ML20079P813
| ML20079P813 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 10/18/1991 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20079P812 | List: |
| References | |
| NUDOCS 9111140069 | |
| Download: ML20079P813 (6) | |
Text
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.S..E9ppypp, pp,CL,E,AF,,P,L,AN,T,,, UNITS 1 AND 2 DOCKET t10S. EO-327_!.pp,)p,,3,28 1.0 J N,TRODUCTION By lette, dated October 11, 1991, the Tennessee Valley Authority (the licensee) submitted a request for changes to the Sequoyah Nuclear Plant, Units 1 and 2 Technical Specifications (TS), The
., Jested amendment would change the diesel generator full load reject overshoot limits specified in Technical Specification (TS) Surveillance Pequirement 4.8.1.1.P.d.3.
This specification currently requires that tests be performed every 18 months to verify the capability of the diesel generators (DGs) to reject a load of 4400 kw (i.e., full load) without tripping, and that the voltage not exceed 114 percent of the initial pretest voltage or 8276 volts, whichever is less, during and following the load rejection test.
Specifically, the licensee has requested that the voltage overshoot limits of 114 percent anc 8?76 volts be changed to 120 percent and 8712 volts, respectively.
2.0 DISCUSSION As a result of a full load reject test of the IA-A DG performed on October 7, 1991, the licensee determined that the voltage overshoot was 115 percent above the pretest voltage and, therefore, declared the DG inoperable. Following dis ussion" with the licensee, the NFC staff granted a verbal TWC on October 9, 1991, to raise the acceptance value to 115 percent, the value spccified in the Standard Technical Specifications.
This allowed the DG to be declared operable.
A similar test of the IB-B DG was then performed on October 9, 1991, which resulted in an overshoot of approximately 119 percent. The IB-B DG was, therefore, declared inoperable.
Following further discussions with the licensee, the NRC staff granted a verbal TWC of compliance on October 10, 1991, which raised the limit to 120 percent. The licensee determined that the slightly higher overshoot resulting from the IB-B full load reject test was primarily due to additional adjustments of the generator voltage that were necessary to obtain the desired pretest conditions. There are four dicsel generators associated with the two units, and all four must be operable to satisfy the technical soecification requirements for each unit's operation.
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l-2 With Unit 2 operating at full power in Mode 1, limiting Condiiion for Operation (LCO) 3.8.1.1 was applicable. This LC0 requires that the DG be returned to the operable status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> or that the plant be placed in at least the hot standby condition within the next six bcurs and in the cold shutdown condition within the next 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />.
Also, with Unit 1 in Mode 5 for the Cycle 5 refueling outage, scheduled activities associated with the outage were immediately ard severely ircpacted with the DG in the inoperable status.
By letter dated October 10, 1991, the licensee requested that the staff issue a Terporary Vaiver of Con:pliance (TWC) to waive the criteria until such time as the f:RC staff could act on the licensee's amendnent application.
This TWC was issued on October 11, 1991.
3.0 EVALUAT10tj Analysis of the voltage spikes resulting from the full load reject tests determined that failure to meet the overshoot acceptance criteria stated in the TS was primarily due to changes to the test procedure that in.plemented Informa-tior. Notice 91-13 and due to nodifications to the generator exciter control system that changed the excitation current transformcr (CT) tap settings to provide a boost of the field current for a given DG kva load.
The Information Notice indicated that the worst-case conditions of voltage, electrical power factor, frequency and environment, should be considered when DG tests are performed. Therefore, procedures had been revised to require that the generator loadirg be increased to rated electrical kw and kva (4400 kw and 5000 kva).
This resulted in an increase in both the internal generator voltage setpoint (excitation) that is required to overexcite the generator to rated reactive load, and an increase in generator field current. These conditions tended to create a higher voltage overshoot when the load was suddenly removed.
In addition, a more accurate test instrument (high speed visicorder) was used to determine the magnitude and duration of the voltage overshoot.
Previously, the voltage overshoot-had been determined using the installed panel meter, which significantly reduced the accuracy of the transient data.
Also, the TS value of 114 percent was based on past test results prior to the changes described above and when less accurate techniques were used to measure the voltage spike, not on criteria associated with equipment damage.
The effects of these changes on the full load reject test had not been recognized or analyzed by the licensee until the performance of the test.
As a result of discussions with the appropriate vendors, the licensee has determined that the proposed overshoot voltage limits have not and will not adversely affect DG equipment or the capability of the DG to perform its intended function. The overvoltage condition exists for such a very brief I
duration (measured at 13 cycles and E3 cycles for DG 1A-A and IB-B respectively) and is of such a small magnitude, that the effects on the generator, the insulation, the DG controls, and the instrumentation is negligible.
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3 In summary,-the cause of the voltage overshoots being in excess of the TS lir.its and previously measured values is the combination of the revised pro-cedural, requirements to test at rated generator load, the CT modifications, and the use of more accurate test instrumentation. The individual contributions of each are not knrun ar.d would require additional testing to determine, t.ddi-tional testing is not desired due tn potential reliability concerns. The effects of these changes was not quantified at the time of initial irplementa-tion and, therefore, was not identified until performance of the full load reject test.
i Yhe staff has evaluated the safety significance associated with failure to meet the voltage overshoot criteria presently specified in the TS related to the PG full load re,iect test, the proposed increase to the overshoot criteria, and the effects of the increase. He have determined that the proposed change to Surveillance Requirement 4.8.1.1.2.d.3, as it pertains to the JG overvoltage limit of 114 percent and 8276V (to 120 percent and 8712V respectively), is acceptable.
4.0 EMERGENCY CIRCUMSTANCES The licensee, in its October 11, 1991 application.. renuested that the proposed TS change be approved on an emergency basis.
The licensee stated that failure to satisfy the current TS requirement would require a forced outege of SCN Unit 2. unnecessary additional testing of the DGs, and delays in critical path activities related to the Unit 1 Cycle 5 refueling outage.
In addition, the situation could not heve been avoided without additional (and undesirable) testing of the DGs. and was not recognized by the licentee as a consecuence of the DG changes in procedure, components, and testing eouipment prior to perfor-mance of the tests. Once the condition was identified by the licensee, an intensive investigation and evaluation was conducted with the determination that the intent of the TS requirement was satisfied by the test that was conducted, and the DGs were considered inoperable only because of the specific wording of the TS, not for technical reasons.
l The licensee was granted a verbal TWC on October 9, 1991, based on the results l
of-the analysis following testing of the IA-A DG. Another verbal TWC was l
granted on October 10, 1991, following analysis of the additional information l
that was developed following tests of DG 1B-B.
In granting the temporary waivers, the NRC staff recognized that emergency circumstances existed that warranted prompt approval, since failure to act would result in extending the Unit 1 refueling outage and could result in a forced shutdown of Unit 2, that the situation could not have been avoided, and that the licensee applied for the amendment in a timely fashion. Thus, pursuit to 10 CFR 50.91(a)(5), the staff finds tnat an emergency situation exists which would result in shutdown of the operating plant (Unit 2).
5.0 FINAL NO SIGNIFICANT HAZARDS CONSIDERATION
DETERMINATION The Commission's regulations in 10 CFR 50.92 state that the Commission may make a final determination that the license amendment involves no significant hazards consideration if operation of the f acility, in accordance with the I
anendment, would not:
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Involve a significant increase in the probability or consequences of an
.cident previously evaluated.
The request proposed a TS change fur Surveillance Requirement (SR) 4,8.1.1.2.d 3 to allow a voltage overshoot of 120 percent of pretest voltage or 8712 volts, whichever is less, it has been determined that these increased values would not result in damage to the DC and U ll continue to ensure operability. The revised limits will ensure that safety-related functions are adequately verified. Therefore, the DGs will cor.tinue to be verified to reet safety requirements and the subject changes will not increase the consequences of an accident.
In addition, the DGs are not postulated to be the source of any design basis accident and, therefore, cannot increase the probability of an accident.
2.
Create the possibility of a new or different kind of accident from any previously analyzed.
The request only provides an increase in the alloweble voltage overshoot on a total DG load rejection. The change does not create any new type of accident because alteration of test requirtments for the DG sr any other DG test or operation cannot create an accident.
The DGs only provide accident mitigation functions.
3.
Involve a significant reduction in a margin of safety.
The revised voltage values will not result in damage to any DG components and therefore the DG safety functions will be naintainEd. Since the DGs will centinue to provide full accident mitigation capabilities, a margin of safety will not be reduced.
Accordingly, the NRC staff concludes that the proposeo amendment involves no significant hazards considerations.
6.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Tennessee State official was notified of the proposed issuance of the amendment. The State official had no comments.
7.0 ENVIRONMENTAL CONSIDERATION
The amendment changes a requirement with respect to a surveillance requirement.
The NRC staff has determined that the anendment involves no significant increase in the amounts, and no sionificant 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.
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.
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. 8.0 C0HCLUSION The Comission has concluded, based on the consideratioris discussed above, that:
(1) tiiere is reasonabit: assur er co t hat the t evith ar>d safety of the public will not be endangered by operaticri in the proposed manner, (?) such activities will be conducted in cerrpliance with the Cor:rission's regul6tions, and (3) the issuar ce c' ire amendr'ent will not be inimical to the cormon defense and security or to the health and safety of the public.
Principal Coi.tritutor:
D. LaBarge Date: October 18, 1991 n
, 5.0 CONCLllS10N The Comission har concluded, Dased 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 oublic.
Principal Contributors:
F. Allenspach, LPEB, DI.PQ K. Jabbour, PDIl-3, DRPE F. Rinaldi, PDII-3, DRPE Date: October 15, 1991 2
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