ML18067A633
| ML18067A633 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 07/10/1997 |
| From: | Bordine T CMS ENERGY CORP. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| Shared Package | |
| ML18067A634 | List: |
| References | |
| NUDOCS 9707180071 | |
| Download: ML18067A633 (14) | |
Text
A CMS Energy Company July 10, 1997 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555 DOCKET 50-255 - LICENSE DPR PALISADES PLANT Thoma* t:. BordlaB Manager Licensing RESPONSE TO NRC REQUEST FOR ADDITIONAL INFORMATION REGARDING CABLE AMPACITY ADJUSTMENT METHODOLOGY On May 27, 1997, the NRC requested additional information to facilitate their assessment of Consumers Energy Company's application of the Harshe-Black ampacity methodology, and Palisades' conformance to the ampacity design basis provided by FSAR Section 8.5.2. Consumers Energy Company provides the requested additional information in the attachment to this letter.
Consumers Energy Company has used a modified Harshe-Black methodology to predict operating temperatures of cables installed in filled cable raceway, and has used the results to determine if cable insulation thermal limits are exceeded. With the cable ampacity analyses, Palisades complies fully with FSAR Section 8.5.2. Cable
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replacements completed by the end of the 1998 refueling outage will extend the life of
/ J affected cables beyond the end of the Palisades licensed operating term.
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Palisades Nuclear Plant
- 27780 Blue Star Memorial Highway
- Cove;/, Ml 49043
- Tel: 616 764 2913
- Fax: 616 764 2490 C-.,
SUMMARY
OF COMMITMENTS This letter contains one new commitment and no revisions to existing commitments.
The new commitment is:
In order to extend the remaining life of cables in tray XK456, the two 75°C cables evaluated to have a remaining life of four years will be replaced, as will the 90°C cable which is driving the overall calculated tray temperature above the 75°C value. These three cables will be replaced prior to startup from the 1998 refueling outage.
Thomas C. Berdine Manager, Licensing CC Administrator, Region Ill, USNRC Project Manager, NRR, USNRC NRC Resident Inspector - Palisades Attachment 2
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I ATTACHMENT CONSUMERS ENERGY COMPANY PALISADES PLANT DOCKET 50-255 RESPONSES TO NRC REQUESTS FOR ADDITIONAL INFORMATION REGARDING CABLE AMPACITY ADJUSTMENT METHODOLOGY 11 Pages
RESPONSES TO NRC REQUESTS FOR ADDITIONAL INFORMATION REGARDING CABLE AMPACITY ADJUSTMENT METHODOLOGY NRC QUESTION 1:
Please explain how the population of cables analyzed was selected. What assurances exist that all cables requiring adjustment in accordance with Final Safety Analysis Report (FSAR) Section 8. 5. 2 have been identified for analysis?
CONSUMERS ENERGY RESPONSE 1:
FSAR, Revisions 18 and 19, (Section 8.5.2) state, in part:
"Cables installed in ventilated trays, conduit or underground ducts are thermally sized in accordance with NEC or IPCEA/ICEA ampacity values (depending on cable physical size) of concentric stranded insulated cable for the conductor operating temperature of the insulation. Insulation type may be of thermosetting, rubber or plastic. Ampacities are adjusted based on actual field conditions when possible. These adjustments may include, but are not limited to, conductor operating temperature, ambient temperature, cable overall diameter, tray depth of fill, conduit percent fill, and fire-stops."
Revision 4 of the Palisades FSAR stated in part:
"Tray fill will generally be limited to 30% by cross section. Tray fill greater than 30% by cross section is carefully reviewed to assure that cable damage, either mechanical or thermal will not take place."
In order to ensure compliance with the FSAR commitments, it was the objective of the Palisades Cable Ampacity program to analyze all continuously energized power cables (energized at least 100 hours0.00116 days <br />0.0278 hours <br />1.653439e-4 weeks <br />3.805e-5 months <br /> per year) that require adjustments in accordance with the FSAR. In order to accomplish this, it was necessary to identify power cables in cable trays with excessive (greater than 30%) depth of fill, power cables in conduits with multiple cables, power cables in duct runs and power cables routed in raceways passing through fire stops.
Installed cable information at Palisades is contained in the Circuit and Raceway Schedule (CRS). The CRS is a sortable electronic database which contains data for cables such as cable size, diameter, number of conductors, load information and routing data. For a cable route, the CRS identifies cable tray routing points and conduit numbers. The database also includes raceway information such as raceway type, size and tray width. The CRS is sortable to provide reports such as cables per raceway, percent fill of raceways, conduits with multiple cables, cables per duct, etc.
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RESPONSES TO NRC REQUESTS FOR ADDITIONAL INFORMATION REGARDING CABLE AMPACITY ADJUSTMENT METHODOLOGY The Palisades CRS was the tool used to identify all cables which required derating as a result of multiple cables per conduit, cables in ducts and cables in overfilled trays.
Fire stop information is not included in the CRS. Therefore, in order to identify power cables which pass through fire stops, information from walkdowns performed as part of the Palisades Cable Ampacity Program in accordance with controlled procedures was included in the analysis. The walkdowns identified raceways containing power cables which pass through penetrations containing fire stop material, and recorded penetration size, fire stop material and configuration. The data collected during the walkdowns was applied to specific raceways (cable trays and/or conduits) and appropriate derating factors were assigned in the analysis.
The above selection criteria for raceways at the Palisades plant ensures that the cables requiring adjustment in accordance with the FSAR have been included in the various ampacity adjustment analyses.
NRC QUESTION 2:
Palisades FSAR Section 8.5.2 (pg. 8.5-2) states, in part, that:
Cables installed in ventilated trays, conduit or underground ducts are thermally sized in accordance with NEC or IPCEA/ICEA ampacity values (depending on cable physical size) of concentric stranded insulated cable for the conductor operating temperature of the insulation. Insulation type may be of thermosetting, rubber or plastic.. Ampacities are adjusted based on actual field conditions when possible. These adjustments may include, but not be limited to, conductor operating temperature, ambient temperature, cable overall diameter, tray depth of fill, conduit percent fill, and fire-stops.
Insulated Power Cable Engineers Association/Insulated Cable Engineers Association (IPCEAllCEA) or National Electrical Code (NEC) standards permit the modification of various design parameters for cables (typical design parameters are cited in the subject FSAR section). However, each parameter modification is adjusted in accordance with the IPCEAllCEA or NEC procedure that reflects a well understood technical basis between the parameter and the allowable ampacity value. Please provide responses to the following questions:
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RESPONSES TO NRC REQUESTS FOR ADDITIONAL INFORMATION REGARDING CABLE AMPACITY ADJUSTMENT METHODOLOGY (a)
How are ampacity values adjusted consistent with the FSAR design basis using the Harshe-81ack ampacity methodology?
(b)
What is the technical basis for using the Harshe-81ack methodology versus the methodology specified by NEC or IPCEAllCEA standards?
(c)
Is the Harshe-81ack ampacity methodology more conservative in terms of ensuring that the applicable cables remain within their thermal limits than the approach taken by NEC or IPCEAllCEA?
CONSUMERS ENERGY RESPONSE 2:
(a)
Consistent with the FSAR, ampacity values were adjusted in accordance with the IPCEA/ICEA and NEC standards where the specific cable or raceway configurations and field conditions being analyzed are addressed in these standards. In evaluating cases where actual field conditions are not specifically addressed in the ICEA standards, the ICEA methodologies were supplemented with approaches based on testing results and based on detailed analysis of conservative thermal models of the field.condition:
The Harshe-Black methodology has limitations as identified in the IEEE paper entitled "Ampacity of Cables in Single Open-Top Cable Trays" by Harshe and Black, 1994. Therefore, the Harshe-Black methodology has been modified to address these limitations as described in the response to question 3(b). The analytical models used in the analysis borrow from l:larshe-Black the concept
- of layering cables based on their thermal loading. The parameters associated with these layers is different from the Harshe-Black method. In the modified method used in the analysis, layer parameters and tray thermal models were developed based on conservative representation of the configuration of the cables in the cable mass.
The layering method (modified Harshe-Black) was used in two cases:
- 1. Cables in cable tray: In this case, the layering method was used to calculate the maximum cable temperature at the evaluated tray sections..
The calculated temperature for trays in open air was compared to the cable rated temperature. In cases where the tray ampacity needed to be derated due to tray covers or firestops, the calculated temperature was adjusted based on the applicable derating and the resulting temperature was compared to the cable temperature rating.
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RESPONSES TO NRC REQUESTS FOR ADDITIONAL INFORMATION REGARDING CABLE AMPACITY ADJUSTMENT METHODOLOGY
- 2. Multiple cables in conduit: The layering techniquewas used twice in the ampacity assessment of conduits; first to establish a derating factor for power and control cables in the same conduit, and second to determine the maximum conductor temperature in a conduit containing both power and control cables.
The derating factors provided in the NEC and ICEA do not address the presence of control cables routed with power cables in the same conduit. To establish a derating factor, the layering technique, which models the cables in the conduits as a cylindrical layered cable mass, was used. This model groups the power cables in the central cylinder and the control cables form the outer cylindrical shell. The maximum calculated temperature at the center of the cylinder is then used to calculate the applicable derating factors. These conservative derating factors are then applied to the NEC/IPCEA ampacities to determine the appropriate derated ampacity.
The derating factors were then applied to the NEC/IPCEA ampacities and compared to the full load of the power cable. It was noted that some cables had ampacity values below the full load current. The disposition of these conduits used the layering technique in specific calculations employing actual loading of power cables to determine the maximum conductor temperature. For these calculations, the heavily loaded power cables are grouped in.a central cylinder and the lightly loaded cables are modeled as a cylindrical shell surrounding the inner cables. The control cables are located at the outermost cylindrical shell. The maximum temperature in the center of the cable mass is calculated and compared. to the rated conductor temperature.
Methodologies used for other cable ampacity adjustments were as follows:
- 1. Fire-stops: Fire stop deratings are not available from ICEA or the NEC. We have therefore conducted tests at Palisades to determine
- the required derating factors for cable trays and conduits passing through Kaowool or silicone foam fire-stops. In one case, the derating factor for a conduit passing through a fire stop made of concrete grout was calculated using a computer program that has been developed and calibrated using the results of published tests 1.
. 1 Haddad, S. Z.; Bloethe, W. G.; Lamkin, D. C.; Stolt, H. K.; and Sykora, G. 1982. Tests at Braidwood Station on the Effects of Fire Stops on the Ampacity Rating of Power Cables. Chicago: Proceedings of the American Power Conference.
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RESPONSES TO NRC REQUESTS FOR ADDITIONAL INFORMATION REGARDING CABLE AMPACITY ADJUSTMENT METHODOLOGY (Because of the low thermal resistivity of concrete grout compared to other fire stop materials, the derating associated with grouted fire stops is minor.)
- 2. Tray covers: The derating of cables in trays with tight covers is determined based on the results of the tests reported in the 1982 American Power Conference paper cited above 1. This derating factor is more conservative than the factor listed in the NEC.
- 3. Underground duct banks: The ampacity of cables in ducts was adjusted by applying a duct bank derating factor. The derating factors were calculated using the Neher - McGrath method and were based on the worst case duct bank configuration.
(b)
The modified Harshe-Black method provides a more realistic, yet conservative thermal model of the configuration under analysis. ICEA Standard P-54-440 (NEMA WC-51) treats the cables in a tray as a uniform mass that generates an equal amount of heat per cross sectional area throughout the tray.
Therefore, all cables are assumed to be loaded to their ampacity rating. In.a normal power plant installation, while some cables may be heavily loaded, others are lightly loaded or carry no current at all. The layering approach (modified Harshe - Black method) creates a conservative thermal model of the tray section being analyzed. In this model, the heavily loaded cables are lined up in a compact group at the center of the tray cable mass. These compact, heavily loaded cables are blanketed at their top and bottom by cables that are more lightly loaded. The width of the cable tray section thermal model is limited to the sum of the diameters of the heavily loaded cables plus half of the depth of fill of the actual tray section. This severe limit on the allowable width of the tray thermal model provides a very conservative representation of the actual layout of the cables in the tray because it does not take full credit for the full heat transfer area actually available in the physical tray, while at the same time maintaining the thickness of the heavily loaded cable layer to that comparable to the average diameter of the heavily loaded cables. The next step in the methodology is to apply conventional heat transfer equations to the developed layered thermal model. The heat transfer formulations used follow the same principles adopted in the Stolpe method which forms the basis of ICEA P-54-440.
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RESPONSES TO NRC REQUESTS FOR ADDITIONAL INFORMATION REGARDING CABLE AMPACITY ADJUSTMENT METHODOLOGY (c)
Although the layering method used (modified Harsh-Black) is not more conservative than the approach taken by the NEC or the IPCEA/ICEA, the method does promote a more realistic assessment with adequate conservatism to provide assurance that applicable cables operate within their thermal limits.
The layering method removes some of the extreme conservatism inherent in industry Standard Ampacity Tables. The actual reduced loading on the majority of cables in a raceway is used to increase the allowable loading on other cables in the same raceway beyond their standard ampacities.
NEMA WC 51 I ICEA P-54-440, which implements the original Stolpe method, assumes that all cables in the tray are loaded to operate at their rated conductor temperature. This approach provides large margins, which is appropriate when the amount of diversity of the cables installed in a raceway is not known. This method is preferred for use in the design stage of a power plant when detailed cable loading information is unavailable. However, almost all power plant cable trays have a substantial amount of ioad diversity. I PC EA/IC EA and the NEC allow for increase in ampacity for load factors lower than unity in some installations but do not deal with load diversity in cable trays. The layered cable mass technique (modified Harshe-Black), provides a consistent conservative estimate of the effects of cable loading diversity when the cable loadings are known. This technique is appropriate for use at an operating plant where the amount of diversity in the cable loading can be determined.
NRC QUESTION 3:
The Harshe-Black ampacity methodology as detailed in the referenced IEEE [Institute of Electrical and Electronic Engineers] paper, 94WM100-8PWRD, "Ampacity of Cables in Single, Open-Top Cable Trays," describes the following limitations to the applicable thermal model:
(a)
Since the model is one-dimensional and steady state, it should not be used where significant fill variation exists along the length of the tray or in situations where changes with respect to time exist.
(b)
The Uniform Model temperature assumes the individual power conductors are spread uniformly across the cable bundle. This model predicts the average temperature of the cable bundle in the tray, but it is not a good model for grouped power cables..
(c)
The maximum cable ampacity should still be restricted to 80 percent of the free-air ampacity as stated in the IPCEAllCEA Standard.
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RESPONSES TO NRC REQUESTS FOR ADDITIONAL INFORMATION REGARDING CABLE AMPACITY ADJUSTMENT METHODOLOGY For each of the points above confirm that the application of the Harshe-Black ampacity methodology has been implemented within the stated limitations or provide a technical justification for any deviations to the application of the subject model.
CONSUMERS ENERGY RESPONSE 3:
(a)
The use of a one-dimensional model is conservative given the selection of the routing points and the cables within each routing point to be included at the routing point. The list of the cables in each routing point evaluated was constructed in such a way that the maximum number of cables in the routing point were included, such that the depth of fill used in the model was the highest possible for the routing point. Since all routing points exceeding 30% loading were analyzed, the worst case depth of fill for any cable was included in the analysis. The presence of nearby areas with lower depth of fill allows heat to flow towards the areas with lower depth of fill, reducing the temperature in the area with the highest depth of fill.
(b)
The question addresses the concern that the effect of the most heavily loaded cables in the Harshe-Black model can be diluted. However, the layered model used for the Palisades ampacity assessment has modified the Harshe-Black method to address this concern and to result in a calculated maximum cable temperature at the tray section rather than a calculated average temperature.
The following major features of the layering method used address these concerns and reflect conservatism in the calculated conductor temperature:
Only the most heavily loaded cables are placed in the "hot" layer. The*
cables in the "hot" layer are all loaded above their ICEA ampacity. If no.
cabl~ in the tray is loaded to more than its ICEA ampacity, the cables in the "hot" layer are the most heavily loaded cables in the tray.
A cable tray thermal model is developed for each tray section analyzed. The width of the cable tray thermal model is restricted to the sum of the hot cable diameters plus half of the depth of.fill. This reduced width for tray sections containing a small number of hot cables prevents the temperature rise in the hot layer from being reduced by being spread over the entire tray width in a thin layer. The width of the cable tray thermal model for sections containing a large number of hot cables is limited to the actual width of the tray section.
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RESPONSES TO NRC REQUESTS FOR ADDITIONAL INFORMATION REGARDING CABLE AMPACITY ADJUSTMENT METHODOLOGY The temperature of the cables in the "hot" layer whose loading exceeds the average loading of the cables in the "hot" layer by a threshold amount is adjusted upward in proportion to the actual loading. This allows the determination of the maximum cable temperature in the tray section; rather than the average temperature of the hot layer.
The "hot" cables are assumed to lay side by side in the cable tray section. In an actual randomly filled cable tray, the relative positions of the cables will change over the length of the cable tray. Therefore, the heavily loaded cables are expected to be in a concentrated area, as assumed by the modified Harshe - Black model, for only a short distance. As a result, the heating effect of the heavily loaded cables in an actual cable tray will be lower than assumed in the model.
(c)
The loading of-the cables in the cable trays was compared to 80% of the cables' free air ampacity criterion. This comparison indicated that the loading of the cable_s in the trays does not exceed 80% of their free air ampacity.
NRC QUESTION 4:
You identified a number of cables that could exceed their derated ampacity limits and indicated that the cables should be considered operable as the calculated elevated temperatures could result in premature cable jacket aging, but not catastrophic failure.
Please address the following questions:
(a)
A number of cases involve cables in conduits, openings (cables in air), sleeves, and ducts that did not meet the acceptance criteria. Given that the Harshe-Black ampacity methodology is based on cable trays, please explain how the subject methodology is applied for other electrical raceway types.
(b)
It is not clear whether any of the identified cables have a rated temperature of 75°C versus the 90°C rating assumed in your analyses. Given the elevated temperatures calculated, please estimate the remaining qualified life for the identified cables.
(c)
Specify the schedule for final disposition of corrective action for cables that have been identified as not being in compliance with the ampacity design basis as stated in FSAR Section B. 5. 2; and provide justification for continued operability of the affected cables, including consideration of the potential impact of cable failures on adjacent circuits.
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RESPONSES TO NRC REQUESTS FOR ADDITIONAL INFORMATION REGARDING CABLE AMPACITY ADJUSTMENT METHODOLOGY CONSUMERS ENERGY RESPONSE 4:
The results of the Palisades Ampacity Program reveal that all continuously energized power cables, with the exceptions noted in 4(b), operate within their allowable, derated ampacity and temperature limits.
(a)
As stated in our response to question 2(a), the layering technique was applied in cable tray evaluations and in addressing multiple cables in conduits. In this latter case, the multiple cables were modeled as concentric cylinders inside the conduit with the heavily loaded cables in the inner cable mass cylinder. The other cases were addressed as described in our response to question 2(a).
(b)
For continuously energized power cables with a temperature rating of less than 90°C, the revised analyses were run using an acceptance criteria of 75°C. 165 installed power cables have been identified that have insulation rated for less than 90°C (85°C or 75°C). These analyses concluded that each of these cables were acceptable as installed, with the exception of three routing points which have calculated temperatures that exceed 75°C.
The temperatures of these tray sections are due to the loads on 90°C cables routed in the same trays. The 75°C cables alone are sized correctly. These revised analyses are still in the review process.
Tray sections XP020 and XP310 exceed 75°C by less than 3°C each. Each of these trays has three 75°C rated power cables (two of which are common to both trays, and each of the other two cables unique to one tray; resulting in four unique cables total considering both trays). XK456 exceeds 75°C by less than 11°C. It has two power cables assumed to be 75°C rated (one of the cables is common to all three trays previously identified, and the second cable is unique to XK456). Thus, there is a total of five unique power cables rated for 75°C which are in trays that exceed 75°C.
These calculated temperatures are conservative since the program utilized for the analysis assumes that all energized power cables are producing heat continuously and does not account for the time when equipment is not required to run. The analyses also conservatively assume that the entire routing point is at the temperature of the calculated hottest cable. The three cable tray points are located in mild environment areas of the plant. Therefore, the cables in these raceways are not exposed to a Design Basis Accident, and as such are not required to be in the EQ Program. As a result, the 75°C cables routed in these three cable tray points do not have a defined expected life.
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v RESPONSES TO NRC REQUESTS FOR ADDITIONAL INFORMATION REGARDING CABLE AMPACITY ADJUSTMENT METHODOLOGY Sandia Report, SAND 96-0344 "Aging Management Guideline for Commercial Nuclear Power Plants - Electrical Cable and Terminations", printed September 1996, provides numerous references that show a 60 to 80 year life is achievable for low voltage power cables with an insulation operating temperature of 50 to 75°C. The fuel pool circulating pump whose cable is producing the heat in XP020 and XP310 has an approximate 60% duty cycle.
The pump's run is alternated with a sister pump except for several days after fuel has been removed from the core. The plant has been operating for 26 years. A conservative qualified life of 60 years is assumed for these 75°C cables, and the cables are subjected to a maximum temperature of less than 3°C higher than their rating; 60% of the time. The 10°C rule discussed in NP1558 indicates that a qualified life of 42 years can be expected for the cables in trays XP020 and XP310, of which 16 years remain. As such, replacement is not necessary for cables located in trays XP020 and XP310 as the cables are, and will remain, operable throughout plant operating life.
For the two cables in tray XK456 that have possibly been subjected to a predicted 10.9°C temperature higher than their rating, it is estimated that they have a remaining life of approximately four years in the existing environment.
(c)
Corrective action will be taken for the following:
- 1.
Cable tray XP600 - This is a covered tray that has a calculated temperature of 93.6°C. This tray has a cover to keep debris from (a stairway landing constructed of grating is located above) falling into it.
Raising the cover slightly and allowing air to circulate will reduce its calculated temperature to 84.4°C. A Work Order is in place to do this work. The work order is 'in the maintenance schedule and is currently due to be worked during the third quarter of 1997. This is one of the trays that was instrumented for temperature. The hottest recorded temperature with the plant at full load was 30.3°C; well below the predicted 93.6°C. This data indicates that there are no remaining life issues associated with cables in this tray, and the cables are considered operable.
- 2.
Cable tray XK456 - This tray contains two power cables that are rated for 75°C. The calculated temperature of the tray is 85.9°C. As estimated in Response 4 (b), there is approximately four years of life remaining for these cables (based on the current best estimate). Therefore, the cables are considered operable.
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RESPONSES TO NRC REQUESTS FOR ADDITIONAL INFORMATION REGARDING CABLE AMPACITY ADJUSTMENT METHODOLOGY In order to extend the remaining life of cables in tray XK456, the two 75°C cables evaluated to have a remaining life of four years will be replaced, as will the 90°C cable which is driving the overall calculated tray temperature above the 75°C value. These three cables will be replaced prior to startup from the 1998 refueling outage.
Since none of the cables evaluated have exceeded their estimated life, no formal evaluation of potential impact of cable failures on adjacent circuits has been performed. As the cables that had an estimated remaining life performed for them reside in mild environments, the worst-case failure that can be predicted is an internal cable fault or fault to ground. This type of fault would result in opening an upstream -fuse or breaker thereby interrupting the fault prior to any physical impact on cables adjacent to the faulted cable in the raceway.
NRC QUESTION 5:
Please provide typical calculations for each raceway type (e.g., tray, conduit, duct, etc.)
where the Harshe-Black ampacity methodology was utilized to adjust ampacity values in accordance with FSAR Section 8.5.2. The sample calculations should represent worst-case configurations in terms of uncertainty of calculated results using the Harshe-Black
- ampacity methodology.
CONSUMERS ENERGY RESPONSE 5:
A typical calculation from the text portion of each of the following analyses is enclosed.
Revision 1 of these analyses, currently in review, has been used. Since they are in review, they are subject to change.
EA-ELEC-AMP-032, "Ampacity Evaluation for Open Air Cable Trays with a Percent Fill Greater than 30% of the Useable Cross Sectional Area" (Enclosure 1 ).
EA-ELEC-AMP-039, "Ampacity Evaluation for Continuously Energized Power Cables in Open Air Conduits" (Enclosure 2).
EA-ELEC-AMP-040, "Ampacity Evaluation for Duct Runs Containing the Continuously Energized Power Cables" (Enclosure 3).
EA-ELEC-AMP-041, "Ampacity Evaluation for Continuously Energized Power Cables Routed Through Fire Stops" (Enclosure 4).
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