Kinetic study of corrosion product activity in primary coolant pipes of a typical PWR under flow rate transients and linearly increasing corrosion rates

Transient behavior of activated corrosion products on inner surfaces of primary coolant pipes have been studied for a typical pressurized water reactor (PWR) under flow rate perturbations and linearly increasing corrosion rate conditions. Computer program CPAIR-P [27] has been modified to accommodate for time dependent corrosion rates. Results, for 24Na, 56Mn, 59Fe, 58Co, 60Co and 99Mo, show that the specific activity in primary loop approaches equilibrium value under normal operating conditions fairly rapidly. Predominant corrosion product activity during operation is due to 56Mn while 58Co and 60Co dominate the activity after shutdown of reactor. Flow rate perturbations for various linearly increasing corrosion rates were introduced in the system and effects on saturation activity were studied. For a linear decrease in flow rate and a constant corrosion rate, the total coolant activity and activity on pipe scale approaches higher saturation values as compared to the normal condition values. With a linearly increasing corrosion rate, the behavior of specific activity changes considerably and the effect of flow rate perturbations on specific activity for pipe scale is smeared by a linearly rising corrosion rates. The saturation value of activity depends on both the changes in flow rate (Δw) and equilibrium corrosion rate (Cs) values while, the time taken to reach the saturation activity depends on the slope of corrosion rate. The peak value and the subsequent decay of the corrosion product activity after scram is a strong function of flow rate and efficiencies to remove corrosion products from pipe scale.