Abstract Simulations of fission product activity in primary circuits of a typical PWR under flow rate transients have been performed by using a two-stage model for release of fission products from fuel into coolant region. A one-dimensional nodal-scheme has been developed for modeling the behavior of fission products in the primary circuit. For constant-power operation at constant flow rate, results for 15 major fission products show that the activity due to fission products in the primary coolant circuit of PWRs is dominated by 133Xe and it is followed by 135Xe, 131MXe and 129Te which contribute 40%, 12.9%, 11% and 8.2%, respectively, to the total fission product activity. The results of these simulations have been found to agree well with the corresponding values found in ANS-18.1 Standard as well as with some available power-plant operation data. These simulations indicate a strong dependence of saturation values of specific activity on primary coolant flow rate. For pump coastdown having a characteristic time tp ∼ 2000 h, a 8.6% increase has been observed in the value of total specific activity due to fission products. For increasing tp values, the value of maximum specific activity due to fission products shows a rise followed by an approach towards a saturation value.