Information Notice 1992-18, Potential for Loss of Remote Shutdown Capability During a Control Room Fire
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
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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 When thermal overload protection devices are bypassed and valve operators are
wired as shown in Figure 1, hot shorts can result in sustained abnormal
circuit currents. Even if thermal overload protection devices are not
bypassed, hot shorts can cause loss of power to MOYs by tripping the thermal
overload protection devices because of the demand of the motors for excessive
current. In this case, hot shorts impair the licensee's capability to
efficiently shut its plant down and maintain it in a safe shutdown condition.
Rewiring the control circuitry for the valve operators to put them between
the control room, the remote or alternate shutdown panel, and the motor
control centers prevents bypassing of the torque and limit switches by hot
shorts in the control room.
Related-Generic Communications
The staff addressed hot shorts in Generic Letter 86-10, "Implementation of
Fire Protection Requirements," Enclosure 2, Section 5.3.1.
This information notice requires no specific action or written response.
If
you have any question 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.
.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:
1. Figure 1, "Conceptual Control Circuitry for MOYs"
Figure 2, "Postulated Short Occurring While an MOV is Closed"
Figure 3, "Postulated Short Occurring While an MOV is Open"
Figure 4, "Conceptual Modification of Control Circuitry for MOYs"
Figure 5, "Postulated Short Occurring While an MOV with Modified Control
Circuitry is Closed"
Figure 6, "Postulated Short Occurring While an MOv with Modified Control
Circuitry is Open"
2. List of Recently Issued NRC Information Notices
Attachment 1
IN 92-18 February 28, 1992
CONTACT
ACTUATION POI NTS
..
LEGEND
CR
VO
MC
MO
C
0
T
0
R
CONTROL ROOM
REMOTE SHUTDOWN PANEL
MOTOR CONTROL CENTER
VALVE OPERATOR
RELAY COIL - CLOSE VALVE
RELAY COIL - OPEN VALVE
PUSH BUTTON - CLOSE VALVE
PUSH BUTTON - OPEN VALVE
LIMIT SVITCH - CLOSE VALVE
LIMIT SWITCH - OPEN VALVE
TORQUE SWITCH
GREEN LMP
RED LAIP
a
I
I
I
I
III
I
1 - -
OPEN
VALVE STROKE
CLOSED
- THE LINES INDICATE THAT THE SWITCH
CONTACT
S
ARE CLOSED.
THE POINTS INDICATE THE VALVE
POSITIONS WHERE THE SWITCH
CONTACT
S OPEN AND.
CLOSE.
FOR THE TORQUE SWITCH, THE
CONTACT
S
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
CONTACT
ACTUATION POINTS *
LEGEND
CR
VO
mC
MO
C
0
T
0
R
S
CONTROL ROOM
REMOTE SHUTDOWN PANEL
MOTOR CONTROL CENTER
VALVE OPERATOR
RELAY COIL - CLOSE VALVE
RELAY COIL - OPEN VALVE
PUSH BUTTON - CLOSE VALVE
PUSH BUTTON - OPEN-VALVE
LIMIT SWITCH - CLOSE VALVE
LIMIT SWITCH - OPEN VALVE
TORQUE SWITCH
GREEN LAMP
RED LAMP
a
-.
T
___
OPEN
I
I
I
I
I
l.
VALVE STROKE
I
I
'CLOSED
- THE LINES INDICATE THAT THE SWITCH
CONTACT
S
ARE CLOSED.
THE POINTS INDICATE THE VALVE
POSITIONS WHERE THE SWITCH
CONTACT
S OPEN AND
CLOSE.
FOR THE TORQUE SWITCH. THE
CONTACT
S
ARE ACTUATED BY THE POSITION OF THE VALVE
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
CONTACT
ACTUATION POINTS -
T
LEGEND
CR
VO
MC
MO
C
0
T
0
R
S
CONTROL ROOM
REMOTE SHUTDOWN PANEL
MOTOR CONTROL CENTER
VALVE OPERATOR
RELAY COIL - CLOSE VALVE
RELAY COIL - OPEN VALVE
PUSH BUTTON - CLOSE VALVE
PUSH BUTTON - OPEN VALVE
LIMIT SWITCH - CLOSE VALVE
LIMIT SWITCH - OPEN VALVE
TORQUE SWITCH
GREEN LAMP
RED LAMP
I
I
I
I
I
-1 I
I
I
I
CLOSED
OPEN
VALVE STROKE
X THE LINES INDICATE THAT THE SWITCH
CONTACT
S
ARE CLOSED.
THE POINTS INDICATE THE VALVE
POSITIONS WHERE THE SWITCH
CONTACT
S OPEN AND
CLOSE.
FOR THE TOROUE SWITCH.
THE
CONTACT
S
ARE ACTUATED BY THE POSITION OF THE VALVE
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
CONTACT
ACTUATION POINTS '
LEGEND
CR
VO
mC
MO
C
0
T
0
R
CONTROL ROOM
REMOTE SHUTDOWN PANEL
MOTOR CONTROL CENTER
VALVE OPERATOR
RELAY COIL - CLOSE VALVE
RELAY COIL - OPEN VALVE
PUSH BUTTON - CLOSE VALVE
PUSH BUTTON - OPEN VALVE
LIMIT SWITCH - CLOSE VALVE
LIMIT SWITCH - OPEN VALVE
TORQUE SWITCH
GREEN LAMP
RED LAM
p
OPEN
I
I
I
I
I
u
VALVE STROKE
.
.
.
I
I
I
I
CLOSED
- THE LINES INDICATE THAT THE SWITCH
CONTACT
S
ARE CLOSED.
THE POINTS INDICATE THE VALVE
POSITIONS WHERE THE SWITCH
CONTACT
S OPEN AND
CLOSE.
FOR THE TORQUE SWITCH. THE
CONTACT
S
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
CONTACT
ACTUATION POINTS 6
T
LEGEND
CR
VO
Mc
MO
C
0
T
G
R
S
CONTROL ROOM
REMOTE SHUTDOWN PANEL
MOTOR CONTROL CENTER
VALVE OPERATOR
RELAY COIL - CLOSE VALVE
RELAY COIL - OPEN VALVE
PUSH eUTTON - CLOSE VALVE
PUSH BUTTON - OPEN VALVE
LIMIT SVITCH - CLOSE VALVE
LIMIT SWITCH - OPEN VALVE
TORQUE SWITCH
GREEN LAMP
RED LAMP
OPEN
r
.
.
l
l
l
l
l
I
I
I
I
I
VALVE STROKE
I
I
I
I
CLOSED
- THE LINES INDICATE THAT THE SWITCH
CONTACT
S
ARE CLOSED. THE POINTS INDICATE THE VALVE
POSITIONS WHERE THE SWITCH
CONTACT
S OPEN AND
CLOSE.
FOR THE TORQUE SWITCH, THE
CONTACT
S
ARE ACTUATED BY THE POSITION OF THE VALVE
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
CONTACT
ACTUAT I ON PO INTS *
T
LEGEND
CR
VO
MC
MO
C
0
T
0
R
S
CONTROL ROOM
REUOTE SHUTDOWN PANEL
MOTOR CONTROL CENTER
VALVE OPERATOR
RELAY COIL - CLOSE VALVE
RELAY COIL -
OPEN VALVE
PUSH BUTTON - CLOSE VALVE
PUSH BUTTON - OPEN VALVE
LIMIT SWITCH - CLOSE VALVE
LIMIT SWITCH - OPEN VALVE
TOROUE SWITCH
GREEN LAMP
RED LAMP
I
I
I
I
I
I
I
I
I
.
.
.
.
.
OPEN
VI
'
I
I
VALVE STRC
I
I
I
C S
0KE
CLOSED
U THE LINES INDICATE THAT THE SWITCH
CONTACT
S
ARE CLOSED.
THE POINTS INDICATE THE VALVE
POS IT I ONS WHERE THE SW I TCH
CONTACT
S OPEN AND
CLOSE.
FOR THE TORQUE SWITCH, THE
CONTACT
S
ARE ACTUATED BY THE POSITION OF THE VALVE
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
February 28, 1992 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
Information
Date of
Notice No.
Subject
Issuance
Issued to
92-17
92-16
92-15
92-14 NRC Inspections of Pro- grams being Developed at
Nuclear Power Plants in
Response to Generic
Letter 89-10
Loss of Flow from the
Pump during Refueling
Cavity Draindown
Failure of Primary System
Compression Fitting
Uranium Oxide Fires at Fuel
Cycle Facilities
Relap5/Mod3 Computer Code
Error Associated with the
Conservation of Energy
Equation
Inadequate Control Over
Vehicular Traffic at
Nuclear Power Plant Sites
Effects of Cable Leakage
Currents on Instrument
Settings and Indications
Soil and Water Contamina- tion at Fuel Cycle Facil- ities
02/26/92
02/25/92
02/24/92
02/21/92
02/18/92
02/18/92
02/10/92
02/05/92
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 fuel cycle and uranium
fuel research and development
licensees.
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 uranium fuel fabrica- tion and conversion facil- ities.
92-02, Supp. 1
92-13
92-12
92-11 OL = Operating License
CP = Construction Permit