ML20059F112
| ML20059F112 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 01/06/1994 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20059F110 | List: |
| References | |
| NUDOCS 9401130154 | |
| Download: ML20059F112 (6) | |
Text
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'o UNITED STATES
[ %.,, I g NUCLEAR REGULATORY COMMISSION t
WASHINGTON. D. C. 20555
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RE. LATED TO AMENDMENT NO. 86 TO FACILITY OPERATING LICENSE NO. DPR-80 AND AMENDMENT N0. 85 TO FACILITY OPERATING LICENSE NO. DPR-82 PACIFIC GAS AND ELECTRIC COMPANY DIABLO CANYON NUCLEAR POWER PLANT. UNITS 1 AND 2 DOCKET N05. 50-275 AND 50-323
1.0 INTRODUCTION
Pacific Gas and Electric Company (PG&E), by letter dated December 22, 1992, as supplemented July 19, 1993, proposed that the Facility Operating License Nos.
DRP-80 and DRP-82 be amended to modify Diablo Canyon Unit I and 2 Technical Specification (TS).
Specifically, TS Section 3/4.3.2, " Engineered Safety Features Actuation System Instrumentation," would be revised to change the second level undervoltage trip setpoint and allowable values.
Technical Specification 3/4.8.1, "A.C. Sources," would also be changed to revise the diesel generator (DG) steady state voltage surveillance requirements.
The second level undervoltage relay TS setpoint and allowable values will be changed to maintain acceptable voltages at the 480 volt and 120 volt buses during sustained degraded voltage conditions. The DG steady state voltage surveillance requirements will be changed to ensure that the diesel generators provide adequate voltage when required to power the vital loads.
The supplemental letter dated July 19, 1993, provided additional information and did not affect the initial federal Reaister notice and proposed no significant hazards consideration.
2.0 BACKGROUND
The Diablo Canyon Power Plant (DCPP) electrical power system consists of an offsite system and an onsite system.
The offsite power system is comprised of a 230 kV and 500 kV transmission system. The onsite power system consists of a distribution system normally supplied by the offsite power system.
In the event of a loss of offsite power, the onsite power system will be available to supply power to the safety loads via emergency diesel generators (EDGs).
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The onsite 4160 volt power system is a three-phase system that serves motors from 200 to 3000 hp and transformers for the smaller loads at the lower voltages.
The 4160 volt vital buses can be supplied power from the main generator (or the offsite 500 kV system) or from the emergency diesel generators (EDGs).
During normal operation, the 4160 volt vital buses are c
9401130154 940106 PDR ADOCK 05000275 P
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. supplied from the auxiliary _ transformer, which is powered from the main generator.
In the event of a loss of vital bus voltage or degraded grid voltage conditions, the 4160 volt vital buses are provided with two levels of undervoltage protection, the First Level Undervoltage Relays (FLURs) and the Second Level Undervoltage Relays (SLURS). The FLURs detect the loss of bus voltage (less than 69% bus voltage). The SLURS provide a second level of undervoltage protection which protects all Class IE loads from short-term or long-term degradation in the offsite power system. TS 3/4.3.2 currently requires the SLURS setpoint and allowable value to be greater than or equal to 3600 volts.
3.0 fyALUATION SLURS Setooint in 1985, the original SLURS T.S. minimum setpoint was 3600 volts at the 4160 volt buses, or 90% of the 4000 volt rating of motors. This setpoint was found acceptable in the NRC's supplement No. 9 to the Safety Evaluation Report (SSER9), section 4(3). Although the SLURS setpoint was actually set conservatively at 3700 volts and ensured that the safety-related 4000 volt motors would not experience sustained degraded voltage, the maintenance of adequate voltage had not been assured for the 460 volt motors and essential 120 volt control circuits for the Class IE components.
As part of a "re-establish the design basis program" PG&E performed an evaluation to determine the effect of the 3700 volt SLURS setpoint on plant safety and provide the basis for operation during reevaluation and implementation of the new setpoint.
PG&E determined that under normal operations the worst case voltage the 4160 volt buses cculd experience is 3916 volts or 98% of the 4000 volt motor rating. Thus, the SLURS setpoi,ts would not be approached on a sustained basis when the buses are in the Urmal-configuration. However, during start-up and shutdown of the units, the buses
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are connected to the start-up transformer, which is powered from the 230 kV switchyard.
It was determined that in this configuration a safety concern l
could result if the grid voltage was to degrade to a point just above the SLURS setpoint and maintain that same voltage level for a lengthy period of i
time.
Inadequate voltages may be seen at the 480 volt and 120 volt levels and could impact the ability of safety equipment to operate.
PG&E determined that a new SLURS setpoint needed to be calculated but the consequences of this scenario on present plant operations was not significant based on the following raticnale:
1.
For the 480 volt motor starters, the contactor drop-out and inadvertent de-energizing of a device is not a concern during a degraded voltage i
condition based on the drop-out characteristics of the contactor.
Typically, the drop-out characteristics of the contactor are in the i
voltage range of 60% of nominal voltage. The postulated degraded voltage condition would maintain a minimum steady state voltage in the j
range of 85% to 86% at the 480 volt buses.
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. 2.
For the 480 volt motor starters, the contactor pick-up and ability to energize a service is reasonably assured for motors and MOVs based on the design of the control circuits which utilize low current inrush along with larger size control wires. At DCPP, #10 and #12 AWG wire sizes and a minimum of 100VA control power transformers are used, thereby minimizing voltage drop in the transformers and cable.
Based on PG&E calculations, a maximum length of 1100 ft. and 1800 ft. for #12 AWG and #10 AWG wire sizes respectively could be allowed for a NEMA size 1 starter to pickup at 86% of 480 volts at the motor control center (HCC).
A majority of control circuits for loads located within the power block would typically not exceed a circuit length of 500 ft.
3.
The potential for the control power transformer secondary fuses to open due to degraded voltage at MCCs is also mitigated because of oversized fuses selected by the MCC vendor. The maximum continuous current for a 100VA control power transformer under a degraded voltage condition is no-more than 1.5 ampere.
4.
The grid would have to remain in this condition for 20 to 30 minutes before any of the motors at the 480 volt level would trip on thermal overload. Taermal ove..oads for continuous duty motors would see higher current at reduced voltage and, because of the inverse time tripping characteristics, it would require a 20- to 30-minute heatup before actuation.
Furthermore, this is not considered to be a likely event and, therefore, not a safety concern while the new SLURS setpoints are being calculated because of the following:
1.
Based on the operating record at DCPP, a sustained degraded grid voltage is in itself not a likely event. There have been no cases of degraded grid conditions at DCPP during the 6 years the plant has been operating.
PG&E standard practices require a minimum of 230kV to be maintained on the DCPP switchyard.
voltages below these values can only occur as a mlt of a major system disturbance. The design of the system teporates frequency controlled automatic load shedding combined with uneration excitation to assure a stable operation and restoration to the minimum voltage within 10 to 15 minutes.
2.
Even if the above were to occur, the system voltage would have to degrade from 227.5kV to 217kV (just above the SLUR setpoint) and remain there for this scenario to proceed.
3.
Also during this scenario, an accident has to occur so that Class IE loads, such as motors and MOVs, are called upon to start and run under a degraded bus voltage condition.
Nonetheless, as a conservative measure, interim actions were taken by PG&E to establish administrative controls to ensure adequate voltage for all vital
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. motor loads until a new SLURS setpoint could be calculated and implemented.
Emergency Procedure EP E-0, " Reactor Trip c' Safety Injection," was changed to r
confirm 3870 volts (this voltage was based on preliminary calculations and includes inherent inaccuracies in the meter and reading the meter),
corresponding to 111 volts as read from the control room vertical board, is maintained on the 4160 volt vital buses following bus transfer to 230kV source during a loss-of-coolant accident (LOCA). Should the 4160 volt vital bus voltage drop below 3870 volts, Emergency Procedure EP E-0 directs the operator l
to transfer the 4160 volt vital bus loads to the diesel generators.
The adequacy of the DCPP SLUR setpoint was reviewed in the NRC electrical I
distribution safety functional inspection (EDSFI) at DCPP.
PG&E had performed calculations to determine the new SLUR setpoint prior _ to the NRC EDSFI. The EDSF1 report indicated that the interim actions discussed in the previous paragraph ensure operation of all Class IE equipment until the SLURS setpoint change is implemented. Also, it is important to note, that after the setpoint l
change has been implemented the interim measure of manually transferring to the emergency diesel generator is no longer considered to be an appropriate action. The transfer from the offsite power source to the EDG should be auto-atic.
PG&E performed a new calculation to establish an allowable value of 3785 for the SLURS setpoint, replacing the old value of 3700 volts. The new value will ensure adequate voltage to 480 volt and 120 volt levels, including the 460 volt motor-operated valves and 120 volt control circuits.
Field verification results obtained in 1985 will be used to support the new calculation because the licensee stated that there have been no significant design changes since 1985 that would invalidated the data.
For any future 1
design changes the licensee will use both administrative and computer programs i
to monitor changes and update the SLURS setpoint calculation.
EDG Steady State Voltaae Surveillance l
The licensee is proposing to make two additional changes regarding EDG I
surveillance testing. The first change would be to raise the minimum voltage requirement (from 3740 to 3785) and lowering the maximum voltage requirement (from 4580 to 4400) for the EDG steady state voltage TS requirement. Raising the minimum EDG steady state voltage TS requirement from 3740V to 3785V would ensure the EDGs provide adequate voltage to all levels in the unlikely event of a sustained EDG operation to support vital loads. Lowering the niaximum EDG steady state voltage TS requirement from 4580V to 4400V would ensure the 1
EDGs do not exceed the allowable voltage limits on motor terminals, in the j
event of sustained EDG operation. Test data from EDG surveillance testing l
indicated that the EDGs currently meet the proposed maximum and minimum EDG steady state voltage TS surveillance requirement of 4160 +240/-375 volts.
Therefore, there are no current or past safety concerns with EDG operability with respect to maximum and minimum steady state voltage.
The second change to the EDG surveillance test would be the elimination of the voltage and frequency limit en the EDG during the 24-hour test. During the 1
e g
, 24-hour test, the EDG is loaded to greater than or equal to 2750 kW during the first 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> and greater than or equal to 2484 kW for the subsequent 22 hours2.546296e-4 days <br />0.00611 hours <br />3.637566e-5 weeks <br />8.371e-6 months <br />.
For the EDG to be loaded to this capacity, it is necessary to parallel the EDG to either the auxiliary or startup transformer.
In this alignment, the EDG voltage regulator does not control the 4160 volt vital bus voltage. The 4160 volt vital bus voltage is controlled by either the 500 kV or 230 kV system voltage, and by the loads on the system.
Since the 24-hour test is performed during a unit's refueling outage, when plant loads are lower, the 4160 volt vital bus is expected to be higher than the 4160 volt vital bus voltage during normal operation. Assurance that adequate voltage is available to vital loads when powered by a EDG is verified by the proper response of the EDG voltage regulator during the surveillance tests.
In the alignment used for the 24-hour test, however, the EDG is paralleled to the auxiliary or startup transformer which would have more control over the voltage and frequency for the EDG than the voltage regulator.
Since voltage and frequency are not controlled by the EDG, PG&E is proposing to delete this requirement in their current TS for the 24-hour test when the EDG is paralleled to the auxiliary or startup transformer. This deletion from the TS i
is consistent with the NRC-approved Westinghouse Owners Group Standard TS (NUREG-1431, Revision 0) for the 24-hour test.
j CONCLUSION The proposed changes to increase the SLURS TS minimum setpoint and allowable value provides additional conservatism for the 4160 volt vital buses to provide safety-related equipment adequate voltage at the 4160 volt, 480 volt and 120 volt levels, and therefore, reduce the risk of tripping or damaging the vital equipment associated with accident mitigation. The proposed changes to increase the minimum and decrease the maximum EDG steady state voltage TS surveillance requirement would ensure that (1) adequate voltage is provided by the EDGs for all voltage levels and (2) allowable voltage limits at the motor terminals are not exceeded.
Finally, the elimination of the voltage and frequency limit for the EDG during the 24-hour test will not prevent the EDG from providing the adequate voltage to the vital buses and is consistent with the NRC-approved Westinghouse Owner Group Standard TS for the 24-hour test.
Therefore, the proposed changes are found to be acceptable to the staff.
4.0 STATE CONSULTATION
In accordance with the Commission's regulations, the California State official-was notified of the proposed issuance of the amendments. The State official had no comments.
5.0 ENVIRONMENTAL CONSipERATION These amendments change a requirement with respect to the installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 and change surveillance requirements. The NRC staff has
-l 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 i
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offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission has previously issued a proposed finding that the amendments involve no significant hazards considera-tion, and there has been no public comment on such finding (b8 FR 7002).
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.
6.0 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 issiknce of the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor:
H. Pratt Date: January 6, 1994 I
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