Study on the influence of power reduction process on the leakage rate of steam generator

The steam generator tubes in PWR unit are prone to mechanical damage and corrosion hazard, tube cracks and steam generator leakage often occurs on alloy 600 tubes. Due to the better corrosion resistance of alloy 690, the tube leakage has never occurred in CPR1000 unit. However, based on the systematic study of the alloy 690 behavior in the PWR operation condition, cracks will also occur at the end of burnup life (EOL); in addition, whether the limit leakage value for steam generator operation originating from EDF units under full power operation is valid for CPR1000 unit should be investigated. For the same critical crack size, in the process of unit power reduction, the change of the pressure difference between the primary and secondary sides of steam generator may cause the leakage rate to exceed the operating limit. An overall model for CPR1000 unit was developed, the changes were investigated of the pressure difference and leakage rate on the primary and secondary sides of the steam generator under different unit load reduction rates, the results showed that for the critical size crack at the bottom of the hot leg, during the load reduction transient process, the calculated leakage rate will exceed the limit value of 44 L/h in the Operating Technical Specifications (OTS) of CPR1000 unit. Finally the corresponding operation strategy was proposed.