Information Notice No. 94-36: Undetected Accumulation of Gas in Reactor Coolant System


May 24, 1994



All holders of operating licenses or construction permits for nuclear power


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

Description of Circumstances

Sequoyah Unit 1 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 [120F].
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
the reactor coolant system.  However, they failed to recognize the magnitude

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                                        May 24, 1994
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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 RVLIS 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;


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.
IN 94-36                                                             May 24,
1994                                         Page 3 of 4

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

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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                                        Page 4 of 4

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.

 /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, RII
                     (615) 842-8001

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