Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
February 25, 1992
NRC INFORMATION NOTICE 92-16: LOSS OF FLOW FROM THE RESIDUAL HEAT REMOVAL
PUMP DURING REFUELING CAVITY DRAINDOWN
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a recent event involving the loss of flow from
the residual heat removal pump during refueling cavity draindown. 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 are not
NRC requirements; therefore, no specific action or written response is
required.
Description of Circumstances
On October 26, 1991, the Vogtle Electric Generating Plant, Unit 1, was in
Mode 6 (Refueling) with the reactor vessel head removed. The Georgia Power
Company (the licensee) had reloaded the core and reinstalled the upper
internals. The licensee was using the 1B residual heat removal (RHR) pump
to provide shutdown cooling and the 1A RHR pump to drain the refueling
cavity by taking suction from one of the reactor coolant system (RCS) hot
legs and discharging to the refueling water storage tank (RWST). The RCS
temperature was approximately 87øF. The water level in the refueling cavity
was at 210 feet 4 inches. Operations personnel were preparing to lower the
level to 192 feet, 2 feet below the reactor vessel head flange, to allow the
reactor vessel head to be reinstalled. The mid-loop elevation of the RCS
for Unit 1 is 187 feet. An assistant plant operator (APO) in the Unit 1
containment was directed to establish a watch at a tygon tube to monitor the
RCS level during draindown and mid-loop operations. During the outage, the
licensee had installed a permanent sight glass in the Unit 1 containment for
monitoring the RCS level. This new sight glass had neither been tested nor
aligned for the operators to use. The APO assumed that the new sight glass
was operable and established communications with the control room at the
permanent sight glass, rather than at the tygon tube, to monitor the
draindown. The licensee then started the draindown.
When the day shift ended, a night shift plant equipment operator (PEO)
relieved the day shift APO who was monitoring the permanent sight glass.
The PEO discovered that the valves for the permanent sight glass were not
aligned correctly. The PEO informed the control room and the operators
stopped the
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IN 92-16
February 25, 1992
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draindown while the problem was investigated. The PEO and APO then filled
and vented the sight glass without using a procedure. In their attempt to
place the permanent sight glass in service, the upper isolation valve, which
was not readily visible, was not opened as required.
The licensee resumed the cavity draindown and, approximately 2 hours later,
received a control room annunciator which indicated a high level, 192 feet 6
inches, in the reactor vessel. The control room operator observed that the
control room level indicator was at the top of scale (100 percent) and
tapped on the indicator, causing it to drop to a reading of 60 percent (190
feet 9 inches). The licensee again stopped the draindown. The PEO
monitoring the sight glass level reported that reactor vessel water level
appeared to be even with the reactor vessel head flange (194 feet), which
agreed with the level indicated by the permanent sight glass and the
temporary tygon tube. The licensee assumed that the control room level
indicator was inaccurate and continued the draindown, believing that it had
three reliable indications of the RCS level, i.e., visual vessel water
level, the permanent sight glass, and the temporary tygon tube.
When the level in the RCS reached approximately 193 feet, as indicated by
the sight glass, a control room operator observed discharge pressure, flow,
and motor current oscillations for the 1B RHR pump, indicating that the
coolant was forming a vortex on the suction side of the pump or that the
pump was cavitating. The operators closed the discharge valve for the
1B RHR pump, thus putting the 1B RHR pump on the miniflow line. Although
the electrical current reading for the motor of the 1B RHR pump became more
stable, the discharge pressure remained low.
The licensee again stopped the draindown by shutting down the 1A RHR pump
and realigning its suction to the refueling water storage tank (RWST) to
refill the refueling cavity. Shortly after beginning to refill the RCS, the
licensee noted that the discharge pressure of the 1B RHR pump began to
improve. When the flow of the 1B RHR pump reached approximately 2600
gallons per minute, the licensee again observed indications of vortex
formation or cavitation. The licensee reduced the flow from the 1B RHR pump
to 1800 gallons per minute and found that the pump operated satisfactorily
with no indication of vortex formation or cavitation. The licensee used the
1A RHR pump to refill the refueling cavity from the RWST and stopped
refilling when the sight glass indicated a level of 194 feet 10 inches. The
licensee increased the flow from the 1B RHR pump to approximately
3000 gallons per minute and found that the pump operated satisfactorily with
no further indication of vortex formation or cavitation.
When operators performed a walkdown inspection of the tygon tube and the
sight glass level indicators, they found the upper isolation valve for the
sight glass closed with a tag on it which indicated that the new sight glass
had not been released for use. The licensee later determined that a similar
tag had also been installed on the lower isolation valve but apparently had
fallen off the valve.
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IN 92-16
February 25, 1992
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The licensee also discovered that a high efficiency particulate absorber
(HEPA) filter unit was connected, by means of a flexible duct, to the
opening from which a pressurizer safety valve had been removed to provide a
vent path for all level instrumentation. The licensee found that the HEPA
unit was running and the flexible duct was collapsed, apparently caused by
the vacuum created by the running HEPA filter unit and the RCS draindown.
This resulted in an inadequate vent path from the pressurizer. (LER
50-424/91-09 and NRC Inspection Report 50-424,425/91-30)
Discussion
False high RCS level indications led to the RCS level being inadvertently
lowered to the point at which the coolant formed a vortex in the RHR pump
suction line. The false high level indications were caused by an inadequate
vent path from the pressurizer and by the closed upper isolation valve for
the sight glass. When conditions in the pressurizer changed, it affected
all of the reactor vessel level instruments, because their reference legs
connected to the pressurizer. The system installed at Vogtle did not meet
the intent of two independent continuous water level indications as
discussed in Generic Letter 88-17, "Loss of Decay Heat Removal."
Procedures for the initial RCS draindown during refueling operations
provided sufficient steps to ensure that the level instrumentation was
installed properly and the vent paths were adequate. However, the
procedures for the subsequent draindowns did not include sufficient steps to
reverify these actions. Administrative controls were inadequate in
addressing the reviews and documents required for attaching HEPA filter
units to plant equipment. In this case, the HEPA filter unit was installed
without a temporary modification or a work order, and consequently the
control room was not aware of the installation.
During the event, the 1B RHR pump was not available to provide recirculation
shutdown cooling for approximately 16 minutes. Core temperature as
indicated at the RHR pump discharge increased from approximately 87øF to
107øF. There was no radiological release to the environment. The licensee
reviewed available data further and found that the coolant on the suction
side of the 1B RHR pump had formed a vortex but the pump did not cavitate.
Air may have begun entering the 1A RHR pump shortly before the pump's
discharge valve was closed. This resulted in a slightly reduced discharge
pressure and flow. The coolant in the RCS reached the lowest level, 186 to
187 feet, when the discharge valve for the 1A RHR pump's heat exchanger was
closed. After the event, the licensee performed an inservice test on the 1A
and 1B RHR pumps and found that the performance of neither pump was
degraded.
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February 25, 1992
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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
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts: Doug Starkey, Region II
(404) 554-9901
Pierce Skinner, Region II
(404) 331-6299
Attachment: List of Recently Issued NRC Information Notices
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