ML20136C140
| ML20136C140 | |
| Person / Time | |
|---|---|
| Site: | LaSalle  |
| Issue date: | 11/13/1985 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML20136C137 | List: |
| References | |
| NUDOCS 8511210034 | |
| Download: ML20136C140 (5) | |
Text
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UNITED STATES g
E NUCLEAR REGULATORY COMMISSION o
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wAsmNGTON, D. C. 20555
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SAFETY EVALUATION AMENDMENT NO.16 TO NPF-18 LA SALLE COUNTY STATION, UNIT 2 DOCKET NO. 50-374 Introduction By letter dated August 28, 1985, Comonwealth Edison Company (the licensee) requested a one-time technical relief, during the present, first refueling outage for La Salle Unit 1, to allow the two diesel generators required by La Salle Units 1 and 2 Technical Specifications, 0 and 1A, be inoperable for a 30 day period without perfonning surveillance of the other operable diesel generators every 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> as required by the Technical Specifications. This one time change will allow the installation of the diesel generator lube oil modification required by the La Salle Unit 1 License Condition 2.C.(2)(1)(b) to be installed on Unit 1 prior to startup after the first refueling outage.
Because of the fact that Diesel Generator 0 is shared between La Salle Units 1 and 2 and Diesel Generator IA of Unit 1 is designed to supply power to the Unit 2 equipment (e.g. standby gas treatment, hydrogen recombiner, and emergency filtration for the control room) the present Technical Specifications for Unit 2 require that Diesel Generators 0,1A, 2A, and 2B be operable. Otherwise, if one of the three diesel generators (0, 2A, or 28) is inoperable, the action statement permits 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> of reactor operation before action must be taken to shut down the unit. For Diesel Generator 1A, the action statement allows a maximum of 10 days of reactor operation before action must be taken to shut down the unit. Furthermore, each action requires the other operable diesel generators to be tested every 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> during this interval of inoperable diesel generator. Thus, the proposed relief from the requirements of the Technical Specifications would extend the present three-day limiting condition for operation to a duration of 30 days for Diesel Generator 0 modification, while for Diesel Generator IA from 10 days to 30 days.
The licensee proposes to modify the Diesel Generators 0, IA, and 18 during the upcoming Unit I refueling outage while the changes for the remaining diesel generators (2A and 28) will be completed during the Unit 2 refueling outage.
Furthermore, the licensee states that no maintenance is to be performed on the other diesel generators required for unit operation while a diesel generator is being modified.
EVALUATION l
The Comonwealth Edison Company is currently a member of the Mid-America Interpool Network for the purpose of ensuring that the transmission system is reliable and adequate. La Salle County Station is interconnected to the above electrical grid system through four 345 kV overhead transmission lines which enter the station by way of two separate rights-of-way. These four 345kV 8511210034 851113 PDR-ADOCK 05000374 P
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l circuits in the switchyard are connected to ten circuit breakers in a ring-bus arrangement. This ring bus configuration provides for quickly locating and isolating any system fault or equipment failure. Therefore, any single failure would only affect one source of offsite power. The staff has reviewed the data i
base for loss of offsite power (LOOP) events compiled in NUREG/CR-3992, " Loss of Off-site Power at U.S. Nuclear Plants," and found that La.Salle County Station has not experienced a LOOP event since its initial criticality in 1982.
The primary supply (an inanediate access circuit) to the three onsite Engineered Safety Feature (ESF) buses for Unit 2 is provided by a 345/4.16 kV system auxiliar The alternate supply (delayed access circuit)y transformer (SAT), SAT-242.is manually connected to the onsite system ESF buses) from the Unit 1 ESF buses through the unit auxiliary transformers (UAT), UAT-141, or the system auxiliary transfomer (SAT-142).
1 The onsite power system is designed such that five diesel generators (0,1A, 18 for Unit 1 and 2A, 2B for Unit 2) are used to provide onsite power to six Class 1E ESF buses (three for each unit). Among the five diesel generators, i
Diesel Generators IA and 2A are assigned to Division 2. Diesel Generators 18 and 28 to Division 3, and Diesel Generator 0 is shared between Division 1 of two units. With Unit 1 in cold shutdown, only three of the four diesel generators (0, lA, 2A, and 28) are required to satisfy the standby AC onsite power requirement for Unit 2; however, the Technical Specifications require i
fcur functional diesel generators.
As for the onsite power sources, the licensee states that La Salle's diesel generator reliability exceeds 0.99 while the U.S. average diesel generator reliability is 0.98, according to NUREG-1032, " Evaluation of Station Blackout Accidents at Nuclear Power Plants," Table 1.1, "Sunenary of Station Blackout Program Technical Results." Further, the licensee states that the station i
i record indicates only seven failures in cver 600 diesel generator starts.
Within the last three years, only one failure has occurred. Thus, La Salle presently maintains a 30 day test interval for their diesel generators, as l
required by the Technical Specifications because of this failure rate. The staff has reviewed the licensee's probability assessment that a LOOP occurs j
concurrently with one or two ciesel generators failures during a 30 day period as requested by the licensee. We concur with the licensee's finding that the probability of such 'an event during that period is sufficiently small.
In addition, we corroborated the licensee's conclusion by performing our own assessnent and arrived at the same conclusion (see Attachment). Also, the.
licensee has perfonned a loop transient analysis with any combination of diesel generators being inoperable and they have detemined that the unit can be shut-down with one operable ESF division.
Aside from the required monthly diesel generator surveillances, the licensee will perfom the following additional tests:
1.
The diesel generators needed.for the operation will be demonstrated to be operable by performing the surveillance test required by Specification 4.8.1.1.2a.4 prior to taking the other ciesel generator out-of-service.
In addition, the licensee, by letter dated October 2,1985 has committed to perfom the above surveillance test (Specification 4.8.1.1.2a.4) again at the 14th day.of the 30-day period.
If any diesel generator fails to E
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start properly, the appropriate action called for in the Technical Specifi-cations would be taken.
2.
The control circuit for the unit cross-tie breakers between Diesel Generators IA and 2A will be temporarily modified to allow the unit tie breakers to be manually closed (from the control room) if such a need arises.
l Based on:
(1) the favorable operating history of the offsite power system and the diesel generators. (2) the number of available paths to the onsite buses from the offsite sources, (3) the probability assessment, and (4) power t
the fact that one ESF division can safely shutdown the unit for LOOP, the LOOP does 1
not represent a significant threat to the health and safety of the public for the period proposed.
For the accident situation, it is our judgement that a large break loss-of-coolant accident concurrent with a LOOP and the loss of an additional diesel generator is a very unlikely event over the 30 day period for Diesel Generators 0 and 1A outages; and, therefore, does not represent a signif-icant danger to the health and safety of the public. We will request that the NRC Resident Inspector, before any action is taken by the licensee, is assured that adequate procedures have been developed for the shedding of nonessential loads from Diesel Generator IA and the operation of the tie breakers between 142Y and 242Y buses are in place. The staff, therefore, concludes that the proposed 4
one time relief from the requirements of the Technical Specifications is acceptable.
ENVIRONMENTAL CONSIDERATION i
This amendment changes requirements with respect to use of a facility component located within the restricted area as defined in 10 CFR Part 20 and changes surveillance requirements. We have determined that the amendment involves no significant increase in the amounts, and no significant change in the types, of i
any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission has previously issued a proposed finding that this amendment involves no significant hazards consideration and the only coments received have been i
addressed in the safety evaluation. Accordingly, this 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 p'repared in connection with the issuance of this amendment.
i CONCLUSION We have concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the i
will not be endar.5Ered by operation in the proposed manner, and (2) public such activities will be conducted in compliance with the Comission's regulations l
. and the issuance of this amendment will not be inimical to the comon defense l
and security or to the health and safety of the public.
l Dated: NOV la S i
l
i ATTACHMENT i
I The staff has conservatively estimated the probability for severe core damage i
for La Salle Unit 2 fr of an order of 3 X 10-gm this one-time change in Technical Specifications to be This probability was arrived at by the following.
h Two cases must be considered: either Diesel Generator 0 is taken out of service for the lube oil modification, or Diesel Generator IA is taken out of service. Of these two cases, the worst case is that where Diesel Generator i
0 is taken out of service; Diesel Generator 0 is the swing diesel. The con-servative estimate of severe core damage probability was obtained by assuming 4
that if (1) a loss of offsite power of duration greater than 1/2 hour were to take place during the 30 day period, with Diesel Generator 0 out of service, (2) Diesel Generator 2A fails to start, and (3) there is failure to power bus 242Y (nomally powered by Diesel Generator 2A) by Diesel Generator IA; then j
l severe core damage occurs.
In actuality, if the Reactor Core Isolation Cooling System or the High Pressure Core Spray System were operable, severe core damage would not occur for some hours, during which time either offsite or onsite power might be recovered. The amount of time before severe core damage would occur i
j has not been investigated.
The estimate of severe core damage probability associated with the proposed change to the Technical Specifications used the following data:
4 (1) Loss of offsite power frequency for periods greater than 1/2 hour
.03/ year (from NUREG-1032. " Evaluation of Station Blackout Program Technical Results," Figure A.10.
This figure gives the l
frequency of losses of offsite power exceeding specified durations j
for Zion (another Coninonwealth Edison nuclear station).
It was assumed that Zion and La Salle have similar loss of offsite power frequency versus duration curves).
(2) Probability Diesel Generator 2A fails to start
.01/ day (plant specific data given by the licensee in its August 28, 1985 letter),
(3) Fraction of Diesel Generator 1A or Diesel Generator 0 is inoperable 1
for the lube oil modification - 30/365 (we are assuming in actuality 1
the Diesel Generator 0 is out for thirty days since that is thi worstcase,).
l (4) Probability of failure to power bus 242Y (nomally powered by Diesel Generator 1A) - 0.1 (This estimate of the probability of failure to power bus 242Y by Diesel Generator IA is a conservative estimate which includes the conditional probability of failure of Diesel Generator IA, given Diesel Generator 2A fails to start; and the probability of human error of failing to properly close the tie breakers between buses 142Y and 242Y, or improperly shedding the nonessential loads on Diesel Generator IA. Since the tie breakers between buses 142Y and 242Y can be closed from the control room, f
f this is a conservative estimate.)
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The prguct of these four quantities yields our conservative estimate of 3 X 10 of probability for a severe core damage at La Salle Unit 2 due to the one-time change in the Technical Specifications. Since the increase in core melt probability from this one-time change in Technical Specifications is estimated to be small, even on a conservative basis, and since the diesel generator lube oil modification to be made will likely improve the long terin reliability of the diesel generators, we find the proposed change to be acceptable.
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