Information Notice No.90-55: Recent Operating Experience on Loss ofReactor Coolant Inventory While In a Shutdown Condition
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
AUGUST 31, 1990
Information Notice No. 90-55: RECENT OPERATING EXPERIENCE ON LOSS OF
REACTOR COOLANT INVENTORY WHILE IN A
All holders of operating licenses or construction permits for nuclear power
This information notice is being provided to alert addressees to recent
operating experience involving losses of reactor coolant inventory caused by
deficiencies in human performance while in a shutdown condition. 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 do not
constitute NRC requirements; therefore, no specific action or written
response is required.
A number of NRC generic communications have been written describing events
at operating reactor facilities involving inadvertent loss of reactor
coolant inventory while in shutdown conditions (e.g., IN 84-81, "Inadvertent
Reduction In Primary Coolant Inventory In Boiling Water Reactors During
Shutdown and Startup" and IN 86-74, "Reduction of Reactor Coolant Inventory
Because of Misalignment of RHR Valves"). IN 90-25, "Loss Of Vital AC Power
With Subsequent Reactor Coolant System Heat-up" describes a March 20, 1990
event at Vogtle Unit 1 in which a significant event occurred during shutdown
conditions in which human performance played an important role. Generic
Letter 88-17, "Loss Of Decay Heat Removal", stated that these types of
events indicated an apparent lack of a complete industry understanding of
the potential seriousness of such events with the reactor in a shutdown
Description of Circumstances:
The following are examples of recent events that were caused or initiated by
deficiencies in human performance with the reactor in a shutdown condition.
CATAWBA 1, 6/11/90
At Catawba Unit 1, on June 11, 1990, 5000 gallons of water from the
reactor coolant system (RCS) were transferred inadvertently to the
August 31, 1990
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refueling water storage tank (RWST). This coolant was transferred
because the reactor operator ordered isolation valves to be opened
while performing Residual Heat Removal (RHR) check valve testing before
ensuring that the manual isolation valve to the RWST was closed. The
loss of RCS inventory occurred over a 30 second period while a
motor-operated cross-tie RHR isolation valve was partially open.
MAINE YANKEE, 6/4/90
While performing the RCS fill and vent procedure with the reactor in
cold shutdown, the licensee inadvertently transferred 6000 gallons of
coolant to the RWST. The RCS was being pressurized with nitrogen for
venting of the reactor coolant pumps and loops. Pressurizer level was
being monitored by means of one cold- and two hot-calibrated level
instruments. As the RCS pressure increased, the hot- and
cold-calibrated level instruments were not in agreement. The operators
requested that a calibration be performed on the pressurizer level
instruments while continuing to fill the pressurizer. With the
hot-calibrated level instruments indicating 67 percent, the operators
decided to reduce the level to 25 percent by draining to the RWST.
Because the hot-calibrated level instruments were not reading
correctly, the actual RCS level was below the bottom of the pressurizer
for 3 hours.
BRAIDWOOD 2, 3/20/90
While performing activities for defueling with the reactor in cold
shutdown, the reactor operators inadvertently transferred about 9500
gallons of reactor coolant to the RWST while preparing to switch
operation of the RHR trains. RHR train B was in operation with train A
operating in the recirculation mode to the RWST. A licensed operator
inadvertently opened the RHR train A hot leg suction valve in
preparation for shifting from train B operation to train A operation.
Opening this valve established a flow path from the RCS to the RWST.
The operator immediately recognized the error and started to close the
hot leg suction valve. However, the valve takes 4 minutes to close.
About 9500 gallons of reactor coolant were transferred to the RWST. In
addition, this error allowed a rapid reduction in pressurizer level and
RCS pressure to occur, resulting in the pressurizer exceeding its
technical specification limit on the pressurizer cooldown rate
(200 degrees F/hour).
The events described above are examples where RCS inventory was
inadvertently reduced as a result of deficiencies in human performance with
the reactor in a shutdown condition. These types of events are of concern
because with the reactor in a shutdown condition, many pieces of ESF
equipment can be removed from service at the same time unusual valve lineups
are performed. Human performance then becomes a major factor in maintaining
safety and system configuration control during shutdown conditions.
August 31, 1990
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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 NRR project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: John Thompson, NRR
Attachment: List of Recently Issued NRC Information Notices
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