(A) study on the evaluation of radioactive corrosion product behavior at PWR for extended fuel cycle

Abstract

Nowadays nuclear power plant operation practice shifts to extended fuel cycle such as from 12 month operating cycle to 18~24 month operating one. It is important to emphasize that the current trend to longer fuel cycles has complicated the dilemma of finding optimum pH range for the primary coolant chemistry. Typically, current 12-month fuel cycles start with no more than 1200ppm boron concentration at the start of a cycle, so a maximum value of 2.2ppm lithium is supplied to satisfy the pH6.9 requirement. The occupational radiation exposure (ORE) increases due to radioactive corrosion products. The recent ICRP recommendation (ICRP publication No. 60) for the radiological protection requires more strict reduction of ORE in the nuclear power plant. The major sources of the radiation are generated by the neutron activation of the non-radioactive corrosion products, that is called CRUD(Chalk River Unidentified Deposit). To optimize the chemical conditions at the extended fuel cycle, COTRAN code which was developed to simulate the behavior of the radioactive corrosion product(CRUD) was used. COTRAN code was developed to estimate CRUD during only one cycle. This study, however, is to predict corrosion product not for one cycle, but for multi-cycle. In order to predict corrosion products for multi-cycle, COTRAN code is modified for considering decontamination, refueling and applied to KNGR. The results show that not only selecting the optimum pH but also cycle duration are important parameters reducing the generation of CRUD. It turned out that the activities of CRUD decreases as pH of the coolant increases, and for the same period of different fuel cycle, as the fuel cycle duration increased, the generation of the CRUD increases. As operation cycle duration is increased, the ratio of $Co^{58}/Co^{60}$ becomes small. Activities of $Co^{60}$ and $Co^{58}$ shows similar trend to measured activities of those at steam generator tube surface of Millstone Point 3 PWR. It is...