Information Notice 1992-16, Loss of Flow from the Residual Heat Removal Pump During Refueling Cavity Draindown
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 tnternals. 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 870F. 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 installeda 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 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 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> 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 lB 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.
a
IN 92-16 February 25, 1992 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. Adminis- trative 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 870F to 1070F. 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.
February 25, 1992
This information notice requil
you have any questions about l
of the technical contacts lis- Reactor Regulation (NRR) proji
Technical contacts: Doug Star
res no specific action or written response. If
the information in this notice, please contact one
ted below or the appropriate Office of Nuclear
ect manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
I
-kov - Region II
(40i) 554-9901
Pierce Skinner, Region II
(404) 331-6299 Attachment:
Cowm~-
List of Recently Issued NRC Information Notices
Pa.'~.
A
x
9-
Attachment
February 25, 1992 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
Information
Date of
Notice No.
Subject
Issuance
Issued to
92-15
92-14
92-02, Supp. 1
92-13
92-12
Failure of Primary System
Compression Fitting
Uranium Oxide Fires at Fuel
Cycle Facilities
Relap5/Mod3 Computer Code
Error Associated with the
Conservation of Energy
Equation
Inadequate Control Over
Vehicular Traffic at
Nuclear Power Plant Sites
Effects of Cable Leakage
Currents on Instrument
Settings and Indications
Soil and Water Contamina- tion at Fuel Cycle Facil- ities
Brachytherapy Incidents
Involving Iridium-192 Wire
Used in Endobronchial
Treatments
Overloading and Subsequent
Lock Out of Electrical
Buses During Accident
Conditions
02/24/92
02/21/92
02/18/92
02/18/92
02/10/92
02/05/92
01/31/92
01/30/92
All holders of OLs or CPs
for nuclear power reactors.
All fuel cycle and uranium
fuel research and developme
licensees.
All holders of OLs or CPs
for nuclear power reactors.
All holders of OLs or CPs
for nuclear power reactors.
All holders of OLs or CPs
for nuclear power reactors.
All uranium fuel fabrica- tion and conversion facil- ities.
All Nuclear Regulatory Com- mission (NRC) licensees
authorized to use
iridium-192 for brachy- therapy; manufacturers and
distributors of iridium-192 wire for use in brachy- therapy.
All holders of OLs or CPs
for nuclear power reactors.
92-11
92-10
92-09 OL = Operating License
CP = Construction Permit