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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. 8303040012 . IN 83-18 April 1, 1983 Page 2 of 4 (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 . IN 83-18 April 1, 1983 Page 3 of 4 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, . IN 83-18 April 1, 1983 Page 4 of 4 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 Attachment: List of Recently Issued Information Notices .
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