Scale Model Testing Confirms Adequate Refueling Water Storage Tank Vortex Allowance

Drew Robb

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Recent Nuclear Regulatory Commission (NRC) Component Design Bases Inspection activities have scrutinized empirical approaches used to determine vortex allowances for emergency core cooling system (ECCS) suction sources. In 2006, the NRC asked nuclear plant operators to review their analysis of the tanks in their ECCS and containment spray systems (CSS). The NRC wanted to verify that plant designs were adequate to ensure that any vortices formed during draw-down of the fluid would not result in pump air ingestion (see sidebar). Duke Energy Corp. of Charlotte, N.C., opted to perform rigorous physical scale model testing to demonstrate adequate vortex allowances were present in its designs.

NRC Issues Notice on Air Entrainment

On September 21, 2006, the U.S. Nuclear Regulatory Commission (NRC) issued NRC Information Notice 2006-21 Operating Experience Regarding Entrainment of Air into Emergency Core Cooling and Containment Spray Systems. As the notice stated, air can severely compromise the cooling system operation: "Air entrained by more than a few percent by volume may degrade or cause failure of centrifugal type pumps. A degraded pump which successfully expels small amounts of entrained air (or other gases), may become air (or gas) bound to such a degree that it will not restart after being stopped due to the coalescing of air into the pump casing. Additionally, voids in the pumped fluid can cause excessive vibration and wear of important internal parts."

The information notice was prompted by events at several nuclear power facilities. Each plant in question had different conditions that put it at risk, but the fact that there were multiple potential problem sources emphasized the need for a review of all individual nuclear plants.

As the report states: "The above described events highlight the importance of proper engineering analysis in ensuring that entrained air will not enter suction supply lines and impair the ability of the ECCS and CSS pumps to perform their specified safety functions. The conditions which could have allowed air to be entrained had not been revealed by routine surveillance testing of pumping capability because it is not always practicable for the surveillance test to replicate the accident conditions that could cause air entrainment."

"The ECCS includes a variety of different pump designs: some are single-stage centrifugal, some are multi-stage pumps — and the latter are very intolerant to air ingestion," says Bryan Meyer, principal engineer in the Primary Systems Engineering Group at Duke Energy’s McGuire Nuclear Station.

To demonstrate that its cooling systems could operate safely, Duke Energy engaged Alden Research Laboratories (Alden) of Holden, Mass., to conduct physical modeling of its refueling water storage tanks (RWST). The tanks were found to be well within the margin of safety.

"The physical modeling allowed us to recover additional margin in our usable tank inventory and reduce our vortex allowance dramatically," says Meyer.

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