NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
April 23, 1999
|NRC INFORMATION NOTICE 88-23, SUPPLEMENT 5:||POTENTIAL FOR GAS BINDING OF HIGH PRESSURE SAFETY INJECTION PUMPS DURING A LOSS-OF-COOLANT ACCIDENT|
- Description of Circumstances
All holders of operating licenses for nuclear power reactors, except those who have permanently ceased operations and have certified that fuel has been permanently removed from the reactor.
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform addressees of industry experience regarding the transport and accumulation of gases in high-pressure safety injection (HPSI) systems. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required.
Numerous NRC and industry notifications have been issued to alert licensees of the potential to gas bind HPSI pumps in pressurized-water reactors (Attachment 1). Many pressurized water reactors are designed with centrifugal charging pumps that also serve as HPSI pumps. The pumps are equipped with minimum flow recirculation lines. A flow-restricting orifice is installed in each recirculation line, and the recirculation flow is directed back to the pump suction header. During normal operation, hydrogen gas is dissolved in the fluid from the volume control tank to reduce oxygen and to inhibit corrosion. In addition, nitrogen is used to pressurize safety injection (SI) accumulators. It has been shown that there are numerous physical plant configurations that can promote the transportation and accumulation of gases within the HPSI system, specifically at local high points in stagnant or low-flow sections of the system. The HPSI pumps are relatively intolerant of gas ingestion. Starting an idle HPSI pump with gas in the suction line or the pump casing may result in gas binding or mechanical damage to the pump. The HPSI pumps are risk-significant components that could significantly affect the probability of core damage if they are unavailable.
In 1988, the licensee for Unit 2 of the Beaver Valley Power Station experienced gas binding of HPSI pump A. At that time, the licensee performed ultrasonic testing examinations of the HPSI pump suction piping to determine gas growth rates and to establish acceptable gas void fraction criteria and pump venting frequencies. The gas void fraction criteria and venting frequencies did not completely address the issue, however, as HPSI pump C experienced gas binding in 1993, 1996, and 1997. On September 12, 1997, an operational surveillance test showed that HPSI pump C could not meet the required performance criteria. The pump was declared inoperable, and the subsequent investigation found that the pump shaft was cracked. The crack resulted from the cyclic impact loading on the shaft attributed to the presence of gas voids during pump starts. The investigation found that significant amounts of hydrogen gas were accumulating in the pump suction piping. The source of the gas was the orifices in the pump recirculation lines. The orifices were, in effect, stripping gas from the recirculation flow. The orifice design was found to contribute to the rate of gas stripping. Eleven-stage orifices, installed in two of the three recirculation lines, were found to generate significantly more gas than the 14-stage orifice installed in the recirculation line for the third pump (HPSI pump B). The HPSI suction header configuration also was found to have a significant effect on the location and the accumulation rate of the gas. The configuration of the piping allowed more significant gas accumulation on HPSI pumps A and C. The investigation also determined that the installed suction piping vents were deficient in that they lacked the ability to positively verify adequate venting and provided minimal differential pressure. Specifically, the vents were routed back to the volume control tank, which allowed for only a few pounds of differential pressure. Although the licensee had established a pump venting frequency, the licensee had not performed ultrasonic testing after pump venting to verify that all gas had been removed. In addition, the licensee had not performed ultrasonic testing between 1988 and 1997 to verify that the estimated gas growth rates and established venting frequency remained valid.
The licensee implemented several corrective actions to address this event. Twenty-two-stage orifices were installed to reduce the gas stripping in the recirculation lines. The licensee has not detected any gas generation since the twenty-two-stage orifices were installed. Appropriate gas void fraction criteria were established to replace the nonconservative criteria established in 1988. Gas accumulation rates and venting frequencies have been revised to ensure that the void fraction of gas remains below the established criteria. In addition, the licensee performs ultrasonic examination of the HHSI piping just prior to venting to ensure that the venting frequencies are adequate to prevent significant gas accumulation.
On November 16, 1998, the licensee for Diablo Canyon determined that the emergency core cooling system may have been outside its design basis because of gas voiding in the system suction piping. Recent analysis and calculations indicated that past occurrences of gas voiding could have affected the operability of either both SI pumps or both HPSI pumps during the recirculation phase of a loss-of-coolant accident by exceeding a 5-percent void fraction limit at the pump suctions. The cause of the voiding has not been determined. The licensee is currently performing ultrasonic testing and venting as necessary of the subject piping to ensure operability of the system. The licensee discovered the condition during surveillance testing and system monitoring performed as a result of reviewing industry operating experience.
On January 5, 1999, during the performance of a surveillance procedure, the 4B HPSI pump did not develop adequate discharge pressure and the motor amperes were low. An estimated 8 cubic feet of gas was vented from the pump casing. The licensee's investigation noted that during periodic venting of the system header on December 5, 1998, a small quantity of gas had been identified in the discharge header. The licensee increased the venting frequency on the discharge header from once a month to weekly. During the periodic venting of the header, the licensee continued to identify gas. The licensee suspected back leakage from reactor coolant system cold legs through normally closed motor-operated valves. These valves were stroked on December 28, 1998, in an attempt to better seat the valves. This activity did not resolve the problem. The licensee subsequently initiated a number of actions to address the issue. The licensee vented the other three pumps and headers and ran the pumps daily. The licensee performed ultrasonic testing and radiographs to identify any piping containing gas. The licensee also conducted a review of maintenance performed on the system. Analysis of the gas indicated that the source of gas was nitrogen from the SI accumulators. The leakage path was from the SI accumulators through a 3/4-inch-diameter inservice testing (IST) test line, into the recirculation line, and into the 4B HPSI pump casing. This path involved nitrogen leakage past multiple barriers (closed isolation valves and check valves). The licensee had performed an engineering evaluation to respond to recent industry operating experience, at which time the licensee considered leakage past multiple barriers. The licensee concluded that leakage past multiple barriers was not credible. The licensee continued to experience gas intrusion into the SI system, but periodic venting and pump runs maintained the operability of the system. The licensee intends to cut and cap the 3/4-inch IST lines in the upcoming refueling outage to eliminate this problem.
Recently, several licensees have reported that a postulated fire may result in gas binding of the HPSI pumps. Specifically, the postulated fire may result in the loss of reactor coolant system letdown flow to the volume control tank (VCT). The VCT outlet isolation valves may remain open during this event because either: (1) the valves cannot be closed because of fire damage to the valves or associated electrical equipment, or (2) licensee operating procedures do not direct operators to close the valves. If the VCT outlet isolation valves remain open, hydrogen from the VCT could be drawn into the HPSI pump suction.
Despite significant NRC and industry information (Attachment 1) describing gas accumulation in emergency core cooling systems, licensees continue to identify situations in which gas accumulation possibly rendered HPSI equipment inoperable. Recent industry events indicate that HPSI system configurations and interfaces and HPSI pump recirculation line orifice design significantly influence the accumulation of gas in the HPSI system. The NRC continues to be concerned about the occurrence of such situations because degraded HPSI pumps can have a significant impact on overall plant risk. This information notice highlights various approaches that licensees have taken in response to past industry operating experience. It is expected that licensees will review this information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems.
This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact one of the technical contacts listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.
|/s/'d by S. F. Newberry|
David B. Matthews, Director
Division of Regulatory Improvement Programs
Office of Nuclear Reactor Regulation
|Technical contacts:||Christopher P. Jackson
|Eric J. Benner|
|Attachment:||1.||Previous NRC Information Concerning the Potential for Gas Binding Safety Injection Pumps|
|2.||List of Recently Issued NRC Information Notices|
(NUDOCS Accession Number 9904200058)
IN 88-23, Supp. 5
April 23, 1999
Page 1 of 1
Previous NRC Information Concerning the Potential
for Gas Binding Safety Injection Pumps
Information Notice 88-23, "Potential for Gas Binding of High-Pressure Safety Injection Pumps During a Loss-of-Coolant Accident," May 12, 1988.
Information Notice 88-23, Supplement 1, "Potential for Gas Binding of High-Pressure Safety Injection Pumps During a Loss-of-Coolant Accident," January 5, 1989.
Information Notice 89-67, "Loss of Residual Heat Removal Caused by Accumulator Nitrogen Injection," September 13, 1989.
Information Notice 88-23, Supplement 2, "Potential for Gas Binding of High-Pressure Safety Injection Pumps During a Loss-of-Coolant Accident," January 31, 1990.
Information Notice 90-64, "Potential for Common-Mode Failure of High-Pressure Safety Injection Pumps or Release of Reactor Coolant Outside Containment During a Loss-of-Coolant Accident," October 4, 1990.
Information Notice 88-23, Supplement 3, "Potential for Gas Binding of High-Pressure Safety Injection Pumps During a Loss-of-Coolant Accident," December 10, 1990.
Information Notice 88-23, Supplement 4, "Potential for Gas Binding of High-Pressure Safety Injection Pumps During a Loss-of-Coolant Accident," December 18, 1992.
Information Notice 91-50, Supplement 1, "A Review of Water Hammer Events After 1991,"July 17, 1997.
Information Notice 97-38, "Level-Sensing System Initiates Common-Mode Failure of High-Pressure-Injection Pumps," June 24, 1997.
Information Notice 97-40, "Potential Nitrogen Accumulation Resulting from Back leakage from Safety Injection Tanks," June 26, 1997.