United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-23: 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

                                  May 12, 1988


Information Notice No. 88-23:  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. 

Purpose: 

The purpose of this information notice is to alert addressees to potential 
problems resulting from hydrogen transport from the volume control tank and 
accumulation in emergency core cooling system piping.  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 do not constitute NRC 
requirements; therefore, no specific action or written response is required. 

Description of Circumstances: 

On February 26, 1988, the licensee was operating Farley 1 at power near the 
end of the current fuel cycle.  Valves 8706A and B (see Figure 1) had been 
stroked as required for surveillance testing.  After testing was completed, 
the licensee noted that the boron concentration in the reactor coolant system 
was higher than expected and suspected that there was leakage through one of 
these valves.  To determine whether valve 8706A was seated, the licensee took 
a coolant sample from a vent downstream from the valve in order to analyze the 
sample for boron concentration.  Fifty cubic feet of gas was vented from the 
line before a coolant sample could be obtained.  The gas was 98 percent 
hydrogen. 

Discussion: 

Farley 1 has three centrifugal charging pumps and two centrifugal residual 
heat removal (RHR) pumps.  During a loss-of-coolant accident (LOCA), the 
charging pumps function as high-pressure safety injection (HPSI) pumps, and 
the RHR pumps function as low-pressure safety injection (LPSI) pumps.  When a 
safety injection signal is generated, the discharge sides of the HPSI and LPSI 
pumps are aligned to supply coolant to the cold legs of the reactor coolant 
system, the suction sides of the HPSI and LPSI pumps are aligned to the 
refueling water storage tank (RWST), and the pumps are started.  At the low 
water level setpoint for the RWST 





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                                                                May 12, 1988 
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when transferring to the recirculation mode, the suction sides of the LPSI 
pumps are realigned from the RWST to the sump.  At the same time, the 
discharge sides of the LPSI pumps are aligned to supply coolant to the suction 
header for the HPSI pumps as well as the cold legs of the reactor coolant 
system.  In addition, the two block valves in the HPSI suction header between 
HPSI pumps B and C close to establish independent trains. 

Had a small-break LOCA occurred before the line downstream of valve 8706A was 
vented, 50 cubic feet of hydrogen would have been swept through HPSI pumps A 
and B when valve 8706A opened.  The licensee stated that if 6 cubic feet of 
hydrogen were swept through one HPSI pump as a single gas bubble, the pump 
might be damaged significantly.  Whether or not hydrogen gas causes 
significant damage to HPSI pumps A and B depends on the amount of mixing of 
hydrogen and water before the mixture enters the pumps and on the distribution 
of hydrogen bubbles between HPSI pumps A and B.  Because of this uncertainty, 
the operability of HPSI pumps A and B would be in question with hydrogen 
trapped in lines upstream of the pumps. 

Figure 1 shows the layout for piping and components on the suction side of the 
HPSI pumps for Units 1 and 2.  For Unit 1, to provide physical separation of 
the lines from the LPSI pumps to the suction header for the HPSI pumps, the 
architect-engineer routed the line containing valve 8706A so that part of the 
line is 32 feet above the line from the volume control tank (VCT).  Figure 2 
shows the elevations of horizontal piping on the suction side of the HPSI 
pumps.  The lines for Unit 2 were arranged in a similar way. 

Hydrogen is normally used for the cover gas in the VCT; thus, water flowing 
through the VCT is saturated with hydrogen while it is in the VCT.  If the 
local pressure in the piping at some point between the VCT and HPSI pump 
suction nozzles is less than VCT pressure, the dissolved hydrogen will come 
out of solution and will not immediately go back into solution even if the 
pressure downstream from that point is greater than VCT pressure.  The 
licensee believes that the pressure distribution in some elbows and tees 
downstream from the VCT is such that some hydrogen comes out of solution in 
those fittings and that these hydrogen bubbles are swept through the pumps 
without damaging them.  In the suction piping of pump A, however, some of the 
bubbles are trapped in the vertical section of line that runs to the high 
point vent.  Data obtained by the licensee indicate that the collection rate 
is approximately 5 cubic feet per day. 

At Unit 2, there is a second hydrogen collection point in the suction piping 
for HPSI pump B (see Figure 3).  With pump B out of service, hydrogen gas 
collects between the tee and the closed valve. 

The licensee has taken interim corrective action to avoid declaring HPSI pump 
A in Unit 1 and HPSI pumps A and B in Unit 2 inoperable.  In Units 1 and 2, 
the line downstream from valves 8706A is vented once each shift to remove 
accumulated hydrogen gas.  In Unit 2, only pump B is normally operated so that 
hydrogen gas bubbles are swept through the pump and cannot accumulate. 

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                                                                May 12, 1988 
                                                                Page 3 of 3 


No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical 
contact listed below or the Regional Administrator of the appropriate regional 
office. 




                                 Charles E. Rossi, Director 
                                 Division of Operational Events Assessment 
                                 Office of Nuclear Reactor Regulation 

Technical Contact:  Roger Woodruff, NRR
                    (301) 492-1180 

Attachments:
1.  Figure 1 - Farley 1 & 2 Charging Pump Schematic 
2.  Figure 2 - Farley 1 Horizontal Piping Elevations 
3.  Figure 3 - Farley 2 Horizontal Piping Elevations 
4.  List of Recently Issued NRC Information Notices 
.                                                            Attachment 4
                                                            IN 88-23 
                                                            May 12, 1988 
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                            NRC INFORMATION NOTICES 
_____________________________________________________________________________
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

88-22          Disposal of Sludge from       5/12/88        All holders of OLs
               Onsite Sewage Treatment                      or CPs for nuclear
               Facilities at Nuclear                        power reactors. 
               Power Stations 

88-21          Inadvertent Criticality       5/9/88         All holders of OLs
               Events at Oskarshamn                         or CPs for nuclear
               and at U.S. Nuclear                          power reactors. 
               Power Plants 

88-20          Unauthorized Individuals      5/5/88         All holders of OLs
               Manipulating Controls and                    or CPs for nuclear
               Performing Control Room                      power, test and 
               Activities                                   research reactors,
                                                            and all licensed 
                                                            operators. 

88-19          Questionable Certification    4/26/88        All holders of OLs
               of Class 1E Components                       or CPs for nuclear
                                                            power reactors. 

88-18          Malfunction of Lockbox on     4/25/88        All NRC licensees 
               Radiography Device                           authorized to 
                                                            manufacture, 
                                                            distribute, and/or 
                                                            operate radio-
                                                            graphic exposure 
                                                            devices. 

88-17          Summary of Responses to NRC   4/22/88        All holders of OLs
               Bulletin 87-01, "Thinning of                 or CPs for nuclear
               Pipe Walls in Nuclear Power                  power reactors. 
               Plants" 

88-16          Identifying Waste Generators  4/22/88        Radioactive waste 
               in Shipments of Low-Level                    collection and 
               Waste to Land Disposal                       service company 
               Facilities                                   licensees handling
                                                            prepackaged waste, 
                                                            and licensees 
                                                            operating 
                                                            low-level waste 
                                                            disposal 
                                                            facilities. 
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit 
Page Last Reviewed/Updated Tuesday, November 12, 2013