United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-65: Undetected Accumulation of Gas in Reactor Coolant System and Inaccurate Reactor Water Level Indication During Shutdown

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                         WASHINGTON, D.C.  20555-0001

                               December 11, 1996


NRC INFORMATION NOTICE 96-65:   UNDETECTED ACCUMULATION OF GAS IN REACTOR
                                COOLANT SYSTEM AND INACCURATE REACTOR WATER
                                LEVEL INDICATION DURING SHUTDOWN


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 ongoing issues related to an undetected loss of
reactor coolant inventory at Haddam Neck caused by an accumulation of nitrogen
in the reactor coolant system.  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.  

Background

Information Notice (IN) 94-36, "Undetected Accumulation of Gas in Reactor
Coolant System," issued on May 24, 1994, discussed an event at the Sequoyah
Nuclear Power Plant in September 1993 where gas accumulated in the reactor
coolant system (RCS).  The gas accumulated because the temperature in the
volume control tank was much lower than normally expected.  (This lower
temperature resulted from unusually low component cooling water temperatures
and from a maintenance problem with a cooling water valve that reduced 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 there
was more dissolved gas in the water.  When the water was transferred to the
RCS by the charging system and heated up in the reactor vessel, the gas came
out of solution and collected in the RCS.  This information notice also
discussed a similar issue which occurred at the Salem Nuclear Generating
Station on April 12, 1994.

IN 96-37, "Inaccurate Reactor Water Level Indication and Inadvertent Draindown
During Shutdown," issued on June 18, 1996, discussed an event which occurred
on September 13, 1995, at Surry Unit 1.  The plant was in cold shutdown and
depressurized.  Operators had closed the reactor head vent to install the
reactor vessel cavity seal ring.  After the seal ring was in place, the
reactor head vent was not reopened.  This resulted in a loss of function of
the only reactor vessel water level indication.  As pressurizer relief tank
nitrogen pressure was gradually being reduced, an operator saw the standpipe
indicated level increase as the .                                                            IN 96-65
                                                            December 11, 1996
                                                            Page 2 of 6


gas bubble trapped in the reactor vessel head expanded, forcing water out of
the reactor and up the surge line and standpipe.  Unaware of the closed head
vent and believing the standpipe level indication, the operator increased
letdown from the reactor coolant system cold leg piping to maintain indicated
level.  This effectively reduced the inventory in the RCS by approximately
17.0 cubic meters [4500 gallons] over a period of 5 hours.

Description of Circumstances

The reactor at Haddam Neck was in cold shutdown and decay heat was being
removed by the residual heat removal (RHR) system using the "A" RHR pump.  The
RCS was depressurized and a temporary primary vent header was connected to the
reactor vessel head.  The volume control tank was being maintained at a
pressure of 310 kPa [30 psig].  The RCS loop stop valves had been closed to
isolate the reactor vessel from the steam generators.  The Technical
Specification required boration flow path was from the boric acid metering
tank, through the chemical and volume control system metering pump, and into
the RCS.  The reactor vessel level indication system (RVLIS) and core exit
thermocouples were disconnected in preparation for removal of the reactor
vessel head.  The operators were using pressurizer level indication and cavity
level indication to monitor reactor vessel level (see Attachment 1). 

On August 28, 1996, the metering pump was to be declared inoperable because of
an upcoming surveillance test on the emergency power supply for the pump,
requiring the alignment of a different boric acid flow path.  During the
alignment, the operator opened both valve BA-V-354 (blended makeup to the
volume control tank) and BA-V-355 (blended makeup to the charging pump
suction).  It appears that following this operation the operator may have
failed to fully seat one isolation valve (BA-V-355) between the top of the
volume control tank (nitrogen gas space) and the RCS (see Attachment 2). 
Note:  the procedure in use during the alignment did not allow valve BA-V-355
and BA-V-354 to be open at the same time.  This allowed nitrogen from the
volume control tank to leak from the volume control tank, through several
closed, leaking valves in the chemical and volume control system, and into the
reactor vessel.  The installed vent system was unable to vent off nitrogen as
fast as the nitrogen was being added to the reactor vessel and a nitrogen
bubble accumulated in the vessel head region.  As the nitrogen gas displaced
water from the reactor vessel into the pressurizer, the water level in the
reactor vessel decreased and the water level in the pressurizer increased.

The pressurizer level instrumentation erroneously indicated that the reactor
vessel was full of water.  Pressurizer level slowly increased because of the
displacement of water from the reactor vessel by nitrogen gas.  The operators
believed that the level increase was caused by water leaking into the RCS
through the loop stop valves.  The operators tightened the valves to stop the
leakage.  Water continued to be displaced from the reactor vessel by the
growing nitrogen bubble.  However, this was masked by the operators removing
RCS inventory to remove the reactor vessel conoseals.  For approximately four
days, control room operators were unaware that nitrogen gas was leaking into
the reactor vessel and causing the level in the reactor vessel to decrease.  
.                                                            IN 96-65
                                                            December 11, 1996
                                                            Page 3 of 6

On September 1, 1996, the nitrogen gas supply to the volume control tank was
isolated in an attempt to identify the source of the relatively high nitrogen
usage.  The isolation of the    nitrogen gas stopped the nitrogen leakage
into the RCS.  The vent header system capacity now exceeded the rate of gas
intrusion and the vent began to release the nitrogen which had accumulated in
the reactor vessel.  The volume of nitrogen being removed from the reactor
vessel was replaced by the water in the pressurizer.  The pressurizer level
rapidly decreased until the level indication decreased off scale (low).  Six
additions of water to the RCS, totalling approximately 18.9 cubic meters [5000
gallons], were required to stabilize pressurizer level within the normal
range.

Discussion

The event at Haddam Neck revealed a number of weaknesses.  However, there are
three issues which the NRC has determined warrant particular notice.  These
issues are:  (1) Inaccurate reactor vessel level instrumentation; (2)
Inadequate reactor coolant inventory balance; and (3) Non-condensible gas
intrusion into the RCS.  These issues are discussed below.  An NRC augmented
inspection team evaluated this event and reported its findings in NRC
Inspection Report 50-213/96-80.

Inaccurate Reactor Vessel Level Instrumentation

Lack of accurate reactor vessel level instrumentation and lack of adequate
inventory balances while shutdown are issues of generic concern.  The NRC has
issued several generic communications on the issue:  these are listed in the
"Related Generic Communications" section.  These generic communications
discuss numerous events where safety-related equipment operability was
challenged because of inaccurate level instrumentation.

At Haddam Neck, the absence of direct reactor vessel monitoring
instrumentation had an adverse affect on the operators' ability to monitor
reactor vessel conditions.  The available level instruments (pressurizer level
and cavity level) did not provide a direct indication of reactor vessel level.

The level instrumentation measured actual level in the pressurizer which the
operators believed was representative of reactor vessel level because the
pressurizer air space was vented to the same vent header as the reactor vessel
head (see Attachment 1).  During this event, pressurizer level indication and
cavity level indication were not representative of actual reactor vessel level
because of a difference in pressure between the reactor vessel head and the
pressurizer air space.  The difference in pressure was caused by the inability
of the installed reactor vessel head vent system to remove nitrogen gas as
fast as it was being introduced into the RCS.      

RVLIS, which does provide a direct reactor vessel level indication, was
disconnected in preparation for refueling maintenance activities.  The core
exit thermocouples are another direct indication of reactor vessel conditions.

The core exit thermocouples were also disconnected during the duration of this
event.

The RVLIS indicates reactor vessel level at discrete elevations.  During this
event, the RVLIS would have indicated abnormal reactor vessel level when the
actual level dropped below the .                                                            IN 96-65
                                                            December 11, 1996
                                                            Page 4 of 6


reactor vessel flange.  Following this event, local RVLIS readings were
collected and a temporary jumper was purchased and installed to provide RVLIS
indications in the control room.  A second jumper was installed to provide
core exit thermocouple indication in the control room.

Inadequate Reactor Coolant Inventory Balance

The licensee had not maintained an RCS inventory balance to account for the
transfer of water into and out of the RCS.  No detailed procedural guidance
existed which required an inventory balance for draindown operations.  The
operators compensated for a lack of detailed procedural guidance by writing
instructions in accordance with administrative control procedure 1.2-5.3,
"Evaluation of Activities\Evolutions Not Controlled by Procedure."  The
instructions written in accordance with administrative control procedure 1.2-
5.3 do not require the same level of review and approval that other plant
procedures receive.  The guidance written for the RCS draindown did not
require RCS inventory balances or specify reference levels.

On August 29, 1996, plant management made the decision to suspend refueling
activities over the weekend.  The operators were directed to and did refill
the RCS.  However, where 5000 gallons had been removed from the RCS earlier,
only 1000 gallons were added to the RCS to reach essentially the same
indicated level.  (The resulting 8-inch difference in P2r level indication
between before and after only represented 500 gallons of the inventory
difference not the 4000 gallons which existed.)

The magnitude of the apparent RCS inventory discrepancy was not explained to
operations management nor did the operators solicit engineering and technical
support to assist in resolving this discrepancy.  The licensee had experienced
leakage of the loop stop valves during past outages, however, an inventory
balance may have alerted the operators to the actual magnitude of the
inventory discrepancy, and may have caused the operators to question their
initial conclusion that the increase in pressurizer level was caused by
leaking stop valves.

Non-Condensible Gas Intrusion into the RCS

Gas intrusion into the RCS and safety-related cooling system piping is an
issue of generic concern.  The NRC has issued several generic communications
on the issue:  these are listed in the "Related Generic Communications"
section.  These generic communications discuss numerous events where safety-
related equipment was potentially rendered inoperable because of gas
intrusion.  The generic communications discuss the various processes by which
non-condensible gases have accumulated unknown to the reactor operators in the
RCS and safety-related cooling system piping.  The event at Haddam Neck
illustrates that gas intrusion events continue to occur in spite of the
operational experience available to the industry.

At Haddam Neck, nitrogen intrusion into the RCS could potentially result in
gas binding and common mode failure of the decay heat removal pumps and the
charging pumps.  In .                                                            IN 96-65
                                                            December 11, 1996
                                                            Page 5 of 6


addition, nitrogen present in the RCS could potentially interfere with the
ability of the steam generators to remove heat from the RCS via natural
circulation cooling.  The significance of the loss of the charging pumps at
Haddam Neck is that a method of injecting water into the reactor vessel would
be lost and that the charging pumps are needed to fill the RCS before opening
the loop stop valves.  The loop stop valves would need to be opened to allow
the RCS to communicate with the steam generators, a condition necessary for
the steam generators to remove heat from the RCS via natural circulation.  In
addition, the volume of the RCS increases significantly with the loop stop
valves open, thus providing a larger passive heat sink which would slow the
heating of the RCS.

At Haddam Neck the overpressure in the volume control tank was purposely
maintained by the licensee as an independent source of motive force to inject
water into the RCS in the event of a loss of other injection methods.  The
potential hazard of this condition is that nitrogen may intrude into the RCS
if valves relied upon to isolate the volume control tank from the RCS either
leak or are not properly controlled.

During the event, the operators attempted to start the "B" RHR pump, however,
it was found to be seized.  The licensee believes that the seizure occurred at
the end of the last surveillance test and was not related to the gas intrusion
event.  The operators were unaware that the "B" RHR had seized following its
last operation on August 19, 1996.  After the event, the licensee determined
that the "B" RHR pump was inoperable throughout the event.  The failure of the
"B" RHR pump added to the significance of this event.  If the "A" pump was
damaged, the RHR system would not be available to remove decay heat.  A
mitigating condition at Haddam Neck is that two trains of the low pressure
injection system were available.

The abnormal operating procedure for a loss of RHR requires that a cavitating
RHR pump be secured and vented.  However, the location of the RHR pump vents
is not optimal and significant difficulty was encountered during venting the
"B" RHR pump following maintenance to address its seizure.  Therefore, an
effective venting of a RHR pump may not have been easy to achieve during an
event if a pump had become gas bound.

Related Generic Communications

Information Notice 88-23, "Potential for Gas Binding of High-Pressure Safety
Injection Pumps During a Loss-of-Coolant Accident," May 12, 1988.

Information Notice 88-23, Supplement 1, "Potential for Gas Binding of High-
Pressure Safety Injection Pumps During a Loss-of-Coolant Accident,"
January 5, 1989.

Information Notice 89-67, "Loss of Residual Heat Removal Caused by Accumulator
Nitrogen Injection," September 13, 1989.

Information Notice 88-23, Supplement 2, "Potential for Gas Binding of High-
Pressure Safety Injection Pumps During a Loss-of-Coolant Accident,"
January 31, 1990.
.                                                            IN 96-65
                                                            December 11, 1996
                                                            Page 6 of 6


Information Notice 90-64, "Potential for Common-Mode Failure of High Pressure
Safety Injection Pumps or Release of Reactor Coolant Outside Containment
During a Loss-of-Coolant Accident," October 4, 1990.

Information Notice 88-23, Supplement 3, "Potential for Gas Binding of High-
Pressure Safety Injection Pumps During a Loss-of-Coolant Accident,"
December 30, 1990.

Information Notice 88-23, Supplement 4, "Potential for Gas Binding of High-
Pressure Safety Injection Pumps During a Loss-of-Coolant Accident,"
December 18, 1992.

Information Notice 90-55, Recent Operating Experience on Loss of Reactor
Coolant Inventory While in a Shutdown Condition," August 31, 1990.

Information Notice 93-93, "Inadequate Control of Reactor Coolant System
Conditions During Shutdown," December 8, 1993.

Information Notice 94-36, "Undetected Accumulation of Gas in Reactor Coolant
System," May 24, 1994.

Information Notice 95-03, "Loss of Reactor Coolant Inventory and Potential
Loss of Emergency Mitigation Functions While in a Shutdown Condition,"
January 18, 1995.

Information Notice 96-37, "Inaccurate Reactor Water Level Indication and
Inadvertent Draindown During Shutdown," June 18, 1996.

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.

                                             signed by D.B. Matthews

                                       Thomas T. Martin, Director
                                       Division of Reactor Program Management
                                       Office of Nuclear Reactor Regulation

Technical contacts:  

        Eric J. Benner, NRR          Warren C. Lyon, NRR
        (301) 415-1171               (301) 415-2947 (correction 301/415-3892)
        E-mail: ejb@nrc.gov          E-mail: wcl@nrc.gov

Attachments:
1.  The Undetected Nitrogen Gas Introduction 
      into the Reactor Vessel from the Charging Line
2.  Chemical and Volume Control System (see file IN96065.WP1 for figs. 1 & 2)
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