ML20214A249
| ML20214A249 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 11/07/1986 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20214A241 | List: |
| References | |
| NUDOCS 8611190341 | |
| Download: ML20214A249 (7) | |
Text
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o EhCLOSURE SAFETY EVALUATION REQUEST FOR PROPOSED CHANGES TO TECHNICAL SPECIFICATIONS FOR Fl'PVEILLANCE REQUIREMENTS OF MOLDED CASE CIRCUIT BREAKERS FE000YAH UNITS 1 AND 2 BACKGROUND Tennessee Valley Authurity (TVA) by letter dated August 8,1986 to B.J. Youngblood, requested changes to the Technical Specifications (TS) of Sequoyah Nuclear Plant, Units 1 and 2. The proposed changes uculd revise Surveillance Requirements
( SR ) 4.8. 3.1.a .1. a 4. 8. 3.1.a .1. b , 4. 8. 3.1.a .1. c , 4. 8. 3.1. a . 2, 4. 8. 3.1. a . 3, 4.8.3.1.b, and 4.8.3.3.a for Units 1 and 2 Technical Specifications. The proposed revisions would accomplish the followino: (11 delete testing of the instantanecut elements of the molded case circuit breakers; (2) delete referer.ccs to specific surveillance instruction (SI) numbers for implementing surveillarce requirerents; and (3) describe the activities necessary te verify the fuses in lieu of testing them. This safety evaluation addresses testing of the molded case circuit breakers only.
EVALUATION To meet 10 CFR 50, Appendix A, GDC 18, " Inspection and testing cf electric power systems", the Seqcoyah Nuclear Plant Units 1 and 2 TS, Sections 4.8.3.1.a.2 and 4.8.3.3.a. require functional testing of at Itast 10 percent of each type of low voltage circuit breakers. Thr low voltage molded case circuit t,reckers (MCCBs) are used for containment penetraticn protection and as isolation aevices protecting IE buses frem cercualified loads. The MCCB trip units consist of two separate trip clements, thermal and instantaneous; these provide 8611190341 861107 PDR i
P ADOCK 05000327 PDR l
protection for the full range of expected current values (overleads ar.d fault currents). The thenpal element provides protection for overloads and 0.cdcrate fault currents (1.25X to 10X breaker currer.t rating). The instantanecus element provides protection for the high fault currents (above 10X breaker current rating). The therr'ai elenent usually is not qualified by the manufacturer for thenoal capability (I T) at high currents (above 10) breaker current rating). The operating characteristics of the thermal and instantaneous eierents of each type of MCCB are tested per UlaPo and published by the manufacturer.
These publisheo data clearly show the operating characteristics and capabilities 2
(1 T) of the therral ard instantaneous elerrents. To ensure that the MCCB performs its intended function over its life, both the thenral erd instantaneous elcr'erts are periodically tested with current injection. Industry standards (such as NEf'A /E-2 and NETA specifications) and manufacturer's maintenance guides provide recorrendations for periodically testing MCCBs.
It was roted during a March 20, 1986, NRC review of containment penetration overcurrent protection devices at the Sequoyah plant that TVA was not _ testing the MCCB instantaneous clcrents and the thermal elements were being tested in series rather than individually as required by irdustry standards. TVA has contended that the MCCBs could to tested sufficiently by testing the thenrel overcurrent elements without the ir.stantaneous trip test. Subsequent to the March 20, 1986 findings, TVA has conducted lir,ited testing of some MCCBs and has offered the following reasons for not testing the instantaneous elements.
Et, CLOSURE SAFETY EVALUATION REQUEST FOR PROPOSED CHANGES TO TECHfMCAL SPECIFICATIONS FOR ,
SUPVEILLAf:CE REQUIREMENTS OF M0LDED CASE CIRCUIT BREAKERS SEOUOYAH UNITS 1 AND 2 BACKGROUND Tennessee Valley Authority (TVA) by letter dated August 8,1986 to B.J. Youngblood, requested changes to the Technical Specifications (TS) of Sequoyah Nuclear
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Plant, Units 1 and 2. The proposed changes would revise Surveillance Requirements
( SR ) 4. 8. 3.1. a .1. a , 4. 8. 3.1. e . l . b , 4. 8. 3.1. a .1. c , 4. 8. 3.1. a . 2, 4. 8. 3.1. a . 3, 4.8.3.1.b, and 4.8.3.3.a for Units 1 and 2 Technical Specifications. The proposed revisions would accomplish the followino: (11 delete testing of the instantanceus elements of the molded case circuit breakers; (2) delete references -
to specific surveillance instruction (SI) numbers for implementing surveillerce requirements; and (3) describe the activities necessary te verify the fuses in lieu of testing them. This safety evaluation addresses testing of the molded case circuit breakers only.
EVALUATION To meet 10 CFR 50, Appendix A, GDC 18, " Inspection and testing cf electric power systems", the Sequoyah Nuclear Plant Units I and 2 TS, Se:tions 4.8.3.1.a.2 and 4.8.3.3.a, require functional testing of at leert 10 percent of each type of low voltage circuit breakers. The low voltage molded case circuit treakers (MCCBs) are used for containment per.etretion protection and as isolation devices protecting IE buses from rercualified loads. The MCCB trip units consist of two separate trip clements, thennal and instantaneous; these provide 8611190341 861107 PDR P
ADOCK 05000327 PDR
protection for the full range of expected current values (overicads and fault currents ) . The thermal element provides protection for overloads and r.,cdcrate fault currents (1.25X to 10X breaker current rating). The instantaneous element provides protection for the high fault currents (above 10X breaker current rating). The thernal elenent usually is not qualified by the manufacturer 2
for thenoal capability (1 T) at high currents (above 107 breaker current rating) . The operating characteristics of the thermal and instantaneous clerents of each type of MCCB are tested per l!Ld89 and published by the manufacturer.
These publisheo data clearly show the operating characteristics and capabilities 2
(1 T) of the therral ard instantaneous elements. To ensure that the MCCB performs its intended function over its life, both the thermal and instantaneous elcrents are periodically tested with current injection. Industry standards (such as NEPA /E-2 and NETA specifications) and manufacturer's maintenance guides provide recormendations for periodically testing f! CCBS.
It was noted during a March 20, 1986, NRC review of containment penetration overcurrent protection devices at the Sequoyah plant that TVA was not testing the MCCB instantaneous cletents and the thermal elements were being tested in series rather than individually as required by irdustry standards., TVA has contended that the MCCBs could to tested sufficiently by testing the thermal overcurrent elements without the instantaneous trip test. Subsequent to the March 20,1986 findings, TVA has conducted limited testing of some MCCBs and has offered the following reasons for not testing the instantaneous elements.
- 1. TVA contends that testing of the instantaneous elements damages the breaker contacts due to the high current values needed to ensure in-stantaneous tripping only. These high current values are approximately 50 to 60 times tne load rating of the breaker and are unique to the type of breakers used at Sequoyah. In most circuit breakers the in-stantaneces elements generally require pick-up currents in the neighborhood of 20X the breaker current rating. Based on the TVA test results, which are documented in their report of June 3,1986, "Sequoyah Nuclear Plant Units 1 and 2 - Molded Case Circuit Breakers Testing," the staff concludes that no significant contact damage would occur when testing MCCBs with current values required to test the instantaneous trip.
Minor pitting and discoloration of contacts is normal in all types of circuit breakers. Based upon the Sequoyah TS, each MCCB would be tested about 3 times over its service life. Therefore, the MCCBs at Sequoyah Plant would not be subjected to the same number of tests as stated in TVA's June 3, 1986 report. A MCCB is a reuseable device (i.e., it can be used again after fault interruption), and it is designed to be reused after it has been subjected to currents many times beyond the 50 to 60X used in the test conducted by the licensee and currents delineated in the industry standards (UL 489). The staff's review of the manu-facturer's (GE & Westinghouse) literature on testing of MCCBs and industry standards (UL 489 and NEMA AB-2) has not revealed any reference to possible contact damage as indicated by TVA. The nomal practice employed by industry and recomrrended by standards (NEMA AB-2) is to l
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test the instantaneous trip elements at a value slightly above its pick-up value and not at higher values as used in the TVA tests.
- 2. TVA contends that thermal trip elements of MCCBs can perfonn without degradation beyond their published values; i.e., the thennal element can operate well into the instantaneous region without degradation.
Since the MCCBs are tested with the instantaneous element operable (per UL 489), there is no industry test data or manufacturer's pub-lished infonnation to substantiate that the thermal elements could provide protection in the instantaneous current region without de-gradation. The staff discussed this matter with TVA (Telecon, June 9, 1986) and suggested that TVA contact the manufacturer of each type of MCCB used at the Sequoyah plant and have them substantiate TVA's position regarding the use of the thennal element beyond the pub-lished values. As documented in TVA's letter to Electrical Engineer-ing Branch Files from J. K. Greene, dated June 24, 1986, the manu-facturer (GE) would not quantify the capability of the thermal element beyond published values. In support of their position TVA decided to perform a test to determine the capability of the thermal element. As documented in the above referenced letter, the thermal 2
trip elements' I T capability of the GE type breakers are considerably 2
lower than the 1 T expected due to available fault currents. Although the results of the TVA tests appear to be favorable in support of their position, these tests were limited in scope. The test procedure has l
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shortcomings in that it did not follow many of the UL 489 procedures as outlined in the standard. Therefore the results of the TVA tests cannot be applied generically for all MCCBs. Another factor not con-sidered in TVA's evaluation of using the thermal element in the in-stantaneous region is the coordination with the upstream protective devices. It is not clear how the coordination of the backup protective devices will be affected as a result of increased time in tripping the circuit breaker by the thermal element. There is no doubt it increases the potential that upstream breakers will operate, thus increasing the possibility that other safety related loads may be lost.
- 3. TVA has stated in their justification that the instantaneous trip function mechanism has an extremely low probability of failure.
, TVA reasons that the mechanism is fixed and has only one simple moveable part. Although the above statement may be true for a given type of MCCB, it can't be applied to all circuit breakers.
We believe this reasoning is very subjective and is not sub-stantiated by published data. In addition, while it may be true that a simple trip device is employed, there are two distinct means to initiate the tripping (thermal and instantaneous), each of which acts independently on the trip mechanism and, therefore, the testing of the thermal cannot be said to be equivalent to testing the in-stantaneous.
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- 4. TVA has alluded that they have conducted a review of the industry and found that the r;ajcrity of those asked do not perfort. instan-taneous testing. However, no overtitttive data is offered to sup-port their findings. The staff kncvs of several plants in Region E performing the instantaneous trip tests as required by TS. Also it is common practice in the non-nuclear industry to perform these tests.
In addition, major verders of PCCBs have guides on the various field tests for these breakers including testing cf the instantaneous trip.
CONCLUSION Although the staff in'.tially troposed a determination of "no significant hazards consideration" (hSHC) regarding the acendrent request, upon additieral censider-etions reached based upon a full safety review, the staff has deterr. fred that the request for removire the testing of the instantaneous elements of the MCCBs will s not be beneficial for the reasons as stated in our evelvation. It is the staff position that testing of the MCCBs ircluding demonstration of the instantar. ecus trip is required and that no reascrable basis to eliminate such testing has been provioeo by the licensee. These devices are beirg relied upon as allowed by Regulatory Guide 1.63 " Electrical Penetration Asserrblies In Containment Structures For 1.ight-l'ater Cooled huclear Power Plants" to limit fault current to eithin I
the I T ratings of containnent electrical penetrations to avoid failure of con-tainment durfre eccident conditions. This protection is required to be single failure proof and to be testable and tested in conferrance with IEEE 279-1971 and General Design Criterion 18. These devices are considered of sufficient scf ety significance as to require periodic demonstration (each refueling outage) of a
percentdge (10? ) cf these devices to give some assurance of their reliability brid capability to perforni their design function. These devices arc designed tc te rcusable after experiencing faults within their design rating and ticrc-fore oc scund reason to exclude testing has been provided. The staff cannet agree with the TVA position without the manufacturer's substantietion and/or industry corsersus that the testing of the instantaneous element is indeed harmful and that the thermal element can be used in the instantaneous region without degradatien for all MCCBs. Therefore, the proposed an,endrent for deleting the testing of the instantaneous clerrent of the MCCBs is denied.
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