ML20070D738
| ML20070D738 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 02/25/1991 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20070D730 | List: |
| References | |
| NUDOCS 9103010126 | |
| Download: ML20070D738 (3) | |
Text
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 159 TO FACILITY OPERATING LICENSE NO. OPR-50 METROPOLITAN EDISON COMPANY JERSEY CENTRAL POWER & LIGHT COMPANY PENNSYLVANIA ELECTRIC COMPANY GPU NUCLEAR CORPORATION THREE MILE ISLAND NUCLEAR STATION, UNIT NO. 1
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DOCKET NO. 50-289 INTRODUCTION i
The NRC has, for many years, had concerns regarding nuclear safety problems that could result from degraded electrical grid system voltage.
A generic letter to all power reactor licensees, " Adequacy of Station F.lectric Distri-bution Systems Voltages," was issued by the NRC on August 8, 1979.
In response to that U;tter, GPU Nuclear Corporation, the licensee for Three Mile Island Unit 1 (TMI-1), performed a voltage drop study and made certain plant modifi-cations to remedy inLiequacies identified by the study.
One of those modifi-cations was an undervoltage relay on the 4,160 volt buses which, if bus voltage dropped to 3,595 volts, would disconnect the plant 1E electrical system from the offsite power system (grid), trip certain nonessential loads and automatically 1
start the emergency diesel generators.
License Amendment No. 70 was issued on July 29, 1981 to incorporate the setpoint for thS undervoltaca M p into the Technical Specifications (TSs).
The Safety Evaluation (SE) supporting
.a amendment approved the 3,595 volt setpoint for this trip but noted it was lower than might have been expected.
The NRC senc Genetic Letter No. 88-15 to all licensees on September 12, 1988, noting that various problems with plant electrical systems, including potential i
for degraded grid voltage, were being identified with increasing regularity.
In respnnse to the generic letter, the licensee commissioned an updated degraded grid voltage drop study by Gilbert / Commonwealth, Inc.
This study (TOR No. 995 dated 2/20/90) indicated that the degraded voltage trip setpoint was too low to protect all motors under all scenarios postulated in the study.
The recommendations resulting from the study included raising the degraded voltage trip setpoint to a nominal value of 3,748 volts.
Consequently, GPU Nuclear submitted Technical Specification Change Request No. 203 on November 20, 1990 to implement this recommendation.
EVALUAl g One of the prime objectives of the degraded grid voltage effort was to ensure adequate voltage to motors driving critical components.
The electrical industry (i.e, NEMA) gemerally uses the guideline that voltage should be at least 90% of c103010126 91022S PDR ADOCK 05000289 P
2 the motor's nameplate voltage to prevent overheating and thermal degradation of insulation.
As discussed above, License Amendment No. 70 incorporated an undervoltage trip on the 4,160 volt buses. The staff's SE supporting the amendment noted that voltage at the terminals of 460 volt motors could be as low as 393 volts (85% of nominal voltage) before the undervoltage trip occurs separating the buses from the grid. The SE stated tnat the licensee justified this seemingly low setpoint partly on the basis that all 460 volt motors have s
a service rating of_1.15. As noted in TDR No. 995, however, some 460 voit c
motors base a service factor of 1.0 and that under degraded grid voltage
'nnditions with one unit auxilicy transformer not in tryice, 46 ES motors would see less than 90% of rate.' voltage.
Since the undervoltage tri would not have actuated at the iissumed voltage of 227 kilovolts (Kv) p relays on the 230 Kv system, manual lead shedo# 'g would be relied upon to restore voltage.
Raising the setpoint of the under w itage relays to about 3,750 volts would provide separation from the grid and initiate load sheddtng automatically under those conditions after a 10 second time delay.
The analysis reported in TDR No. 995 utilized the MPPER computer program and calculated actual load current for each motor under.he assumed degraded voltage conditions.
Thi_s current was then compared to the rated (nameplate) full loed current for each motor, taking-into consideration the service factor of that motor. The revised setpoint for the undervoltage trip is based on preventing any motor exceeding an actual current equal to its nameplate current multiplied by its service factor.
be acceptable.
The staff considers this approach to
'3 The _ revised undervoltage protection setpoint provides the necessary protection of 480 volt safety rela'ted electrical loads for the worst case electrical lineup'and loading assuming a degraded grid condition, one (1) auxiliary transformer in operation., and a design basis LOCA.
In addition to the existing first and second level of undervoltage protection, existing relays on the 480 volt safety-buses are used-to sound an annunciator in the control room at approximately 92% of the nominal ratings of-the motors (460V) connected to these buses. This alarm alerts the operators to a low voltage condition to allow them time to shed unnecessary loads to restore and preclude-trips if possible for this_ low probab_ility event.
The revised undervoltage protection setpoint will result in an increased probability of separation from the 230Kv system during a postulated accident coincident.with_ single auxiliary. transformer operation and a degraded grid event while~ the.offsite-system is still capable of providing power to the
_ plant.
However, neither_the accident nor the degraded grid result in any-event-related electrical e stem transfers that would cause loss of an auxiliary transformer or the automatic transfe" of loads. Existing Technical
- Specification Section 3.7.2 restricts single auxiliary transformer operation to a period'of 30 days only, during;which-b',th emergency diesel generators (EDG) must be operable with one EDG running continuously. Accordingly, it is concluded that simultaneous occurrence of these unrelated-events is a very low probability, and the intent of requiring an adequate level of undervoltage protection is maintained by the1 revised degraded voltage setpoint.
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3 Based upon the above ditsession, the staff finds the proposed change to the degraded voltage retsy cetpc nt to be acceptable.
[NVIRONMENTAL CONSIDfRA,T.
The amendnent changes a re,uiremes..
'th rett'et to ir.stallation or use of a facility component lo;ated within the restricted areas as defined in 10 CFR Part 20.
The NRC staff has detcemined that the amendment involves 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 Ccmmission has previously issued a proposed finoing that the amendnent involves no significant hct rds consideration, and there has been no public conwent on such finding.
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 statenent or environmental assessment need be prepared in connection with the issuance of the amendment.
1 CON t utigN The Commission has concluded, based on the considerations discussed above.
that: (1)-there is reasonable assurance that the health a"id safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be cotducted in compliance with the Commission's regulations, end (3) the issuance of the anendment will not be inimical to the common g
defense and security or to the health and safety of the public.
Principal Contributor: Ronald W. Hernan Date:
February 25, 1991 i
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