CONCLUSIONS AND RECOMMENDATIONS

This section provides conclusions and recommendations of the research effort.

CONCLUSIONS

Significant operational problems do not exist with the general population of power plant cable. The number of failures has been low in proportion to the amount of installed cable.

Those cable insulation problems that have occurred have predominantly been limited to the effects of localized hot spots resulting in premature aging of insulation systems. A review of reported cable failures listed in the nuclear industry (contained in the Licensee Event Reports (LERs) and Nuclear Plant Reliability Data System (NPRDS)) databases and 1.

discussion with industry experts indicates that the bulk of the reported cable failures are primarily termination failures. Whereas the majority of such failures are random in nature (i.e., use of wrong crimp tool, etc.), a recurring problem was recognized. This problem relates to the use of poor cable-to-connector terminations (solder joints) at coaxial and triaxial cable connections. The total quantity of such cable/ connector applications is rather limited as these cables/connectors typically service only ex-core or out-of core

detectors and radiation monitoring instrumentation. Therefore, this problem appears to be significant and deserves further attention (See Recommendations below).

2. As indicated in Section 8, electrical application stresses on low-voltage cable are such that electrical breakdown under normal conditions is all but precluded. Even substantial reductions in the insulation wall thickness of low voltage cables will not cause cable failures under normal conditions. Essentially, if the integrity of the insulation is retained (i.e., no punctures or cracks), the insulation will continue to perform its electrical function.

On the other hand, medium-voltage cable and its terminations have a potential for

electrical stress-induced failure if significant cable insulation voids and imperfections exist.

The substantial theoretical basis indicating that only a thin insulation wall is required for low voltage cabling, coupled with design basis accident simulation testing, reasonably assures the capability of low-voltage cable to withstand electrical operating stress.

3.

With regard to environmental qualification:

4.

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a. Environmental qualification practices, including qualification margins, provide an adequate basis for assuring that common-mode-failure mechanisms from aging and accident conditions do not exist.

b. Conservative design practices provide further assurance that cables will function

adequately under accident conditions. In many cases, design conservatisms (e.g., use of larger ampacity cables than necessary for the application, use of 600 V rated cable for 125 V and less applications) provide additional conservatisms beyond the built-in qualification margins because the actual application of the cables does not stress the cables to the extent that they were stressed in the qualification tests.

Current environmental qualification practice is based on the assumption that cables are properly installed. Wide spread cable installation damage would invalidate this assumption and require remedial action. Unavoidable random installation damage to a limited number of cables in a plant would not invalidate this assumption because redundancy built into the plant design would assure safety function.

c.

With regard to condition monitoring and troubleshooting of low-voltage cables:

5.

a. Industry standards have not previously addressed condition monitoring of low-voltage cable as (a) there has been no historical need, (b) no completely cost-effective method exists, and (c) the need for proving cable capability after more than 40 years of service is new.

b. Electrical high-voltage or insulation resistance tests without a conductive medium (shield, water, conductive gas) surrounding the insulation cannot reliably indicate even gross cable insulation degradation except for extreme cases (e.g., dead short).

High-voltage testing methods are undesirable for condition monitoring of non-shielded c.

cable and may damage sound cable.

d. Insulation resistance testing of low-voltage unshielded cable is a go/no go test for severe insulation degradation (i.e., shorts to ground). It is not useful for trending of cable condition. All other testing based on IR such as polarization index is also of little value.

e. Test methods based on Time Domain Reflectometry (TDR) technology are effective for series impedance problems such as poor terminations. It has not as yet been

demonstrated to be effective for determining general cable insulation conditions.

Of all the physical properties that are available to determine acceptability of cable insulation, retained tensile elongation to break appears the most appropriate. On-going research into non-destructive test methods indicates that compressive modulus can be a f .

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useful measure of the condition of insulations. The Indenter Polymer Aging Monitor under development by EPRI measures modulus.

6. The number of failures of medium-voltage cables within nuclear power plants does not warrant the application of condition monitoring of cables for all applications. Recent research results indicate that ac and dc high-potential testing will not detect significant deterioration in most cases. In the case of cross-linked polyethylene medium voltage insulation having water-trees, do high-potential testing will not identify the condition but appears to significantly reduce the remaining life of the cable (Note: The same research program determined that dc high-potential testing of unaged cross-linked polyethylene does not appear to affect service life.). Partial discharge testing appears to be the most

promising method for evaluating deterioration of medium voltage cables, but the system is not fully perfected for in plant use.

RECOMMENDATIONS

Many within the nuclear industry do not have a sufficient understanding of the cable population, cable practice, cable installation, cable assessment efficacy, and related areas.

It is recommended that industry organizations hold workshops on cable practice to promote 1.

a better understanding of cable applications and cable operability and to correct

misconceptions. Other workshops could concentrate on cable installation practice to reflect the recently identified needs in the area of cable installation.

Many of the industry "standard good practices" for cable installation (e.g., sidewall

pressure, pulling tension, pulling friction factors) are very conservative as demonstrated by EPRI research. The data from this research should be factored into the existing generation of national consensus guidelines (e.g., IEEE Std 690-1984 [17]). However, care must be taken not to remove all the conservatism that exists between good practice and the ultimate capabilities of cables.

2.

3. Present industry standards (e.g., IEEE Std 690-1984) for cable installation are incomplete in regard to documenting good installation practice. Without such good practice guidelines, utilities are forced to establish their own programs to ensure adequate installations.

Furthermore, the present standards provide little guidance on methods to assess or evaluate operability of low-voltage cables that are suspected of being damaged. The only

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method included, mentioned, or implied is insulation resistance testing, which is of little value for unshielded low-voltage cable operability assessment.

The IEEE's Insulated Conductors Committee has proposed development of two new standards to address these issues. These are:

To enhance coverage of IEEE Std 6901984 to add, "Committee Report

Recommended Practice on Specific Aspects of Cable. Installation in Power Generating Stations," an ICC Task Force 14-1 Report.

To develop a new standard P1186, "Recommended Practices for the Evaluation of Installed Cable Systems for Class BE Circuits in Nuclear Power Generating Stations"

Research activities sponsored by EPRI, DOE, NRC, and others are recommended to produce the input to the national consensus standards writing efforts needed by industry.

The use of high-level constant volts/mil test voltages to assess cable operability is discouraged since it may severely over-test heavy wall cable.

4 .

Suspect installations may be of less concern in the future due to the generation of new or enhanced installation guidelines. Conversely, currently available test techniques to assess suspect non-shielded cable do not appear effective. It appears prudent for industry to continue research into the development of tools for cable assessment.

As stated above under Conclusions, poor cable to connector terminations (solder joints) at coaxial and triaxial cable connections appear to be causing more problems than expected.

The total quantity of such cable/ connector applications is rather limited as these cables/connectore typically service only ex-core or out-of core detectors and radiation monitoring instrumentation. Physical inspection of these connections may not be prudent (i.e., due to radiation exposure) or effective (i.e., poor termination may be hidden within a potted connector). Further research may be appropriate in this area. It is possible that Time Domain Reflectometry may be useful to evaluate defective coaxial connections.

To assure operability of cable systems, identification and control of hot spots is recommended; this should include, but not be limited to, in-panel local temperatures, instrumentation mounted on process pipes, such as primary loop RTDs, and cables installed near hot proeess piping.

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8. Utilities should add a visual inspection requirement to maintenance procedures for cables and leads in the vicinity of equipment that is being maintained to identify unexpected cable deterioration. This effort should be done for components that are in high ambient temperatures, produce high temperatures, or are connected to hot process piping.

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Section 12.0

References

Cable Operability Report

Section 12.0 REFERENCES

Technical Evaluation Report, TER-C5506-649 (Sequoyah), "Evaluation of Sequoyah Units 1 and 2 Cable Pulling and Cable Bend Radii Concerns," Prepared for Nuclear Regulatory Commission by Franklin Research Center, February 19, 1987, by G. J. Toman, W. A. Thue, S. P. Carfagno with consulting input from J. B. Garner.

1.

White, S. A., Manager of Nuclear Power, TVA Letter to James G. Keppler, Director Office of Special Projects, USNRC, "Sequoyah Nuclear Plant Units 1 and 2 - Docket Nos. 60-327 and 50-328 - Facility Operating Licensees DPR 72 and 77 - Preliminary 10 CFR 21 Report On Silicone Rubber-Insulated Cables," dated September 10, 1987.

2.

USNRC Information Notice, IE Notice 87-52, "Insulation Breakdown of Silicone Rubber " dated October 16, 1987.

3.

Insulated Single Conductor Cables During High Potential Testing,

Toman, G. J. and J. B. Gardner, "Development of a Nondestructive Cable-Insulation Test,"

Presented at the EPRI Workshop on Power Plant Cable Condition Monitoring, February 16-18, 1988.

4.

Bustard, L. D., SAND 86-1897 Sandia National Laboratories, "Definition of Data Base, Code, and Technologies for Cable Life Extension," Printed March 1987.

5.

American Heritage Dictionary of the English Language; William Morris, Editor, Copyright 1980, Published by Houghton-Mifflin Company.

6

ANSI/IEEE Std 100-1984, "IEEE Standard Dictionary of Electrical and Electronics Terms,"

Published by IEEE.

7.

McGraw-Hill Dictionary of Scientific and Technical Terms, Copyright 1976, Published by McGraw-Hill.

8.

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