United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 93-12: Off-Gassing in Auxiliary Feedwater System Raw Water Sources

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C.  20555

                               February 11, 1993

                               SYSTEM RAW WATER SOURCES


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to problems that could result from off-gassing in
safety-related raw water sources.  It is expected that recipients will review
this 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.

Description of Circumstances

In March and April 1992, personnel at the McGuire Nuclear Station discovered
air pockets in the nuclear service water discharge piping supplying the safety
grade auxiliary feedwater for both Units 1 and 2.  Duke Power Company (the
licensee) determined the source of this air to be the off-gassing of the raw
lake water caused by the warming of the water by the components cooled by the
nuclear service water system.  The air intrusion is a safety concern because
it could cause air binding or damage to the auxiliary feedwater pumps.

The auxiliary feedwater system at McGuire Nuclear Station consists of two
450 gallon per minute (gpm) motor-driven pumps and one 900 gpm turbine-driven
pump that take suction from either of two sources.  The normal, condensate
grade source of water for auxiliary feedwater is either the upper surge tank,
the condenser hotwell, or the condensate storage tanks.  The safety grade,
seismic category I source of auxiliary feedwater is the nuclear service water
system.  When aligned to take suction from the nuclear service water system,
train "A" and "B" of the auxiliary feedwater system take suction from the
corresponding nuclear service water system train "A" and "B" (Figure 1).  The
turbine driven auxiliary feedwater pump can take suction from either train "A"
or "B" of the nuclear service water system.  In addition to the train "A" and
"B" nuclear service water, nuclear service water can be supplied to the
suction of the auxiliary feedwater system through a supply line controlled
from the standby shutdown facility.  (The standby shutdown facility is used to
mitigate certain fire and security events.) 


                                                      IN 93-12
                                                      February 11, 1993
                                                      Page 2 of 3

The licensee discovered two different sources of air in the nuclear service
water system that could have potentially affected the operability of the
auxiliary feedwater system.  On March 27, McGuire personnel discovered air at
a high point in the Unit 1 standby shutdown system line upstream of the
isolation valves that separate the nuclear service water and the auxiliary
feedwater systems (Valves 1CA161C and 1CA162C, Figure 2).  McGuire personnel
repeatedly vented this line over the following days, but air continued to be
produced at the high point.  After thoroughly searching for air inleakage and
performing a detailed engineering and chemical evaluation of the system,
McGuire staff concluded that the air was produced by off-gassing of the heated
raw lake water.  As a result of this discovery, the licensee searched for and
found the second source of air at high points in the "A" train of the nuclear
service water discharge header (Detail A-A, Figure 2).  Under certain
operating conditions, these additional sources of air could have affected the
"A" auxiliary feedwater pumps and the turbine-driven auxiliary feedwater pumps
on both units.

The auxiliary feedwater line from the "A" train on each unit is attached to
the discharge line of the emergency diesel cooling heat exchanger, near the
point where this line discharges into the main "A" train service water
discharge header.  The cooling water from the diesel generator heat exchanger
flows at the rate of 900 gpm.  With the diesel running and only one auxiliary
feedwater pump drawing water from the diesel line, sufficient diesel coolant
would be supplied to the feedwater pump suction without drawing water from the
main "A" train discharge line.  However, if the "A" diesel generator cooling
is lost, or if both a motor-driven and a turbine-driven auxiliary feedwater
pump are operated simultaneously, water could be drawn from the high point on
the main "A" train discharge line along with any air that might have collected
in this discharge line.  The "B" train on each unit was not affected because
it had a different piping configuration.


The events discussed in this information notice highlight the potential for
off-gassing in auxiliary feedwater system raw water sources.  According to
Henry's law, the solubility of a gas in water at a constant temperature is
approximately proportional to the partial pressure of the gas.  The solubility
at a constant pressure, however, generally decreases with increasing
temperature.  The solubility of air in water at atmospheric pressure, for
example, is reduced from 29 milliliter per liter at 0 C (32 F) to
16 milliliter per liter at 30 C (86 F).  Raw water that has become saturated
with air in the outside environment is not likely to emit gas in the service
water system supply piping because the pressure of the water has been
increased by pumping while the temperature of the water remains near its
original value.  As this water is heated by the various plant systems, and as
the pressure of the water decreases to near atmospheric levels in the
discharge piping, some of the air can be expected to evolve from the water and
collect above any elevated, slow moving water volumes in the discharge piping.

                                                      IN 93-12
                                                      February 11, 1993
                                                      Page 3 of 3

Upon discovering air in the auxiliary feedwater supply line for the Unit 1
standby shutdown system, the licensee declared all three of the auxiliary feed
water pumps to be inoperable.  The licensee concluded that these pumps had
technically been inoperable since initial plant operation.  The licensee
isolated the auxiliary feedwater line by closing one of the isolation valves
in this line and removing the power supply to this valve.  Although this
action made the standby shutdown line inoperable, the action was initially
considered to be sufficient to return the rest of the auxiliary feedwater
system to operability.  However, after discovering the second source of air in
the high points of the "A" nuclear service water train discharge lines of both
units, the licensee isolated the turbine-driven pumps of both units from the
"A" service water lines and declared the turbine-driven pumps to be
inoperable.  This action returned the "A" motor driven pumps to operability,
because these pumps alone could not draw sufficient water to ingest air from
the main nuclear service water discharge lines.

The licensee considered several options to correct the problem permanently. 
One possible solution was to reroute the safety-grade auxiliary feedwater
lines to take suction from the nuclear service water supply piping.  A second
possible solution was to reroute the suction of the auxiliary feedwater lines
to sections of the nuclear service water discharge piping that were not
susceptible to the formation of air pockets.  However, the licensee chose a
third alternative, which was to install permanent vent valves at the high
points where air was accumulating (Valves 1RN1060, 1RN835 and 2RN815,
Figure 2).  The licensee piped these vent valves to drains and throttled the
valves to provide a continuous drainage flow that was sufficient to vent any
air that might accumulate in the emergency feedwater supply.

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.

                                      ORIGINAL SIGNED BY

                                   Brian K. Grimes, Director
                                   Division of Operating Reactor Support
                                   Office of Nuclear Reactor Regulation

Technical contacts:  P. Kim Vandoorn, RII
                     (704) 875-1681

                     Don Kirkpatrick, NRR
                     (301) 504-1849

1.  Figure 1
2.  Figure 2
3.  List of Recently Issued NRC Information Notices
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