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

NUCLEAR REGULATORY COMMISSION

P/-3Y

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 JEote qq-f3tn 9t1S EsoI

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..~-IN94-36 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, Switchyard Problems that 05/19/94 All holders of OLs or CPs

Supp. 1 Contribute to Loss of for nuclear power reactors.

Offsite Power

94-35 NIOSH Respirator User 05/16/94 All holders of OLs or CPs

Notices, "Inadvertent for nuclear power reactors, Separation of the Mask- and all licensed fuel

Mounted Regulator (MMR) facilities.

from the Facepiece on the

Mine Safety Appliances (MSA)

Company MMR Self-Contained

Breathing Apparatus (SCBA)

and Status Update

94-34 Thermo-Lag 330-660 05/13/94 All holders of OLs or CPs

Flexi-Blanket Ampacity for nuclear power reactors.

Derating Concerns

94-33 Capacitor Failures in 05/09/94 All holders of OLs or CPs

Westinghouse Eagle 21 for nuclear power reactors.

Plant Protection Systems

93-53, Effect of Hurricane 04/29/94 All holders of OLs or CPs

Supp. 1 Andrew on Turkey Point for nuclear power reactors.

Nuclear Generating

Station and Lessons Learned

94-32 Revised Seismic Hazard 04/29/94 All holders of OLs or CPs

Estimates for nuclear power reactors.

94-31 Potential Failure of 04/14/94 All holders of OLs or CPs

Wilco, Lexan-Type HN-4-L for nuclear power reactors.

Fire Hose Nozzles

90-68, Stress Corrosion Cracking 04/14/94 All holders of 01 or CPs

Supp. 1 of Reactor Coolant Pump for pressurized water

Bolts reactors.

94-30 Leaking Shutdown Cooling 04/12/94 All holders of OLs or CPs

Isolation Valves at for nuclear power reactors.

Cooper Nuclear Station

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- wBw

concurrence

-_. _ .

--- - 1 _._

OFFICE *OEAB:DORS *DRP:RII *OGCB:DORS *SC/DRP:RII *TECH ED

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 *C/OEAB:DORS *AC/OGCB:DORS D/DORS

TECollins MVirgilio AChaffee AJKugler 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. Stone) have been

informed of the development of this information notice and they have

reviewed the draft information notice.

Peter Wen, 5/10/94

IN 94-xx

May xx, 1994 This information notice requires no specific action or written response.

you have any questions about the information in this notice, please contactIf

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 *AC/OGCB:DORS D/DORS

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NOTE: The PMs for Sequoyah (D.LaBarge) and Salem (J.Stone) have been

informed of the development of this information notice and they have

reviewed the draft information notice.

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

NAME } RBenedict SMShaeffer PCWen MLesser MFMejac

DATE 1 05/04/94 1_05/05/94 1 05/05/94 _ 05/05/94 1 05/05/94

• AC/SRXB:DSSA *AD/DSSA j *C/OEAB:DORS AC/OGCB:DORS D/DORS

TECollins MVirgilio AChaffee AJKugler i(: BKGrimes

05/06/94 = 05/09/94 05/09/94 5/1V94 / /94 DOCUMENT NAME: SEQGASIN.RV1 NOTE: The PMs for Sequoyah (D. LaBarge) and Salem (J. Stone) have been

informed of the development of this information notice and they have

reviewed the draft information notice.

Peter Wen, 5/10/94

IN 94-xx

May xx, 1994 This information notice requires no specific action or writttten 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, RII

(615) 842-8001 Attachment:

List of Recently Issued NRC Information Notices

OFFICE IOEABDOR5 DRP:RIIpcyJ OGCB__ _ SC/DRP:RI TECH ED

NAME Benedict 'MShaeffer PCWen Lesser

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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

E

• See Previous concurrence

OFFICE

NAME

• OEAB:DORS

RBenedict

• DRP:RII

SMShaeffer

• OGCB:DORS

PCWen

• SC/DRP:RII

MLesser

• TECH ED

MFMejac

DATE j05/04/94 05/05/94 05/05/94 05/05/94 05/05/94 l*AC/SRXB:DSSA *AD/DSSA 5*C/OEAB:DORS *AC/OGCB:DO RS*

TECollins MVirgilio AChaffee IvAJKugler _s

05/06/94 05/09/94 i 05/09/94 05/12/4 05//f/94 UuUMtrII NAMt: vq-io. IN

NOTE: The PMs for Sequoyah (D. LaBarge) and Salem (J. Stone) have been

informed of the development of this information notice and they have

reviewed the draft information notice.

Peter Wen, 5/10/94