Information Notice No. 89-67: Loss of Residual Heat Removal Caused by Accumulator Nitrogen Injection
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
September 13, 1989
Information Notice No. 89-67: LOSS OF RESIDUAL HEAT REMOVAL CAUSED
BY ACCUMULATOR NITROGEN INJECTION
Addressees:
All holders of operating licenses or construction permits for pressurized
water reactors (PWRs).
Purpose:
This information notice is intended to alert addressees to potential problems
resulting from the loss of residual heat removal (RHR) caused by the injection
of nitrogen from an accumulator into the reactor coolant system (RCS). 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.
Description of Circumstances:
Salem Unit 1 lost both RHR pumps for about 50 minutes on May 20, 1989, as a
result of an injection of nitrogen from an accumulator into the RCS and, sub-
sequently, into the RHR system. This injection occurred while the licensee
was conducting full-flow testing of the accumulator check valves. The reactor
was in Mode 5 (cold shutdown) after a recent refueling with the reactor head
installed. The RCS was filled to a cold calibrated pressurizer level of
10 percent with air contained in the reactor vessel head and in the steam
generator U-tubes. All accumulators were filled to normal operating level and
were pressurized to approximately 600 psig.
At 9:25 a.m., while performing a post-maintenance full-flow test of the check
valves for accumulator 13, the accumulator isolation valve remained open for
about 70 seconds as a result of an operator error. During this time, approxi-
mately 1800 cubic feet of nitrogen at about 62 psig entered the RCS. As the
nitrogen expanded into the RCS, the pressurizer level went off-scale high, and
the reactor pressure rose from 14 psig to about 51 psig. The operator, not
realizing that nitrogen had been injected into the RCS, initiated pressurizer
level restoration by draining the RCS to the refueling water storage tank
(RWST). At 9:35 a.m., the operator observed zero RHR flow and a reduction of
the pump motor current from 44 amps to about 21 amps.
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September 13, 1989
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The operator, assuming that the pump was mechanically damaged, secured the
pump and started the second RHR pump. When this pump exhibited the same
characteristics as the other pump, the operator realized that the pumps were
gas bound and started venting the RHR system. Venting was slow because of the
size of the vent lines and an RHR system configuration that allowed air
entrapment. Operators also had difficulty in locating one vent and drain
valve; when it was located, an installed cap had to be removed. Slow venting
continued until 10:18 a.m. when the operators initiated filling of the RHR
system, using gravity feed from the RWST. At this time, the core exit
temperature had increased to 122 F from a pre-event value of 92 F. At 10:23
a.m., the RHR system was filled, and RHR pump 11 was successfully started. At
10:37 a.m., RHR pump 12 was placed in service, and RHR pump 11 was secured to
return the system to normal operation. Because of an inadequate abnormal
operating procedure and emergency classification guide, a 10 CFR 50.72 report
for this loss of RHR pumps was not made until May 22, 1989.
Discussion:
An assessment of this loss-of-RHR-pump event at Salem Unit 1 identified the
following concerns:
1. The operators appeared to believe that nitrogen injection into the RCS
was not possible during the full-flow test of the accumulator check
valves. This mindset caused the operators to drain the RCS when the
pressurizer level kept increasing due to the expansion of injected
nitrogen.
2. The abnormal operating procedures and the emergency classification guide
did not adequately address the potential for loss of RHR cooling. The
symptom-oriented procedures did not address the reactor parameters during
a loss of shutdown cooling while in Modes 5 and 6. Consequently, the
event was not reported immediately under 10 CFR 50.72.
3. The operators were not adequately trained to differentiate gas binding of
the RHR pumps from mechanical damage. This caused the operators to start
RHR pump 11 when they erroneously concluded that RHR pump 12 was mechani-
cally damaged.
4. Elevation differences in the RHR suction pipe, the difficulty in locating
one vent and drain valve, and the size of the vent lines caused the
venting operation to be extremely slow. In addition, complete system
venting was not possible because gas was trapped in the high points of
the RHR system.
5. The appropriateness of performing a full-flow test of the accumulator
check valves with fuel in the reactor vessel had not been fully assessed.
An event similar to the one described in this information notice would
not occur to this extent if this test had been conducted while the core
was off-loaded. Furthermore, if this test had been performed with either
reduced accumulator nitrogen pressure or with the reactor vessel head
removed, the effects of nitrogen injection into the RCS would have been
minimized.
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September 13, 1989
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It is important to note that if the accumulator check valve full-flow tests
had been conducted when they are more commonly performed, during the shutdown
prior to refueling, and an event similar to the one described in this
information notice had occurred, the consequences could have been much more
significant. It is also important to note that operator training provided in
response to NRC Generic Letter 88-17, "Loss of Decay Heat Removal," did assist
the operators in restoring the RHR system once gas binding of the system was
recognized.
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 NRR project
manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Daniel Prochnow, NRR
(301) 492-1166
Warren Lyon, NRR
(301) 492-0891
Attachment: List of Recently Issued NRC Information Notices
. Attachment
IN 89-67
September 13, 1989
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
______________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to_________
89-66 Qualification Life of 9/11/89 All holders of OLs
Solenoid Valves or CPs for nuclear
power reactors.
88-46, Licensee Report of 9/11/89 All holders of OLs
Supp. 4 Defective Refurbished or CPs for nuclear
Circuit Breakers power reactors.
89-65 Potential for Stress 9/8/89 All holders of OLs
Corrosion Cracking in or CPs for PWRs.
Steam Generator Tube
Plugs Supplied by
Babcock and Wilcox
89-64 Electrical Bus Bar Failures 9/7/89 All holders of OLs
or CPs for nuclear
power reactors.
89-63 Possible Submergence of 9/5/89 All holders of OLs
Electrical Circuits Located or CPs for nuclear
Above the Flood Level Because power reactors.
of Water Intrusion and Lack
of Drainage
89-62 Malfunction of Borg-Warner 8/31/89 All holders of OLs
Pressure Seal Bonnet Check or CPs for nuclear
Valves Caused By Vertical power reactors.
Misalignment of Disk
89-61 Failure of Borg-Warner Gate 8/30/89 All holders of OLs
Valves to Close Against or CPs for nuclear
Differential Pressure power reactors.
88-48, Licensee Report of Defective 8/22/89 All holders of OLs
Supp. 2 Refurbished Valves or CPs for nuclear
power reactors.
89-60 Maintenance of Teletherapy 8/18/89 All NRC Medical
Units Teletherapy
Licensees.
______________________________________________________________________________
OL = Operating License
CP = Construction Permit
..
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