Information Notice No. 94-29: Charging Pump Trip During a Loss-Of-Coolant Event Caused by Low Suction Pressure

UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C.  20555

April 11, 1994


NRC INFORMATION NOTICE 94-29:  CHARGING PUMP TRIP DURING A LOSS-OF-COOLANT
EVENT CAUSED BY LOW SUCTION PRESSURE


Addressees

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

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the possibility of a charging pump trip caused
by inadequate suction pressure because too many pumps may be aligned to a
single suction path during accident mitigation.  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 are not NRC requirements;
therefore, no specific action or written response is required.

Description of Circumstances

During a steam generator tube rupture event at Palo Verde Unit 2 on March 14,
1993 (NRC Inspection Report 50-279/93-14), a charging pump tripped off on low
suction pressure when the operators switched the charging pump suction source
from the volume control tank to the refueling water tank.  During the
subsequent evaluation of the event, the licensee determined that four pumps,
three charging pumps and a boric acid makeup pump, were all taking suction
through the same 7.6-cm [3-in.] diameter pipe.  The licensee also determined
that the high rate of flow through the suction pipe that resulted from the
simultaneous operation of all four pumps caused a sufficient pressure
reduction at the suction of the pumps to lower this pressure below the low
suction trip setpoints of the charging pumps.

Two charging pumps were already running when the tube rupture occurred.  In
addition, the operators were running a boric acid pump to circulate the
refueling water through filters to purify it for an upcoming outage.  The
boric acid pump flow was being recirculated back to the refueling water tank
and did not contribute to the makeup water going into the reactor coolant
system.  The operators started the third charging pump early in the event in
an effort to restore the pressurizer level, which was decreasing because of
the tube rupture.  At that time all of the charging pumps were taking suction
from the volume control tank, which is the normal source of makeup water for
the reactor coolant system.  Approximately 30 minutes later, in accordance
with emergency procedures, the operators opened the suction valve from the


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refueling water tank and then closed the suction valve from the volume control
tank.

At this point one of the charging pumps tripped off.  The operators were able
to restore the tripped charging pump to operation by realigning the suction of
the charging pumps to other suction paths from the refueling water tank.

Discussion

Following this event the licensee calculated that, with the boric acid pump
running, the remaining flow capacity of the refueling water tank suction
source that was in use was just sufficient for two charging pumps.  With all
four pumps running the suction pressure was below the low suction trip set
points (81.0 kPa to 87.2 kPa [11.75 psia to 12.65 psia]) for the charging
pumps.  Because the trip set points are set at slightly different pressures,
only one pump tripped.  Once this first pump tripped, the resulting rise in
suction pressure allowed the remaining pumps to keep running.
The licensee concluded that, if recirculation by the boric acid pump is not in
progress, sufficient suction pressure exists for all three charging pumps.
Consequently, the licensee revised the emergency procedures that involve the
charging pumps to specify that any ongoing recirculation of the refueling
water by the boric acid pump is to be terminated when refueling water is
needed for emergency makeup.

Licensees of pressurized-water reactors use the charging systems in a number
of ways for accident mitigation, such as including the charging system as a
part of the automatic safety injection system.  Charging systems are also used
as a source of pressurizer spray to depressurize the reactor when the reactor
coolant pumps are shut down.  Emergency operating procedures often refer to
the charging systems for other uses as well.

Some licensees do not isolate the volume control tank (or its equivalent) when
the charging system source is switched to the refueling water tank (or its
equivalent), but depend on the static pressure in the refueling water tank to
keep the volume control tank from draining.  Should the volume control tank
empty in response to lower-than-expected suction pressure, hydrogen cover gas
normally contained in this tank could enter the charging system suction and
cause the pumps to malfunction..                                        IN
94-29                                         April 11, 1994
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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 Office of Nuclear
Reactor Regulation project manager.

                        /s/'d by CIGrimes


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

Technical contact:  Warren C. Lyon, NRR
              (301) 504-3892

Attachment:
List of Recently Issued NRC Information Notices