A mathematical model of corrosion product transport in the boiling water reactor (BWR) primary system has been developed. The model, which can be characterized as a semi-empirical phenomenological model, is capable of reproducing the observed data obtained in many BWRs with a variety of operational histories and a wide range of radiation levels. The results of parametric studies confirm the successful experience that the radiation fields in operating plants can be controlled and reduced by close control of the water quality in the primary system. The radiation field measured at recirculation piping of a new plant can be controlled below 200 mR/h over its entire plant life.