Building the Model
Duke Energy Corp. owns and operates three nuclear stations — Catawba, McGuire, and Oconee — and opted to perform physical scale model testing for all three stations.
"The system design requires that the RWST suction piping remain water-filled during drawdown, such that no air ingestion occurs," says Meyer. "We must transfer pumps from the tank prior to the pipe becoming voided."
The company considered using CFD but eventually opted for hydraulic modeling. Alden was ultimately selected to perform the scale model testing. "Alden proposed the most rigorous modeling and technical approach," says Meyer.
Alden, which has been conducting hydraulics research for more than a century, has an established reputation with the NRC and has an on-site calibration laboratory to ensure that all measurements are accurate. Flow meter calibrations are done using equipment that is NIST (National Institute of Standards and Technology) traceable and accurate to within 0.25%.
Because several different tanks with different geometries needed to be modeled, Alden utilized a circular tank with a diameter of approximately 10 feet at the bottom and a depth of approximately 5.5 feet, which would allow simulation of different depths. For the McGuire station, which consists of two 1,100-MW Westinghouse pressurized water reactors with wet/ice containment, a model-scale of 1:4.073 was used.
The 24-inch primary outlet pipe, which is installed at a 45 degree angle, with the elliptical entrance 12 inches above the tank bottom, was modeled using acrylic pipe. The installation of clear acrylic piping enabled visual observations of air entrainment. A flow loop could operate closed, fully open, or partially open. Partial return flow controls the rate of draw-down in the water tank, with the rest of the water going to the laboratory sump. Because water and acrylic have nearly identical refractive indexes, a rectangular acrylic viewing box was installed around the outlet pipe to compensate for the visual distortion of the curved pipe and to allow for good viewing and videotaping of the air bubbles (Figure 5).
5. Scale models. Alden Labs designed a scale model test of the pump suction at the Duke Energy nuclear plants. Duke Energy elected to develop scale models to determine vortex allowances for its three plants’ emergency core cooling systems and containment spray systems to demonstrate that their pump suction designs ensure that any vortices formed during drawdown of the fluid would not result in pump air ingestion. Courtesy: Alden Labs
A series of 10 tests were run at prototype flow rates from 1,600 gpm to 19,700 gpm. Five of the tests were conducted with a return flow rate representing a water level drop of about 0.5 inch per minute, and five were done with the return pipe closed.
The tests showed that the tank could safely operate at much lower water levels than required by ANSI’s Hydraulic Institute Standards (HIS). Whereas the HIS specified a submergence of 2.85 feet for a flow of 1,600 gpm and 8.70 feet for 19,700 gpm, the hydraulic model testing showed that the tank was free of air entrainment to depths as low as 0.045 feet at 1,600 gpm and 0.705 feet with no return flow at 19,700 gpm.
Similar results were achieved on the tests for the tanks at the two other Duke stations. As a result, Duke was able to demonstrate that prior vortex allowances were conservative and avoided the need for additional vortex suppression devices priced at roughly $50,000, without plant down-time expenses included.
— Contributed by Drew Robb, a Los Angeles – based writer specializing in engineering and technology issues.
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