Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 83-08
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
March 9, 1983
Information Notice No. 83-08: COMPONENT FAILURES CAUSED BY ELEVATED DC
CONTROL VOLTAGE
Addressees:
All holders of a nuclear power reactor operating license (OL) or
construction permit (CP).
Purpose:
This information notice is provided as a notification of a potentially
significant problem pertaining to premature degradation failure of
equipment, caused by elevated DC control voltage in safety-related circuits.
Because of the potential safety significance and related generic
implications of this problem, addressees are expected to review the
information for applicability to their facilities. No specific action or
response is required
Description of Circumstances:
The folloWing three events, covered in Licensee Event Reports (LERs),
indicate problems in safety-related DC control circuits where equipment
degraded prematurely and caused short circuits and control problems,
apparently as a result of DC voltages that exceeded the design voltage.
1. On October 3, 1982, at the Trojan Nuclear Plant, the indicating lamp
socket associated with a control switch on a 125V DC system broke and
fell into the panel, shorting out associated control circuitry. This
caused the fuse to blow, resulting in the loss of control power for the
startup of an emergency diesel generator, and thus the diesel startup
capability was lost.
This event occurred as a result of an excess voltage condition which
led to thermal breakdown of the lamp socket. The 125V DC system
continuously subjected the lamp socket to its maximum rated voltage of
130V DC. In this case a higher than nominal operating voltage caused
the lamp socket to become brittle because excessive heat was generated
and also necessitated more frequent lamp replacement.
On the basis of the results of an engineering evaluation, two cells
were removed from each station battery; this reduced the system voltage
and battery charger output voltage. The licensee then conducted a load
profile test to demonstrate adequate ampere-hour capacity for the
design load. The bus voltage did not drop below the minimum acceptable
level.
8212060372
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IN 83-08
March 09, 1983
Page 2 of 2
2. On August 30, 1982, during preoperational testing of hydrogen monitors
at the Fort Calhoun Nuclear Plant, a control room operator noticed that
two associated containment isolation valves had no position indication.
One of the solenoid valves had an internally shorted coil; the other
valve had a coil shorted to ground. These shorts blew the fuse. The
blown fuse caused the loss of valve position indication and allowed the
valves to fail open. Followup evaluation indicated that the solenoid
valves were intended for service at 125V DC +/- 10 percent. During the
month preceding the failures, the station batteries had been placed on
an equalizing charge of 140V DC. The licensee concluded that this
elevated voltage caused the valves to fail since they remained
energized for the entire period of time.
The licensee intends to replace all solenoid valve coils with coils
designed to operate at a higher voltage.
3. On March 15, 1982, at the Zion Nuclear Generating Station, a relay coil
in a safety related reactor trip relay burned up and failed in a
nonconservative mode. The failed relay coil was a replacement and had
been rated at 120V DC; whereas the original relay coil had been rated
at 125/130V DC. The licensee concluded that the replaced relay coil
failed as the result of overheating that had been caused by five years
of operation at elevated voltage. During a follow-up evaluation, five
additional relays with 120V DC coil rating in a 130V DC system were
identified. These relays were replaced.
These events show that DC safety-related control components and indicating
circuit components which operate for a sustained period of time at elevated
voltages or voltages above their rated design voltage are subject to
accelerated degradation which may have some impact on plant safety. A
careful balance of rated voltage for components in DC systems must be
maintained to assure maximum voltage during equalizing charging doesn't
adversely affect components and that those components which are required to
function in an emergency remain operable at minimum battery voltages at
design ampere-hour capacity.
No written response to this notice is required. If you have any questions
regarding this matter, please contact the Regional Administrator of the
appropriate NRC Regional Office, or this office.
Edward L. Jordan Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: W. Laudan
301-492-9759
Attachment:
List of Recently Issued Information Notices
