Information Notice 1995-35, Degraded Ability of Steam Generators to Remove Decay Heat by Natural Circulation

Degraded Ability of Steam Generators to Remove Decay Heat by Natural Circulation

ML031060293
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, 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:	08/28/1995
From:	Crutchfield D Office of Nuclear Reactor Regulation
To:
References
IN-95-035, NUDOCS 9508220031
Download: ML031060293 (11)

K-,

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001 August 28, 1995 NRC INFORMATION NOTICE 95-35: DEGRADED ABILITY OF STEAM GENERATORS

TO REMOVE DECAY HEAT BY NATURAL CIRCULATION

Addressees

All holders of operating licenses or construction permits for pressurized- water reactors (PWRs).

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees to conditions that may degrade the ability of the

steam generators (SGs) to remove decay heat by natural circulation of the

reactor coolant in Mode 5, cold shutdown. 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.

DescriDtion of Circumstances

During a refueling outage in September 1994 on Vogtle Unit 1, the licensee

found that the ability to remove decay heat by natural circulation through the

steam generators was degraded. On September 11, 1994, Vogtle Unit 1 entered a

refueling outage. On September 16, 1994, the unit was in Mode 5 with the

reactor coolant system (RCS) being drained down. The reactor coolant pumps

were tagged out of service and residual heat removal (RHR) system Train B was

in service removing decay heat. RHR Train A was operating intermittently but

had been administratively removed from service to adjust the suction valve

limit switch and allow stroking of local valves. During this period, the SGs

were relied upon to meet the technical specification requirement for a second

source of decay heat removal. While taking credit for the steam generators as

a heat sink, the RCS was vented to the containment atmosphere when the

pressurizer code safety valves were removed and a conoseal on the reactor

vessel head was disassembled. The licensee later determined that the heat

removal capability by natural circulation through the SGs was degraded. When

the RCS was vented, the inability to pressurize the RCS reduced the natural

circulation cooling capacity.

On February 13, 1995, the licensee for Turkey Point Units 3 and 4 reported

that during previous refueling outages it had relied upon the SGs for decay

heat removal when the SGs may not have been able to perform that function.

Specifically, the licensee relied upon the SGs as one of the means of decay

heat removal while testing one of the RHR loops with the RCS vented. The

PD& If E IUo4K. 7S-o3s- 5¢g

9508220031 QII

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-' IN 95-35 August 28, 1995 licensee later concluded that the RCS cannot support subcooled natural

circulation decay heat removal through the SGs while the RCS is vented.

Discussion

These two examples illustrate plant conditions that were not adequate to fully

support natural circulation through the SGs as a method of decay heat removal

during operation in Mode 5 with loops filled and the reactor coolant pumps out

of service.

Technical specifications generally require two methods of decay heat removal

in Mode 5 with loops filled. When this is the case, they generally go on to

indicate that this requirement can be satisfied by two loops of RHR or one

loop of RHR and a minimum water level in the SGs. Decay heat can be removed

either through the RHR system or through the SGs by natural circulation after

the reactor coolant pumps are secured. The heat removal mechanism with

residual heat removal is through forced circulation through the RHR heat

exchanger. Heat removal with natural circulation of reactor coolant through

the SGs occurs because of the differential pressure created between the heated

water in the reactor core and the cooler water in the SG tubes. This

differential pressure is created through temperature differences that in turn

create fluid density differences between these two locations.

When the RCS is being depressurized and cooled down, the reactor coolant pumps

are stopped, the RCS is depressurized and vented, and level is decreased in

preparation for Mode 6 (refueling) entry. In Mode 6, both RHR trains must be

operable. During the transition from Mode 5, with no reactor coolant pumps

running, to Mode 6, plant conditions may exist that are not adequate to

support natural circulation. The second train of RHR may need to be operable

before proceeding with plant cooldown and depressurization to provide a second

method for RCS cooling.

During natural circulation, the SG secondary side water boils and steams off

through the atmospheric relief valves or other openings that may exist during

shutdown conditions. The minimum temperature at which boiling will begin in

the SG is 100lC [2120 F]. A minimum temperature differential of 28- C

[500 F] between the RCS and the SG secondary water is routinely used for

evaluating conditions that would ensure sufficient natural circulation flow to

prevent boiling in the core. The heat transfer rate across the steam

generator tubes is less for lower RCS-to-SG secondary temperature

differentials but still may be adequate to promote sufficient natural

circulation and prevent core boiling. Adding the differential temperature of

280 C [50 F] to 1000 C [2120 F] results in a minimum RCS temperature of

128- C [262 F] to maintain sufficient natural circulation flow. The lowest

pressure point in the RCS, at the top of the SG tubes, should therefore be

maintained above the saturation pressure for 128' C [262 F]. If the RCS

pressure at the top of the SG tubes is allowed to fall below the primary fluid

saturation temperature, flashing and steam voiding may occur, interrupting or

degrading the natural circulation flow path. Additionally, when system

pressure is dropped with elevated water temperatures, gases may come out of

solution.

I i I

<-' IN 95-35 August 28, 1995 When relying on the ability of the SGs to remove decay heat by natural

circulation of reactor coolant in Mode 5, the following factors are worthy of

consideration: (1)the ability to pressurize and control pressure in the RCS,

(2)secondary side water level in the SGs relied upon for decay heat removal,

(3)availability of a supply of feedwater, and (4) availability of an

auxiliary feedwater pump capable of injecting into the relied-upon SGs.

Consideration should also be given to avoiding the potential for

pressurization of the SG secondary side. It is also important to note that

during the decay heat removal scenario for the natural circulation process, a

mode change (Mode 5 to Mode 4) could occur due to heat up of the RCS.

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.

Crutch el Director

D sM.

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical contact: Brian R. Bonser, RII

(706) 554-9901 Attachment:

List of Recently Issued NRC Information Notices

47/TW%4~ fSjACkeY

"" ttachment

IN 95-35 August 28, 1995 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Notice No. Subject Issuance Issued to

95-34 Air Actuator and Supply 08/25/95 All holders of OLs or CPs

Air Regulator Problems in for nuclear power reactors.

Copes-Vulcan Pressurizer

Power-Operated Relief Valves

93-83, Potential Loss of Spent 08/24/95 All holders of OLs or CPs

Supp. 1 Fuel Pool Cooling After a for nuclear power reactors.

Loss-of-Coolant Accident

or a Loss of Offsite Power

95-33 Switchgear Fire and 08/23/95 All holders of OLs or CPs

Partial Loss of Offsite for nuclear power reactors.

Power at Waterford

Generating Station, Unit 3

95-10, Potential for Loss of 08/11/95 All holders of OLs or CPs

Supp. 2 Automatic Engineered for nuclear power reactors.

Safety Features Actuation

95-32 Thermo-Lag 330-1 Flame 08/10/95 All holders of OLs or CPs

Spread Test Results for nuclear power reactors.

95-31 Motor-Operated Valve 08/09/95 All holders of OLs or CPs

Failure Caused by Stem for nuclear power reactors.

Protector Pipe Inter- ference

95-30 Susceptibility of Low- 08/03/95 All holders of OLs or CPs

Pressure Coolant Injection for nuclear power reactors.

and Core Spray Injection

Valves to Pressure Locking

94-66, Overspeed of Turbine- 06/16/95 All holders of OLs or CPs

Supp. 1 Driven Pumps Caused by for nuclear power reactors.

Binding in Stems of

Governor Valves

OL - Operating License

CP - Construction Permit

IN 95-35 August 28, 1995 When relying on the ability of the SGs to remove decay heat by natural

circulation of reactor coolant in Mode 5, the following factors are worthy of

consideration: (1) the ability to pressurize and control pressure in the RCS,

(2) secondary side water level in the SGs relied upon for decay heat removal,

(3) availability of a supply of feedwater, and (4) availability of an

auxiliary feedwater pump capable of injecting into the relied-upon SGs.

Consideration should also be given to avoiding the potential for

pressurization of the SG secondary side. It is also important to note that

during the decay heat removal scenario for the natural circulation process, a

mode change (Mode 5 to Mode 4) could occur due to heat up of the RCS.

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.

[Original signed by]

Dennis M. Crutchfield, Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical contact: Brian R. Bonser, RII

(706) 554-9901 Attachment:

List of Recently Issued NRC Information Notices

DOCUMENT NAME: 95-35.IN

• See previous concurrence

To receive a copy of this document, Indicate In the box: AC = Copy without attachment/enclosure 'E'

• Copy with attachmentlenclosura N' = No copy

-

OFFICE OECBIDOP5 E SC/OECB:DOPS NI

NADM:PUB I N SRXB:DSSA IE C/OTSB:DOPS lE

NAME . JTappert* EGoodwin* Tech Ed* RJones* ICGrimes*

DATE 04/20/95 04/27/95 04/28/95 04/25/95 05/16/95

- . _ . . __ _ _. . . ........... _ ._ I

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4 NAME RKiessel* 1AChaffee* IDCPV((I4fif elI d

DATE 108/10/95 108/17/95 108/1/95 OFFICIAL RECORD COPY

IN 95-35 August 28, 1995 When relying on the ability of the SGs to remove decay heat by natural

circulation of reactor coolant in Mode 5, the following factors are worthy of

consideration: (1)the ability to pressurize and control pressure in the RCS,

(2)secondary side water level in the SGs relied upon for decay heat removal,

(3)availability of a supply of feedwater, and (4) availability of an

auxiliary feedwater pump capable of injecting into the relied-upon SGs.

Consideration should also be given to avoiding the potential for

pressurization of the SG secondary side. It is also important to note that

during the decay heat removal scenario for the natural circulation process, a

mode change (Mode 5 to Mode 4) could occur due to heat up of the RCS.

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.

[Original signed by]

Dennis M. Crutchfield, Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical contact: Brian R. Bonser, RII

(706) 554-9901 Attachment:

List of Recently Issued NRC Information Notices

DOCUMENT NAME: 95-35.IN

• See previous concurrence

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NAME JTappert* EGoodwin* Tech Ed* RJones* CGrimes* I

DATE 04/20/95 04/27/95 04/28/95 04/25/95 05/16/95 OFFICE PECB:DRPM IE IC/PECB:DRPM INID/DAEJA, / SN1_.

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UFFILIAL MUCMV LUPY

IN 95-XX

August xx, 1995 When relying on the ability of the SGs to remove decay heat by natural

circulation of reactor coolant in Mode 5, the following factors are worthy of

consideration: (1) the ability to pressurize and control pressure in the RCS,

(2) secondary side water level in the SGs relied upon for decay heat removal,

(3) availability of a supply of feedwater, and (4) availability of an

auxiliary feedwater pump capable of injecting into the relied-upon SGs.

Consideration should also be given to avoiding the potential for

pressurization of the SG secondary side. It is also important to note that

during the decay heat removal scenario for the natural circulation process, a

mode change (Mode 5 to Mode 4) could occur due to heat up of the RCS.

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.

Dennis M. Crutchfield, Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical contact: Brian R. Bonser, R11

(706) 554-9901 Attachment: List of Recently Issued NRC Information Notices

DOCUMENT NAME: G:\JRT\NATCIRC.BRB

• See previous concurrence

F. cnoov with attchmntl~enclosure 'N = No copy

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OFFICE OECB:DOPS lE SC/OECB:DOPS IN ADM:PUB IN SRX A C/OTSB:DOPS IE

NAME JTappert*_JEGoodwin* Tech Ed* RJones* CGrimes*

DATE 04/20/95 104/27/95 04/28/95 104/25/ 05/16/95 OFFTCEF IPECB:DRPM

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IN 95-XX

May xx, 1995 128 0 C

therefore be maintained above the saturation pressure for is

(2620 F). If the RCS pressure at the top of the SG tubes

allowed to fall below the primary fluid saturation temperature, flashing and steam voiding may occur, interrupting or degrading

the natural circulation flow path. Additionally, when system may

pressure is dropped with elevated water temperatures, gases

come out of solution.

When relying on the ability of the SGs to remove decay heat by

natural circulation of reactor coolant in Mode 5, the following

factors are worthy of consideration: (1) the ability to

pressurize and control pressure in the RCS, (2) secondary side (3)

water level in the SGs relied upon for decay heat removal, of an

availability of a supply of feedwater, and (4) availability

auxiliary feedwater pump capable of injecting into the relied- upon SGs. Consideration should also be given to avoiding the

potential for pressurization of the SG secondary side. It is

also important to note that during the decay heat removal (Mode

scenario for the natural circulation process a mode change

5 to Mode 4) could occur due to heat up of the RCS.

This information notice requires no specific action or written in

response. If you have any questions about the information or

this notice, please contact the technical contact listed below

the appropriate Office of Nuclear Reactor Regulation (NRR)

project manager.

Brian K. Grimes, Director

Division of Project Support

Office of Nuclear Reactor

Regulation

Technical contact: Brian R. Bonser, RII

(706) 554-9901 Attachment: List of Recently Issued NRC Information Notices

DOCUMENT NAME: G:\JRT\NATCIRC.BRB

'E' - Copy wIth attachmentlenclosure

To recelv a copy of this document, Indicato In the box: *C - Copy without attachment/enoloue

'N - No copy I

on noon I

OFFICE OECB:DOPS SC/OECB:DOPS ADM:PUB l SRXB:DSSA l C/uiT:uuwr

NAME JTappert * EGoodwin * Tech Ed * RJones * CGr mes*

DATE 4/20/95 4/21/95 4/28/95 4/25/95 5/16/95 OFFICE j BO C/OECB:DOPS D/DOPS

NAME Wessel AChaffee BGrimes

DATE A t J

I/95 1/01 I /95 =============

OFFICIAL RECORD COPY

• See previous concurrence

K)

IN 95-XX

May xx, 1995 the RCS pressure at the top of the SG tubes is allowed to fall

below the primary fluid saturation temperature, flashing and

steam voiding may occur, interrupting or degrading the natural

circulation flow path . Additionally, when system pressure is

dropped with elevated water temperatures, gasses may come out of

solution.

When relying on the ability of the SGs to remove decay heat by

natural circulation of reactor coolant in Mode 5, the folowing

factors are worthy of consideration: (1) the ability to

pressurize and control pressure in the RCS, (2) secondary side

water level, at or above the top of the tubes in the SGs relied

upon for decay heat removal, (3) availability of a supply of

feedwater, and (4) availability of an auxiliary feedwater pump

capable of taking suction from the feedwater source and injecting

into the relied-upon SGs. Consideration should also be given to

avoiding the potential for pressurization of the SG secondary

side. It is also important to note that a mode change (Mode 5 to

Mode 4) could occur during the decay heat removal scenario for

the natural circulation process.

Brian K. Grimes, Director

Division of Project Support

Office of Nuclear Reactor

Regulation

Technical contact: Brian R. Bonser, RII

(706) 554-9901 Attachment: List of Recently Issued NRC Information Notices

DOCUMENT NAME: G:\JRT\NATCIRC.BRB 0i

Cu = Copy without attachment/enclosure 'E' Copy attachment/enclosure

To receive a copy of this document, Indicate In the box:

'Nt = No copy __

OFFICE OECB:DOPS SC/OECB:DOPS ADM:PUB SPB DSSA C/OTSB:DOP

NAME JTappert * EGoodwin * Tech Ed * RJones * CGrimes

DATE 4/20/95 4/27/95 4/28/95 4/25/95 OFFICE OECB:DOPS C/OECB:DOPS L D/DOPSIL

NAME RKiessel AChaffee BGrimes

DATE I/I / /95 / /95 OFFICIAL RECORD COPY

• See previous concurrence

K>

IN 95-XX

May xx, 1995 maintained above the saturation pressure for 2620 F (1280 C].

Flashing and steam voiding may occur interrupting or degrading

the natural circulation flow path if the RCS pressure at the top

of the SG tubes is allowed to fall below the primary fluid

saturation temperature. An added complication of gasses coming

out of solution can occur when system pressure is dropped with

elevated water temperatures.

When relying on the decay heat removal capability of the SGs via

natural circulation of reactor coolant in mode 5 certain criteria

should be considered. These criteria include: 1) the ability to

pressurize and control pressure in the RCS; 2) the SGs relied

upon for decay heat removal have secondary side water level at or

above the top of the tubes; 3) a supply of feedwater is

available; and 4) an auxiliary feedwater pump is available

capable of taking suction from the feedwater source and injecting

into the relied upon SGs. Consideration should also be given to

avoiding the potential for pressurization of the SG secondary

side. It is also important to note that a mode change (mode 5 to

mode 4) will occur during the decay heat removal scenario for the

natural circulation process.

Brian K. Grimes, Director

Division of Project Support

Office of Nuclear Reactor

Regulation

Technical contact: Brian R. Bonser, RII

(706) 554-9901 Attachment: List of Recently Issued NRC Information Notices

DOCUMENT NAME: G:\JRT\NATCIRC.BRB

To receive a copy of this document, Indicate In the box: 'C' - Copy without attachment/enclosure E' - Copy with atachment/enclosure

• N'

a No copy

OFFICE IOECB:DOPS SC/OECB:DOPS lAMPUB l [SPLB:DSSA J I

NAME JTappert Voodwin Tech Ed __ones_____

DATE 1q/ Q $ 1 /a) 2I i 1 / f__ I/

OFFICE OECB:DOPS C/OECB:DOPS D/DOPSL

NAME RKiessel IAChaffee BGrimes l

DATE // / /95 / /95 OFFICIAL RECORD COPY

IN 95-XX

May xx, 1995 maintained above the saturation pressure for 128 0 C [2620 F]. If

the RCS pressure at the top of the SG tubes is allowed to fall

below the primary fluid saturation temperature, flashing and

steam voiding may occur interrupting or degrading the natural

circulation flow path . Additionally, when system pressure is

dropped with elevated water temperatures gasses may come out of

solution.

When relying on the decay heat removal capability of the SGs via

natural circulation of reactor coolant in mode 5 the folowing

factors are worthy of consideration. These include: 1) the

ability to pressurize and control pressure in the RCS; 2) the SGs

relied upon for decay heat removal have secondary side water

level at or above the top of the tubes; 3) availability of a

supply of feedwater; and 4) availability of an auxiliary

feedwater pump capable of taking suction from the feedwater

source and injecting into the relied upon SGs. Consideration

should also be given to avoiding the potential for pressurization

of the SG secondary side. It is also important to note that a

mode change (mode 5 to mode 4) could occur during the decay heat

removal scenario for the natural circulation process.

Brian K. Grimes, Director

Division of Project Support

Office of Nuclear Reactor

Regulation

Technical contact: Brian R. Bonser, RII

(706) 554-9901 Attachment: List of Recently Issued NRC Information Notices

DOCUMENT NAME: G:\JRT\NATCIRC.BRB

To raceive a copy of this document, Indicate In Xt box: 'C' - Copy wihout attachmentlenclosure T

E Copy with attachmentlenclosure ¶N'

• No copy

OFFICE

NAME

DATE

OECB:DOPS

JTappert

/ /

IEGoodwin

1/

SC/OECB:DOPS

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AMPUB

Tech Ed

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SPLB:DSSA

RJones

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OFFICE OECB:DOPS C/OECB:DOPS _

__D/DOPS

NAME RKiessel AChaffee BGrimes

DATE I / / /95 / /95 OFFICIAL RECORD COPY