Abstract
The largest known Service Water System (SWS) chemical cleaning ever performed at a nuclear plant was successfully completed at Gulf States Utilities Company, River Bend Station. Corrosion product buildup was observed during system inspections in the first operating cycle and the first refueling outage in 1987. Under deposit corrosion was followed with microbiologically influenced corrosion (MIC) occurring as a later stage under deposits. The heavy corrosion caused blockage of heat exchanger tubes, fouling of valve seats, and general flow blockage throughout the system. Due to concerns regarding the flow obstructions and the potential for a shortened SWS operating life, a corrective action plan was developed and successfully implemented to mitigate the problem.
Various options were evaluated for restoring the Service Water System back to an acceptable long term operating condition. A major part of the overall restoration plan was to chemically clean the SWS. The large scale chemical cleaning performed arrested the corrosion by removing the deposits down to the bare metal surfaces and leaving behind a protective passivation layer. After the cleaning, the open recirculating SWS was converted to a closed system. The implementation of a molybdate/nitrite water treatment program with a copper corrosion inhibitor maintained at a high pH (8.5 - 10.5) has significantly reduced corrosion rates in the closed system. This should extend the life of the SWS piping for the remaining life of the plant.
Chemical cleaning of a large cooling water system inside a nuclear power plant presented many unique problems which had to be resolved. The selection, qualification, and performance of the cleaning process were done using a phased approach. The piping was inspected to determine the extent of the problem. Samples of deposits were removed from the SWS pipe and tested in the laboratory to determine the most effective cleaning solvent for deposit removal. Actual SWS pipe samples were used on a “Dynamic Test Loop” to develop and ensure effectiveness of the specialized chemical cleaning process. An engineering study was also performed to define the design parameters that were required to implement the system wide chemical cleaning.
Several field tests were conducted to qualify the process and demonstrate its ability to achieve acceptable cleaning results prior to being used on a larger scale. In the summer of 1992, temporary and permanent modifications were installed to divide the SWS into two separate cleaning loops for the system wide cleaning. The SWS chemical cleaning was successfully performed and completed on schedule during the fourth refueling outage. Post cleaning inspections at various locations throughout the Service Water System showed the process to be very effective at complete deposit removal. Comparison of Figure 1 (SW Piping Pre-cleaning) to Figure 2 (SW Piping Post Cleaning) clearly demonstrates the excellent results achieved from the SWS Chemical Cleaning at River Bend Station.
