Computer simulation of corrosion product activity in primary coolants of a typical PWR under flow rate transients and linearly accelerating corrosion

Computer simulation of behavior of coolant activation due to corrosion products have been investigated in a typical pressurized water reactor (PWR) under flow rate perturbations for linearly accelerating corrosion. The computer program CPAIR-P (Deeba et al., 1999) has been modified to accommodate for time dependent corrosion. Results for 24Na, 56Mn, 59Fe, 60Co and 99Mo show that the specific activity in primary loop approaches equilibrium value under normal operating conditions fairly rapidly. During reactor operation, predominant corrosion product activity is due to 56Mn and after shutdown cobalt activity dominates. These simulations suggest that the effect of flow rate perturbations on specific activity in the form of a depression in the activity curve can be smeared by a linearly rising corrosion. Such a dip can only be seen in activity when corrosion rate approaches to an equilibrium value well before the initiation of the transient. The time period to reach minimum coolant activity during transient is a function of the slope of flow rate perturbation parameter, g(t). The new saturation value for activity depends on changes in flow rate (Δw) and equilibrium value (Cs) for the corrosion rate. For linearly accelerated corrosion and a pump coastdown condition, the activity does not show an initial drop when flow starts decreasing. It monotonically rises and follows the slope of corrosion rate. If the slope is greater than 9×10−6 μg/s2, then the activity crosses the normal saturation value of 0.22 μCi/cm3 before the reactor scram occurs. These results also indicate that the pump coastdown does produce a coolant activity spike before the reactor scram; however it can only be observed when the corrosion acceleration is fast.