United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 92-18: 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 
configuration exists to ensure that the thermal overload protection does not 
prevent MOVs 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 
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                                                        IN 92-18
                                                        February 28, 1992
                                                        Page 2 of 3


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 sufficient to prevent reactor operators from manually operating the 
valve.  A similar problem can occur for MOVs 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 
MOVs.  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.

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                                                        IN 92-18
                                                        February 28, 1992
                                                        Page 3 of 3


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 MOVs 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.




                                  Charles E. Rossi, Director
                                  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 MOVs"
    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 MOVs"
    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
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