ML20056H630
| ML20056H630 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 08/27/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20056H629 | List: |
| References | |
| NUDOCS 9309100230 | |
| Download: ML20056H630 (3) | |
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UNITED STATES
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j NUCLEAR REGULATORY COMMISSION gv y WASHINGTON, D.C. 20555-000 ENCLOSURE 3 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGGLATION RELATED TO AMENDMENT NO.170 TO FACILITY OPERATING LICENSE NO. DPR-77 AND AMENDMENT N0.160 TO FACILITY OPERATING LICENSE NO. DPR-79 TENNESSEE VALLEY AUTHORITY SE000YAH NUCLEAR PLANT. UNITS 1 AND 2 DOCKET NOS. 50-327 AND 50-328
1.0 INTRODUCTION
By application dated March 1, 1993, the Tennessee Valley Authority (TVA or the licensee) proposed an amendment to the Technical Specifications (TS) for Sequoyah Nuclear Plant (SQN) Units 1 and 2.
Under the TS Surveillance Requirement (SR) 4.8.1.1.2.d.3, SQN is required to perform a full lead rejection surveillance test of each emergency diesel generator (EDG) every 18 months during shutdown by verifying that the generator is capable of rejecting a load of 4400kw without tripping.
In addition, the acceptance criteria states that the generator voltage cannot exceed 120 percent of the initial pretest voltage or 8712v, whichever is less, during and following the load rejection test.
The proposed amendment would increase the maximum voltage (i.e., the overshoot) limit for an acceptable test to 8880v and remove the 120 percent of the initial pretest voltage requirement. This would be accomplished by removing the phrase " voltage shall not exceed 120 percent of the initial pretest voltage or 8712v, whichever is less" from the SR and replacing it with a new requirement of " voltage shall not exceed 8880v." Thus, the revised SR 4.8.1.1.2.d.3 would read, " Verifying the generator capability to reject a load of 4400kw without tripping. The generator voltage shall not exceed 8880V during and following the load rejection."
2.0 EVALUATION During the 18 month full load rejection test performed during the Unit 1 Cycle 5 refueling outage, the EDG voltage overshoot exceeded the acceptance criteria for a very brief duration (measured at 13 and 23 cycles for EDGs IA-A and IB-8, respectively). To address this voltage overshoot condition, TVA requested an emergency TS amendment and a Waiver of Compliance to caage the EDG voltage overshoot limits from 114 percent or 8276v to 120 percent or 8712v. NRC approved this emergency TS amendment on October 18, 1991.
Since then, the licensee has performed a more detailed study of EDG components affected by the voltage overshoot that occurs during the full load rejection test. As a result, TVA determined that the 120 percent of the initial pretest 9309100230 930827 PDR ADOCK 05000327 P
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2 generator voltage acceptable value requirement for a full load rejection test j
should be removed entirely from the SR and the 8712v limit for voltage overshoot should be increased to 8880v.
The purpose of a full load rejection test as performed under SR 4.8.1.1.2.d.3 is designed tu demonstrate that the EDG is capable of rejecting a full load (4400kW) without causing an overspeed trip and exceeding the predetermined voltage limits.
TVA has evaluated the EDG components potentially impacted by the voltage overshoot from the initial EDG 6.9ky nominal bus voltage during a full load rejection test and has provided the following rationale for the increased voltage overshoot limits.
The generator "high potential (HyPot)" voltage tests at the factory were performed at 14.8kv and the initial testing (preoperational) after installation were performed at 75 percent (i.e.,11.lkv) of the factory test i
voltage. The in-service high potential tests are recommended at 60 percent (i.e., 8880v or 8.88kv) of the factory test voltage. The EDG manufacturer has determined that the engine and/or generator controls would not experience detrimental effects for transient voltages up to 9000v. The medium voltage cables from the EDG to 6900v shutdown board switchgear (safety bus) are rated at 8000v and have a recommended high-potential test voltage for maintenance of 30.0-kv dc voltage and 17.6-kv ac voltage.
In addition, a voltage overshoot reduction device (VORD) limits EDG voltage by i
shunting the field current when the voltage exceeds 103 percent of nominal voltage. This device is active only in the isochronous mode; i.e, when the EDG is paralleled to the bus. Therefore, the maximum voltage applied to the cables would be limited by the VORD to a smaller value should an overspeed occur during actual EDG operation than the voltage that would result from the EDG test when the output breaker is opened to conduct the test.
Based on the EDG manufacturer's information, TVA determined that the proposed limit of 8880v is acceptable based on the ability of components to withstand this transient voltaqe level without damage. Therefore, the magnitude and duration of the overshoot condition expected during a full load rejection test would not damage the connected EDG, its control system components, or distribution components.
HyPot tests for an EDG are performed during major maintenance or if an event occurs that causes the bus voltage to reach 13kv de for 3 minutes, since this has the potential for causing electrical insulation damage.
By considering a de to ac conversion factor of 1.7, the 13kv dc (phase-to-ground) is equivalent to 7.647kv ac (i.e, 13kv/1.7). Since the maximum voltage allowed during the full load rejection test (8880v) is measured phase-to-phase, while the HyPot test is measured phase-to-ground, 8880v phase-to-phase is equivalent to i
5.127ky (i.e., 8.88kv/1.7321) phase-to-ground value. By comparing the EDG's insulation voltage (7.647kv) with the maximum voltage allowed during load rejection test (5.127kv), the voltage level specified before insulation damage can be expected is significantly higher than the voltage overshoot expected during the full load rejection test. Therefore, no insulation damage is expected to occur at the new test voltage level.
3 Regarding the concern for EDG control equipment to withstand the proposed Full full load rejection test, the proposed transient voltage (i.e., 8880v) expected during a full load rejection test was determined to be within the equipment voltage rating of 9000v. The 9000v rating is based on considering i
the potential transformer ratio (60:1) and 150v rating for all the control components on the low side of the transformer (i.e., 150v X 60 - 9000v).
Therefore, the proposed 8880v is an acceptable limit for the EDG control i
equipment during the full load rejection test.
Based on the staff's evaluation of the information provided by the licensee, i
the staff agrees with the licensee that the proposed voltage overshoot limit during a full load rejection test once every 18-month refueling outage would not adversely affect the EDG equipment or the capability of the EDG to perform its intended safety function. We have also determined that the 120 percent i
provision is not specified in the current Standard TS (Revision 4A) or i
improved TS (NUREG-1431, dated September 1992, " Standard Technical Specifications Westinghouse Plants"). Therefore, there are no technical requirements for the 120 percent initial pretest generator voltage overshoot provision during a full load rejection test.
Based on this analysis, the staff concludes that the increase of the EDG voltage overshoot limit from l
8712v to 8880v, and the removal of the 120 percent provision from the current SR 4.8.1.1.2.d.3 are acceptable.
3.0 STATE CONSULTATION
l In accordance with the Comission's regulations, the Tennessee State official was notified of the proposed issuance of the amendments. The State official had no comments.
4.0 ENVIRONMENTAL CONSIDERATION
The amendments change a surveillance requirement.
The NRC staff has determined that the amendments involve no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Comission has previously issued a proposed finding that the amendments involve no significant hazards consideration, and there has been no public coment on such finding (58 FR 19487). Accordingly, the amendments meet 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 amendments.
5.0 CONCLUSION
I The Comission 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 Comission's regulations, and (3) the issuance of the amendments will not be inimical to the comon defense and security or to the health and safety of the public.
Principal Contributor:
P. Kang Dated:
August 27, 1993