycle, the effect of rise in pH value on specific activity is smeared by the accelerating corrosion rate. For multiple long-term fuel cycles the corrosion product activity shows an initial rise to a maximum value and then it fall back to low saturation values due to high pH concentration for enriched boric acid (10B 40%) as chemical shim. Flow transient during an operating cycle leads to higher peak and saturation values of specific activity in the next cycle. In case of power-following flow rate transient, only small rise in the saturation values is observed. It has been observed that the use of enriched boric acid as chemical shim actually lowers the primary coolant activity when higher pH values are employed in coolant.