Information Notice 1992-18, Potential for Loss of Remote Shutdown Capability During a Control Room Fire

Potential for Loss of Remote Shutdown Capability During a Control Room Fire

ML031200481
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:	02/28/1992
From:	Rossi C Office of Nuclear Reactor Regulation
To:
References
IN-92-018, NUDOCS 9202240025
Download: ML031200481 (10)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 February 28, 1992 NRC INFORMATION NOTICE 92-18: POTENTIAL FOR LOSS OF REMOTE SHUTDOWN

CAPABILITY DURING A CONTROL ROOM FIRE

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 conditions found at several reactors that could

result in the loss of capability to maintain the reactor in a safe shutdown

condition in the unlikely event that a control room fire forced reactor

operators to evacuate the control room. 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.

Description of Circumstances

On July 26, 1991, the Washington Public Power Supply System, the licensee for

Unit 2 at the Washington Nuclear Plant.(WNP-2), discovered an unanalyzed

condition regarding fire protection and the safe shutdown capability for the

plant. The licensee found that a fire in the control room could cause hot

shorts, i.e. short circuits between control wiring and power sources, for

certain motor-operated valves (MOVs) needed to shut the reactor down and to

maintain it in a safe shutdown condition. If a fire in the control room

forces reactor operators to leave the control room, these MOVs can be

operated from the remote/alternate shutdown panel. However, hot shorts, combined with the absence of thermal overload protection, could cause valve

damage before the operator shifted control of the valves to the remote/

alternate shutdown panel.

Thermal overload protection is absent for some valves at WNP-2. This configu- ration exists to ensure that the thermal overload protection does not prevent

MOYs from performing their safety-related functions during an accident. As a

result, the thermal overload protection is configured to be either

continuously bypassed or bypassed only during an accident. Regulatory Guide

(RG) 1.106, Revision 1, 'Thermal Overload Protection for Electric Motors on

Motor-Operated Valves,* provides guidance in this area.

Figure 1 of Attachment 1 shows the control circuitry for-MOVs in a conceptual

manner, and the figure includes the relay coils which operate the contactors

9202240025 PDR 'LT T 0r% 9L-co0 I

Abbe- I/

IN 92-18 February 28, 1992 in the power circuitry for the motors. Figure 2 provides an example of the

manner in which the motor of an MOV that is closed can be energized and

damaged by a hot short if its overload protection is bypassed. The hot short

bypasses the push button that is normally used to close the MOV and thus

provides power to the relay coil, which closes those contactors that provide

power to drive the motor in the closed direction. Power will not be

disconnected from the motor although it is stalled, because the same hot

short bypasses the torque switch. With the motor stalled, current and torque

are abnormally high, possibly causing the motor windings to fail and possibly

causing mechanical damage to the valve. This mechanical damage may be suffi- cient to prevent reactor operators from manually operating the valve. A

similar problem can occur for MOYs that are open (see Figure 3). Shorts to

other sources of power can also cause failure of MOVs.

The licensee for WNP-2 determined that up to .15 MOVs in the residual heat

removal system might be adversely affected in this scenario. After

identifying the problem while the reactor was shut down, the-licensee

notified the NRC Operations Center and took corrective action. After con- ducting an initial analysis and before restarting the reactor, the licensee

rewired the control circuitry for the MOVs so that the torque and limit

switches in the valve operators are now located electrically between the

control room, the remote or alternate shutdown panel, and the motor control

center. Figure 4 is a diagram of the concept. Figures 5 and 6 show how hot

shorts can still cause the modified control circuitry to open or close the

MOYs. However, now the torque and limit switches are not bypassed by the hot

short, and the MOVs are protected from damage.

On November 20, 1991, the Pennsylvania Power and Light Company, licensee for

the Susquehanna Steam Electric Station, learned of the problem at WNP-2, determined that a similar condition existed for both units at the Susquehanna

Steam Electric Station, and notified the NRC Operations Center. Later, the

licensee stated that 37 MOVs which would be required to place the plant in a

safe shutdown condition could possibly be damaged by hot shorts occurring

during a fire in the control room. One of the MOVs is a reactor recirculation

suction valve, 15 are in the reactor core isolation cooling system, 16 are in

the residual heat removal system, and 5 are in the service water system.

On December 10, 1991, Northern States Power Company also found a similar

condition at the Monticello Nuclear Generating Plant.

Discussion

In RG 1.106, Revision 1, the staff stated that if thermal overload protection

devices are bypassed, it is important to ensure that the bypassing does not

result in Jeopardizing the completion of the safety function or in degrading

other safety systems because of any sustained abnormal circuit currents that

may be present.

IN 92-18 February 28, 1992 and valve operators are

When thermal overload protection devices are bypassed in sustained abnormal

wired as shown in Figure 1, hot shorts can result devices are not

circuit currents. Even if thermal overload protection

MOYs by tripping the thermal

bypassed, hot shorts can cause loss of power to of the motors for excessive

overload protection devices because of the demand capability to

current. In this case, hot shorts impair the it licensee's

in a safe shutdown condition.

efficiently shut its plant down and maintain operators to put them between

Rewiring the control circuitry for the valve

panel, and the motor

the control room, the remote or alternate shutdownand limit switches by hot

control centers prevents bypassing of the torque

shorts in the control room.

Related-Generic Communications

86-10, "Implementation of

The staff addressed hot shorts in Generic Letter 5.3.1.

Fire Protection Requirements," Enclosure 2, Section

or written response. If

This information notice requires no specific action notice, please contact

this

you have any question about the information inthe appropriate Office of

one of the technical contacts listed below or

Nuclear Reactor Regulation (NRR) project manager.

.Rosir Dere~recto=r!

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical contacts: James E. Knight, NRR

(301) 504-3264 Roger W. Woodruff, NRR

(301) 504-1152 Attachments:

for MOYs"

1. Figure 1, "Conceptual Control CircuitryWhile

Figure 2, "Postulated Short Occurring an MOV is Closed"

Figure 3, "Postulated Short Occurring While an MOV is Open"

Figure 4, "Conceptual Modification of Control Circuitry for MOYs"

Modified Control

Figure 5, "Postulated Short Occurring While an MOV with

Circuitry is Closed"

with Modified Control

Figure 6, "Postulated Short Occurring While an MOv

Circuitry is Open"

2. List of Recently Issued NRC Information Notices

Attachment 1 IN 92-18 February 28, 1992 LEGEND

CONTACT

ACTUATION POI NTS

CR CONTROL ROOM LO

..

RSP REMOTE SHUTDOWN PANEL

MCC MOTOR CONTROL CENTER

a LC

VO VALVE OPERATOR

MC RELAY COIL - CLOSE VALVE

MO RELAY COIL - OPEN VALVE I I I I III I 1 --

OPEN VALVE STROKE CLOSED

C PUSH BUTTON - CLOSE VALVE

0 PUSH BUTTON - OPEN VALVE

LC LIMIT SVITCH - CLOSE VALVE

• THE LINES INDICATE THAT THE SWITCH

CONTACT

S

LO LIMIT SWITCH - OPEN VALVE ARE CLOSED. THE POINTS INDICATE THE VALVE

T TORQUE SWITCH POSITIONS WHERE THE SWITCH

CONTACT

S OPEN AND.

0 GREEN LMP CLOSE. FOR THE TORQUE SWITCH, THE

CONTACT

S

R RED LAIP ARE ACTUATED BY THE POSITION OF THE VALVE

MI5K AT MID STROKE AND BY THE PRESET TORQUE

AT THE END OF THE CLOSING STROKE.

FIGURE 1. CONCEPTUAL CONTROL CIRCUITRY FOR MOVs

I

Attachment 1 IN 92-18 February 28, 1992 LEGEND

CONTACT

ACTUATION POINTS *

CR CONTROL ROOM LO a

RSP REMOTE SHUTDOWN PANEL -. T

MCC MOTOR CONTROL CENTER

LC ___

VO VALVE OPERATOR

mC RELAY COIL - CLOSE VALVE

I I I I I l.

MO RELAY COIL - OPEN VALVE

OPEN VALVE STROKE

I I

C PUSH BUTTON - CLOSE VALVE 'CLOSED

0 PUSH BUTTON - OPEN-VALVE

LC LIMIT SWITCH - CLOSE VALVE

• THE LINES INDICATE THAT THE SWITCH

CONTACT

S

LO LIMIT SWITCH - OPEN VALVE ARE CLOSED. THE POINTS INDICATE THE VALVE

T TORQUE SWITCH POSITIONS WHERE THE SWITCH

CONTACT

S OPEN AND

0 GREEN LAMP CLOSE. FOR THE TORQUE SWITCH. THE

CONTACT

S

R RED LAMP ARE ACTUATED BY THE POSITION OF THE VALVE

S HOT SHORT DISK AT MID STROKE AND BY THE PRESET TORQUE

AT THE END OF THE CLOSING STROKE.

F I GURE 2. POSTULATED SHORT OCCUR I NG WH I LE AN MOV I S CLOSED

I

Attachment 1 IN 92-18 February 28, 1992 LEGEND

CONTACT

ACTUATION POINTS -

LO

CR CONTROL ROOM T

RSP REMOTE SHUTDOWN PANEL

MCC MOTOR CONTROL CENTER LC

VO VALVE OPERATOR

MC RELAY COIL - CLOSE VALVE I I I I I -1 I I I I

MO RELAY COIL - OPEN VALVE OPEN VALVE STROKE CLOSED

C PUSH BUTTON - CLOSE VALVE

0 PUSH BUTTON - OPEN VALVE

LC LIMIT SWITCH - CLOSE VALVE X THE LINES INDICATE THAT THE SWITCH

CONTACT

S

LO LIMIT SWITCH - OPEN VALVE ARE CLOSED. THE POINTS INDICATE THE VALVE

T TORQUE SWITCH POSITIONS WHERE THE SWITCH

CONTACT

S OPEN AND

0 GREEN LAMP CLOSE. FOR THE TOROUE SWITCH. THE

CONTACT

S

RED LAMP ARE ACTUATED BY THE POSITION OF THE VALVE

R

S HOT SHORT OtSK AT MID STROKE AND By THE PRESET TORGUE

AT THE END OF THE CLOSING STROKE.

FIGURE 3. POSTULATED SHORT OCCURRING WHILE AN MOV IS OPEN

Attachment 1 IN 92-18 February 28, 1992 LEGEND

CONTACT

ACTUATION POINTS '

CR CONTROL ROOM LO

RSP REMOTE SHUTDOWN PANEL

MCC MOTOR CONTROL CENTER

VO VALVE OPERATOR LC

mC RELAY COIL - CLOSE VALVE p

MO RELAY COIL - OPEN VALVE I I I I I u I

.

I

.

I I

.

C PUSH BUTTON - CLOSE VALVE OPEN VALVE STROKE CLOSED

0 PUSH BUTTON - OPEN VALVE

LC LIMIT SWITCH - CLOSE VALVE

• THE LINES INDICATE THAT THE SWITCH

CONTACT

S

LO LIMIT SWITCH - OPEN VALVE ARE CLOSED. THE POINTS INDICATE THE VALVE

T TORQUE SWITCH POSITIONS WHERE THE SWITCH

CONTACT

S OPEN AND

0 GREEN LAMP CLOSE. FOR THE TORQUE SWITCH. THE

CONTACT

S

R RED LAM ARE ACTUATED BY THE POSITION OF THE VALVE

DISK AT MID STROKE AND BY THE PRESET TORQUE

AT THE END OF THE CLOSING STROKE.

FIGURE 4. CONCEPTUAL MODIFICATION OF.CONTROL CIRCUITRY FOR MOVs

Attachment 1 IN 92-18 February 28, 1992 LEGEND

CONTACT

ACTUATION POINTS 6

CR CONTROL ROOM LO

RSP REMOTE SHUTDOWN PANEL T

MCC MOTOR CONTROL CENTER

LC

VO VALVE OPERATOR

Mc RELAY COIL - CLOSE VALVE r

l l. l. l

MO RELAY COIL - OPEN VALVE

lI I I I I

I I I I

OPEN VALVE STROKE CLOSED

C PUSH eUTTON - CLOSE VALVE

0 PUSH BUTTON - OPEN VALVE

LC LIMIT SVITCH - CLOSE VALVE

• THE LINES INDICATE THAT THE SWITCH

CONTACT

S

LO LIMIT SWITCH - OPEN VALVE ARE CLOSED. THE POINTS INDICATE THE VALVE

T TORQUE SWITCH POSITIONS WHERE THE SWITCH

CONTACT

S OPEN AND

G GREEN LAMP CLOSE. FOR THE TORQUE SWITCH, THE

CONTACT

S

R RED LAMP ARE ACTUATED BY THE POSITION OF THE VALVE

S HOT SHORT DISK AT MID STROKE AND BY THE PRESET TORQUE

AT THE END OF THE CLOSING STROKE.

I

FIGURE 5. POSTULATED SHORT OCCURRINO WHILE AN MOV WITH

MODIFIED CONTROL CIRCUITRY IS CLOSED

Attachment 1 IN 92-18 February 28, 1992 LEGEND

CONTACT

ACTUAT I ON PO INTS *

LO

CR CONTROL ROOM

RSP REUOTE SHUTDOWN PANEL T

MCC MOTOR CONTROL CENTER LC

VO VALVE OPERATOR

MC RELAY COIL - CLOSE VALVE . I

. I

. I

. I II

. I

I I

I

I

C S

I

' VII I

MO RELAY COIL - OPEN VALVE OPEN VALVE STRC0KE CLOSED

C PUSH BUTTON - CLOSE VALVE

0 PUSH BUTTON - OPEN VALVE

LC LIMIT SWITCH - CLOSE VALVE U THE LINES INDICATE THAT THE SWITCH

CONTACT

S

LO LIMIT SWITCH - OPEN VALVE ARE CLOSED. THE POINTS INDICATE THE VALVE

T TOROUE SWITCH POS IT I ONS WHERE THE SW I TCH

CONTACT

S OPEN AND

0 GREEN LAMP CLOSE. FOR THE TORQUE SWITCH, THE

CONTACT

S

R RED LAMP ARE ACTUATED BY THE POSITION OF THE VALVE

S HOT SHORT DISK AT MID STROKE AND BY THE PRESET TORQUE

AT THE END OF THE CLOS ING STROKE.

FIGURE B. POSTULATED SHORT OCCURR INO WHILE AN MOV WITH

MODIFIED CONTROL CIRCUITRY IS OPEN

I

Attachment 2 IN 92-18 February 28, 1992 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Notice No. Subject Issuance Issued to

92-17 NRC Inspections of Pro- 02/26/92 All holders of OLs or CPs

grams being Developed at for nuclear power reactors.

Nuclear Power Plants in

Response to Generic

Letter 89-10

92-16 Loss of Flow from the 02/25/92 All holders of OLs or CPs

Residual Heat Removal for nuclear power reactors.

Pump during Refueling

Cavity Draindown

92-15 Failure of Primary System 02/24/92 All holders of OLs or CPs

Compression Fitting for nuclear power reactors.

92-14 Uranium Oxide Fires at Fuel 02/21/92 All fuel cycle and uranium

Cycle Facilities fuel research and development

licensees.

92-02, Relap5/Mod3 Computer Code 02/18/92 All holders of OLs or CPs

Supp. 1 Error Associated with the for nuclear power reactors.

Conservation of Energy

Equation

92-13 Inadequate Control Over 02/18/92 All holders of OLs or CPs

Vehicular Traffic at for nuclear power reactors.

Nuclear Power Plant Sites

92-12 Effects of Cable Leakage 02/10/92 All holders of OLs or CPs

Currents on Instrument for nuclear power reactors.

Settings and Indications

92-11 Soil and Water Contamina- 02/05/92 All uranium fuel fabrica- tion at Fuel Cycle Facil- tion and conversion facil- ities ities.

OL = Operating License

CP = Construction Permit