United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 97-38: Level-Sensing System Initiates Common-Mode Failure of High-Pressure-Injection Pumps

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                         WASHINGTON, D.C. 20555-0001

                                June 24, 1997


NRC INFORMATION NOTICE 97-38:  LEVEL-SENSING SYSTEM INITIATES COMMON-MODE  
                               FAILURE OF HIGH-PRESSURE-INJECTION PUMPS 


Addressees

All holders of operating licenses or construction permits for nuclear
power reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a recent incident in which two high-
pressure-injection (HPI) pumps were damaged as a result of an inaccurate
letdown storage tank (LDST) level-sensing system.  The inaccuracy was
caused by a drained reference leg.  As a result, an incorrect level was
displayed, permitting the pumps to take suction from an empty tank.  It is
expected that recipients will review this information for applicability to
their facilities and consider actions, as appropriate.  However,
suggestions contained in this information notice are not NRC requirements;
therefore, no specific action or written response is required.

Description of Circumstances

On May 3, 1997, the licensee for Oconee Unit 3 was conducting a planned
shutdown to inspect an HPI line.  During cooldown of the plant, HPI pump
3B was operating in the reactor coolant system (RCS) makeup mode.  The
pump's discharge pressure dropped to the "low" pressure setpoint
initiating the automatic start of standby HPI pump 3A.  The operators
later secured pump 3A when reactor coolant pump (RCP) seal injection flow
indicated "high".  The HPI header pressure decreased again.  HPI pump 3A
automatically restarted.  The pump's motor current began oscillating.  The
operators secured HPI pump 3B because of an indication of low motor
current.  Shortly afterwards, HPI pump 3A was also secured when its motor
current decreased sharply.  The licensee issued a Notification of an
Unusual Event because of the loss of two of the three HPI pumps.

The licensee later concluded that both HPI pumps became hydrogen bound and
possibly damaged when they took suction from an empty LDST even though
adequate tank inventory was indicated.  

 
9706200203.                                                           IN 97-38
                                                           June 24, 1997
                                                           Page 2 of 3

Discussion
 
The HPI pumps at Oconee perform the dual functions of RCS makeup and
high-pressure safety injection.  These pumps normally take suction from
the LDST (also referred to as the makeup or volume control tank by other
vendors).  A 25-psi (172.4 kPa) hydrogen overpressure is maintained in
this tank to scavenge oxygen from the RCS.  

During this event, two level transmitters monitored tank level.  These
transmitters produce level alarms in the control room.  The alarms alert
operators to initiate makeup to the LDST.  Both level transmitters shared
a common reference leg.  Because the shared reference leg was partially
drained, the indicated letdown tank level remained at 9 inches above the
low-level alarm setpoint.  The tank was actually empty, its contents
having been depleted during normal charging.  Letdown tank level was
investigated only after both HPI pumps developed problems.  

Indicated level in the control room was derived from the equivalent
pressure difference between the drained reference leg and the back
pressure from the 25 psi hydrogen overpressure in the LDST.  Because both
level instruments shared the common reference leg, both gave the same
erroneous indication.  Therefore, operators failed to provide makeup
inventory to the LDST.  Two of the three HPI pumps became gas-bound,
cavitated, and, as a result, were structurally damaged. 

If an actual safety injection actuation occurs, the suction isolation
valve to the LDST remains open while the suction isolation valve to the
borated water storage tank (BWST) opens on the safety injection signal. 
All three HPI pumps receive a start signal and take suction on both the
LDST and the BWST.  With the two tanks cross-tied, the potential existed
for this event to have caused the loss of all HPI pumps as a result of gas
binding.  Loss of all three HPI pumps could prevent the successful
mitigation of a loss of coolant accident.  During normal operations, loss
of all HPI pumps would result in the loss of reactor coolant pump seal
injection, normal RCS makeup, and normal boration.     

The level transmitters were last calibrated in February 1997.  Sometime
between that last calibration and the event, a leak developed from a
scored cap on a test connection that drained the reference leg.  The cap
was used to isolate the test connection.  A drained reference leg or a
reference leg with entrained air can result in incorrect level indication. 
Incorrect level indication will prevent appropriate setpoints being
reached when process conditions demand, defeat critical automatic safety
functions, and could mislead plant operators into taking less than
conservative actions.  

Pressure and level sensors, in any safety-related application, that rely
on a reference leg for a differential pressure reading, are sensitive to
changes in reference leg level.  Normal surveillances that include only
channel checks with other instruments sharing the reference leg or
transmitter calibrations with external hydrostatic test sets will not
reveal the true condition of the reference leg.  In the absence of an
automatic reference leg fill system, periodic verification of reference
leg level based on the plant-specific environment is needed to ensure the
operational integrity of the critical systems that are relied upon for
manual and automatic functions. .                                                           IN 97-38
                                                           June 24, 1997
                                                           Page 3 of 3


This information notice requires no specific action or written response. 
If you have any questions about information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.



                                         signed by S.H. Weiss for

                                   Marylee M. Slosson, Acting Director
                                   Division of Reactor Program Management
                                   Office of Nuclear Reactor Regulation


Technical contacts:  Thomas Koshy, NRR
                     301-415-1176 
                     E-mail:  txk@nrc.gov

                     Nick Fields, NRR
                     301-415-1173
                     E-mail:  enf@nrc.gov
Page Last Reviewed/Updated Tuesday, December 03, 2013