Mathematical models have been developed to describe the activities of I-129 and Cs-137 in the primary coolant and resin of the chemical and volume control system (CVCS) during constant power operation in a pressurized water reactor (PWR). The models, which account for the source releases from defective fuel rod(s) and tramp uranium, rely on the contribution of CVCS resin and boron recovery system as a removal process. and differences in behavior for each nuclide. The current models were validated through measured coolant activities of Cs-137. The resultant scaling factors agree reasonably well with the results of the test resin of the coolant and the actual resins from the PWRs of other countries. (c) 2006 Elsevier B.V. All rights reserved.