Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 87-46
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
September 30, 1987
Information Notice No. 87-46: UNDETECTED LOSS OF REACTOR COOLANT
Addressees:
All pressurized water reactor facilities holding an operating license or a
construction permit.
Purpose:
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 195øF, 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 140øF, (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
110øF, 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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September 30, 1987
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.
Discussion:
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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September 30, 1987
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
