United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 95-03: Loss of Reactor Coolant Inventory and Potential Loss of Emergency Mitigation Functions While in a Shutdown Condition

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                         WASHINGTON, D.C.  20555-0001
                                       
                               January 18, 1995


NRC INFORMATION NOTICE 95-03:  LOSS OF REACTOR COOLANT INVENTORY AND POTENTIAL 
                               LOSS OF EMERGENCY MITIGATION FUNCTIONS WHILE IN 
                               A SHUTDOWN CONDITION


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 (IN) to alert addressees to a recent incident involving a loss of
reactor coolant inventory while in a shutdown condition at a Westinghouse
pressurized-water reactor.  The incident is unique because the initiating
event has the potential to create an accident and cause a loss of accident
mitigation capability.  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 notice do not constitute NRC requirements; therefore, no specific action
or written response is required.

Background

NRC has issued a number of generic communications describing events at reactor
facilities involving inadvertent loss of reactor coolant inventory while the
facility was in a shutdown condition.  In Generic Letter 88-17, "Loss of Decay
Heat Removal (DHR)," the staff requested several actions to address loss of
DHR events that occurred while the reactor was in a shutdown condition.  In
two information notices (IN 90-55, "Recent Operating Experience on Loss of
Reactor Coolant Inventory While in a Shutdown Condition," and IN 91-42, "Plant
Outage Events Involving Poor Coordination Between Operations and Maintenance
Personnel During Valve Testing and Manipulations"), the staff discussed
inadvertent loss of inventory events.  A document issued by the NRC Office for
Analysis and Evaluation of Operational Data (AEOD/E704), "Discharge of Primary
Coolant Outside of Containment at PWRs While on RHR Cooling," reported six
additional events having similar characteristics.  

This IN deals with the Wolf Creek draindown event of September 17, 1994.  A
similar event occurred at Braidwood in 1990.  Both events involved operators
inadvertently transferring more than 9000 gallons of reactor coolant system
(RCS) inventory to the refueling water storage tank (RWST) while preparing to
shift operation of the residual heat removal (RHR) trains.  However, the Wolf
Creek event occurred when the RCS was pressurized to 340 psi at a temperature


9501110412.                                                            IN 95-03
                                                            January 18, 1995
                                                            Page 2 of 4


of 300 �F.  As a result, more than 9000 gallons of RCS inventory was drained
from the RCS to the RWST in about 1 minute.  

The Wolf Creek design incorporates a 24 inch header pipe from the RWST from
which the RHR pumps, the charging pumps, the high-head injection pumps, and
the containment spray pumps take suction.  If an RCS to RWST fluid transfer
filled the 24 inch header with steam and pump operation were attempted, the
pump could have been severely damaged and lost as a means of supplying RCS
makeup and core cooling.  In addition, such steam would create conditions
favoring water hammer, which could be destructive to involved components. 

Description of Circumstances

On September 17, 1994, the Wolf Creek Generating Station experienced a loss of
reactor coolant inventory while in a shutdown condition when operators
performed two incompatible activities concurrently.  Preceding the event,
operators were controlling the reactor coolant system in Mode 4 (hot shutdown)
at approximately 300 �F and 340 psig.  Operations personnel found that during
the latter part of the cycle leaking check valves had diluted the boron
concentration in the RHR train B piping.  Licensee procedures require that the
water in the piping be reborated before the RHR B train is put into operation
in this circumstance.  This was routinely done by recirculating the RHR piping
water through the RWST using a containment spray pump.

Maintenance personnel were repairing a packing leak and performing valve
motor-operator diagnostic testing on the train A RHR discharge crossover
isolation valve.  The shift supervisor decided that it would be acceptable to
stroke the train A discharge crossover isolation valve provided that the RHR
train B discharge crossover isolation valve and the RHR crossover return to
the RWST manual isolation valve remained shut.  To continue the cooldown to
Mode 5, operators began preparations to start RHR train B so that reactor
coolant pumps could be secured.  While personnel continued the repair and
valve motor-operator diagnostics on the train A crossover isolation valve, an
auxiliary operator opened the RHR crossover return to the RWST manual
isolation valve for the reboration.  When both valves were opened, the reactor
coolant system had a draindown flowpath through RHR train A into the 24-inch
pipe that leads from the RWST.  After approximately 1 minute, operators
recognized an unintended flowpath and shut the train A discharge crossover
isolation valve to terminate the draindown. The event transferred
approximately 9200 gallons of RCS water to the RWST, depressurized the RCS to
approximately 225 psig, and allowed the RCS temperature to increase by
approximately 7 �F.

Discussion

The Office for Analysis and Evaluation of Operational Data (AEOD) reviewed
this event at the Wolf Creek site from November 7 through 10, 1994, and plans
to issue a report.  The following discussion is, in part, based on the AEOD
review and, in part, on the Office of Nuclear Reactor Regulation (NRR) and
regional staff view of the potential safety implications.
.                                                            IN 95-03
                                                            January 18, 1995
                                                            Page 3 of 4


Two incompatible activities were performed concurrently, causing this event:
(1) alignment of the RHR train B crossover isolation valve (to adjust the
boron concentration) required the valve to be opened and (2) motor-operator
diagnostic testing required the train A RHR discharge crossover isolation
valve to be stroked open and closed.  These two activities inadvertently
created the flow path for the draindown.  The failure of several individuals,
including the reactor operator, supervising operator, and shift supervisor, to
recognize that these activities were incompatible resulted in a loss of
control of plant configuration and directly caused the draindown event.

The shift supervisor initially recognized the potential for diverting RHR flow
from the RCS to the RWST; however, the shift supervisor failed to establish a
positive barrier, such as tagging or padlocking, to ensure that the manual
crossover return to the RWST manual isolation valve remained shut.  Repair and
diagnostic testing of the train A discharge crossover isolation valve
represented work on the only available train of a safety system.  These
decisions permitted work on a safety system required for safe operation of the
plant without proper controls in place to prevent an inappropriate system
configuration. 

In recognition of additional challenges to plant operators during outage
conditions, the licensee had established an outage emergent work process to
evaluate unscheduled work.  This process was intended to relieve some of the
additional burden that might distract the operators from properly monitoring
the safe condition of the plant.  It was also intended to provide additional
assurance that potential adverse impacts on plant operation were fully
considered.  However, this emergent work process was not used to evaluate the
motor-operated valve work on RHR train A.

The presence of the deborated water in the RHR train B piping, the attempt to
reborate, and the concurrent repair and testing of the RHR train A crossover
isolation valve raises the following concerns:

1.   There is a possibility of introducing hot RCS water into the common    
     24-inch suction header supplied from the RWST.  The RHR pumps, the
     charging pumps, the high-head injection pumps, and the containment spray
     pumps take suction from this header.  Introducing hot RCS water into this
     header could create steam conditions and interfere with RHR pump
     operation.  If the RHR pump were lost, alignment of the other pumps to
     this header as a response to the loss of RHR cooling could have also
     interfered with the operation of these other pumps.

2.   There is the potential that if the draindown continued, the RHR piping
     could become filled with steam.  This would create conditions favoring
     water hammer, which could damage valves, pumps, piping, or pipe supports. 
     The RWST supply to the ECCS and containment spray could be jeopardized.
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                                                            January 18, 1995
                                                            Page 4 of 4


3.   There was the possibility for inadvertent injection of relatively cold,
     lightly borated water from the RHR train B piping into the RCS.  In the
     hot shutdown condition (Mode 4), where the reactor is being borated as it
     cools down, introduction of cold, lightly borated water reduces the
     margin to criticality.  The licensee determined that the injection of the
     water from the RHR train B piping into the RCS would not have brought the
     reactor to a critical state.  However, the potential for criticality may
     exist at other facilities under similar conditions. 

The Wolf Creek draindown event has been classified by the NRC staff as a
significant event for the NRC Performance Indicator Program.


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 project manager.


                                    Signed by

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

Technical contacts:  J. Frederick Ringwald, RIV
                     (316) 364-8653

                     Lambros Lois, NRR
                     (301) 504-3233

Attachment:  
List of Recently Issued NRC Information Notices

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