Maintenance Log: Troubleshooting A Nuke Power Station’s Valve Problems with Acoustic Emission

Failure is not an option for most plants, but even more so if you’re talking about the emergency cooling system of a nuclear power-gen operation.

Wolf Creek Nuclear Generating Station in Burlington, KS, operates a single pressurized water reactor to produce a rated electrical output of about 1150 megawatts. If normal reactor coolant system circulation is lost, the plant’s safety injection pumps draw borated water from pressurized accumulators and pump it into the reactor. The boron stops the nuclear reaction and the water cools the reactor.

The accumulators are kept pressurized to 650 psi by a nitrogen blanket, so the borated water becomes saturated with nitrogen. Any decrease in pressure causes the gas to come out of solution. Voids of gas in the suction piping for the safety injection pumps can cause those pumps to malfunction.

Technical Specification Surveillance Requirement 3.5.2.3 requires that Emergency Core Cooling System (ECCS) piping be verified full of water once every 31 days. A combination of ultrasonic testing (UT) and/or venting is used to ensure that the piping in the ECCS pump casings and accessible discharge piping high points are full of water.

The problem
During the January 2, 2008, performance of this surveillance test procedure, the guidance for maximum vent times was not met. The operators vented 110 seconds of air from a vent valve between Safety Injection Pump A and the Reactor Cooling System—110 seconds is significantly greater than the guidance of two seconds outlined in the procedure. A condition report was written for Engineering to evaluate the potential impacts on system operability. Testing of the void gas indicated 99.4% nitrogen

To see if there were other voids of gas in the piping, 105 locations on the discharge and suction sides of ECCS pumps were inspected using UT. On January 10, voids exceeding the industry-accepted value of 5% were discovered in the piping between the safety injection pump suctions and residual heat removal (RHR) discharge for both trains. The suction-side void gas was determined to be air, with minor amounts of hydrogen. Both trains of ECCS were declared inoperable. The operators shut down the unit on January 11, 2008.

Wolf Creek’s plant manager and Plant Safety Review Committee commissioned an Incident Investigation Team to examine the causes behind this event. The event included the intrusion of nitrogen in the safety injection pump discharge piping and the accumulation of entrapped air in safety injection pump/centrifugal charging pump (CCP) recirculation phase suction piping. The scope of the investigation included:

• Root cause determination of the initiating event, any related pre-events and extent of condition findings;
• Investigation of organizational performance regarding Leak Detection Services’ (LDS) capacity to have recognized the vulnerabilities of ECCS voiding;
• Review the design of systems for venting effectiveness;
• Review of precursor signs (i.e. Safety Injection Accumulator inventory); and
• Sensitivity of the organization to gas intrusion.

The fix
During the troubleshooting phase, LDS® was used to determine where the gas voiding was originating. About 30 air-operated valves and several check valves were identified as possible sources of the gas intrusion. Dismantling and internally inspecting all of those valves would have taken time and resulted in an unnecessary dose of radiation for the maintenance personnel. To pinpoint the actual problem, all valves were tested with LDS’ acoustic emission ValveAlyzer® System.

Transducers on the outside of valves and piping detect ultrasonic acoustic emissions that are characteristic of internal leakage. The ValveAlyzer® System amplifies and processes the signals from the transducers to separate the leakage signals from the background noise. Analysis of the recorded signatures of amplitude vs. frequency also enables an estimate of the damage severity as follows:

• (LRG)—Large. Indicates that the soft metal is being attacked. Body damage is likely. Seats and plugs may be cut deeply.
• (MED)—Medium. Indicates damage to the hard metal only. Lapping is the most likely repair required.
• (SML)—Small. The leak leaves no visible damage in valves.

The damage estimates are used to predict the extent of repairs required. The test data showed there were only nine valves leaking—and also correctly predicted the extent of internal damage to each valve.

The nine valves were disassembled and damage—as indicated by pits and signs of seat leakage—was found on both the seats and the plugs. The valves were repaired and returned to service. When the ECCS system was returned to service, extensive testing confirmed the absence of gas voids in the piping.

The result
Luckily, the plant was back in operation in less than a week. The cost of lost output alone for a facility the size of Wolf Creek is over a million dollars a day. Using LDS to find which valves actually leaked saved the operation several days of downtime. Today, Wolf Creek calls on LDS whenever valve seat leakage is suspected. This technology has proven successful in identifying even small amounts of leakage.

Joseph G. Dimmick is president of Leak Detection Services, Inc., based in Chester, MD. E-mail: joedimmick@mindspring.com

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