Information Notice No. 84-74: Isolation of Reactor Coolant System from Low-pressure Systems Outside Containment
SSINS No.: 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
September 28, 1984
Information Notice No. 84-74: ISOLATION OF REACTOR COOLANT SYSTEM FROM
LOW-PRESSURE SYSTEMS OUTSIDE CONTAINMENT
All nuclear power reactor facilities holding an operating license (OL) or a
construction permit (CP).
This information notice is provided as a notification of potentially
significant problems in maintaining isolation boundaries between the
high-pressure reactor coolant system (RCS) and low-pressure piping systems
outside containment. These problems contribute to an increased likelihood of
an intersystem loss-of-coolant accident (LOCA) which would bypass primary
It is expected that recipients will review the information for applicability
to their facilities and consider actions, if appropriate, to preclude
similar problems occurring at their facilities. However, suggestions
contained in this information notice do not constitute NRC requirements and,
therefore, no specific action or written response is required.
Description of Circumstances:
At Pilgrim, on September 29, 1983, the low-pressure section of the high
pressure coolant injection (HPCI) suction piping was overpressurized during
functional testing of the HPCI system logic. The cause was personnel error
while conducting more than one surveillance test at the same time. The
prerequisites and initial conditions were not met for all steps. This led to
the simultaneous opening of two motor-operated HPCI discharge valves.
A testable injection check valve should have isolated the HPCI from the RCS.
However, the movable internals of this valve were bound by rust. Apparently,
this condition held the valve partially open during normal operation, but
did not prevent closing when pressure was applied. As a result, a sudden but
brief overpressurization of the HPCI piping occurred.
The Pilgrim check valve was repaired and tested. The licensee held a
critique with operators and instrumentation and control technicians and
initiated administrative actions to ensure strict compliance with
surveillance procedure action steps.
September 28, 1984
Page 2 of 4
Hatch Unit 2
At Hatch Unit 2 in October 1983, it was found that a testable check valve
had been held open for about 4 months by an incorrectly assembled actuator.
This is a swing-type testable check valve with an air actuator controlled by
a four-way pilot solenoid valve. It is installed on a 24-inch low pressure
coolant injection (LPCI) line. The second isolation device on this line is a
normally closed motor-operated gate valve. The gate valve automatically
receives a signal to open upon a LPCI actuation signal but has independent
diverse interlocks to prevent opening at high differential pressure.
The Hatch event resulted from a series of errors. On June 7, 1983, during
maintenance on the valve actuator, the two air supply lines were installed
backwards. The air supply line to the right-hand cylinder of the actuator
was incorrectly connected to the left-hand cylinder, and vice versa. Failure
to use a vendor maintenance manual appears to have contributed to this
error. Inadequate post-maintenance functional testing of the valve allowed
the initial error to go undetected. The check valve position is indicated in
the control room. It is not known with certainty why this did not lead to
early detection. However, it appears likely that, after maintenance, the
indication was readjusted to show a closed position in the belief that the
check valve must actually be closed.
A Hatch plant maintenance worker was counseled by utility management on the
importance of performing correct maintenance and the importance of using
maintenance manuals and performing thorough post maintenance testing before
returning components to service, particularly for components that are
safety-related. For the longer term, the licensee is considering alternative
methods of testing the check valve using shutdown cooling flow. This could
allow permanently deactivating the actuator without interfering with check
valve operability or position indication.
Browns Ferry Unit 1
At Browns Ferry Unit 1, on August 14, 1984, the core spray system was
overpressurized. In December 1983 or earlier, during maintenance on the
pilot solenoid valve for the testable check valve, a plunger with reversed
air ports was apparently installed in the solenoid valve. This resulted in
the check valve being held open. Then in August, while performing a
semi-annual logic functional test, the operators failed to electrically
disarm the motor-operated injection valve. The motor-operated valve opened
with the testable check valve open. The core spray system, designed for 500
psi, was pressurized above 500 psi, lifting the small relief valve installed
on the line. The maximum core spray system pressure is not known. Operators
observed that the control room pressure gauge read off-scale (above 500 psi)
and the pressure might have approached primary coolant system pressure
(about 1050 psi).
The piping was not damaged, probably because of substantial design margins.
The pump discharge check valves in combination with open pump suction valves
apparently prevented overpressurizing the pump suction piping which is
designed for 150 psi.
September 28, 1984
Page 3 of 4
The Browns Ferry occurrence was caused by errors similar to those in the
Hatch event. The solenoid valve had been reassembled incorrectly and without
using a maintenance manual. The proximity switch and the air actuator had
been readjusted providing an erroneous closed position indication.
Post-maintenance testing was inadequate. Finally, during the logic
functional test, there was a failure to properly follow the test procedure
and to disarm the outboard motor-operated valve. The licensee's final
corrective actions are yet to be determined.
The events described above are considered to be significant because they
substantially reduced safety margins for preventing an intersystem LOCA that
bypasses containment. When the testable check valve is open, a postulated
failure or inadvertent opening of, the motor-operated valve could allow
discharge of high-pressure reactor coolant into low-pressure systems.
The consequences of such an event are not certain. The flowrate through the
motor-operated valve could vary from a small amount of leakage to a massive
discharge. If the flow force were moderate, it could close the check valve
despite the actuator. This would effectively terminate the event. If,
however, the forces were large, the movable internal portions of the check
valve could be severely damaged. A substantial failure of the low-pressure
system, if it were to occur, would lead to a LOCA that bypasses the
containment and could flood the low-pressure ECCS pumps. This would be an
accident exceeding current design basis with radioactive material discharged
outside the primary containment.
Other plants were thought to have a valve configuration similar to that of
Hatch. Following the Hatch event, the NRC's Office of Analysis and
Evaluation of Operational Data (AEOD) prepared Engineering Evaluation Report
E414, "Stuck Open Isolation Check Valve on the Residual Heat Removal System
at Hatch Unit 2," on May 31, 1984. This report confirmed that a number of
BWRs have a similar residual heat removal (RHR) system configuration (i.e.,
a testable check valve inside primary containment and a motor-operated
injection valve outside primary containment). Additional plants found with
this configuration include Duane Arnold, Brunswick 1 and 2, Cooper, Dresden
2 and 3, Hatch 1, FitzPatrick, Monticello, Peach Bottom 2 and 3, Pilgrim,
and Quad Cities 1 and 2.
In the Pilgrim and Browns Ferry events, the low-pressure section of the HPCI
suction piping and one loop of the core spray system, respectively, were
actually overpressurized. The potential for an accident was increased by
degradation of the barriers.
It is suggested that licensees of nuclear reactor facilities consider
reviewing their practices and controls in the area of maintenance activities
involving air-operated testable check valves, especially where such valves
provide isolation barriers for the reactor coolant system. It is also
suggested that licensees consider reviewing their practices and controls to
ensure that instrument and logic tests do not permit the inadvertent opening
of motor-operated valves, especially where these valves also provide
isolation barriers for the reactor coolant system.
September 28, 1984
Page 4 of 4
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 NRC Regional Office or this
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contacts: M. S. Wegner, IE
S. Newberry, NRR
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