Entergy's River Bend Station is a 991 MWe nuclear plant on the Mississippi River in St. Francisville, just north of Baton Rouge, La. The plant has been operating since 1986. Its engineering personnel raised concerns recently when their evaluation of condenser efficiency reported declining performance, which was tied directly to MWe output of the plant. According to Ed Deweese, Sr. Lead Engineer, if the condenser continued on a linear progression of degradation, it would soon fall below a 65 percent minimum cleanliness/efficiency factor. This also resulted in secondary performance issues because the plant was getting moisture in the air removal system, causing "chugging" in the steam jet air ejectors, a sign of dirty condenser tubes. Plant management contracted with an outside specialist firm to conduct an inspection of the condensers and perform what would most likely be a tube cleaning operation.

Personnel considered various service contractors and cleaning methodologies—hydro-lasing, chemical cleaning and mechanical tube scraping. Because River Bend features a boiling water reactor (BWR), its condensers are in a radiation dose field, unlike a pressurized water reactor (PWR), and minimization of dose was a key factor in deciding the method of inspection and cleaning activities. River Bend Station has a hydrogen water chemistry system at the plant, which also increases the dose rates at the condenser. In addition, the plant's chemistry personnel were concerned about taking the system out of service for a long time because continuous hydrogen injection into the feed water system provides long-term integrity protection to the reactor vessel. Based on these factors and other dose-related considerations, it was important for personnel to spend as little time as possible in the condenser areas for inspection and cleaning.

Plant personnel selected the mechanical tube cleaning method due to the reduced time required and some inefficiencies that occur with other methods. They selected Conco Systems Inc., Verona, Penn., based in part on its experience with condenser tubes in nuclear plants, including River Bend. It was important to the River Bend Station condenser team that the people coming to do the work be already badged for work inside a nuclear plant to mitigate the time needed to train unfamiliar crews. Conco management put together two crews, with a supervisor and three technicians on each crew. All personnel were badged and experienced in inspecting and cleaning nuclear plant condensers. Most were ready to start working the morning they arrived, following completion of site-specific training. The River Bend Station employs the Institute of Nuclear Power (INPO) Nantel Training Process, which added the benefit of certifying the Conco crew to perform tube cleaning operations at any nuclear facility using the Nantel Process. Personnel who complete the Nantel Training Process, which is used at most nuclear power plants today, are certified for a full year.

There are four condenser water boxes serving River Bend, each containing 17,452 Admiralty 3/4 inch diameter brass tubes, 18 gauge and more than 42 feet long. The inspection revealed that the Delta water box was suffering significantly degraded performance and the reason was quickly apparent. Plastic "fill," V-shaped perforated sections located in the circulating water cooling towers had deteriorated and began sloughing off in small pieces. A missing section of screening, since repaired, allowed these pieces of plastic to travel through the circulating water system pumps, eventually winding up in the condenser, lodged against the inlet holes of the tubesheet. That, in turn, created a low flow condition at the inlet side of the water box.

Water for the plant's condensers is pumped from the Mississippi River up to two large clarifiers and then into the four cooling towers, which in turn send 125,000 gallons of water per minute through each of the four water boxes. The water at this stage is basically clean, but still contains particles of silt and mud that normally would pass through the condenser tubes without a problem. Because of the low flow condition caused by the plastic fill pieces, the condenser tubes were severely fouled with mud that had settled in thick deposits.

To clean the two water boxes that contained the deposit buildup, power was reduced to 75 percent. For the first time, River Bend conducted an on-line cleaning operation. With the two crews working around the clock, they proceeded to "shoot" the tubes with Conco C4S tube cleaners. These cleaners have a four-metal-bladed center rivet design that provides overlapping tube I.D. coverage. A tarp was hung at the far end of the water box to catch the exiting cleaners, which were each cleaned and re-used several times. The crews were able to clean the more than 17,000 condenser tubes in each water box in only three days, completing the 34,000 plus tubes in less than a week. In all, more than a thousand pounds of mud was removed from the first water box and almost as much from the second, along with the plastic cooling tower fill pieces and sponge balls (tube cleaners) that were blocked by the buildup of mud and debris on the tube sheets.

Randy Glueck, River Bend's Senior Systems Engineer, maintains the condenser efficiency ratings, averaged hourly, for the plant. He says they are up to 80 percent and that they have realized an increase in power output of 2.5 MWe per hour, or 60 MWe per day. New circulating water screens have been procured and complete overhauls of the cooling towers are in progress at this time to prevent a recurrence of the problem in the future.