Management of PWR Steam Generator Tube Plugging and Primary Flow-Rate Predictions

The tubes of PWR steam generators are part of the second barrier between the nuclear fuel and the environment. The integrity in operation of the tubes is addressed with Non Destructive Examinations (NDE) and flaw allowances criteria. If a tube does not match the criteria, it is plugged. As a consequence, the steam generators tube plugging (SGTP) may increase during the maintenance outages. This increase has to be managed properly because it basically affects the heat exchange capacity of the Nuclear Steam Supply System (NSSS). This can be managed by performing long-term predictions in order to prepare in advance the possibility of steam generator replacements. But this “long-term operation” management is to be completed with an intermediate term management considering the real operating conditions of the NSSS. Intermediate term predictions, based on a simulation of the mechanisms leading to the degradation of the tubes, are annually compared with the evolution of real NDE and real SGTP. These predictions are completed with the set-up of a model, for each Reactor Coolant System (RCS), considering the relation between the average SGTP and the primary flow-rate. The predictions are used to check that the real operating conditions of each NSSS can be matched with an existing safety file.