ML20199G816

SER Accepting Util 970707 Response to NRC 970509 RAI Re GL 92-08, Thermo-Lag 330-1 Fire Barriers. NRC Finds No Significant Safety Hazards Based on Application of Util Ampacity Derating Methodology

ML20199G816
Person / Time
Site:	South Texas
Issue date:	01/19/1999
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20199G808	List: ML20199G805 ML20199G816 ML20199G840
References
GL-92-08, GL-92-8, NUDOCS 9901250034
Download: ML20199G816 (3)

Text

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i om l p UNITED STATES g j NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20066-0001 o,

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMPACITY DERATING ISSUES STP NUCLEAR OPERATING COMPANY (STPNOC)

DOCKET NOS. 50-498 AND 50-499 SOUTH TEXAS PROJECT. UNITS 1 AND 2 (STP)

1.0 BACKGROUND

By letter dated July 7,1997, Houston Lighting & Power Company (the former licensed operator; STPNOC is the current licensed operator) submitted a response to the Nuclear Regulatory i Commission's (NRC's) May 9,1997, request for additional information (RAI) related to Generic Letter 92-08, "Thermo-Lag 330-1 Fire Barriers," for STP. ,

The licensee had previously id entified a total of 24 cable trays that did not pass the " Step 1 Ampacity Analysis," as described in.its November 6,1996, letter. For these 24 trays, the licensee has performed a " Step 2 Heat Analysis." Based upon the information provided in the 1996 letter the staff concluded that the " Watts per foot" methodology had been applied in the l Step 2 Heat Analysis. Earlier staff reviews associated with other licensees has lead to the l conclusion that the " Watts per foot" methodology has fundamental weaknesses in providing an adequate assessment of the ampacity performance limits for individual cables.  !

The staff's May 9,1997, RAI ic'entified a number of open issues and concerns associated with the " Watts per foot" methodology which required further clarification by the licensee. The licensee's July 7,1997, letter contained a reassessment of the subject cable trays using an alternate methodology. Further, the licensee's April 15,1998, letter stated the thermo lag fire barrier material will be removed from the subject cable trays by the end of 1998 in order to eliminate any concerns regarding cable ampacity.

l The staff's evaluation of the ampacity derating methodology for STP follows. 1 2.0 EVALVATIQN After reviewing the licensee's letters and Sandia National Laboratories (SNL) Technical Evaluation (see Enclosure 2), the staff agrees with the SNL analyses and conclusions. The ampacity derating analysis questions, the licensee's responses, and the staff's evaluation of the responses follow.

ENCLOSURE 1 i 9901250034 990119  ;

PDR ADOCK 05000498 '

P PDR I

2 Amoacity Deratina Analysis Review

. Question 1 What test data supports and demonstrates the appropriatenecs of this approach?

Licensee Response By letter dated July 7,1997, the licensee provided an ampacity derating analysis reassesement for the subject cables based upon alternate considerations.

3 Staff Response The information provided by the licensee is fully responsive to the subject question.

. Question 2 in general, the ' Watts per foot" methodology provides an inadequate treatment of the impact of cable loading on the allowable heat loads and assumes those effects are largely irrelevant to the overall heat rejection capacity of the cable tray or conduit system.

Licensee Resoonse By letter dated July 7,1997, the licensee provided an ampacity derating analysis reassessment for the subject cables based upon alternate considerations.

Staff Response The information provided by the licensee is fully responsive to the subject question.

. Question 3 See SNL's discussion in Section 2.5 of the SNL Letter Report to NRC, Rev. O, dated April 24,1997, on cable diversity effects (excerpted and restated below):

Cable Diversity Effects: The Watts /ft analysis method provides for no significant treatment of cable diversity effects and how this would impact the total allowable heat loads for the cable tray system. All of the available ampacity tests typically cited as supporting the method are based on cable trays in which all of the cables are powered uniformly. In real applications, cable trays contain a mixture of loaded and unloaded cables. It is unrealistic to assume that a diverse cable load would have the same overall heat rejection capacity as a uniform cable load. Recall that the objective is to ensure a hot spot of no greater than 90 C.

Concentrating the heat generation in just a few cables clearly will create significant localized heating effects that would lead to higher hot spot temperatures. This is not accounted for in the " Watts /ft method."

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Licensee Resoonse j

i By letter dated July 7,1997, the licensee provided an ampacity derating analysis j reassessment for the subject cables based upon alternate considerations.

l Staff Response The information provided by the licensee is fully responsive to the subject question.

AppJintion of Amoacity Deratina Methodoloov lt is important to note that the licensee's April 15,1998, letter states that the Thermo-Lag material will be removed from the 24 Thermo-Lag enclosed cable trays. Hence, in the current -

context of this review, the primary objective is to ensure that formerly protected cables have not been subjected to excessive premature aging.

In its ampacity assessments for cable trays, the licensee has applied a 32% derating factor for 1-hour barriers (based on testing by Texas Utilities Electric (TUE)) and a 48% derating for 3-hour barriers (based on testing by Tennessee Valley Authority (TVA)). The staff has previously approved these values for use. Given that licensee has indicated that the STP Thermo-Lag installations are representative in design and construction of the configurations tested by TVA and TUE, the cited ampacity derating factors can be utilized by the licensee.

The licensee had provided specific ampacity assessments for the 24 cable trays that were previously accepted on the basis of a Watts-per-foot method. Screening ampacity margins assessments identified a total of 30 individual cables (13 cables for Unit 1 and 17 cables for Unit 2) requiring further consideration. The initial assessment uniformly includes a load factor (LF) of 1.25 for all cables.

i As noted in the SNL report dated April 24,1997, one outstanding concern was the finding that i

the Watts-per-foot approach for cable tray ampacity assessment was inadequate and inappropriate for use. The current licensee assessments continue to use a cable tray overall .

heating rate derived from the Watts-per-foot methodology. However, by letter dated July 7, 1997, the licensee provided an attemate approach for its final assessments. Individual reassessments were provided for the subject cables. This reassessment action fully resolves the staff concern regarding the application of the Watts-per-foot-method for these specific STP configurations. The staff finds that all of the cable reassessments were acceptabla. q 1

The staff finds that the licensee has resolved all of the outstanding technical items which had ,

been identified in the previous reviews. In particular, the licensee has abandoned the Watts-  !

per-foot methodology in favor of a more conventional margins approach. The staff does not credit the use of the Watts-per-foot approach in the context of the STP assessments.

The staff takes exception to two aspects of the licensee assessments:

- The licensee states that " typical industry practice does not include consideration of undervoltage or overload conditions in ampacity assessments." As a result, for several motor and pump cases, the licensee resolved a nominal ampacity overload by relaxing the LF from 1.25 to 1.0. The staff does not cone'ir with this assumption. Indeed, load

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4 factors have no other purpose than to ensure that the ampacity assessment allows for all conditions of operation and they are considered in typical industry practice.

Ultimately, this finding had no impact on the final assessment of aceptability. Givar.

that the fire barriers are being removed, in all of the impacted cases it was determined that the available "LF margin" was adequate to ensure that no adverse life-to-date aging has been realized for the subject cables.

The licensee occasionally referred to a given cable as not being an Appendix R cable; hence,"the adequacy of the cable ampacity is not a concern from an Appendix R standpcint? The staff does not agree with this argument since ampacity limits are a necessary design parameter applicable to all electric power circuits, not just to the Appendix R safe shutdown systems. However, in all such cases, the licensee has provided alternate arguments for acceptability of the subject cables, in all cases there was sufficient information provided by the licensee to conclude that no adverse aging impact had been experienced by the cables independent of the above two points.

It should be noted that SNL has recommended that certain cables that have been operated at overload conditions in the past will remain acceptable for future operation. This assessment is acceptable to the staff based on two specific considerations. First, the licensee has stated that the subject fire barriers will be removed and this action will relieve any potential overload problems during future operations. Second, an assessment made by SNL of the worst-case impact of the cables for life-to-date operation at the stated overload conditions indicates that no adverse equipment aging impact was likely to have occurred in the specific cases.

l Given the above findings and the licensee commitment to remove the Thermo-Lag fire barriers  !

l from the subject cable trays, the staff finds that the licensee has provided adequate information I L

to resolve the arnpacity-related points of concern raised in GL 92-08.

3.0 CONCLUSION

S l

Given the above evaluation and the licensee commitment to remove the applicable thermo-lag fire barrier material, the staff concludes that no significant safety hazards exists based upon the l application of the licensee's ampacity derating methodology. Therefore, there are no ampacity I

related safety concerr.s at STP. '

Principal Contributo': R. Jenkins l Date: January 17, 1999 i

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