Information Notice 1994-36, Undetected Accumulation of Gas in Reactor Coolant System
UNITED STATES
P/-3Y
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C.
20555
May 24, 1994
NRC INFORMATION NOTICE 94-36: UNDETECTED ACCUMULATION OF GAS IN REACTOR
COOLANT SYSTEM
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 problems that could result from unrecognized
evolution and accumulation of gas in reactor coolant system high points. It
is expected that recipients will review this 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
Sequoyah Unit I had been shut down in March 1993.
In early September 1993, after the unit had been refueled, the reactor vessel was reassembled and the
reactor coolant system was degassed, filled, and vented. The reactor coolant
system was then depressurized to atmospheric pressure. One pressurizer power- operated relief valve (PORV) was open and the reactor coolant system average
temperature was being maintained at approximately 49C [120'F]. Nitrogen
cover gas was being supplied to the volume control tank at approximately
239 kPa [20 psig] and reactor coolant was being circulated by the charging
system and the residual heat removal (RHR) system.
The reactor vessel level
indication system (RVLIS) was functional for a significant portion of the
event. Before this event, instrument maintenance stickers were placed on the
RVLIS indicators for other maintenance work. Although the RVLIS information
was available, the operators were not monitoring this information because it
was not procedurally required to be used in shutdown modes of operation.
Reactor coolant system inventory was being monitored using pressurizer cold
calibration level indications.
On December 17, 1993, the operators began to pressurize the containment to
191 kPa [13 psig] in order to perform a containment integrated leak rate test.
As containment pressure increased, the operators noticed a decrease in
pressurizer water level and, over a period of time, added approximately 31,400
liters [8,300 gallons] of water to maintain the pressurizer level.
Licensee
personnel evaluated this situation and recognized that gas was accumulating in
However, they failed to recognize the magnitude
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94-36 May 24, 1994 or cause of the problem.
Three days later, as the containment was being
depressurized upon completion of the containment leak test, the operators
noted that approximately the same amount of water had to be removed from the
reactor coolant system to maintain the level.
On December 21, 1993, on the
basis of operators' observations of the changing coolant system Inventory
during the containment leak test, the reactor vessel head was vented. It was
subsequently vented several times to maintain appropriate reactor vessel
inventory based on RYLIS information. The reactor vessel gas voiding
conditions went undetected from early September 1993, when the reactor coolant
system was verified to be filled, until December 21, 1993, when the reactor
head was vented.
The gas evolution resulted from the temperature in the volume control tank
being much lower than that normally expected.
(This lower temperature was due
to unusually low component cooling water temperatures and to a maintenance
problem with a cooling water valve that resulted in the reduction of the heat
sink temperature in the letdown heat exchanger.) The lower temperatures
increased the solubility of gas in the volume control tank water so that more
gas was dissolved in the water. This gas evolved when the water was
transferred to the reactor coolant system by the charging system and heated up
in the reactor vessel.
Further details concerning this event are in NRC Inspection Report No. 50-327;
50-328/94-04.
Discussion
The licensee completed a final evaluation in January 1994 and determined that
during this event the reactor vessel water level had decreased to slightly
below the top of the hot leg, and that the steam generator tubes were nearly
empty. The reactor coolant system inventory was being monitored solely on the
basis of pressurizer level, which was not indicative of the reactor vessel and
steam generator levels. Calculations by the licensee indicated that an
equilibrium had been reached during the event so that the water level was
approximately 1.6 meters [5.25 feet] above the top of the core. Any
additional gas evolved in the reactor vessel would be expected to be vented
through the hot leg and surge line to the pressurizer and out through the open
pressurizer PORV.
The reactor water level at which equilibrium was established at Sequoyah was
sufficiently high so as not to interfere with reactor coolant flow through the
RHR cooling system.
However, at other plants, the equilibrium level, which is
related to pressurizer surge line geometry and RHR suction line location, might be such that a similar event could interfere with shutdown cooling.
Another potential safety concern is related to the capability of the steam
generators to transfer heat from the reactor coolant to secondary coolant. In
the equilibrium condition at Sequoyah, the steam generator tubes were almost
empty during at least a portion of the duration of this event. During this
--JMay 24, 1994 period, the steam generators were assumed to be available as an alternate
means for shutdown cooling.
It is not clear that cooling through the steam
generators could have been established, if needed, especially because the
plant operators were unaware that the tubes were empty.
Several factors involving licensee performance contributed to the delay in the
identification and evaluation of this event. They included (1) lack of
consideration on the part of licensee personnel with respect to compression of
gas in the system caused by containment pressure transmitted to the reactor
coolant system water through the open pressurizer PORV; (2) misunderstanding
by the operators about the operability of RVLIS; (3) failure to stop the
containment leak test when water level changes were first noted so that the
situation could have been adequately evaluated; (4) an apparent lack of
thorough evaluation of previously published information on similar events.
The event discussed in this information notice highlights the potential for
gas evolution and accumulation in the reactor coolant system in locations and
quantities that may not be evident to operators. Changes in temperature and
pressure can have significant effects on the solubility of gas in water, especially at or near atmospheric pressure conditions. During cold shutdown, this phenomenon can permit significant quantities of cover gas to be dissolved
in lower-temperature, higher-pressure volumes such as the volume control tank
and to evolve into a large gas bubble in the reactor vessel (a lower-pressure, higher-temperature area). This phenomenon can occur without a sudden change
in pressurizer level, which is the indicator normally used by the operators to
monitor water inventory.
Use of available instrumentation in shutdown modes
to monitor reactor vessel water level can provide detection of unexpected
system conditions when reduced vessel inventory is not planned as well as in
reduced inventory evolutions.
On April 12, 1994, a similar gas accumulation event was identified at the
Salem Generating Station (Unit 1) shortly after it entered Mode 5 (cold
shutdown) operation. Although this occurrence did not involve amounts of gas
accumulation as large as in the Sequoyah event, many similarities existed
regarding both the process of gas formation and the lack of operator awareness
of the abnormal condition. In both events, reactor vessel level information
was or could have been made available for prompt identification of the
problem.
Related Generic Communications
NRC Information Notice 93-12, "Off-Gassing in Auxiliary Feedwater
System Raw Water Sources," discusses similar solubility
considerations related to air pockets in piping at the McGuire
Nuclear Station.
NRC Information Notice 87-46, "Undetected Loss of Reactor
Coolant," discusses undetected loss of reactor coolant inventory
at North Anna Unit 1 resulting from inadequate use of available
indications and failure to perform mass inventory balances.
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<_IN 94-36 May 24, 1994 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 Of ice of
Nuclear Reactor Regulation (NRR) project manager.
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts: R. Benedict, NRR
(301) 504-1157
S. M. Shaeffer, RII
(615) 842-8001 Attachment:
List of Recently Issued NRC Information Notices
I
At,.thment
May 24, 1994 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
Information
Date of
Notice No.
Subject
Issuance
Issued to
91-81, Supp. 1
94-35
94-34
94-33
93-53, Supp. 1
94-32
94-31
90-68, Supp. 1
94-30
Switchyard Problems that
Contribute to Loss of
Offsite Power
NIOSH Respirator User
Notices, "Inadvertent
Separation of the Mask- Mounted Regulator (MMR)
from the Facepiece on the
Mine Safety Appliances (MSA)
Company MMR Self-Contained
Breathing Apparatus (SCBA)
and Status Update
Thermo-Lag 330-660
Flexi-Blanket Ampacity
Derating Concerns
Capacitor Failures in
Westinghouse Eagle 21
Plant Protection Systems
Effect of Hurricane
Andrew on Turkey Point
Nuclear Generating
Station and Lessons Learned
Revised Seismic Hazard
Estimates
Potential Failure of
Wilco, Lexan-Type HN-4-L
Fire Hose Nozzles
of Reactor Coolant Pump
Bolts
Leaking Shutdown Cooling
Isolation Valves at
Cooper Nuclear Station
05/19/94
05/16/94
05/13/94
05/09/94
04/29/94
04/29/94
04/14/94
04/14/94
04/12/94
All holders of OLs or CPs
for nuclear power reactors.
All holders of OLs or CPs
for nuclear power reactors, and all licensed fuel
facilities.
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 holders of OLs or CPs
for nuclear power reactors.
All holders of OLs or CPs
for nuclear power reactors.
All holders of 01 or CPs
for pressurized water
reactors.
All holders of OLs or CPs
for nuclear power reactors.
OL - Operating License
CP - Construction Permit
IN 94-xx
May xx, 1994 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.
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts:
R. Benedict, NRR
(301) 504-1157
S. M. Shaeffer, R11
(615) 842-8001 Attachment:
List of Recently Issued NRC Information Notices
- See Dnrvinim- concurrence
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OFFICE
- OEAB:DORS
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NAME
RBenedict
SMShaeffer
PCWen
MLesser
MFMejac
lDATE
05/04/L94
05/05/94
05/05/94
05/05/94
05/05/94
[*AC/SRXB:DSSA
- AD/DSSA
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TECollins
MVirgilio
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BKGrimes
05/06/94
05/09/94
05/09/94
05.12/94
/ /94 DOCUMENT NAME:
SLQGAbIN.KV1
NOTE: The PMs for Sequoyah (D. LaBarge) and Salem (J.
informed of the development of this information
reviewed the draft information notice.
Stone) have been
notice and they have
Peter Wen, 5/10/94
IN 94-xx
May xx, 1994 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 Office of Nuclear
Reactor Regulation (NRR) project manager.
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical Contacts:
R. Benedict, NRR
(301) 504-1157 S. M. Shaeffer, RI!
(615) 842-8001 Attachment:
List of Recently Issued NRC Information Notices
- See previous concurrence
OFFICE
- OEAB:DORS
- DRP:RII
- OGCB:DORS
- SC/DRP:RII
- TECH ED
NAME
RBenedict
SMShaeffer
PCWen
MLesser
MFMejac
DATE
05/04/94
05/05/94
05/05/94
05/05/94
05/05/94
- AC/SRXB:DSSA
- AD/DSSA
- C/OEAB:DORS
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NOTE: The PMs for Sequoyah (D. LaBarge) and Salem (J.
informed of the development of this information
reviewed the draft information notice.
Stone) have been
notice and they have
Peter Wen, 5/10/94
IN 94-xx
May xx, 1994 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 Office of Nuclear
Reactor Regulation (NRR) project manager.
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical Contacts:
R. Benedict, NRR
(301) 504-1157
S. M. Shaeffer, R11
(615) 842-8001 Attachment:
List of Recently Issued NRC Information Notices
See previous concurrence
OFFICE l *OEAB:DORS
- DRP:RII
- OGCB:DORS l *SC/DRP:RII
- TECH ED
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PCWen
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NOTE: The PMs for Sequoyah (D. LaBarge) and Salem (J.
informed of the development of this information
reviewed the draft information notice.
Stone) have been
notice and they have
Peter Wen, 5/10/94
IN 94-xx
May xx, 1994 This information notice requires no specific action or writttten response.
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.
If
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical Contacts:
R. Benedict, NRR
(301) 504-1157
S. M. Shaeffer, RII
(615) 842-8001 Attachment:
List of Recently Issued NRC Information Notices
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IN 94-36 May 24, 1994 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.
orig /s/'d by BKGrimes
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts:
R. Benedict, NRR
(301) 504-1157
S. M. Shaeffer, R11
(615) 842-8001 Attachment:
List of Recently Issued NRC Information Notices
- See Previous concurrence
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OFFICE
- OEAB:DORS
- DRP:RII
- OGCB:DORS
- SC/DRP:RII
- TECH ED
NAME
RBenedict
SMShaeffer
PCWen
MLesser
MFMejac
DATE
j05/04/94
05/05/94
05/05/94
05/05/94
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NOTE: The PMs for Sequoyah (D. LaBarge) and Salem (J.
informed of the development of this information
reviewed the draft information notice.
Stone) have been
notice and they have
Peter Wen, 5/10/94