ML18033B097
| ML18033B097 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 12/19/1989 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML18033B098 | List: |
| References | |
| NUDOCS 8912280191 | |
| Download: ML18033B097 (8) | |
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NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 ENCLOSURE SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION CABLE AMPACITY PROGRAM SECTION III.13.2 OF THE BROMNS FERRY NUCLEAR PERFORMANCE PLAN TENNESSEE VALLEY AUTHORITY BROWNS FERRY NUCLEAR PLANT, UNIT 2
1.0 INTRODUCTION
In 1986, the Institute of Nuclear Power Operations performed an audit on the Bellefonte plant that revealed inadequacies in Tennessee Valley Authority's (TVA's) electrical design standards DS-E 12. 1. 1 through DS-E 12.1.4.
Because these standards have been used to size all the insulated power cable ampacities (auxiliary and control) throughout TVA's nuclear plants, the potential exists for undersizing of safety-related cables in the Browns Ferry Nuclear Plant (BFN).
TVA developed a
new design standard, DS-E 12.6.3, "Ampacity Tables for Auxiliary and Control Power Cables (0 - 15,000 volts)," which corrected all the inadequacies.
This standard was based on various recognized industry standards and test reports on cable ampacity.
The new standard also addresses ampacities for cables in conduits, cable trays, and duct banks, as well as derating fac-tors for cable coatings, fire wraps, cable tray covers, and cable tray bottoms all of which are required by 10 CFR Part 50, Appendix R.
TVA established an ampacity program to determine the extent of nonconformance and to take correc-tive actions for any nonconformances at BFN; 2.0 EVALUATION
- 2. 1 Ampacity Program Plan By letter dated January 25,
In this program, TVA planned to establish a remaining life of at least two years for all auxiliary and control power safety-related
- cables, even though cables may be operating beyond their design rating.
At the BFN site on April 26,
- 1988, a meeting was held between TVA and NRC staff members to discuss the program plan.
In this meeting, TVA agreed with the staff that any cable that does not meet the plant life requirements will be replaced.
The staff expressed concern about operating the cable beyond the design rating of the cable (NRC Meeting Summary dated May 17, 1988).
By letter dated July 7, 1988, TVA submitted Revision 1 of the ampacity program plan.
In accorda'nce with the revised ampacity program plan, safety-related cables will not operate at temperatures greater than their qualified maximum temperature rating or their design rating, whichever is smaller.
The ampacity program plan
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2 for BFN Unit 2 is similar to the previously approved ampacity program plan for Sequoyah Nuclear Plant (SgN), except for two major differences.
The first difference is related to the method of sampling.
At SgN, TVA has used Military Standard 105D as the basis for obtaining a 95/95 assurance level (i.e., giving 95 percent assurance that at least 95 percent of the population is acceptable).
At BFN, TVA proposed using a sampling plan based on NCIG-02, Revision 2, which was issued by Electric Power Research Institute as NP-5380, Volume 2.
It should be noted that the staff had expressed concern about Mili-tary Standard 1050 during the SgN ampacity review.
The staff has reviewed the proposed sampling approach at BFN for the ampacity program and has found it acceptable.
However, during the implementation of the ampacity program, TVA decided to analyze 100 percent of V /V cables and a large quantity (3,500) of V> cables.
- Hence, the question of 3am ling became moot at Browns Ferry.
J The second difference is related to the application of load diversity and time diversity in conjunction with hot spot consideration in the ampacity program at BFN.
At S(N, TVA did not take any credit for the load diversity and'time diversity.
These terms are defined below:
Load Diversit Anal sis.
Load diversity analysis is a conservative tec n que t at recognizes that within a tray system there exists a reserve heat capacity for cables that are loaded below their rated amperage and apportions this margin to other more heavily loaded cables in the system.
Time Diversit Anal sis.
Time diversity analysis is the application of t e oa versity ana ysis for specific operating modes (i.e.,
normal operation, shutdown, accident condition, single unit operation with other unit's shut down, etc).
Hot S ot.
A hot spot can be produced by the unanticipated bundling toget er of a few tightly packed, heavily loaded cables in a tray with many other underloaded cables.
The staff reviewed Revision 1 of the ampacity program plan and issued a request for additional information by letter dated August 10, 1988.
By letter dated September 30,
- 1988, TVA provided the requested information. It should be noted that the current ampacity standards do not allow any credit for load or time diversity, although the standard effective at the time of BFN licensing did allow some credit for diversity.
Based on the staff's review of TVA's re-
- sponses, the staff decided to obtain technical assistance from Sandia National Laboratories (SNL) in the review of the ampacity program.
The staff and its consultant had a meeting with TVA at the BFN site on February 2, 1989, to discuss the ampacity program.
During this meeting, several questions were raised concerning TVA's methodology of application of diversity (NRC Meeting Summary dated February 15, 1989).
By its letters dated March 17, 1989 and April 18,
- 1989, TVA submitted additional information to resolve the questions raised by the staff and its consultant..
The initial review of the TVA's submittal identified a number of assumptions used in the
TVA program.
Some of these assumptions were clearly conservative, some were somewhat nonconservative, but others could not be easily identified as conser-vative or nonconservative.
TVA's ampacity program is based on Stolpe's experi-mental work, which was based on random cable trays.
During the meeting on February 2, 1989, TVA also presented experimental data and compared it with the ampacity program's prediction and the results'f a finite element code (Heating
- 6) for cables in a randomly filled cable tray.
At the initial meeting and in subsequent telephone calls, TVA insisted that the cable trays at Browns Ferry were not randomly filled and that the ampacity program was therefore designed to assess worst possible cases.
Therefore, the staff in consultation with SNL decided that to expeditiously resolve this issue, some additional independent worst-case simulation should be conducted rather than confirming or agreeing with all the assumptions used by TVA in its computer code simulations.
- Hence, by letter dated March 21, 1989, the staff sent two test cases developed by SNL to TVA for analysis by their ampacity computer code.
The test cases were specifically devised to challenge the ampacity program and to take advantage of possible program weaknesses.
By letter dated April 27,
- 1989, TVA submitted the results of its analysis.
SNL used the results of the TVA ampacity analysis as input to a finite element computer code (COYOTE) at SNL, which predicted temperatures as much as 28'C above the allowable 90'C predicted by TVA's ampacity analysis.
A conference call was held between staff members and representatives from TVA and SNL to discuss, the differences between the two analyses.
As a
- result, TVA on its own initiative decided 'to perform a laboratory test using the given test cases.
The test results showed that maximum temperatures were below the 90'C used in the TVA ampacity calculations.
A follow-up meeting was held at the BFN site on July 10, 1989 (NRC Meeting Summary dated August 21, 1989), to discuss the ampacity program.
In this
- meeting, TVA also presented the result of the analysis, using HEATING 6, with results typically 5'C or more lower than SNL's, depending on the assumptions used.
At this meeting, reasons for the differences between the experimental results and the computer predictions were discussed.
The use of the test case was developed to challenge analytical predictions in as simple a manner as possible.
In terms of an experimental
- program, a
different physical configuration (many more cables but of smaller sizes and more tightly packed) would be expected to more closely match the finite element predictions.
At the July 10 meeting, TVA was again questioned about the randomness of cable loadings in actual cable trays.
Upon clarification of the definition of random, TVA indicated that the cables were in fact randomly installed in the trays but that the depth of fill in the trays was not uniform.
Previously, TVA had interpreted randomness to include uniform depth.
Mith a valid randomness assumption, the worst-case analysis is
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V less critical if the probability of existence of actual trays that are "close" to the bounding cases is sufficiently low.
SNL per formed a
computer simulation of a random tray to assess the probability of an actual random tray being near a worst case condition.
The test case given to TVA, using 22 large cables with six of them loaded, was used in the simulation.
The results of the probability analysis, based on a simula-tion of 20,000 random trays, indicated that in more than 95 percent of the simulations, less than four loaded cables were together and in more than 99 percent of the simulations, less than five loaded cables were together.
From this information it can be concluded that in a real random tray, such as one most likely found at BFN, with many smaller cables, the probability of-having loaded cable densities corresponding to the test case would be considerably lower.
Hence,,it can be concluded with 95/95 assurance that based on the TVA test results with six loaded cables together resulting in temperatures below those allowable, demonstrate the adequacy of the TVA load diversity computer code.
The TVA ampacity program at BFN is being carried out in three phases.
During Phase I, design standard DS-E 12.6.3 is used to conservatively determine the adequacy of the cables.
In Phase IIa, for any cables that do not meet DS-E 12.6.3 requirements, the actual installation and load requirements are determined to allow for a reduction of some conservatism.
TVA has performed the plant walkdown to determine the as-built configura-tion of raceways; for example, conduit and tray fill, tray cover and bottom, thickness of flame-retardant coatings, fire wraps, fire stops, pressure
- seals, etc.
If cables do not meet the acceptance criteria after Phase IIa, then load diversity is applied during Phase IIb and time diversity is applied during Phase III. If any safety-related cable does not pass the acceptance criteria after this three-phase evaluation, then it will be replaced with a 90'C-rated cable fully sized to the conserva-tive requirements of DS-E12.6.3.
2.2 Ampacity Analysis By its letter of October 6, 1989, TVA submitted the results of ampacity analysis for BFN, Unit 2, cables for the three voltage levels defined below.
V3 - Auxiliary and control ac and dc power cables operating at a voltage of up to 277 volts and a current of less than 30 amperes.
V4 - Auxiliary ac and dc power cables operating at a voltage up to 600 volts (This includes cables of 277 volts or less with a rated load current of 30 amperes or greater.)
V5 - Medium voltage auxiliary power cables with a nominal voltage of 5, 8, or 15 kV.
The following discussion provides the resu1ts of the ampacity analysis by vol tage levels.
V3 Control Power and Control Function Cables TYA provided justification and documentation for excluding certain Y
cables (control function cables) that carry low-level and/or intermittent signals for which the ampacity rating of the cable is of no concern.
For the 3,500 cables reviewed for-V3-level cables, none of the cables were determined to be control power cable which would have required ampacity evaluation.
- Hence, no defi-ciencies were identified for this group of cables.
V4 V5 Low Volta e and Medium Volta e Cables TVA performed 100 percent evaluation of the cables in this group.
For the 263 safety-related V4/V5 cables, routed in dedicated conduits, no failures were identified.
For the remaining 941 cables, which are primarily routed on trays, and which include 194 safety-related
- cables, 66 safety<<related cables failed to meet the requirement.
A further review of the latest revision of the g-List ident)fied three more V
cables requiring ampacity evaluation.
Of these three cable's, two met the amp)city requirement and the third one will be replaced because of cable separation requirements.
- Hence, no ampacity analysis was performed for this last cable.
TVA will replace all the failed cable before startup of Unit 2.
- 3. 0 CONCLUS ION Based on the staff's review of TVA's submittal of their ampacity program, we conclude that the TVA ampacity analysis provides reasonable assurance that cables will not be running above their rated temperature and wi'll perform their intended safety function under normal, abnormal, and accident conditions.
4.0 REFERENCES
TVA Letters Tennessee Valley Authority, January 25, 1988, letter from R. Gridley to
- NRC, Ampacity Evaluation Program Plan.
-, October 24, 1988, letter from S. A. White to NRC, providing Revision 2 of the Browns Ferry Nuclear Performance Plan.
-, July 7, 1988, letter from R. Gridley to NRC, Ampacity Evaluation Program
- Plan, Revision 1.
-;-, September 30, 1988, letter from R. Gridley to NRC, Responses to Request for Additional Information.
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-, March 17, 1989, letter from R. Gridley to NRC, Responses to NRC questions from February 2, 1989 meeting.
-, April 18, 1989, letter from C.
H.
Fox to NRC, Electrical Cable Ampacity Test Results Report.
-, April 27, 1989, letter from C.
H.
Fox to
- NRC, Responses to the request for additional information.
NRC Meetin Summaries U.S. Nuclear Regulatory Commission, May 17,
- 1988, NRC Summary of April 26, 1988 meeting.
-, February 15, 1989, NRC summary of meetings held on February 1-3 and February 6, 1989.
-, August 2, 1989, NRC summary of meetings held on July 11-12, 1989.
NRC Letters U.S. Nuclear Regulatory Commission, August 10, 1988, letter from S.
C. Black to S.
A. White (TVA), requesting additional information on the cable ampacity program.
-, March 21, 1989, letter from S.
C. Black to 0.
D. Kingsley, Jr (TVA),
regarding additional information on the cable ampacity program.
Principal Contributor:
H. Garg t
Dated:
December 19, 1989
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