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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. IN 88-23
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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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
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