Steam Generator Tube Support Plate Degradation in French Plants: Maintenance Strategy

Abstract

This paper reports on the degradations of Steam Generator (SG) Tube Support Plates (TSPs) observed in French plants and the maintenance strategy adopted to continue operating the plant without any decrease of the required safety level. Only drilled carbon steel TSPs of early SGs are affected. Except the particular damage of the TSP8 of FESSENHEIM 2 caused by chemical cleaning procedures implemented in 1992, two main problems were observed almost exclusively on the upper TSP : - Ligaments ruptured near the aseismic block located at 215". This degradation is perfectly detectable by bobbin coil inspection. It occurs very early in the life of the SG as can be seen from the records of previous inspections and no evolution of the signals was observed. This damage can be detected for 51M model SGs on several sites. - Wastage of the ligaments resulting in enlargement of flow holes with in some cases complete consumption of a ligament. This damage was only observed for SGs of at GRAVELINES. This damage evolved cycle after cycle. Detailed studies were performed to analyze tubing behavior when a tube is not supported by the upper TSP because of missing ligaments. These studies evaluated the risk of vibratory instability, the behavior of both the TSP and the tubing in case of a seismic event or a LOCA and finally the behavior of the TSP in case of a Steam Line Break. Concerning vibratory instability it was possible to define zones where stability could not be demonstrated. Damping cables and sentinel plugs were then used when necessary to eliminate the risk of Steam Generator Tube Rupture (SGTR). For accidental conditions, it could be shown that no unacceptable damage occurs and that the core cooling function of the SG is always maintained if some tubes are plugged. From this analysis, it was possible to define the inspection programs for the different plants taking into account the specific situation of each plant regarding the damages detected. These programs include bobbin coil inspections and special Motorized Rotating Pancake Coil (MRPC) inspections when indication of ruptured ligament is obtained with the bobbin coil. It also includes televisual inspection for GRAVELINES SGs. Prevention of evolving damage observed in GRAVELINES was also dealt with. Erosion corrosion of carbon steel of the TSP was identified as the root cause. The occurrence of erosion corrosion was analyzed as the result of secondary chemistry and thermalhydraulic conditions at the periphery of the upper TSP. In 1996 it was therefore decided to modify the secondary chemistry by using morpholine instead of ammonia and by increasing the pH. Results of inspections carried out at the end of the cycle in 1997 provided positive conclusions about the efficiency of this modification.