ML20117C217
| ML20117C217 | |
| Person / Time | |
|---|---|
| Site: | 05000000, Salem |
| Issue date: | 03/08/1983 |
| From: | Haynes R NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| To: | Heltemes C NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| Shared Package | |
| ML20114F930 | List:
|
| References | |
| FOIA-84-616 NUDOCS 8505090359 | |
| Download: ML20117C217 (4) | |
Text
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8, MAR 0 81983 MEMORANDUM FOR:
C. J. Heltemes, Director, Office of Analysis and Evaluation of Operational Data FROM:
Ronald C. Haynes, Regional Administrator, Region I
SUBJECT:
POSSIBLE ABNORMAL OCCURRENCE - SALEM UNIT 1 FAILURE OF REACTOR TRIP BREAKERS TO OPEN ON TRIP SIGNAL On February 25, 1983, Salem Nuclear Gederating Station, Unit 1, experienced a failure of both reactor trip breakers to open automatically upon receipt of a valid trip signal from the RPS.
The details are contained in the attached i
report.
This event appears to meet criterion 10 of the Abnormal Occurrence criteria in that a major deficiency in plant operation had safety implications requiring immediate remedial action.
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Ronald C. Haynes Regional Administrator, Region I
Attachment:
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DRAFT ABNORMAL OCCURRENCE Reactor Trip Breakers Failed To Open On RPS Trip Signal pate and Place:
On February 25, 1983, Public Service Electric and Gas Company reported an event at Unit 1 of the Salem Nuclear Generating Station, a Westinghouse designed, pressurized water nuclear power plant located in Salem County, New Jersey.
Nature and probable Consequences:
At 12:21 a.m. on February 25, 1983, a low - low water level condition in one of the four steam generators initiated a reactor trip signal in the Reactor Protection System (RPS). The reactor was at 12*4 rated thermal power at the time preparatory to power escalation after a recently completed refueling outage. Upon receipt of a valid reactor trip signai the reactor trip circuit breakers which supply power to the reactor control rods failed to open (opening of either circuit breaker would have caused the reactor to trip). About 25 seconds later, operators manually initiated a reactor trip from the Control Room.
The reactor trip circuit breakers opened as a result of the manual trip signal and this resulted in insertion of all control rods and shutdown of the reactor.
Following the manual trip, the plant was stabilized in the hot standby condition. All other systems functioned as designed.
Later that morning when the cause of the failure had been determined by the licensee, the plant was placed in cold shutdown at.the request of the NRC.
Investigation of this incident on February 26, 1983 by the NRC revealed that a similar failure occurred on February 22, 1983, at Salem Unit 1.
At 9:55 p.m. on February 22, with the reactor at 20'4 power, operators were attempting to transfer the 4160 volt group electrical busses from l
the station power transformers to the auxiliary power transformers, a routine evolution during power escalation.
During the transfer attempt, one of the 4160 busses deenergized resulting in the loss of one reactor c_oolant pump and power for the operating main feed pump control and indication.
At 9:56 p.m., a low - low level condition occurred in one steam generator (due to the loss of the main feed pump), initiating a reactor trip signal.
Due to the abnormal conditions created by the loss of the 4160 volt bus and in anticipation of loss of steam generator water levels, the operator was directed at about the same time to manually initiate a reactor trip.
It was understood by plant personnel and was reported to the NRC that the automatic reactor trip signal due to the low - low water. level in one steam generator had, in fact, caused the reactor to trip. On February 26, 1983, as a result of NRC queries, the sequence of events computer printout for February 22 was again reviewed l
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I and it revealed that the reactor trip breakers actually opened in response to the operator's manual trip signal.
Consequently, it is now evident that on February 22 (as on February 25) the two reactor trip breakers failed to open upon receipt of an automatic trip signal from the reactor protection system.
Since the operators initiated a manual reactor trip shortly after receipt of the automatic trip signals on both February 22 and February 25, no adverse consequences occurred and the reactor was in a safe condition.
Cause or Causes:
On February 25, approximately two hours after the event, the cause of the failure to trip was determined by licensee instrumentation te.chnicians to be failure of the undervoltage (UV) trip mechanism associated with each of the 'two reactor trip circuit breakers to function as designed.
The UV trip mechanism consists of a relay and attached mechanical latches; upon receipt of a trip signal from the Reactor Protection System (RPS) the UV coil is deenergized and the mechanical latches cause the trip breaker to open. Opening of either circuit breaker causes a reactor trip.
(A manual trip signal operates both the UV trip relay and a separate shunt trip relay within each breaker.
The shunt trip relay is energized upon a manual trip signal.
Either relay is designed to cause the circuit breakers to trip; and in the February 22 and 25 events, it was the shunt trip relay which actually caused the reactor trip breakers to open.) The failure of the UV trip mechanism was determined by the licensee and the vendor, Westinghouse, to be excessive friction on a mechanical latch lever in the UV trip mechanism.
The cause of the excessive friction is still under investigation.
The circuit breakers are Westinghouse Type DB-50.
Previous failures of a reactor trip breaker have occurred.
Following a DB-50 reactor trip circuit breaker malfunction at the H. B. Robinson Nuclear Power Station in 1973, Westinghouse issued Technical Bulletin NSD-TB-74-1 in January 1974 recommending certain periodic maintenance measures, including lubrication, to improve the reliability of DB-50 breakers.
In February 19.74, Westinghouse issued a letter (NSD DATA TETTER 74-2) which, among other things, specified that a dry or near dry molybdenum disulfide lubricant should be used in the UV trip mechanism.
It appears that no preventative maintenance was conducted on the Salem Unit 1 DB-50 circuit breakers until January 1983. Additionally, the lubrication recommendations of the Westinghouse 1974 Technical Bulletin and Data letter were not implemented during the January 1983 maintenance, e
since personnel performing the maintenance (including a Westinghouse service rep ~resentative) were not aware of this information.
There have been two previous events at Salem Unit 2 involving a failure of one re. actor trip circuit breaker to trip.
On January 6, 1983, a reactor trip occurred due to a low - low water level condition in one
3 steam generator and only one reactor trip breaker operated.
The second trip breaker finally opened 25 minutes later, although the reactor had already tripped from opening of the other reactor trip circuit breaker.
The failure of this trip breaker was concluded by the licensee to be due to dirt and corrosion interfering with proper operation of the UV trip mechanism.
As a result of this event, maintenance was conducted on all Unit I reactor trip circuit breakers in January 1983, under the supervision of the circuit breaker vendor, Westinghouse. All breakers were satisfact-orily tested after maintenance.
Licensee Event Report (LER) 83-001/03L dated January 27, 1983, provides further details of the January 6 event.
On August 20, 1982, during surveillance testing of the Reactor Trip System on Salem Unit 2, one reactor trip breaker would not trip.
The cause of the breaker malfunction was concluded by the licensee to be failure of the UV relay coil.
The affected coil was replaced, and the breaker was satisfactorily tested.
LER 82-072/03L, dated September 8, 1982, provides further details of the August 2 event.
Actions Taken To Prevent Recurrence Because of the generic implications of this issue, the NRC issued IE Bulletin No. 83-01 on February 25, 1983 to all pressurized water nuclear power plants to inform them of this event.
For all pressurized water reactors having DB type reactor trip circuit breakers using UV trip attachments, certain actions were required.
These actions included prompt surveillance testing of the breakers, ensuring that preventive maintenance programs on the breakers include the recommended Westinghouse program, and reviewing with operators procedures to be followed in the event of a failure of the reactor to trip on receipt of an automatic trip signal.
With respect to Salem, the NRC staff met with the licensee at the site on February 26 and in Bethesda on February 28.
The licensee has proposed certain actions with respect to these breakers including implementing quality assurance requirements, augmenting surveillance test requirements, developing a maintenance program, incorporating the Westinghouse recommen-dations, and revising procedures to require the operator to employ a manual trip whenever an automatic trip signal is received.
The NRC is reviewing these actions to determine if they are sufficient to correct the deficiencies.
An NRC task force has been assigned to review and evaluate the impli:ations of this event. A Region I task force was assigned to collect facts and data on-site to provide the bases for the generic review. Additionai e-corrective actions may be required at Salem and other power reactors as a result of the, task force review.
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