The overall cycle efficiency and the capacity of base-load nuclear power plants can be highly sensitive to varying ambient weather conditions. Such is the case for Grand Gulf Nuclear Station, since condenser inlet water temperature varies widely with changing ambient weather conditions.
To achieve rated generator output, the turbine cycle heat balance requires the circulating water system be capable of supplying a condenser inlet water temperature not greater than 85°F (29°C). Based upon the design of the 520-foot Grand Gulf natural draft cooling tower, its circulating water system is capable of delivering an 85°F (condenser inlet water temperature (and rated generator output) only when the ambient wet-bulb temperature (a measure that takes into consideration water vapour) is less than approximately 60°F (15°C). Based on regional weather patterns, the ambient wet bulb temperature will be less than 60°F only approximately half of the year.
To lower circulating water inlet temperature to the condenser when the weather is hot and humid, a 20-cell auxiliary cooling tower (ACT) started operation in June 2002, with plans to install another eight cells to support the plant’s expected extended power uprate.
In late February 2011 Grand Gulf completed the design and installation of the eight cell expansion.
With the two towers operating, the condenser inlet water temperature was lowered 9°F and plant net output increased by 26 MWe, equating to approximately 2.1 percent on an average annual basis. The implementation of this change results in significant recurring cost savings over the remaining life of the plant, since the increased output involved no added fuel cost.
Design specification
The 28-cell ACT is supplied by two 96-inch (2.4m) diameter pipes tapping off the two existing 120 in. supply lines to the NDCT. Valves in the ACT supply piping make it possible to turn flow to the ACT on and off.
The Auxiliary Cooling Tower supply valves are throttled to supply the ACT at approximately 290,000 gpm (gallons per minute, 18,293 lps), approximately half of total circulating water flow, with the other half supplied to the NDCT. The two 96 in. supply lines connect to a single 96 in. supply header with fourteen 36 in. diameter risers at the ACT.
Water is supplied to pairs of ACT cells through 36 in. manual riser isolation valves. The valves are throttled to pass approximately 20,714 gpm of water each, which equates to 10,357 gpm per cell. The water distribution system spray nozzles allow the water to gravity-drain through a labyrinth of synthetic plastic fill material while the cell fans induce air to draft upward to remove heat from the water. The cooled water falls into the one million gallon ACT basin, and gravity-drains to the NDCT basin via an exposed 0.5 million gallon flume. The combined volume of the NDCT basin and ACT flume is approximately seven million gallons.
The cell dimensions for the 28-cell rectilinear ACT are 54ft x 48ft (17.7m x 14.6m) in a back-to-back cell configuration (that is, one air inlet per cell measuring 48ft long x 18ft high). The fans are 28ft in diameter with single-speed, non-variable frequency drive 200 horsepower motors. The basin foot print dimensions for the 28 cells is 678ft x 122ft.
Modifying the plant and the original natural draft cooling tower and constructing the auxiliary tower to passively operate in parallel was a technically-challenging feat. Specialized components together with NDCT distribution nozzles passively regulate an even distribution of water over the NDCT at the two required circulating water flow rates.
The eight-cell expansion of the ACT is similar to the existing 20 cells in design, configuration, and operation except that the configuration of the heat transfer material (also know as the fill material) of the eight new cells is considered state of the art. It consists of a 5 ft. depth of low-fouling PVC film fill material to minimize the potential for biological fouling and includes a 1 ft. depth overlay of high-efficiency PVC film fill to moderate the loss in thermal efficiency associated with the low-fouling type fill material.
Site-specific makeup water quality (for example, suspended and dissolved solids, microorganisms, etc.), chemical treatment programmes, and the PVC film fill’s structural properties and fabrication quality will continue to govern the life expectancy and replacement schedule for the fill. However, an extended service life of the current NDCT high efficiency fill has been observed. It is primarily attributed to the improved operating margin (11-12°F) in mixed cold water return temperature lowered by operation of the ACT.
With the ACT in service, the useable life of the NDCT fill material has effectively been extended by almost five years.
An estimated minimum 4.8 MWe net improvement in main generator output is expected with the eight-cell ACT expansion prior to the EPU in 2012.
Benefits of the newly expanded 28-cell ACT includes increased condenser vacuum operating margin adding to plant reliability and safety, reactor water chemistry improvement through better condensate polisher performance, and lower condensate temperature, which prolongs condensate polishing resin life and reduces low level radioactive waste generation.
Additionally, there is a reduced potential for algae growth in the shallow ACT cold water return basin of the eight ACT cells due to blocking the sunlight to the previously un-shaded basin floor. The new cooling tower configurations also support future unit thermal power increases without losses of condenser vacuum operating margin.
“We have been planning the EPU for years and these final eight cells were additions that increased our output and efficiency immediately, before the first EPU hour was logged,” said Bryan E. Warren, P.E., senior staff engineer and team lead on the eight cell cooling addition. “This ACT addition is a good first step for our EPU.”
Author Info:
Bryan E. Warren, P.E., senior staff engineer, Entergy Grand Gulf Nuclear Station, Bald Hill Road, PO Box 756, Port Gibson, Mississippi 39150;
John W. Cooper, Jr., P.E., president, John Cooper & Associates, P.A.; Robert Fulkerson, president, Fulkerson & Associates, Inc; Mark D. Locke, P.E., senior lead engineer; Russell W. Richards, senior project manager, Entergy Nuclear Fleet Headquarters.
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