Information Notice No. 85-51: Inadvertent Loss or Improper Actuation of Safety-related Equipment
SSINS No.: 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
July 10, 1985
Information Notice No. 85-51: INADVERTENT LOSS OR IMPROPER ACTUATION OF
All nuclear power reactor facilities holding an operating license (OL) or a
construction permit (CP).
This information notice is provided to alert licensees of potentially
significant reactor safety problems that may be a byproduct of the normal
practice of removing fuses or of opening circuit breakers for personnel
protection during maintenance and plant modification activities. The reactor
safety concern may result when the effects of electrical power interruption
on all circuits powered by the fuse or breaker are not fully reviewed in
advance. Errors in the review have resulted in unknowingly disabling safety
systems and also have caused inadvertent actuation of safety systems. It is
suggested that recipients review this information for applicability to their
facilities and consider actions, if appropriate, to preclude similar
problems at their facilities. However, suggestions contained in this
information notice do not constitute NRC requirements; therefore, no
specific action or written response is required.
Description of Circumstances:
At Susquehanna Unit 2 on July 9, 1984 with the plant at approximately 20% of
full power electricians removed two dc-control power fuses for personnel
protection during modifications involving the core spray isolation logic.
The electricians believed that removing these fuses would provide the
nearest local blocking-point protection needed while performing the
modification. However, the fuses that were removed were considerably
"upstream" of the local blocking point and the following situations resulted
from this improper action:
1. Signals to start the pumps and position valves for the A loop of the
core spray system were lost.
2. One of the diesel generators would not have received a "Start" signal
from the Division 1 core spray logic that is provided for a
loss-of-coolant accident (LOCA) condition associated with Unit 2.
July 10, 1985
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3. The A and C instrumentation channels, sensing reactor water level and
drywell pressure, were made inoperable. Because of this, the residual
heat removal system and high pressure injection system would not have
received an actuation signal from those channels in the event of an
accident. However, the B and D channels remained functional.
4. A partial isolation signal for drywell cooling was generated.
5. The load shedding feature of the A and C 4160 V ac essential buses
associated with Units 1 and 2 were disabled, and the instrument air
compressors for Unit 2 would not have tripped if a LOCA condition had
existed for Unit 2.
As a result of this event, the licensee instituted training sessions for
personnel. The training sessions emphasized review and analysis of the
circuits involved in all current and future construction work orders at the
Susquehanna facility and included a human factors analysis focusing on the
adequacy of the status switch features for the core spray system and other
Following the event at Susquehanna Unit 2, the NRC conducted a search for
other licensee event reports (LERs) from 1981 through 1984 that had similar
cause and effect. This search resulted in the identification of five
additional events which may be indicative that the problem is widespread.
The events described in these reports are briefly summarized in Attachment
1. The event described above and those summarized in Attachment 1 illustrate
how the practice of removing fuses may result in actuation or disabling of
safety-related equipment during any mode of plant operation. At the time the
fuses were removed, the involved plant personnel were unaware of the
resulting actuation and inoperabilities. Similar situations could occur when
electrical circuits are de-energized by operating circuit breakers for
The practice of de-energizing circuitry in order to provide plant personnel
with appropriate protection is unavoidable. Corrective and preventive
actions by licensees have emphasized the following items: identification of
effects on plant equipment or systems, independent verification of the
evaluation of effects, and utilization of the nearest local fuse or circuit
breaker to minimize the number of systems affected.
July 10, 1985
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No specific action or written response is required by this information
notice. If you have any questions about this matter, please contact the
Regional Administrator of the appropriate regional office or this office.
Edward L. Jordan Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: V. D. Thomas, IE
1. Earlier Events Similar to the One at Susquehanna
2. List of Recently Issued IE Information Notices
July 10, 1985
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EARLIER EVENTS SIMILAR IN NATURE TO THE ONE
Surry Station, September 1981
In this event, an electrician was attempting to remove a battery in the
plant's smoke, detector system. The electrician did not wish to leave
energized wiring exposed and therefore he removed a line, fuse. This action
disabled the smoke detector panel that provides early detection of fires,
thereby introducing the potential for damage of safety-related equipment.
The licensee attributed the cause of this event to personnel error in that
the electrician did not realize that removing the line fuse would disable
the smoke detector panel. Corrective action taken to prevent recurrence of
this event was to revise the labeling of the smoke detector battery chargers
and associated circuit panels with a caution tag.
Oyster Creek Station, December 1981
While performing maintenance activities to repair a faulty electromatic
relief valve pressure switch, dc-control power fuses were removed, resulting
in the inoperability of one trip system in the automatic depressurization
system (ADS). The licensee reported that the cause of the loss of ADS trip
system redundancy was the removal of the power fuses by plant personnel,
without realizing the consequences on the ADS control logic circuitry.
However, had a plant condition been present that required the operation of
the ADS, the redundant trip system would have actuated the four remaining
relief valves to depressurize the reactor system.
To prevent recurrence of this reportable occurrence, the licensee
incorporated it in the required reading program for Shift Operations
Supervisors and Instrument Department Personnel. Additionally, the power
fuses that defeat the redundancy of the ADS have been identified with a
Sequoyah Unit 1, September 1982
This licensee reported that during modifications to train "B" of the
solid-state protection system (SSPS), the power fuses were removed to
facilitate work on the output relays. This caused the train "B" reactor heat
removal (RHR) suction valve to close rendering that system inoperable. A
review of the drawings associated with the SSPS showed that the power supply
to the output relays also supplied power to a relay that operates the RHR
suction valve. When this relay is de-energized, the valve automatically
closes. The operator immediately returned the system to normal operating
A change was made to the facility work plan covering SSPS modification to
inform operators that removal of the power fuses isolates the associated
train of the RHR suction valve. The licensee also reports that caution signs
were placed near the location of the fuses in the SSPS cabinets.
July 10, 1985
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Diablo Canyon Unit 1, May 1983
The event at Diablo Canyon Unit 1 during May 1983 was similar to the events
discussed above, in that personnel at the plant removed power fuses to
perform work activity. This action resulted in disabling of radiation
To prevent recurrence, plant personnel have been instructed to ensure that
all effects on plant equipment are known and recognized before approving
clearances for work activity.
Susquehanna Unit 1, April 1984
This earlier event at Susquehanna Unit 1 also was caused by removing power
fuses for personnel protection. Plant personnel removed two fuses associated
with the primary containment isolation logic for Unit 2 to perform a
modification for the logic circuitry. This resulted in the actuation of a
false high drywell pressure signal, which, in turn, actuated the common
control room emergency outside air supply and standby gas treatment systems.
The licensee later discovered that an improperly placed wire jumper in
conjunction with fuse removal actually caused the false actuation.
Subsequently, the wire jumper was installed properly.
To prevent recurrence of this event, the subject work activity and
associated wiring error were reviewed with the work crew involved. During
this review the licensee also instructed personnel to review and verify
circuitry before de-energizing power sources to equipment scheduled for
maintenance or modification.
Page Last Reviewed/Updated Friday, May 22, 2015