Assessment of corrosion degraded steam generator tubes

In nuclear power plants VVER 440 MW, both stress corrosion cracking and pitting are the reasons of the outside surface damage of the steam generator tubes. The damage is determined by parameters of the secondary coolant: water chemistry, temperature, surface conditions resp. deposits, and stress level. Both the initiation and crack growth cause the thinning of the tube wall. That can lead to the creation of the through wall defect, which enables the primary coolant to leak from primary to secondary side. To operate the steam generator with the through-wall cracks, the facility has to fulfill the mandatory limits for the leakage of the primary water to secondary circuit (1). They should involve for Czech power plants: - the permissible crack length determined regarding the critical crack size above which the crack growth is unstable - the maximum activity level corresponding with the requirements for the safety of the secondary circuit. The main aims of the present work are to evaluate the leakage of the primary coolant as a function of the geometry of the axial cracks and on the basis of these values to design the limit values for the leakage regarding the maximum allowable defect and the activity level. On the basis of the proposed model, the calculation of the primary coolant leakage was done and it was correlated with the experimental results. (Author) 3 refs