Information Notice No. 83-18: Failures of the Undervoltage, Trip Function of Reactor Trip System Breakers
SSINS No.: 6835
IN 83-18
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND PROCEDURES
WASHINGTON, D.C. 20555
April 1, 1983
Information Notice No. 83-18: FAILURES OF THE UNDERVOLTAGE, TRIP
FUNCTION OF REACTOR TRIP SYSTEM BREAKERS
Addressees:
All nuclear power reactor facilities holding an operating license (OL) or
construction permit (CP).
Purpose:
This notice describes recent failures of reactor trip system circuit
breakers with undervoltage (UV) trip attachments and provides additional
information related to the UV attachment.
Description of Circumstances:
Reactor Trip System (RTS) breaker failures at Salem Unit 1 on February 22
and 25 and RTS breaker failures at San Onofre Units 2 and 3 reported to the
NRC on March 11, 1983 led to issuance of IE Bulletins (IEB) 83-01 and 83-04
respectively. Results of testing of RTS breakers required by IE Bulletins
83-01 and 83-04 have been reported to the NRC. These results show that
electrical breakers may not be achieving the performance reliability
expected of them. The problem apparently lies with the undervoltage trip
attachment. Failures of Westinghouse DB type, Westinghouse DS type and
General Electric AK-2 type RTS breakers have been reported. To our
knowledge, all currently licensed PWRs which use breakers for the RTS use
one of these types. However, other safety-related systems may be configured
with these breakers or breakers of other manufacturers and types which use
an UV attachment to perform the required safety function. Licensees which
use these breakers in other applications should be aware of the likelihood
of failure to trip via the UV device. It would be prudent to check the
operation and maintenance of these breakers.
IEB 83-01 was directed to plants using Westinghouse DB type breakers with
undervoltage trip devices. A summary of the information submitted by
licensees in response to the bulletin follows:
(1) No fairures of DB type breakers were reported as a result of the
requested tests. Only one unit, which is in an extended maintenance
outage involving ihe reactor protectioh system, has not yet performed
the required tests.
(2) Seven of the twenty-eight plants using the subject breakers had not
been maintaining the breakers per the recommendations in Westinghouse
NSD Data Letter 74-2, a copy of which was attached to the bulletin.
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(3) Some plants reported that they were using the guidelines contained in
the NSD Data Letter with certain exceptions (e.g., the maintenance
interval had been extended from the recommended 9 or 12 month interval
to either 18 months or during refueling)
(4) Because the lubricants recommended in the NSD Data Letter are no longer
commercially available, some plants reported that they were,using
lubricants other than those recommended in the NSD Data Letter.
IEB 83-04 was directed to plants using RTS breakers with undervoltage trip
devices, other than those using DB type breakers, such as General Electric
AK-2 type breakers and the Westinghouse DS-416 type breakers. A summary of
the information submitted to date by licensees in response to IE Bulletin
83-04 follows:
(1) Several failures of the General Electric AK-2 type breakers and the
Westinghouse DS-416 type breakers were experienced during the tests and
were reported to the NRC as required by the bulletin. Failures
involving the AK-2 type breakers were reported at the Maine Yankee
plant and at the Calvert Cliffs Unit 1 and 2 plants. On March 15, 1983,
Maine Yankee reported that three of the eight RTS breakers used at the
plant failed to open in the required times; one breaker tripped in 1.9
seconds, another in 2.7 seconds, and the third in 10.3 seconds. Maine
Yankee further reported that these breakers were subsequently cleaned
and lubricated after which they operated within the acceptable normal
time. On March 15, Calvert Cliffs reported that two of the eight
breakers in Unit 1 required approximately 6 seconds to trip and that
two of the eight breakers in Unit 2 required approximately 1.5 seconds
to trip. In its written report to the NRC dated March 21, 1983, Calvert
Cliffs described three additional failures that were detected during
tests at Unit 1 and three additional breaker failures detected at Unit
2. The report also indicated that these breakers functioned within the
acceptable response time subsequent to performing maintenance on the
breakers. On March 18, McGuire Unit 2 reported that one RTS breaker (DS
type) failed to open upon deenergizing the undervoltage coil and on
March 19, McGuire Unit 1 reported a failure of one RTS breaker. On
March 24, Arkansas Nuclear One (ANO) Unit 1 reported failure to trip of
an AK-2 type breaker.
(2) Failures not previously reported to the NRC and identified thus far in
the bulletin responses include the following: ANO Unit 1 identified one
RTS breaker that failed to open and which was subsequently tested 10
times successfully after which it was surveillance tested daily until
the breaker was ultimately replaced. McGuire Unit 2 identified a
breaker failure that occurred during preoperational testing after which
it was reworked and placed into service on February 18, 1983 (this is
the same breaker for which we received a failure notification on March
18). Calvert Cliffs identified four events involving the undervoltage
trip device. Two of the events occurred on Unit 2 and were described as
sluggish trips which did achieve the protective system trip function.
The other two events occurred one on each unit and resulted in the
failure of the undervoltage device to trip the breaker. Three Mile
Island Unit 1 identified a failure which occurred on November 19, 1976,
during post
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maintenance testing and which was stated to be due to binding caused by
transporting and installing the breaker. Oconee, Unit 3 identified one
failure which occurred on December 17, 1978, for which the failure
mechanism was not determined and the breaker was replaced.
It should be noted that IEB 83-01 failed to request information on previous
failures of DB type RTS breakers which had not been reported to the NRC, as
IEB 83-04 did for other type RTS breakers. Any information on previously
unreported events of sluggish operation of all breaker types would be useful
to the NRC.
The information from these plants shows that in some instances breaker trip
operation and timing may be erratic when breakers are tested by tripping the
UV trip attachment. Although all of the GE AK-2 breakers did trip at Calvert
Cliffs, the breaker trip times varied significantly from one test to the
next. At McGuire, a Westinghouse DS-416 breaker intermittently failed to
trip on successive tests. At ANO 1, an AK-2 breaker which tested
successfully in response to the IE bulletin, failed a few days later during
unrelated activities. The root cause of these failures has not yet been
determined.
NRC meetings and discussions with various regulatory review groups,
licensees, and vendors have pointed out the importance of certain aspects of
breaker operation, maintenance and design. The breaker tests at Calvert
Cliffs 1 and 2 and Maine Yankee showed that although all of the AK-2
breakers did trip, the trip times of some breakers varied significantly from
one test to the next, up to a maximum of approximately 25 seconds. This type
of sluggish operation is not acceptable. There are some indications that if
the pick-up voltage for the AK-2 UV device is not set accurately, sluggish
and erratic response times may occur. It is our understanding that the
proper response time of the AK-2 breaker is less than 100 milliseconds.
General Electric has described problems with lubricants used in breakers
which become sticky after about 100 months of exposure. RTS breaker tests of
the UV attachment which measure trip times may be a means of detecting a
precursor of failure of the breaker to trip.
Preliminary study of breaker problems indicates the necessity of regular,
careful maintenance of RTS breakers. To reduce the likelihood of RTS breaker
failure, Combustion Engineering and Babcox and Wilcox in cooperation with
General Electric), and Westinghouse have developed updated maintenance
procedures which are expected to be available within a short time.
The NRC has met with both General Electric and Westinghouse and learned that
there may be limitations associated with the design life of the UV devices
and with the low design margin in terms of the torque available to trip the
breaker by the UV device. Periodic replacement of UV devices may be
necessary due to wear. Consultation with the vendor is recommended. The
torque available to trip th,e General Electric AK-2 breaker by the UV device
is critical to proper operation. General Electric has indicated that breaker
trip bar torque measurements are needed periodically (quarterly) to detect
the onset of problems.
Preliminary review of the Salem event has pointed to one other area of plant
operations that is important for detecting RTS breaker operational
degradation and for preventing failures. The Salem precursor event of
February 22, 1983,
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was not recognized as a failure until after the February 25 event. A
thorough post-trip analysis including close scrutiny of the events recorder,
could have detected the problem associated with RTS breakers.
It is suggested that holders of operating licenses or construction permits
review this information for applicability to their facilities.
No written response to this notice is required. If you have any questions
regarding this matter, please contact the Regional Administrator of the
appropriate Regional Office, or this office.
Edward L. Jordan, Direcotr
Division of Emergency Preparedness
and Engneering Response
Office of Inspection and Enforcement
Technical Contact: G. Lanik, IE
(301) 492-9636
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