ML20079P865
| ML20079P865 | |
| Person / Time | |
|---|---|
| Site: | Waterford  |
| Issue date: | 11/01/1991 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20079P864 | List: |
| References | |
| NUDOCS 9111140119 | |
| Download: ML20079P865 (3) | |
Text
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 71 TO FACILITY OPERATING LICENSE NO. NPF-38 ENTERGY OPERATIONS, INC.
WATERFORD STEAM ELECTRIC STATION, UNIT 3 DOCKET NO. 50-382
1.0 INTRODUCTION
By application dated September 25, 1991, Entergy Operations, Inc. (the licensee) submitted a request for changes to the Waterford Steam Electric Station, Unit 3, TechnicalSpecifications(TSs,. The requested changes would not require the valid test f ailure of the "A" emergency diesel generator (EDG) on August 20, 1991, to count in the adjustment of test frequency for that diesel.
2.0 EVALUATION On August 20, 1991, Waterford 3 EDG "A" failed to maintain rated speed after it was started for periodic surveillance testing.
The diesel reached full speed only to drop in speed to a low value and then increase in speed again. After several oscillations in speed, the diesel was stopped. Troubleshooting revealed that the malfunction of the engine speed control was caused by the failure of one of two parallel voltage dropping resistors ir the power supply for the electronic speed control circuit of the Woodward Governor. This constituted a second valid failure in the last 20 tests.
In accordance with TS Table 4.8-1, Diesel Generator Test Schedule, this required the test frequency of EDG "A" to be increased from once every 31 days to once every seven days. A footnote in Table 4.8-1, explains that for two or more failures in the last 20 valid demands, an increased frequency shall be maintained until seven consecutive failure free demands have been performed and the number of failures in the last 20 valid demands has been reduced to one. Under the current circumstances, the licensee must perform one test every seven days for 14 weeks in order to exit from the increased test frequency.
Waterford 3 has two EDGs that are manufactured by the Cooper Bessener Company.
These EDGs use Woodward Governors for controlling the speed of the generators.
There are two rethods of controlling the speed of these generators; one electrical and one mechanical.
In the event that the electrical governor fails, the backup mechanical governor assumes the function of speed control.
In the power supply circuit for the electrical governor, a resistor assembly functions to reduce the 125 Vdc system vcltage to 24 Vdc. This resistor, assembly has two parallel, 300 ohm, 70 watt resistors. The August 20, 1991, failure was due to the opening of one of the above two resistors. There have been several occurrences at other plants identical to the Waterford 3 failure where one of these parallel resistors has failed. The licensee had established 9111140119 911101 PDR ADOCK 05000302 P
. preventive maintenance, based on known failures of these resistors, to replace the resistors every 18 months and had replaced them in EDG "A" in June 1990, and EDG "B" in July 1990. Waterford 3 had experienced a previously resistor failure in 1986.
The licensee's preventive maintenance program was in agreement with the NRC Information Notice 90-15 which recognizes that replacing the resistors with like resistors was sufficient for at least 18 months. However, the licensee and the Woodward Governor Company have since brought to light the misapplication of the particular design by nuclear plants. The control power for the governors comes from the 125 Vdc station systems which remain in a float charge mode most, if not all of the time. The increased voltage in the float mode can overload the heat dissipation capacity of the resistors causing one to fail at any time, i.e., the licensee should not be on a replacement schedule every 18 months but should be correcting this problem as now recomended by Woodward Governor. This correction is to replace the two resistors with a single larger resistor with greater total heat dissipation ability.
Failure of this improved resistor assembly causes the electronic governor to speed up rather than slow down.
In this situation, the mechanical governor will be able to assume control and regulate the speed. The licensee replaced the resistors on EDG "A" with the new single resistor and successfully tested and returned the EDG to service. Subsequently, the licensee expedited the delivery of a new resistor for EDG "B" and that modification is also complete.
Posed on the above, the staff concludes that the cause of the August 20, 1991, EDG "A" failure may not have been clearly understood by the Waterford operators, however, appropriate corrective actions have now been implemented by the licensee. The newly designed component now in both EDGs appears to offer certain advantages over the original design. Thus, the failure mechanism associated with the previous failure is no_ longer present.
TS Table 4.8 1 presents a logical corrective action to increase the testing frequency should a diesel generator system fall below the selected target reliability of 0.95 chosen to determine the station blackout duration.
The l
reliability of the diesels is influenced greatly by neglected or otherwise unrecognized problems with the diesel and to the extent failures occur j
unexpectedly, the increased testing should reveal any other similar failures.
In the case of the Waterford 3 event on August 20, 1991 the failure potential was recognized before the event, the licensee was respon, ding in agreement with l
the Information Notice 90-51, and the licensee has been recognized as naving l
one of the better diesel programs which should assure high reliability. The problem has been corrected expeditiously on both EDGs and the increased fre-quency of testing 13 not supported by the rational by which the TS actions were developed. The proposal to not count the failure for frequency adjustment is acceptable.
In light of the current EDG testing practices (slow, startings p"elubing, and prewarming), additional testing is not a major concern, however, the need for the testing should be consistent with the intent of the TS. The licensee actions to replace the two.esistors with one with greater heat dissipation capacity is equivalent to the overhaul of the speed control. The
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-. last footnote on Table 4.8-1 provides a-general approach to diesel overhaul by 2
requiring 14 starts to reestaolish reliability and trans-valve the failure to The licensee has taken the same approach with the proposed footnote to zero.
Table 4.8-1 in that the August 20, 1991, failure will be trans-valved to zero following 14 subsequent starts of the diesel with no failures of the power dropping resistor. This allows the footnote to be removed and is acceptable.
3.0 STATE CONSULTATION
in accordance with the Commission's regulations, the Louisiana State official was notified of the proposed issuance d the amendment. The State official had no comments.
4.0 ENVIRONMENTAL CONSIDERATION
The amendment changes a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20. : The NRC staff has determined that the amendment involves no signifi-cant increase in the amounts, and ne significant change in the types, of any effluents that may be released offsite, and that there is no significant increase i_n individual or cumulative occupational radiation exposure.
The Commission has previously issued a proposed finding that the amendment involves no significant hazards consideration and there has been no public comment on 1
such finding (56-FR 49802). Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendment.
5.0 CONCLUSION
The Commission has concluded, based on the considerations discussed above, that: (1) there is reasonable assurance that the health-and safety of the
_ public will not be endangered by operation = in the proposed manner, (2) such.
activities will be conducted in compliance with_the Commission's regulations, and (3) the-issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor:
D. Wigginton Date:
November 1, 1991 P
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