ML20079N164
| ML20079N164 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 02/17/1983 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20079G766 | List: |
| References | |
| NUDOCS 8303030522 | |
| Download: ML20079N164 (6) | |
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,1 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION ON THE ADEQUACY OF STATION ELECTRIC DISTRIBUTION SYSTEM V0LTAGES AND SUPPORTING AMENDMENT N0. 5 2 TO FACILITY OPERATING LICENSE NO. NPF-3 TOLEDO EDIS0N COMPANY AND CLEVELAND ELECTRIC ILLUMINATING COMPANY DAVIS-BESSE NUCLEAR POWER STATION, UNIT 1 DOCKET N0. 50-346
1.0 INTRODUCTION
Toledo Edison Company (TECo), the licensee, was requested by NRC letter dated August 8,1979, to review the electric power system at Davis-Besse Nuclear Power Station, Unit No.1. The review was to consist of:
a) Determining analytically the capacity and capability of the offsite power system and onsite distribution system to automatically start as well as operate all required loads within their required voltage ratings in the event of:
- 1) an anticipated transient, or 2) an accident (such as Loss of Coolant Accident (LOCA)) without manual shedding of any electric loads.
b) Determining if there are any events or conditions which could result in the simultaneous or, consequential loss of both required circuits from the offsite network to the onsite electric distribution system and thus violating the requirements of GDC 17.
The August 8,1979 letter included NRC staff guidelines for performing the required voltige aulysis, and TECo was further required to perform a test in ortkr to verify the validity of the analytical results. Toledo
,o z Edison Company responded by letters dated October 9,1979, November 16, 1979, December 7,1979, January 2,1981, June 16,1981, March 19,1982, no 88 and June 3, 1982. A detailed review and technical evaluation of the
.m8 submittals was performed by Lawrence Livermore Laboratory (LLL) under contract to the NRC, with general supervision by NRC staff. This work l
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- 86 is reported by LLL in the attached Technical Evaluation Report (TER),
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" Adequacy of Station Electric Distribution System Voltages, Davis-Besse
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Nuclear Power Station, Unit No.1", dated September 27, 1982.
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By letter dated March 23, 1979, TECo proposed an amendment to the gh Appendix A TSs which would permit the use of pushbutton activated circuitry to defeat the undervoltage relays used to sense degraded voltage conditions on the 4.16 kV essential buses. The purpose of the defeat is to permit routine starting of a reactor coolant pump (RCP) or circulating water pump (CWP) without the possibility of causing
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. 31 essential 4.16 kV bus source tripping and attendant diesel-generator
' starting. Additional details of the bypass circuitry and' hardware were submitted with TECo -letters dated August 31, 1981 and March 19, 1982.
4 2.0 DISCUSSION AND EVALUATION 2.1 Station Electric Distribution' System Analysis:
The criteria used by LLL in the technical evaluation of the analysis includes GDC 5, " Sharing of. Structures, Systems, and Components",
' GDC 13. -" Instrumentation and Control", GDC 17 " Electric Power
-Systems" of < Appendix A to 10 CFR 50; IEEE Standard 308-1974, " Class lE Power Systems for Nuclear Power Generating Stations"; ANSI-C84.1-1977,
" Voltage Ratings for Electric Power Systems and Equipment - 60 Hz"; and the staff positions and guidelines in NRC letter to TECo dated August 8, -1979.
The TECo analysis was performed using each source of offsite power i
under extremes of load with the 345 kV grid at maximum (102.2%) and minimum (98.3%) anticipated voltages.
The analyses included the transient effects from starting of large motors.- The worst case Class 1E equipment voltages occur under the_ following conditions:
1.
Maximum bus voltage occurs when the grid is at its maximum r
expected value of 102.2%, both startup and both bus tie-transformers-are in service, and there is no load on the Class lE buses.
2.
Minimum steady state bus voltage occurs when the grid is at its minimum expected value of 98.3%, only one startup and one bus tie transformer _ is in service, and all Class lE equipment is in -
operation.
3.
Minimum transient voltage occurs under the same conditions as Item 2 above except that all level 1, 2 and 3 ESF equipment is starting.
The analysis results show that during the worst case minimum voltage transient condition, the voltage to the 240 volt and 120 volt Class lE-buses c~ould degrade to 62.0% and 61.5% respectively. This is below the 67.1% required to start-the equipment. However, this equip-ment would successfully start in less than 4 seconds after the 4160 and 480 volt motors start and accelerate. The duration of this transient is less than the 5 second stall rating of the 240 volt and 120 volt motors. The cause of this low voltage is attributed to excessive voltage. drop in the'3KVA 480/240 volt transformers that supply MCC's YUE2 and YF2.
The licensee has committed to replace these 3KVA trans-formers with transfonners that have a rating of 30 KVA..Upon comple-tion of the modification, the worst case transient voltage to the 240 V and 120 V Class lE buses will be 70.5% and 69.7% respectively, which is above the equipment minimum. starting voltage rating.
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. In addition to the voltage analysis, the licensee was required to perfonn a review to determine if any event or condition exists that could result in the simultaneous or consequential loss of both offsite circuits to the onsite power system and thus may violate the requirements of GDC.17.
The licensee has completed this review and has determined that one event, the failure of the overall generator differential relay to operate when required to clear a fault on the 345 kV loads to No.1 main transformer.
would result in tripping remote breakers on all incoming lines to the switchyard and thereby cause a temporary loss of all offsite power to the Class lE buses. As a result, the Class lE buses would be isolated from the offsite power system and transferred to the onsite emergency diesel generators. One source of offsite power (Lemoyne line to Davis-Besse) would be automatically restored after a 15 second time delay.
The Davis-Besse design meets the requirements of GDC 17 and has the capability to provide power to' the Class lE equipment from the onsite and offsite power system within the time required to prevent fuel damage.
However, the dependence on successful operation of a single overall generator differential relay to clear a fault and prevent a temporary loss of all offsite power to the Class lE buses does not minimize to the extent practical, as required by GDC 17, the probability of losing electric power from any of the remaining supplies as a result of, or coincident with the loss of power generated by the nuclear unit. This item has been discussed with the licensee. To improve overall offsite power system availability it is recommended that the licensee consider modifications to the presently installed relaying scheme.
The licensee has not performed the voltage tests required to verify the assumptions used in the voltage analysis. As a minimum, we require that these tests meet the following criteria:
a.
Loading the station distribution buses, including all Class lE buses down to the 120/208 volt level, to at least 30%;
b.
Recording the existing grid and Class lE bus voltages and bus loading down to the 120/208 volt level at steady state conditions and during starting of both a large Class lE and non-Class lE motor (not con-currently).
Note: To minimize the number of instrumented locations (recorders) during the motor starting transient tests, the bus voltage l
i and loading need only be recorded on that string of buses which previously showed the lowest analyzed voltages.
c.
Using the analytical techniques and assumptions of the previous voltage analyses, and the measured existing grid voltage and bus loading conditions recorded during conduct of the test, calculate a new set of voltages for all Class 1E buses down to the 120/208 volt level.
d.
Compare the analytically derived voltage values against the test results.
-4 e.
With good correlation between the analytical ni;d test results, the test requirements will be met.
In general, the test results should not be more than 3% lower than the analytical results; however, the difference between the two, when subtracted from the voltage levels detemined in the original analysis, should never be less than the Class lE equipment rated voltages.
We have reviewed the LLL TER and the licensee's submittals and find that:
1.
TECo has provided a voltage analysis to demonstrate that upon completion of the proposed modifications, Class lE equipment voltages will remain within acceptable operating limits for the worst case conditions analyzed.
2.
The Davis-Besse design meets the requirements of GDC 17.
3.
We require that the licensee perform plant tests, which incorporate the criteria and methods outlined in the test features section of this evaluation, to verify the accuracy of the assumptions used in the voltage analysis.
We, therefore, find the Davis-Besse Nuclear Power Station, Unit No.1, acceptable with respect to the adequacy of station electric distribution system voltages subject to completion of the proposed modifications and completion of the verification testing. We will address the verifica-tion testing in a supplement to this report.
2.2 Undervoltage Trip Bypass In a letter dated October 1,1976, the NRC requested that TECo analyze the Davis-Besse facility Class lE electrical distribution system to determine if the operability of safety related equipment could be adversely affected by degradation in the grid system voltage within the range where the offsite power is counted on to supply important equip-ment. This letter also requested that TECo provide a description of any proposed actions or modifications to the facility based upon its i
analyses. As a result of its review, TECo proposed to add a second level of undervoltage protection to the Davis-Besse 4.16 kV buses.
PreviJusly, the only level of protection was an undervoltage relay that would open the feeder breakers when voltage dropped to 59% of the nominal value.
This setting wac oredicated upon a total loss of offsite power, but not upon sustaned low voltage conditions. The second level of protection rcopose _ by TECo would result in separation l
of the 4.16 kV buses from '.neir normal power sources during sustained degraded voltage conditions (less than 90% nominal voltage for 7 +1.5 seconds). This undervoltage protection would assure Emergency Core Cooling System injection within 30 seconds under the conditions of a LOCA coincident with degraded 4.16 kV voltage. This modification was approved by the NRC in Amendment No. 7 to the Davis-Besse operating l
license dated November 29, 1977.
As a result of the implementation of the second level of undervoltage l
protection, TECo stated in its March 23, 1979, letter that starting l
one RCP or CWP can result in a voltage reduction on the 4.16 kV buses
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sufficient to cause automatic opening of the feeder breakers to the buses. This in turn results in automatic load shedding from the buses and starting of the diesel generators. This is because the voltage drop on the 13.8 kV buses which supply power to the motors 4
is fed through to the 4.16 kV buses.
In the March 23,1979, pro-posal, the facility would be allowed to temporarily bypass the 90%
undervoltage relays for sufficient times to permit. starting of these large motors.
The bypass consists of two >ushbuttons, one for each power train -
located on the main control board, that when depressed will bypass the degraded voltage relays. This circuitry incorporates indicating s lights in the control room which provide visual indication when the relays are_ bypassed. Releasing the pushbutton will reinstate the degraded-voltage relays and extinguish the indicating lights.
The licensee's proposal is to allow defeat of the 90% undervoltage relay function on both 4.16 kV buses for up to one minute to allow normal RCP or CWP starts. There are eight pumps involved; TECo estimates that it is unlikely that any of them would be started more i
than once_ per week. With a start time of 10 seconds, this would entail an upper estimate of 80 seconds permeek during which the 4
pushbutton undervoltage relay defeat circuit would be used. This 4.
amounts tc less than 0.02% of the time, or a 2x10-4 vulnerability.
The defeat of the undervoltage relays would have a safety significance only if coincident with a LOCA which would demand loading of safety related equipment on the essential buses.
If a LOCA were not in progress, the undervoltage relay defeat would produce no equipment degradation. Therefore, the actual unreliability imparted to safety relat 2x10-*gd equipment caused by the use of the bypass would be less than multiplied by the likelihood of a LOCA. Given the small likeli-hood of a-LOCA, the added unreliability is an acceptably small number.
In a letter dated August 31, 1981, TEco has stated that the bypr circuitry design is consistent with the standards of the original design including IEEE 279-1971. On this basis, we find acceptable the licensee's proposed use of the degraded voltage relay bypass modification and the associated TS change that allows the operator to manually bypass the degraded voltage relays for up to one minute whicle starting an RCP or CWP.
t 3.0 ENVIR0 MENTAL CONSIDERATION We have determined that the amendment does not authorize a change l
in effluent types or total amounts nor an increase in power. level 1
and will not result in any significant environmental impact. Having made this determination, we have further concluded that the amendment
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involves an action which is insignifica'nt from the stand environmental impact and, pursuant to 10 CFR 551.5(d)(4) point of that an l
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environmental impact statement, or negative declaration and environ-mental impact appraisal need not be prepared in connection with the issuance of this amendment.
4".0" CONCLUSION We have concluded, based on the considerations discussed above, that:
(1) because the amendment does not involve a significant increase in the probability or consequences of an accident previously evaluated, does not create the possibility of an accident of a type different from any evaluated previously, and does not involve a significant reduction in a margin of safety, the amendment does not involve a significant hazards considerc. ion, (2) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (3) such activities will be conducted in canpliance with the Commission's regulations and the issuance of this amendment will not be inimical to the common cafense and security or to the health and safety of the public.
Dated: FEB 171983 The following NRR personnel have contributed to this Safety Evaluation:
R. Prevatte, A. De Agazio.
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