Abstract Extended fuel cycles with 18–24-month duration have complicated effects on primary coolant chemistry and finding optimum pH concentration range is difficult. For extended burnup cycles, study of changes in coolant activity due to corrosion products have been carried out for a typical pressurized water reactor (PWR) under pH and boric acid variations for linearly as well as nonlinearly accelerating corrosion rates. The computer program CPAIR-P ( Deeba et al., 1999 ) has been modified to accommodate for time-dependent corrosion coupled with the effects of coolant chemistry. 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 increase in pH value for an extended 24-month cycle on specific activity in the form of a decrease in the activity smeared by a linearly rising corrosion. The new saturation values for activity at the end of cycle are lower (∼50%) than a reactor operated at constant low pH/natural boric acid in coolant. For linearly accelerated corrosion and an increase in pH value, the coolant activity shows an initial rise and then it falls to a lower saturation level at the end of cycle when pH becomes large and the activity follows the slope of corrosion rate. For nonlinear rise in corrosion rate coupled with the pH rise from 6.9 to 7.4 and use of enriched boric acid (20%–40%) results in coolant activity peak during the cycle and it also approaches much smaller saturation values at the end of cycle when compared with the activity for the system having constant low pH value (6.9) in the coolant. In this paper we show that the use of enriched boric acid as chemical shim actually lowers the primary coolant activity when higher pH values are employed in coolant rather than natural boric acid. 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 (40%) as chemical shim. If the removal rates of ion-exchanger are low (eIQI ∼100–300), then the activity builds up. However, when ion-exchanger performs well (eIQI ∼600–900) then the overall cobalt activity remains low and saturation value does not grow much with cycles.