ML18033B168

Forwards Plans for Resolving Concerns Re Potential Pullby Damage to Electrical Cable at Facility.Cable Installation Concerns at Util Initially Stemmed from Reviews Performed at Plant & Included Variety of Issues,Including Jamming

ML18033B168
Person / Time
Site:	Browns Ferry
Issue date:	02/05/1990
From:	Michael Ray TENNESSEE VALLEY AUTHORITY
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9002130135
Download: ML18033B168 (26)

Text

.ACCELERATED DEMONSTION SYSTEM DISTRIBUTION REGULATORY INFORMATION DISTRIBUTION SYSTEM (RIDS)

ACCESSION NBR:9002130135 DOC.DATE: 90/02/05 NOTARIZED: NO DOCKET FACIL:50-260 Browns Ferry Nuclear Power Station, Unit 2, Tennessee 05000260 AUTH. NAME AUTHOR AFFILIATION RAY,M.J. Tennessee Valley Authority RECIP.NAME RECIPIENT AFFILIATION Document Control Branch (Document Control Desk)

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SUBJECT:

Forwards plans for resolving concerns re potential pullby damage to electrical cable at facility.

DISTRIBUTION CODE: D030D COPIES RECEIVED:LTR ENCL SIZE: D TITLE: TVA Facilities Routine Correspondence 3

NOTES:1 Copy each to: S.Black,D.M.Crutchfield,B.D.Liaw, 05000260 R.Pierson,B.Wilson /

RECIPIENT COPIES RECIPIENT COPIES ID CODE/NAME LTTR ENCL ID CODE/NAME LTTR ENCL D

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INTERNAL: ACRS 1 1 NUDOCS-ABSTRACT 1 1 QC/J 1 0 OGC/HDS2 1 0 EG FILE 1 1 EXTERNAL: LPDR 1 1 NRC PDR 1 1 NSIC 1 1 NOTES 5 5 D

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D NOTE TO ALL "RIDS" RECIPIENTS:

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TENNESSEE VALLEY AUTHORITY CHATTANOOGA. TENNESSEE 37401 5N 157B Lookout Place FEB 05 590 UPS. Nuclear Regulatory Commission ATTN: Document Control Desk Hashington, D.C. 20555 Gentlemen:

In the Matter of ) Docket No. 50-260 Tennessee Valley Authority )

BROHNS FERRY NUCLEAR PLANT (BFN) RESOLUTION OF CABLE INSTALLATION CONCERNS (TAC NO. 62260)

References:

1. Letter from R. L. Gridley to NRC dated July 18, 1988, "Browns Ferry Nuclear Plant Resolution of Cable Installation Issues"

2. Letter from R. L. Gridley to NRC dated September 29, 1988, "Browns ferry Nuclar Plant Resolution of Cable Installation Issues Cable Testing

3. Letter from M. J. Ray to NRC dated July 1.9,. 1989,. "Browns.

Ferry Nuclear Plant Electrical Cable Installation - Revised Halkdown and Summary Reports The purpose of this letter is to transmit a plan for resolving concerns related to potential pullby damage to electrical cable at BFN. The plan defined in Enclosure 1 supplements previous submittals on this issue.

Cable installation concerns at TVA initially stemmed from reviews performed at Hatts Bar Nuclear Plant (HBN) and included a variety of issues (sidewall bearing pressure, jamming, inadequate support of cables in long vertical runs, pulling through 90 degree condulets and mid-run flex conduits, bend radius, and pullbys). Though there was no evidence at that time to indicate that damage actually existed, each plant has undertaken programs of analysis, inspection, and testing to confirm the integrity of its cable systems.

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At BFN, this effort, included extensive walkdowns, calculation, analysis, review of existing plant documentation, and in-situ high-potential dielectric withstand testings These reviews, which considered the successful test program at Sequoyah Nuclear Plant, provided a comparative analysis to SQNs installations, materials, and procedures. The results of those reviews were documented in References 1, 2, and 3 and established a high degree of confidence in the installed cahle systems at BFN.

Subsequent to the completion of those reviews and to the submittal of the above documentation, damage to ca" les at HBN was discovered. Analysis of the damage indicated it was because o" c.ab1e pullbys.

9002130i35 900205 05000260 PDR ADDCK P PDC An Equal Opportunity EmpIoyer

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FEB 05 1990 U.S. Nuclear Regulatory Commission Even though no pullby damage was identified at BFN during recent inspections performed as a part of the cable issues review, additional engineering evaluations are being performed to ensure that such damage has not occurred.

This effort will include further analysis of installed conduits and cables at BFN and a program of high-potential testing of cables which may have experienced sidewall bearing pressures during the pullby process. Successful completion of the program as outlined in the enclosure will resolve this issue for BFN and validate the conclusions previously drawn in the referenced reports. contains the commitment contained in this submittal. If you have any questions, please telephone Patrick Carier, BFN, at 729-3570.

Very truly yours, TENNESSEE VALLEY AUTHORITY Manager, Nucl ar Licensing and Regulatory Affairs Enclosures cc (Enclosures):

Ms. S. C. Black, Assistant Director for Projects TVA Projects Division U.S. Nuclear Regulatory Commission One Hhite Flint, North 11555 Rockville Pike Rockville, Maryland 20852 Mr. B. A. Hilson, Assistant Director for Inspection Programs TVA Projects Division U.S. Nuclear Regulatory Commission Region II 101 Marietta Street, NH, Suite 2900 Atlanta, Georgia- 30323 NRC Resident Inspector Browns Ferry. Nuclear Plant Route 12, Box 637 Athens, Alabama 35609-2000

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ENCLOSURE 1 BROWNS FERRY NUCLEAR PLANT CABLE PULLBYS

1.0 BACKGROUND

1.1 Initial Review During the summer of 1986, the U.S. Nuclear Regulatory Commission (NRC) began a review of concerns related to the adequacy of construction practices at the Tennessee Valley Authority's (TVA) Watts Bar Nuclear Plant (WBN). The review identified concerns for potential damage to electrical cables due to alleged improper or inadequate installation practices. Since TVA's Sequoyah (SQN) and WBN plants are based on the same overall design, the NRC extended the evaluation of the cable installation concerns to the SQN plant.

As a result, TVA performed an extensive and comprehensive evaluation of the SQN issues identified in NRC's Technical Evaluation Report (TER-C5506-649) as requiring implementation prior to that plant's restart. That effort resulted in the successful resolution of'able installation issues at SQN and demonstrated that adequate cable installation practices were used.

As a part of the normal process of generic review of Condition Adverse to Quality Reports (CAQRs), TVA initiated an indepth review .to evaluate the applicability of these same concerns to Browns Ferry Nuclear Plant (BFN).

The BFN evaluation consisted of the following;

'Com arisen to ~industr Dractices: Determine if the BFN cable installation practices and procedures were consistent with those used in the industry during the time period of BFN's construction.

• Plant walkdowns: Perform plant walkdowns to review specific practices and assess the overall quality of the cable installation.

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i by the th SQN tt cable hih issue resolution program.

• Review of test data: Evaluate BFN test documentation to confirm the adequacy of the cables and to assess BFN's failure rate as a measure of the quality of the installation process.

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Enclosure 1 Page 2 The review team noted the existence of various physical evidence and plant documentation which established increased confidence in the acceptability of the BFN cable installation process. The findings, which were described in detail in the Cable Installation Issues Reports (References 1, 2 and 3), are briefly summarized, as follows;

• No evidence was found to indicate that braided ~ull ro es had been used at BFN. In lieu of ropes, which had been noted at HBN during the review for the TER, only insulated pull wires were observed.

Subsequent to the task group review under discussion, interviews were conducted with several electricians who have worked at BFN for

't over a decade. These personnel were aware of the dangers of using abrasive cord during a pullby and indicated that such materials were not used at BFN. The insulated pull wires (typically PVC jacketed) have a much lower coefficient of friction and present a much lower

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Since pull forces vary exponentially with the degrees of bends between pull points, this factor was judged to reflect good workmanship as well as greatly increasing the potential for 1 t d.

successful pullbys.

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~ullb s had been made versus where no ~ullb occurred. This is 1

indicative of good craft practice. Current industry recommendations for performing pulibys have identified liberal lubrication as one of the most important factors in achieving a successful installation (ref; Committee Report Recommended Practice on Specific Aspects of Cable Installation in Power Generating Stations, IEEE / Insulated Conductors Committee, Task Force 14-1).

• Due to the ~h sical ~la out of the plant conduit runs are ~relativel short. In particular, the compact nature of the Hark I containment structure and the fact that the various board rooms are in- set into the reactor building minimizes the footage required to reach load devices. Since SNBP is directly proportional to length, pullbys in shorter conduits present less of a risk to the cables.

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of documentation indicated that the installation process was stems.

~it Documentation of both post- installation t'nstalled and maintenance high-potential testing of medium voltage cables indicated the presence of quality craftsmanship during initial installation. This was further borne out by BFN's low cable failure rate compared to industry norms (reference 2).

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Enclosure 1 Page 3

• ~Flnall the 10CFR50.49 cables located in the three worst case environmental areas of the Riant ~are bein ~re laced this o~uta e for environmental considerations. As a result, that group, which has the highest risk of failure should any damage have occurred, will be known to have been installed according to the latest criteria and constructed with state of the art materials.

The review team identified two areas in which cable integrity required further verification prior to plant restart. These consisted of excess unsupported vertical drop and inadequate bend radius, both as applied to medium voltage (5000 vac) cable'. As a result, a program of testing was undertaken to identify any existing degradation and provide a measure of confidence for further operation of those circuits. All 20 cables were successfully high-potential tested during 1989 at IEEE 400 maintenance levels.

In summary, a review team of engineers and consultants with a broad base of industry experience concluded that BFN compared favorably both with SQN and with other (non-TVA) plants of its vintage in both design and construction for each of the issues identified in the SQN TER.

Subsequent reviews have shown that certain practices, believed to be contributors to the potential for pullby damage were not present at BFN.

In particular, BFN conduit systems are not subject to frequent overfill conditions. When allowable fills are exceeded, the difficulty of obtaining a clear path is greater, thereby increasing the risk during the pullby. In addition, a review of BFN records indicates that bulk, rather than system pulling (which results in frequent pullbys), was the general practice. This is consistent with current industry good practice.

1.2 WBN Findings and BFN Response In July 1989, work was performed to resolve an employee concern related to potential heat damage to cables as the result of alleged improper welding activities on conduits at WBN. Installation damage was discovered to a total of five cables in three segments comprising a single run of conduit. Subsequent visual and laboratory evaluation confirmed the damage was caused by a pullby. An additional 33,500 feet of cable were removed to assess the scope of that damage. One additional instance of damage was noted. Analysis of the raceway systems showed a correlation between the calculated installation forces and the occurrence of damage.

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Enclosure 1 Page 4 Though the issue of pullbys had been extensively reviewed at BFN during the process described above, these findings were deemed to be of sufficient magnitude to warrant reanalysis. As a result of the above mentioned correlation of high forces to installation damage, TVA decided to invoke a screening process and perform pulling force calculations to more rigorously identify i'ts worst case pullbys. That ranking process is described in section 2.0. Once the worst case pullbys are identified, high-potential withstand testing will be conducted as outlined in section 3.0. During TVA presentations to the Staff in both December 1989 and in January of 1990, TVA indicated that three condui ts had already been identified for testing. Since TVA cannot be certain at this point in time that these conduits will be in the final worst case group, the expense and difficulty of testing these 50 cables / 210 conductors (approximate) is not warranted. Therefore, no further special action will be taken .on these three condui ts. Only those condui ts which are part of the final group described in section 2.0 will be tested.

2.0 SELECTION CRITERIA The population of conduits containing safety related circuits at BFN in which the potential exists for cable pullbys to have occurred is, being identified and screened using the criteria outlined below. Nithin 'this bounded population, a further screening will be performed of the top 30 to define and identify a group of 10 'worst'i.e. highest pullby damage potential) conduits. For the purposes of this evaluation that group is defined as those conduits meeting the following criteria; 2.1 Using non-QA databases taken from the Unit 1 and 2 cable and conduit schedules and the Unit 3 cable routing program, sorts were performed to identify those Vl-V4 safety related conduits containing more than one cable number. This population (approximately 1330 conduits) defines the group in which potential pullby operations have occurred.

2.2 This subpopulation was further reviewed to identify those conduits in voltage levels 1 through 3 which contain 8 or more cable numbers and those conduits in voltage level 4 which contain at least 4 cable numbers.,

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Enclosure 1 Page 5 2.3 Since conduits are field routed at BFN and the conduit and cable database does not include conduit lengths, these were estimated by scaling from design drawings. Conduits which were not at least 20 feet in length were eliminated from further consideration. This screening yielded a subgroup of approximately 120 conduits.

2.4 The remaining conduits will initially be ranked using the following formula:

(L*F)/R = ranking factor where:

L = conduit length as determined above F = conduit fill per the referenced database R = conduit bend radius per TVA standards This equation reflects the formulas for determining sidewall bearing

'pressure (SNBP) during pulling and includes several of the key parameters for consideration during a pullby.

Pull tension and SNBP increase directly with length. The potential for complex conduit geometry also increases with increasing conduit length.

For example, the likelihood of finding a high number of degrees of bends between pull points is greater for a 150 foot long conduit than for a 30 foot run.

By virtue of similar reasoning, the industry has recognized that the severity of a pullby goes up with fill due to the increasing difficulty of obtaining a clear path for the pullby cables (ref; IEEE/ICC Task Force 14-1 Report). For instance, a pullby into a conduit which results in a final fill of 10 to 15 percent may not have actually resulted in cable-to-cable interaction. This results from the pre-pullby conduit being relatively empty such that the new cables may have ridden directly on the conduit wall. In a conduit where the final fill is high, the reverse is true. Therefore, with a high degree of certainty it can be concluded that cable- to-cable interaction did occur. Additionally, since the severity of any given pullby is a function of its size, this factor will give preference to conduits with high fill and therefore, the potential for large pullbys. As an example, conduits which are lightly filled (i.e. or 2 cables) ca'nnot have exp'erienced large pullbys whereas 1

conduits which are highly filled (many cables) have a greater potential for having experienced large (5,6,7 . . . cables etc.) pullbys.

Finally, since SNBP is inversely proportional to the radius of bend of its conduit, the ranking will assume that each segment contains bends formed to the minimum radius allowed by TVA and industry standards.

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Enclosure 1 Page 6 The combination of these factors will permit quick and efficient determination of a family of conduit runs which contain the potential for significant pullbys to have occurred. As is described in section 2.6, that larger family will then be screened using a more rigorous methodology.

2.5 A review of this grouping will then be performed, to identify the top 30 conduits in which a pullby has occurred. This review will be performed using existing cable pullslip data and/or change package information.

2.6 The top 30 conduits identified in step 2.5 will then be walked down.

Sketches of each conduit will be prepared to identify all intermediate pull points, the length between bends and the degrees of each bend.

2.7 Detailed calculations will be performed to conservatively estimate the maximum SNBP -that would have been encountered during 'the largest pullby in each conduit. This analysis will be based on the configuration data taken in step 2.6 and utilize standard industry methodologies and formulas for the determination of expected pull tension and expected SNBP derived from the fundamental laws of physics. The calculations performed by TVA utilize variations of the following basic equations; T = L*N'K*EXP(K*A) and SNBP = T/R where:

T = pulling tension in pounds SNBP = sidewall bearing pressure in pounds per foot L = conduit length preceding a bend in feet N = weight per foot of the cables being pulled in pounds K = basic coefficient of friction A = angle of the bend in radians R = conduit bend radius in feet Such calculations by their very nature are approximations of the anticipated forces, yet represent best industry practice for their determination. Typical TVA and industry practice involves only the calculation of the maximum allowable forces and a determination that such allowables were not exceeded without a direct concern for degree of conservatism.

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Enclosure 1 Page 7 2.8 The top 30 conduits from step 2.5 will be reranked according to SNBP.

Since cables of differing construction and SNBP limitations may be involved, the ranking will be based on the percentage of allowable SNBP rather than magnitude of the calculated SNBP.

NOTE: The screening process described in paragraph 2.4 does not include direct consideration for raceway configuration since this can only be known as the result of field walkdowns. Therefore, following completion of the detailed calculation and reranking process, the methodologies will be reviewed to confirm that the screening process has produced viable results. If acceptable correlation exists between the screening process and the detailed calculation, TVA will review the above comparison and results with the NRC prior to proceeding with step 2.9. If such correlation does not exist, the conduits (approximately 120) which remained fol.lowing step 2.3 will be reviewed to identify those in which pullbys have occurred. Those conduits will be walked down in accordance with step 2.6, calculations performed per step 2.7 and the total group will be reranked per step 2.8.

2.9 Cables in the top ten conduits identified by this process will be, subjected to high-potential testing as described in section 3.0. TVA anticipates the ten conduits will contain 100-200 cables comprising 400-700 conductors. Successful tests will confirm that the forces experienced during pullbys have not resulted in the systematic degradation of BFN's cable systems.

Implementation of the above screening and ranking process will be documented in a calculation.

3.0 CABLE TEST PROGRAM CRITERIA The predominant concerns during pullbys are for saw-through and mashing of the jacket and insulation. The postulated failure mechanism is the degradation of dielectric properties resulting from a reduction in insulation wall thickness and possible exposure of the copper conductors. The following test program is specifically designed to identify if such a condition exists.

3.1 Cables will be subjected to a high potential withstand test at 2',0 volts DC per mil (vpm) of insulation. The insulation thickness to be used in this determination shall be the minimum qualified thickness as established by a review of TVA and vendor documentation. Two additional criteria wi 11 be applied in assessing the appropriate voltage levels as a result of a review of specific cable constructions. First, the test voltage shall be limited to a maximum of 7200 volts DC. Of particular concern to TVA would be the application of as much as 15,000 volts to

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Enclosure 1 Page 8 certain coaxial cable constructions if the 240 vpm multiplier were directly applied. These cables are over-insulated to obtain the desired impedance characteristics and typically operate in the millivolt range.

Second, the test voltage shall not be allowed to exceed 801. of the industry standard values for factory testings Review of specific cable types (small gauge signal) has shown that 240 vpm would result in a field test above that required for new cable at the factory.

TVA will notify the NRC before testing any cables at levels other than those determined by the 240 vpm multiplier.

3.2 The test shall be 5 minutes in duration.

3.3 The voltage will be applied conductor-to-ground between the conductor under test and its conduit with the conduit, all other conductors, shields and drain wires tied together and tied to ground.

3.4 The tests will be performe~ after injecting ordinary tap water into the conduit segment in which the high SNBP occurred. Those segments to be tested will be reviewed on a case-by-case bas'is to determine the feasibility of introducing water, into, and removing it from, the conduit system without compromising equipment integrity or personnel safety.

TVA will notify the NRC of any instances where water cannot be introduced and the rationale for its exclusion will be documented.

The purpose of the introduction of water is to ensure, as a minimum, that moisture is present along the entire surface of the installed cables in that segment.

3.5 Leakage currents will be recorded at 1 minute intervals at the specified test voltage.

3.6 The cables must pass the in-conduit DC high-voltage test. The acceptance criteria (from ANSI/IEEE 141- 1986 section 11.11.4) is that a polarization index of 1.0 or greater must be demonstrated. The polarization index is the ratio of the current after minute to the 1

current after 5 minutes at the specified test magnitude. Any conductor with a polarization index of less than 1.0 wi 11 be considered a failure unless it can be demonstrated, by engineering evaluation or by retest, that the conductor is acceptable.

If the cables fail the high-voltage test, the NRC will be notified.

Enclosure 1 Page 9 The location of the failure shall be determined and the faulted section electrically isolated. The tests will be repeated. If the cause of the failure is subsequently determined to be non-pullby related and the cables successfully withstand the retest, then the test shall be considered successful in demonstrating adequate pullby practices. TVA will take the necessary actions to identify and bound any problems which are identified during the performance of the tests.

4.0 Conclusion No pullby damage has been identified at BFN. BFN is performing a study consisting of engineering analysis and high potential testing.

Successful testing of the cables in the 10 conduits identified by the selection criteria will ensure that BFN Unit 2 cables which have experienced a pullby are acceptable for service and further validate the conclusions previously drawn by the Cable Installation Issues Report.

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ENCLOSURE 2 COMMITMENT TVA will submit the results of BFN's study to validate that excessive sidewall bearing pressure was not incurred during cable installation involving pullby, which could result in cable damage.

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