Effect of flow rate transients on fission product activity in primary coolant of PWRs

摘要

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 ~(133)Xe and it is followed by ~(135)Xe, ~(131M)Xe and ~(129)Te 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 t_p ～ 2000 h, a 8.6% increase has been observed in the value of total specific activity due to fission products. For increasing t_p values, the value of maximum specific activity due to fission products shows a rise followed by an approach towards a saturation value.