Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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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IN 90-22
March 23, 1990
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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
March 23, 1990
Page 3 of 3
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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