Information Notice 1988-36, Possible Sudden Loss of RCS Inventory During Low Coolant Level Operation

Possible Sudden Loss of RCS Inventory During Low Coolant Level Operation

ML031150094
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, 05000000, Zimmer, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant
Issue date:	06/08/1988
From:	Rossi C Office of Nuclear Reactor Regulation
To:
References
IN-88-036, NUDOCS 8806020047
Download: ML031150094 (10)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 June 8, 1988 NRC INFORMATION NOTICE NO. 88-36: POSSIBLE SUnDEN LOSS OF RCS INVENTORY

DURING LOW COOLANT LEVEL OPERATION

Addressees

All holders of operating licenses or construction permits for pressurized water

reactors (PWRs).

Purpose

This information notice is being provided to alert addressees to the potential

for a sudden loss of reactor coolant system inventory while conducting steam

generator tube inspections and modifications with hot leg nozzle dams in place.

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 do not constitute NRC

requirements; therefore, no specific action or written response is required.

Description of Circumstances

During the second refueling of Diablo Canyon Unit 1, in the spring of 1988, deficiencies in the procedures to be used during the steam generator tube

Inspections were identified that could significantly increase the probability

of a sudden ejection of reactor coolant followed by core uncovery.

In order for the steam generator tubes to be inspected at Diablo Canyon, they

were drained, by drawing air through reactor and pressurizer vents, until the

reactor coolant inventory was drained down to the mid-level of the hot lea

piping (see Figure 1). Lowering the reactor coolant to this level also un- covers the steam generator primary side manways so that they can be removed

to gain access to the steam generator hot and cold leg plenums and their re- spective hot and cold leg nozzles. Nozzle dams are then placed in these steam

generator plenum nozzles so that the reactor coolant level can be raised to

increase the net positive suction head to the decay heat removal pumps without

refilling the steam generators.

If the hot leg nozzle dams were all Installed before all of the cold leg nozzle

dams were in place, a small increase in reactor vessel pressure would cause

reactor coolant to be rapidly expelled from the open cold leg manways. This

would occur because the increased pressure, unable to vent through the dammed- up hot legs, would force the coolant down in the vessel, through the cold legs, and out of the manways. A pressure increase of only 2-1/? psig in the vessel

88060200477A

IN 88-36

-June 8, 1988 would lower the coolant level to the point where the top of the fuel would

begin to be uncovered, with the level of the remaining coolant in the open

steam generator located at the bottom of the cold leg plenum manway.

Similar mechanisms have been identified at San Onofre Units 2 and 3 in their

response to Generic Letter 87-12 (Reference 1), and by the Westinghouse Owners

Group in an ongoing analysis of reactor behavior during the shutdown condition.

The possibility of ejecting coolant by this mechanism can be eliminated by

ensuring that a steam generator hot leg plenum manway and its associated hot

leg pipe are kept open to provide an adequate vent path whenever any cold leg

openings are made. This can be accomplished by ensuring that a hot leg manway

is the first manway to be opened, and a hot leg nozzle dam is the last dam to

be installed. In addition, not installing the last hot leg nozzle dam until

a sufficient vent path is established in the reactor vessel or pressurizer

will reduce the possibility of developing a pressure differential which could

eject a dam.

Discussion:

On April 10, 1987, the Diablo Canyon Unit 2 reactor vessel became pressurized

to approximately 7 to 10 psig when the residual heat removal flow was lost for

a period of 1-1/2 hours (Reference 2". Fortunately, during this event the man- ways, although loosened, were still in place and the nozzle dams had not yet

been installed. Operating a reactor coolant system that has been drained to a.

low level often involves unusual problems that have a significant probability of

causing a loss of residual heat removal unless special care is taken. NUREG-1269 (Reference 3), the report of the NRC investigation into the Diablo Canyon event, discusses a number of these problems. These include the following:

The level which is established for draining the steam generator tubes

is frequently only slightly above the level which will provide an adequate

suction head for the residual heat removal pumps. This marginal suction

head can lead to air entrainment due to vortexing at the suction point, which may cause a loss of pump suction.

The temporary reactor vessel level measurement system necessary for this

type of operation tends to be inaccurate because of the long lengths of

tubing normally used. The possible air entrainment and the surface level

variations due to fluid flow at this low level provide additional mechanisms

that cause error In the level measurement.

The NRC has documented many instances where residual heat removal has been lost, because of loss of pump suction, while the plant was being operated at reduced

reactor coolant water levels. Generic Letter 87-12 (Reference 1) lists 37 loss-of-decay-heat-removal events, occurrinQ from 1977 to 1987, that were at- tributed to inadequate reactor coolant system level. In four cases, including

the 1987 Diablo Canyon event, boiling is known to have occurred before residual

heat removal could be reestablished.

Although small vents are normally established in the reactor vessel head and in

the pressurizer before the coolant level Is drained down, these are far too

IN 88-36 June 8, 1988 small to prevent pressurization of the reactor coolant system after the boiling

pcint is reached. For the recent steam. generator inspection at Diablo Canyon, which was initiated 10 days after shutdown, the reactor was producing 5 MW of

decay heat. This is sufficient to produce 5 lb of steam per second, which

would require a vent area greater than 12 square inches in order to hold the

pressure rise to less than 25 psi. During the 1987 Diablo Canyon event, the

reactor, which had been shut down for seven days, reached the boiling point

about 1/2 hour after decay heat removal capability was lost. The pressure

increased to the 7-to-10-psig maximum value a short time later even though

small vents were available in the vessel head and pressurizer.

With the hot leg nozzle dams in place the pressure rise would be quite rapid.

Generation of a small amount of steam would be sufficient to produce the

partial pressure of 2-1/2 psi necessary to uncover the core by ejecting the

coolant throuch the open cold leg plenum manway. This amount of steam could

be produced in less than a minute. However, the actual time to produce this

pressure would depend on the time to heat the reactor coolant to the higher

boiling point and on the rate of energy deposition in the cold materials in

the upper part of the reactor vessel and, to a lesser extent, in the pressurizer.

The time required for this to occur would likely be only a few minutes.

Loss of residual heat removal capability after the nozzle dams are installed

and before the vessel level is raised would still result in a hazardous situ- ation, however, more time would be available for operator action before loss

of coolant occurred. The nozzle dams used at Diablo Canyon are designed to

withstand about 50 psi of differential pressure. Approximately 1/2 hour of

additional time would be available before the reactor coolant heated up to the

approximately 3000 F necessary to boll at this higher pressure. However, if a

cold leg dam were to be expelled at this point, coolant ejection through the

affected steam generator manway followed by core uncovery would be very rapid.

For this reason, it is prudent to provide a means of venting the vessel with

the dams installed. At Diablo Canyon, the schedule for detensioning the

reactor vessel head was advanced so that this would be done before the reactor

was drained for the steam generator inspection. Although the pressure neces- sary to lift the detensioned vessel head, in order to vent the vessel, is less

than the pressure required to eject the nozzle dams, this pressure is greater

than that which would be required to uncover the top of the fuel by expelling

coolant through an undammed steam generator cold leg nozzle and the associated

manway. Therefore, even with the head detensioned, the hot leg nozzles should

be left open until all cold leg openings are closed.

Generic Letter 87-12 also identified a comparable mechanism for uncovering the

core by pressurization during low coolant level operation. An opening in a

cold leg, such as one caused by the opening of a reactor coolant system pump

or a loop isolation valve (in some plants), would vent the space of the af- fected cold leg, maintaining this space at atmospheric pressure. Any pressure

increase, such as would be caused by boiling in the reactor vessel, would be

propagated throughout the remainder of the reactor coolant system, including

both hot and cold sides of steam generator primary spaces. This differential

pressure would force the coolant levels in the vessel down while the displaced

coolant would be forced up and out of the affected cold leg opening. As with

the mechanism already discussed, only about 2 1/2 psi would be required to

IN 88-36 June 8, 1088 expel the water down to the top of the core with the coolant in the affected cold

leg at the level of a pump opening. Although in this case some steam condensation

may occur in the steam generators, as the 1987 Diablo Canyon event showed, this

will not prevent pressurization. Note that this mechanism, involving coolant

expulsion through a cold leg opening, does not require plugging the steam gener- ator nozzles. As with the previous mechanism, this hazard might be eliminated

by venting the reactor vessel through a large opening, such as a hot leg steam

generator plenum manway or pressurizer opening, before opening the cold leg.

The loss of residual heat removal capability during low reactor coolant level

operation has proven to be a frequent occurrence; leading in several cases to

boiling in the reactor vessel. If this should occur, pressurization of the

reactor vessel can lead to sudden core uncovery by the expulsion of coolant

through any opening in the cold leg side of the reactor coolant system. This

hazard can be eliminated by providing a large vent for the reactor vessel space

before opening the cold leg.

No specific action or written response is required by this information notice.

If you have any questions about this matter, please contact one of the technical

contacts listed below or the Regional Administrator of the appropriate regional

office.

Charles E. Rossi, Director

Division of Operational Events Assessment

Office of Nuclear Peactor Regulation

Technical Contacts: Paul P. Narbut, RV

(805) 595-2354 Donald C. Kirkpatrick, NRR

(301) 492-1152 Warren Lyon, NRR

(301) 492-0891 Attachments:

1. Figure 1 - Reactor Coolant System

2. List of Recently Issued NRC Information Notices

References:

1. Generic Letter 87-12, "Loss of Residual Heat Removal While the Reactor

Coolant System is Partially Filled," July 9, 1987.

2. IN 87-23, "Loss of Decay Heat Removal During Low Reactor Coolant Level

Operation."

3. NUREG-1269, "Loss of Residual Heat Removal System, Diablo Canyon, Unit 2,"

April 10, 1987.

Figure 1 REACTOR COOLANT SYSTEM

PRESSURIZER

STEAM

GENERATOR

151'-41 A

Cl = CENTER LINE

SG TUBES

SG MANWAYS LEG OA

>~~~~1% 14 -% 'A

108'-11" toZ - l l 5 lREACTOR VESSEL IRECO

LEGAA CA A ONAl A= 4 LEG-!=_

COLD07-1PUMP

Cl OOULMANT

COLD LEG NOZZLE TOP - 102711X"

NOZZLE DAMfn

CROSSOVER LEG Cl = 96"-8%"'

I

DA .CORE

.

3

-- A

Attachment 2 IN 88-36 June 8, 1988 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Notice No. Subject Issuance Issued to

88-35 Inadequate Licensee Performed 6/3/88 All holders of OLs

Vendor Audits or CPs for nuclear

power reactors.

88-34 Nuclear Material Control 5/31/88 All holders of OLs

and Accountability of or CPs for nuclear

Non-Fuel Special Nuclear power reactors.

Material at Power Reactors

87-61, Failure of Westinghouse 5/31/88 All holders of OLs

Supplement 1 W-2-Type Circuit Breaker or CPs for nuclear

Cell Switches power reactors.

88-33 Recent Problems Involving 5/27/88 All Agreement States

the Model Spec 2-T and NRC licensees

Radiographic Exposure authorized to manu- Device facture, distribute

or operate radio- graphic exposure

devices and source

changers.

88-32 Promptly Reporting to 5/25/88 All NRC material

NRC of Significant licensees.

Incidents Involving

Radioactive Material

88-31 Steam Generator Tube 5/?5/88 All holders of OLs

Rupture Analysis or CPs for Westinghouse

Deficiency and Combustion

Engineering-designed

nuclear power plants.

88-30 Target Rock Two-Stage 5/25/88 All holders of OLs

SRV Setpoint Drift or CPs for nuclear

Update power reactors.

88-29 Deficiencies In Primary 5/24/88 All holders of OLs

Containment Low-Voltage or CPs for nuclear

Electrical Penetration power reactors.

Assemblies

OL = Operating License

CP = Construction Permit

IN 88-36 June 8, 1988 expel the water down to the top of the core with the coolant in the affected cold

leg at the level of a pump opening. Although in this case some steam condensation

may occur in the steam generators, as the 1987 Diablo Canyon event showed, this

will not prevent pressurization. Note that this mechanism, involving coolant

expulsion through a cold leg opening, does not require plugging the steam gener- ator nozzles. As with the previous mechanism, this hazard might be eliminated

by venting the reactor vessel through a large opening, such as a hot leg steam

generator plenum manway or pressurizer opening, before opening the cold leg.

The loss of residual heat removal capability during low reactor coolant level

operation has proven to be a frequent occurrence; leading in several cases to

boiling in the reactor vessel. If this should occur, pressurization of the

reactor vessel can lead to sudden core uncovery by the expulsion of coolant

through any opening in the cold leg side of the reactor coolant system. This

hazard can be eliminated by providing a large vent for the reactor vessel space

before opening the cold leg.

No specific action or written response is required by this Information notice.

If you have any questions about this matter, please contact one of the technical

contacts listed below or the Regional Administrator of the appropriate regional

office.

Charles E. Rossi, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical Contacts: Paul P. Narbut, RV

(805) 595-2354 Donald C. Kirkpatrick, NRR

(301) 492-1152 Warren Lyon, NRR

(301) 492-0891 Attachments:

1. Figure 1 - Reactor Coolant System

2. List of Recently Issued NRC Information Notices

References:

1. Generic Letter 87-12, "Loss of Residual Heat Removal While the Reactor

Coolant System is Partially Filled," July 9, 1987.

2. IN 87-23, "Loss of Decay Heat Removal During Low Reactor Coolant Level

Operation."

3. NUREG-1269, "Loss of Residual Heat Removal System, Diablo Canyon, Unit 2,"

April 10, 1987.

Transmitted by memo to C. H. Berlinger from Dennis F. Kirsch dated March 21, 1988.

• SEE PREVIOUS CONCURRENCES

D RR *C/OGCB:DOEA:NRR *PPMB:ARM *D/DEST:NRR

1 CHBerlinger TechEd LCShao

0 05/26/88 05/12/88 05/18/88

• OGCB:DOEA:NRR *R *SRXB:DEST:NRR *C/SRXB:DEST:NRR*SAD/DEST:NRR

DCKirkpatrick PNarbut CLyons MWHodges ACThadani

05/11/88 05/23/88 05/16/88 05/17/88 05/18/88

IN 88-XX

May xx, 1988 coolant would be forced up and out of the affected cold leg opening. As with

the mechanism already discussed, only about 2 1/2 psi would be required to

expel the water down to the top of the core with the coolant in the affected

cold leg at the level of a pump opening. Although in this case some steam

condensation may occur In the steam generators, as the 1987 Diablo Canyon event

showed, this will not prevent pressurization. Note that this mechanism, involving coolant expulsion through a cold leg opening, does not require

plugging the steam generator nozzles. As with the previous mechanism, this

hazard might be eliminated by venting the reactor vessel air space through a

large opening, such as a steam generator manway or pressurizer opening, before

opening the cold leg.

The loss of residual heat removal capability during low reactor coolant level

operation has proven to be a frequent occurrence; leading in several cases to

boiling in the reactor vessel. If this should occur, pressurization of the

reactor vessel can lead to sudden core uncovery by the expulsion of coolant

through any opening in the cold leg side of the reactor coolant system. This

hazard can be eliminated by providing a large vent for the reactor vessel air

space before opening the cold leg.

No specific action or written response is required by this information notice.

If you have any questions about this matter, please contact the technical

contact listed below or the Regional Administrator of the appropriate regional

office.

Charles E. Rossi, Director

Division of Operational Events Assessment

Ofc oNuclear Reactor Regulation

Technical Contacts: Paul P. Narbut, RV -

(805) 595-2354 Donald C. Kirkpatrick, NRR

(301) 492-1152 Attachment: List of Recently Issued NRC Information Notices

References:

1. NUREG-1269, "Loss of Residual Heat Removal System, Diablo Canyon, Unit 2",

April 10, 1987.

2. Generic Letter 87-12, "Loss of Residual Heat Removal While the Reactor

Coolant System is Partially Filled", July 9, 1987.

3. IN 87-23, "Loss of Decay Heat Removal During Low Reactor Coolant Level

Operation".

Transmitted by memo to C. H. Berlinger from Dennis F. Kirsch dated

March 21, 1988.

• SEE PREVIOUS CONCURRENCES

D/DOEA:NRR CACB-EA:NRR *PPMB:ARM *D/DEST:NRR

CERossi CHBerlinger TechEd LCShao

05/ /88 05/,Pb/88 05/12/88 05/18/88

• OGCB:DOEA:NRR *RV *SRXB:DEST:NRR *C/SRXB:DEST:NRR*SAD/DEST:NRR

DCKirkpatrick PNarbut WCLyons MWHodges ACThadani

05/11/88 05/23/88 05/16/88 05/17/88 05/18/88

IN 88-XX

May xx, 1988 be propagated throughout the remainder of the reactor coolant system, including

both hot and cold sides of steam generator primary spaces. This differential

pressure would force the coolant levels in the vessel down while the displaced

coolant would be forced up and out of the affected cold leg opening. As with

the mechanism already discussed, only about 2 1/2 psi would be required to expel

the water down to the top of the core with the coolant in the affected cold leg

at the level of a pump opening. Although in this case some steam condensation

may occur in the steam generators, as the 1987 Diablo Canyon event showed, this

will not prevent pressurization. Note that this mechanism, involving coolant

expulsion through a cold leg opening, does not require plugging the steam

generator nozzles. As with the previous mechanism, this hazard can be eliminated

by venting the reactor vessel air space through a large opening, such as a steam

generator or pressurizer manway, before opening the cold leg.

As this discussion indicates, loss of residual heat removal during low reactor

coolant level operation has proven to be a frequent occurrence, leading in

several cases to boiling in the reactor vessel. If this should occur, pressur- ization of the reactor vessel can lead to sudden core uncovery by the expulsion

of coolant through any opening in the cold leg side of coolant system. This

hazard can be eliminated by providing a large vent for the reactor vessel air

space before opening the cold leg.

No specific action or written response is required by this information notice.

If you have any questions about this matter, please contact the technical

contact listed below or the Regional Administrator of the appropriate regional

office.

Charles E. Rossi, Director

Division of Operational Events Assessment

Office-of Nuclear Reactor Regulation

Technical Contacts: Paul P. Narbut, RV

(805) 595-2354 Donald C. Kirkpatrick, NRR

(301) 492-1152 Attachment: List of Recently Issued NRC Information Notices

References:

1. NUREG-1269, "Loss of Residual Heat Removal System, Diablo Canyon, Unit 2",

April 10, 1987.

2. Generic Letter 87-12, "Loss of Residual Heat Removal While the Reactor

Coolant System is Partially Filled", July 9, 1987.

Transmitted by memo to C. H. Berlinger from Dennis F. Kirsch dated

March 21, 1988.

D/DOEA:NRR C/OGCB:DOEA:NRR PPMB:ARM D/DESR

CERossi CHBerlinger TechEd LCShto

05/ /88 05/ /88 05/ /88 05/I)/88 OGCB:DOEA:NRR RV-043c,0 SRXB:DEST:NRR C/SRXB:DEST: RR SAD/DEST:NRJ

DCKirkpatrick PNarbut WCLyon MWHodgesq*%,~ ACThajdani

05/ /88 O5/ 8 05s//1 f 88

05/n 05 /88

IN 88-XX

May xx, 1988 be propagated throughout the remainder of the reactor coolant system, including

both hot and cold sides of steam generator primary spaces. This differential

pressure would force the coolant levels in the vessel and steam generators down

while the displaced coolant would be forced up and out of the affected cold leg

opening. As with the mechanism already discussed, only about 2 1/2 psig would

be required to expel the water down to the top of the core with the coolant in

the affected cold leg at the level of the pump opening. Although in this case

some steam condensation may occur in the steam generators, as the 1987 Diablo

Canyon event showed, this will not necessarily prevent pressurization. Note

that this mechanism, Involving coolant expulsion through a cold leg opening, does not require the plugging the steam generator nozzles. As with the previ- ous mechanism, this hazard can be eliminated by venting the reactor vessel air

space through a large opening, such as a steam generator manway, prior to

opening the cold leg.

As this discussion indicates, loss of residual heat removal during low reactor

coolant level operation has proven to be a frequent occurrence, leading In

several cases to boiling in the reactor vessel. If this should occur, pressur- ization of the reactor vessel can quickly lead to core uncovery by expulsion of

coolant through any opening in the cold leg side of coolant system. This

hazard can be eliminated by providing a large vent for the reactor vessel air

space prior to making the cold leg opening.

No specific action or written response is required by this information notice.

If you have any questions about this matter, please contact the technical

contact listed below or the Regional Administrator of the appropriate regional

office.

Charles E. Rossi, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical Contacts: Paul P. Narbut, RV

(805) 595-2354 Donald C. Kirkpatrick, NRR

(301) 492-1152 Attachment: List of Recently Issued NRC Information Notices

References:

1. NUREG-1269, "Loss of Residual Heat Removal System, Diablo Canyon, Unit 2",

April 10, 1987.

2. Generic Letter 87-12, "Loss of Residual Heat Removal While the Reactor

Coolant System is Partially Filled", July 9, 1987.

Transmitted by memo to C. H. Berlinger from Dennis F. Kirsch dated

March 21, 1988.

D/DOEA:NRR C/OGCB:DOEA:NRR PPMB:ARM D/DEST:NRR

CERossi CHBerlinger TechEd Ma, LCShao

05/ /88 05/ /88 05/11/88 05/ /88 OGCB:DOEA:NRR RV SRXB:DEST:NRR C/SRXB:DEST:NRR SAD/DEST:NRR

DCKIrkpatr,.," PNarbut WCLyons MWHodges ACThadani

05/#/88 05/ /88 05/ /88 05/ /88 05/ /88