Information Notice 1994-36, Undetected Accumulation of Gas in Reactor Coolant System

Undetected Accumulation of Gas in Reactor Coolant System

ML031060539
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant
Issue date:	05/24/1994
From:	Grimes B Office of Nuclear Reactor Regulation
To:
References
-nr IN-94-036, NUDOCS 9405190137
Download: ML031060539 (10)

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 the reactor coolant system. However, they failed to recognize

the magnitude 9405190137 po\ fit JE ote qq-f3tn 9t1S EsoI'I'

..~ -IN 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

IN 94-36--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.

-\ > <_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

IN 94-36 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 Stress Corrosion

Cracking 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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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

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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

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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 OFFICE IOEABDOR5 DRP:RIIpcyJ

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TECH ED NAME Benedict 'MShaeffer

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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

E OFFICE *OEAB:DORS

• DRP:RII *OGCB:DORS

• SC/DRP:RII

• TECH ED NAME RBenedict

SMShaeffer

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