ML20205D952
| ML20205D952 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 03/20/1987 |
| From: | Hukill H GENERAL PUBLIC UTILITIES CORP. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 5211-87-2070, NUDOCS 8703300522 | |
| Download: ML20205D952 (3) | |
Text
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o GPU Nuclear Corporation NQQIg7 Post Office Box 48o Route 441 South Middletown, Pennsylvania 17057-0191 717 944 7621 TELEX 84 2386 Writer's Direct Olal Number:
5211-87-2070 March 20, 1987 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555 Gentlenen:
Three Mile Island Nuclear Station Unit 1 (TMI-1)
Operating License No. DPR-50 l
Docket No. 50-289 10 CFR 50 Appendix R - Associated Circuits Results of Studies In GPUN letter 5211-86-2214 dated December 23, 1986, we committed to the following additional studies to assure that fire induced failure of associated common enclosure circuits will not prevent safe shutdown.
1.
A fault study.
2.
An interrupting device coordination study.
These two studies have been completed.
In addition, we have conducted a comprehensive review of all power circuits fed from the AC auxiliary power system, AC vital power system and DC power system and documented whether they are protected by circuit interrupting devices.
The fusing of the control circuits were also verified against the elementary diagrams.
We believe that these studies have resolved the concern related to safety related and non-safety related circuits which are associated by common power or by common enclosure with the safe shutdown circuits.
A full report of these electrical studies will be available onsite for your inspection and review.
The results of these studies are summarized below.
A.
Short Circuit Analysis A short circuit analysis was performed using the DAPPER computer program.
The main purpose of this analysis was to verify that the fault currents at various electrical buses do not exceed the short circuit interrupting capabilities of the circuit interrupting devices.
Several computer runs were performed to detemine faul t levels at di fferent short circuit time f rame s, based on the most 7
g stringent operating conditions.
GPU Nuclear Corporation is a subsidiary of the General Public Utilities Corporation 8703300522 870320 PDR ADOCK 05000289 F
P{M
.- The short circuit study indicated that the fault currents at the.4160 volt buses exceed the interrupting and the momentary current rating of the 4160. volt circuit-breakers if one' emergency diesel generator is
'. operating in parallel with the offsite power supply while_ one-auxiliary transformer is out. of service for ' maintenance.
'This
-situation occurs during. the engineered safeguards conditions when the maximum number of motors are ' running and -. are contributing short circuit currents.
During power ' operations with :a single-auxiliary transformer, the. diesel generator will not : be paralleled - with the
- system, except. for load tests required by the Technical Specifications.
Operating procedure 1107-3 contains ' restrictions on operating the -diesel in parallel with the system under these conditions.
The short circuit study -also indicated _ that the short circuit levels of the engineered safeguards motor control centers could be as high as six percent above the motor control-center - bus bracing rating of 22,000 amperes.
In all cases,. the available short circuit. currents at 480 volt buses are within the short circuit ratings of the 480 volt circuit breakers (25,000 amps).
A small increase in the short circuit force due to six percent higher fault current should not result in the failure of the bus.
A comparative review of the parameters effecting forces on bus bars was performed for 22,000-ampere and' 42,000-ampere rated motor control centers.
This review showed that the bus bars rated 22,000 amperes have the-required capacity to withstand 25,000 amperes.
This available high short circuit could only occur for a fault at or near r
the motor control center.
We consider this condition acceptable to meet the Appendix R requirements.
B.
Coordination Study A total 'of 49 coordination curves were drawn using the computer program CAPTOR.
A coordination curve was made for each electrical bus showing the time current characteristics of at least two successive interrupting devices to demonstrate that the downstream breaker is coordinated with the upstream breaker.
A separate curve is provided for each large motor rated 100 HP and above.
Smaller motors at the control centers were reviewed along with non-motor loads and characteristic curves for. the. type of overcurrent protection device were drawn on the same graph.
The review of these coordination curves revealed that there are no deficiencies in the existing overcurrent coordination of TMI-1 which would require additional modification.
i C.
Review of Cable Sizes and Breaker Ratings The review of cable sizes and the overcurrent interrupting device j
ratings was completed for the engineered safeguard buses and balance of plant electrical systems.
This review involved all power circuits from the 6900V switchgear, 4160 volt switchgear, 480 switchgear, motor control centers, and distribution panels.
I l
r
=
. The review-determined that there are three (3) circuits that required action to prevent potential threat to Appendix R circuits.
Two (2)
'100 ampere breakers have been replaced with 60 ampere breakers to protect No. 6 AWG power cables.
The electric heater on one (1)50 ampere breaker will be taken.out of service prior to restart to prevent exceeding the allowable ampacity of the feeder cable.
There are other circuits that may -not be protected by the protective device at the source from thermal aging in heavily loaded trays.
However, it is not expected that the magnitude or duration of currents in excess of the cable ampacities will result in damage to adjacent cables in the same enclosure in the time required to bring the plant to safe shutdown from an Appendix R event.
The associated circuits by common power and the associated circuits by common enclosure will not affect the Appendix R shutdown scenario.
The circuit breaker will isolate the faul ted associated circuits before the damage of these cables could affect the shutdown circuits.
D.
Miscellaneous Reviews.
1.
The elementary diagrams were reviewed to verify that all control circuits are adequately protected.
2.
The control circuits for the transfer switches for 1C ES valve MCC and 1M DC Dist. were found to be unprotected.
These fuses have been installed.
We trust this information is sufficient to resolve NRC concerns related to fire induced failure of associated common enclosure circuits.
If any additional information is required, please contact us.
Sincerely, kill Vice President and Director, TMI-l HDH/DJD/jh 0834A cc:
J. Stolz, USNRC J. Thoma, USNRC T..Murley, USNRC Region I R. Conte, USNRC TMI Site D. Kubicki, USNRC PEISCB A. Krasopoulos, USNRC Region I