United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 90-22: Unanticipated Equipment Actuations Following Restoration of Power to Rosemount Transmitter Trip Units

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555

                               March 23, 1990


Information Notice No. 90-22:  UNANTICIPATED EQUIPMENT ACTUATIONS
                                   FOLLOWING RESTORATION OF POWER TO
                                   ROSEMOUNT TRANSMITTER TRIP UNITS


Addressees: 

All holders of operating licenses or construction permits for nuclear power 
reactors.

Purpose:

This information notice is intended to alert addressees to potential 
problems resulting from the reenergization of Rosemount transmitter trip 
units. 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: 

The River Bend Station was operating at 100-percent power on February 11, 
1990.  While maintenance was being performed on a division 2 battery 
charger, a sudden increase in dc bus voltage occurred when the battery 
charger was placed in the equalize mode of operation.  The voltage increase 
had a magnitude of less than 10-percent above the nominal voltage while in 
the equalize mode.  However, this voltage was sufficient to cause the 
automatic shutdown of a Topaz inverter connected to the battery bus that was 
being energized by the charger.  In turn, the Topaz inverter deenergized a 
number of Rosemount transmitter trip units.  The licensee was aware of the 
potential for unwanted equipment actuations upon restoration of power to the 
transmitter trip units and removed what was believed to be the affected 
equipment from service.  The battery charger voltage was then lowered to 
allow the inverter to reset.  Upon resetting the inverter, unexpected trip 
signals occurred that resulted in a number of unanticipated equipment 
actuations.  The most notable of these actuations was the opening of the 
low-pressure coolant injection (LPCI) valves while the plant was at full 
power.  The unanticipated trip signals and actuations occurred because the 
operators did not have procedures and/or a load list available and could not 
predict all components that were affected.  
 
Topaz inverters have an automatic reset feature that operates when the 
supply voltage returns to normal.  When the battery charger voltage was 
lowered, the 




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Rosemount trip units powered by the inverter were immediately reenergized.  
The transmitters required several additional milliseconds to completely 
reenergize.  The transmitter inputs to the trip units decrease to zero on 
loss of power, and therefore the inputs were equal to zero when power was 
restored.  

Those trip units that actuate on a decreasing input (i.e., a trip occurs 
when the input goes below the setpoint value) reenergized in the tripped 
condition, even though the actual value of the monitored variable was above 
the trip setpoint.  In the first few milliseconds after the inverter reset, 
the trip units detected "low" reactor vessel water level which satisfied the 
LPCI initiation logic, and "low" reactor pressure which satisfied the low 
pressure permissive required to open the LPCI valves.  These signals caused 
the division 2 (trains B and C) LPCI valves to automatically open as well as 
numerous other actuations.  This resulted in a single check valve isolation 
of the high-pressure reactor coolant system from the low-pressure residual 
heat removal (RHR) system.  Failure of the single check valve would have 
overpressurized the RHR piping and could have resulted in loss of reactor 
coolant, for example, through the failure of RHR pump seals or other 
components. 

On January 3, 1984, and February 23, 1985, Grand Gulf experienced nearly 
identical events.  In both instances, the plant was at cold shutdown and 
low-pressure coolant injection occurred.  As in the event at River Bend, a 
higher than expected voltage occurred when the battery charger was placed in 
equalize, causing the Topaz inverters to trip.  When the bus voltage was 
lowered, the inverters reset and reenergized the Rosemount transmitter trip 
units.

A similar event occurred at Hope Creek on April 14, 1989.  The licensee 
reported that while attempting to restore the Topaz inverter, a voltage 
transient caused a loss-of-coolant accident initiation signal.  The 
high-pressure coolant injection system was secured before water was injected 
into the reactor vessel; at the time, the plant was at 100-percent power.

Discussion: 

The effects of restoration of power to safety-related equipment that is 
normally energized are typically not as thoroughly analyzed as the effects 
of losses of power.  Since the potential for serious consequences exists 
from unanticipated component actuations, careful consideration of the 
effects of restoration of power to Rosemount transmitter trip units is 
warranted.  The potential may also exist for unanticipated plant response to 
restoration of power to similar equipment from other manufacturers.  
Awareness of the potential consequences of rendering numerous safety-related 
components inoperable during power operation to prevent equipment from 
actuating upon restoration of power is also important.  Degraded transient 
and accident response capability and the potential for human errors during 
these operations are of particular concern. 


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                                                            IN 90-22
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The NRC-approved design described in NEDO 21617, "Analog Transmitter Trip 
Unit System for Engineered Safeguard Sensor Trip Inputs", includes a 
redundant voltage converter leading to the trip units.  The voltage 
converters are auctioneered so if one is lost, the other voltage converter 
supplies power.  The use of the redundant voltage converter would decrease 
the probability of deenergizing the transmitter trip units.  However, other 
power supply failures upstream of the voltage converters may also produce 
events similar to those described above.

This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please contact 
one of the technical contacts listed below or the appropriate NRR project 
manager. 




                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation

Technical Contacts:  P. C. Wagner, RIV
                     (817) 860-8127

                     A. Mattson, NRR
                     (301) 492-1177
               
                     R. Kendall, NRR
                     (301) 492-1192

Attachment:  List of Recently Issued NRC Information Notices
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