Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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
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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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