United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 87-46: Information Notice No. 87-46: Undetected Loss of Reactor Coolant

                                                   SSINS No.:  6835
                                                      IN 87-46

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

                               September 30, 1987

Information Notice No. 87-46:  UNDETECTED LOSS OF REACTOR COOLANT 


All pressurized water reactor facilities holding an operating license or a 
construction permit.   


This information notice is provided to alert recipients to potentially serious 
operational errors that may result in the inadvertent loss of reactor coolant 
system (RCS) inventory during abnormal system conditions.  It is expected that 
recipients will review this information for applicability to their reactor 
facilities and consider actions, if appropriate, to prevent similar problems.  
Suggestions contained in this notice do not constitute NRC requirements; 
therefore, no specific action or written response is required.

Description of Circumstances:

On June 21, 1987, the North Anna Unit 1 operators discovered that 
approximately 17,000 gallons of reactor coolant had been lost from the RCS 
while the unit was in cold shutdown.  The delay in discovering the inventory 
loss resulted from (1) the use of pressurizer level as an indication of 
reactor coolant inventory, (2) failure to use all available indications, (3) 
and failure to perform a mass inventory balance.

On June 17, 1987, while making preparations to start up North Anna Unit 1 
following a refueling outage, a problem developed with a reactor coolant pump 
motor, requiring removal of the motor.  When the problem was discovered, the 
unit was at approximately 195�F, 325 psig, with a bubble in the pressurizer.  
In order to establish plant conditions for removal of the motor (which may 
involve leakage from the RCS), the plant would normally have been (1) cooled 
to less than 140�F, (2) drained to mid-nozzle, and (3) placed on the residual 
heat removal system.  In order to expedite the work, the plant was cooled to 
110�F, and the pressurizer was cooled by filling the pressurizer while venting 
via the power-operated relief valves (PORVs).  The pressurizer level was 
lowered to 80% with the PORVs open.  The PORVs were then shut because the 
vapor space temperature led the operators to believe a bubble still existed, 
and the level was further lowered to 20%.  This was done using a procedure 
that was not specifically intended for draining the system.  The operators did 
not realize that lowering the level with the PORVs shut -- and subsequently 
cooling the pressurizer -- caused a vacuum to form in the pressurizer, 
essentially holding the pressurizer level at 20%.

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                                                            Page 2 of 3

On June 18, 1987, the pump motor was uncoupled and a small amount of expected 
leakage (estimated 2 gallons per minute) up the pump shaft was encountered.  
This leakage was relatively clean water from the seal injection line past the 
pump seals, which did not provide a tight seal when the motor was uncoupled.  
Makeup to the RCS was from the volume control tank (VCT).  The VCT level was 
maintained with VCT pressure greater than reactor pressure.  The operators 
believed that maintaining the pressurizer level and VCT level would maintain 
the reactor coolant inventory by making up for any losses by flow from the VCT 
to the RCS.  Voids consisting of non-condensable gases and vapor formed in the 
RCS and collected in the system high points (reactor vessel head and steam 
generator tubes).  The voids were not indicated by any decrease of the 
pressurizer level.

On June 21, 1987, a decision was made to reduce the pump shaft leakage by 
raising the pressurizer level, cycling the PORVs to vent the pressure, and, 
then lowering the pressurizer level to draw a slight vacuum in the 
pressurizer.  This was a condition that already existed, but the operators 
were unaware of it.  When the PORVs were cycled, the pressurizer relief tank 
pressure dropped, as well as the pressurizer level, indicating that a vacuum 
already existed in the pressurizer.  The reactor vessel level indicating 
system (RVLIS) indication at this time was 79%; however, the operators were 
not monitoring this indication because the system had been modified during the 
previous outage and the operators thought it unreliable.  Because of the 
recorder scale and the time span visible on the RVLIS trend recorder, the 
change in the level indication would only have been noticed (1) with a 
separate level plot or (2) by rolling the chart back 12 to 24 hours to compare 
it with the present indication.  When the condition was discovered, the 
operators took action to make up to the RCS and vent the reactor vessel head, 
as well as to check other available information to account for the system 
inventory.  A total of 17,000 gallons of borated water was required to 
reestablish the RCS inventory.  


The principal error associated with this event is that the operator used only 
pressurizer level as an indication of RCS inventory.  The operator relied on 
this single indication of inventory for an extended period with a known but 
unquantified leak from the RCS, rather than using other available indications 
to confirm reactor inventory.  

The RVLIS level was available but was not used, as discussed previously.  This 
system was not required to be operable while the reactor was shut down.  
Recorded RVLIS readings in the recorder viewing window did not show a downward 
trend in vessel level until the paper was removed from the recorder and evalu-
ated for an extended period, because the level decrease was very gradual.  
Also, several work request stickers on the RVLIS display had not been removed 
following the modifications made during the outage.  In addition, an inventory 
balance was not maintained to show how much water was being collected from the 
leak compared with the amount of water being added to the system.  
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                                                            Page 3 of 3

The plant procedures were not adequate for monitoring the reactor coolant 
inventory.  The procedure used to establish plant conditions for removing the 
motor did not contain appropriate instructions for monitoring and maintaining 
RCS inventory.  

The licensee is changing the procedure to require (1) a review of reactor 
coolant system inventory and (2) routine surveillance of all available level 
indication, including RVLIS.  

No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the Regional 
Administrator of the appropriate regional office or this office.

                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation

Technical Contacts:  R. Croteau, RII
                     (404) 242-4668

                     Sam MacKay, NRR
                     (301) 492-8394

Attachment:  List of Recently Issued NRC Information Notices

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