United States Nuclear Regulatory Commission - Protecting People and the Environment

Supplement 3: Potential for Gas Binding of High-Pressure Safety Injection Pumps During a Loss-Of-Coolant Accident

                                UNITED STATES
                           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


All holders of operating licenses or construction permits for 
pressurized-water reactors (PWRs).


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 

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.


                                                  IN 88-23, Supplement 3 
                                                  December 10, 1990 
                                                  Page 2 of 3 


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 

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) 


                                                  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

1.  Charging Pumps and RHR Crossover for SQN Units 1 and 2
2.  List of Recently Issued NRC Information Notices
Page Last Reviewed/Updated Friday, May 22, 2015