Supplement 3:Potential for Gas Binding of High-Pressure Safety Injection Pumps During a Loss-Of-Coolant Accident
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
December 10, 1990
Information Notice No. 88-23, SUPPLEMENT 3: POTENTIAL FOR GAS BINDING
OF HIGH-PRESSURE SAFETY
INJECTION PUMPS DURING A
LOSS-OF-COOLANT ACCIDENT
Addressees:
All holders of operating licenses or construction permits for
pressurized-water reactors (PWRs).
Purpose:
This information notice supplement is intended to alert addressees to the
potential for common-mode failure caused by hydrogen gas binding of the
high-head safety injection pumps (charging pumps) during a loss-of-coolant
accident (LOCA). 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 supplement do not constitute NRC requirements; therefore,
no specific action or written response is required.
Description of Circumstances:
On August 22, 1990, Unit 2 of the Sequoyah Nuclear Power Plant was at
70-percent power (in coastdown). The licensee was attempting to switch
operation of the charging pumps from the "A" to "B" pump in order to perform
surveillance (see Attachment 1). Upon start of the "B" charging pump, the
licensee observed fluctuation of the pump's motor amperage and rate of flow.
The licensee suspected that gas was accumulating on the suction-side of the
"B" pump and secured the pump. Further investigation and analysis by the
licensee revealed that hydrogen gas was accumulating in the suction piping
of the "B" pump and in the RHR crossover piping to the charging header. The
licensee was able to vent approximately 5.3 cubic feet of gas. An
additional 4.75 cubic feet of gas could not be vented from the RHR crossover
piping.
On September 6, 1990, with Unit 1 at 100-percent power, the licensee
identified the presence of a hydrogen gas bubble on the suction-side of the
charging pumps in Unit 1. The gas was collecting in the piping between the
"A" residual heat removal (RHR) pump and the charging pumps. The licensee
calculated that hydrogen was accumulating at a rate of 0.5 cubic feet per
hour. The gas came out of solution (in part) due to localized reductions in
pressure because of piping elevation differences and eccentric pipe reducers
(see Attachment 1). Immediate corrective action taken by the licensee for
both units included venting the suction piping of the idle charging train
every 8 hours.
9012040239
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IN 88-23, Supplement 3
December 10, 1990
Page 2 of 3
Discussion:
These events at Sequoyah are significant because hydrogen gas accumulation
in the suction piping to the charging pumps has the potential to affect
multiple trains of pumps in the emergency core cooling system (ECCS). Loss
of all high-pressure recirculation capability at Sequoyah during a
small-break LOCA is the dominant risk contributor to the core damage
frequency as identified in Section 5, Sequoyah Plant Results, NUREG-1150,
Volume 1, "Severe Accident Risks: An Assessment For Five U.S. Nuclear Power
Plants."
During a LOCA, suction of the ECCS pumps must be switched from the refueling
water storage tank (RWST) to the containment sump before the RWST is
depleted. If the reactor coolant system (RCS) has not yet depressurized to
the point that the low-pressure injection pumps (i.e., RHR pumps) can inject
into the vessel, then the discharge of the RHR pumps must be directed to the
suction of the centrifugal charging pumps (CCPs) and the safety injection
(SI) pumps. Successful recirculation of water from the containment sump
(with the RCS at high pressure) requires operation of one RHR pump and one
of the high head pumps. At Sequoyah, the "A" RHR pump supplies the suction
of both CCPs and the "A" SI pump. The "B" RHR pump supplies the suction to
the "B" SI pump.
Noncondensible gases accumulating in the piping between the "A" RHR pump and
the charging pump suction header creates the potential for gas binding of
both charging pumps during the switchover from high-pressure injection to
high-pressure recirculation. In addition, because the valves isolating the
"A" RHR and "A" SI pumps from the charging pump suction header are
periodically stroke-time tested, gas may also enter sections of piping
normally isolated from this header. Thus, the gas accumulation in the
charging pump suction header potentially affects three of the four
high-pressure pumps.
In recent NRC information notices, the staff addressed gas binding of ECCS
pumps. Information Notice (IN) 88-23, "Potential For Gas Binding of High-
Pressure Safety Injection Pumps During A Loss-Of-Coolant-Accident (LOCA),"
addressed gas-binding problems in the high-pressure safety injection system
at the Farley Nuclear Power Plant. The staff issued two supplements to that
information notice to address gas accumulation affecting ECCS pumps because
of various root causes. IN 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," discusses another mechanism
that could lead to gas binding of both CCPs.
The two gas-binding events at Sequoyah had root causes that were attributed
by the licensee, in part, to inadequate review of IN 88-23. Although most
gas accumulation in ECCS systems has been hydrogen, in at least one
instance, a mixture of air and hydrogen was found. It is important to
consider all potential sources of gas intrusion to the ECCS suction piping,
such as leaking bladders on the pulsation dampeners for positive displace-
ment charging pumps, ineffective check valves in highpoint venting systems
that lead back to the air space in the volume control tank (VCT), any flow
restrictions (e.g., orifices)
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IN 88-23, Supplement 3
December 10, 1990
Page 3 of 3
which may cause gases to come out of solution, and improper venting and
filling operations following maintenance of ECCS flowpaths. Since most
plants have no technical specification surveillance requirement for periodic
venting of ECCS suction piping (only pump casings and discharge piping), gas
may accumulate and remain undetected for extended periods of time,
subjecting the plant to a possible common mode failure of the ECCS pumps.
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate NRR project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: John Thompson, NRR
(301) 492-1171
Attachments:
1. Charging Pumps and RHR Crossover for SQN Units 1 and 2
2. List of Recently Issued NRC Information Notices
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