Study on Time-Dependent Co-58 and Co-60 Activities in the Primary Coolant of CPR1000 PWRs

Activated corrosion products are the dominate radiation source during PWR maintenance. Co-58 and Co-60 are two important radionuclides due to the high energy emitting gamma rays and high radio activities in the primary circuit. They have contributed to more than 90% occupational radiation exposure during maintenance according to operating experiences from PWRs. Therefore, it is essential to research the behaviour of the two radionuclides. The Co-58 and Co-60 coolant activity for both short-term tendency and long-term tendency are analyzed in this paper. Operating Experience (OPEX) from CPR1000 PWRs show that the Co-58 and Co-60 coolant activity in a fuel cycle usually result in shape of a bowl and the highest activities are at the beginning of the cycle which is consistent with the shape of the core boiling rate. Namely, fuel crud behavior plays a leading role in the formation of activated corrosion products in the primary circuit. For Co-58 and Co-60 coolant activity in long-term period, it is influenced mainly by the corrosion/release rate and the radionuclides’ half-lives. OPEX combined with simulation by CAMPSIS code show that Co-58 coolant activity reach to maximum value at second to third cycle while Co-60 coolant activity reach to maximum value at fifth to ninth cycle. Research on time-dependent Co-58 and Co-60 coolant activities give a better understanding of the radionuclide behaviour as well as provide a basis for developing corrosion product simulating model.