ML20012B894
| ML20012B894 | |
| Person / Time | |
|---|---|
| Issue date: | 03/05/1990 |
| From: | Novak T NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | Rossi C Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20012B895 | List: |
| References | |
| AEOD-S902, NUDOCS 9003190132 | |
| Download: ML20012B894 (3) | |
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-MEMORANDUM FOR:
Charles E. Rossi, Director
' Division of Operational Assessment, NRR b
James E. Richardson, Director Division of Engineering Technology, NRR Lawrence:C. Shao,-Director i
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Division of Engineering, RES a
FROM:
Thomas M.- Novak, Director Division of Safety Programs, AEOD
SUBJECT:
AEOD STUDY ON THERMAL STRATIFICATION ms AE0D has recently completed a broad overview study of the' operating experience associated with thermal stratification and.its effects, including potential safety implications. This study report is enclosed for your information and use. :This report should be of particular interest to staff and licensee personnel who are participating in the resolution of thermal
. stratification issues, such as bulletin closecuts and industry assessment s
9 studies.
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.The report identifies the following findings:
1.
Thermal stratification has caused cracks, damaged supports,.and
. contributed to thermal. fatigue in high energy piping. Safety injection, RHR,_feedwater and the )ressurizer surge line piping. systems have been
'affected.- Some cracks. lave resulted in unisolable' leaks of reactor coolant.. Thermal stratification has. occurred in pressurized-water reactors and boiling-water reactors.
2.
Opera' ting experience indicates that many interfacing systems are potentially-subjected to thermal stratification. Review of operational events. involving cyclic thermal-stratification indicates that through-wall cracks were generally found in the 90-degree elbow connecting the piping section containing stratified fluid and the piping section containing well-mixed fluid. -Apparently, this location-provides l
the minimum margins against. thermal fatigue, j
3..
Searches of data bases.from years 1980 to 1988 found over 2000' reports-I of pipe cracks.and damage-to: pipe supports, snubbers, and pipe wipe restraints. In most of these cases, the failures were attributed to t.
causes such;as vibration induced fatigue or water hammer. Some fraction
.of these failures was probably due to the effects of thermal I
stratification but not identified as such. Detection of thermal stratification is, difficult and usually only confirmed by measurements froni thermocouples placed around a suspected section of pipe.
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Typical monitoring programs to detect thermal stratification monitor only selected portions of lines which are considered to have a potential for thermal stratification, Visual and dye penetrant examination would not detect cracks caused by e
thermal stratification until they are through well since this form of thermal fatigue originates on the inner surf ace with crack depth '
propagating to the outside wall surface. Fatigue cracks are difficult to detect using even the more sophisticated ultrasonic testing and radiographic testing techniques.
5.
As plants age, those locations susceptible to the effects of thermal b>
stratification are subjected to an increasing number of thermal cycles; and the likelihood of cracks increases ercordingly. Experience to date
-suggests that thermal stratification was not considered during the original design of current plants and would have to be a factor in any consideration of life extension.
6.
Interfacing system leakage has a potential to cause thermal stratification in system piping and leakage through check valves, block valves, and isolation valves has been comon. Thus, en approach to minimize the potential for thermal stratification in many locations is to implement effective preventive and corrective maintenance of valves at system boundaries.
=7.
It was noted during the review that the leak-before-break approach has been accepted by the staff in specific instances in piping systems susceptible to thermal stratification, such as the pressurizer surge line. The fatigue and fracture mechanics analyses submitted to justify the application of leak-before-break rely en assumptions regarding the thermal cycles, thermal gradients, material properties, and crack geometry. The validity of the analysis over the long term is best supported by continued monitoring of operating experience related to the effects of thermal stratification, 8.
Existing NRC guidance and industry codes and standards may need to be revised to reflect existing operating experience, the results of ongoing NRC reviews of bulletin responses, and the current two-year Electric Power Research Institute study of thermal stratification. Explicit acceptance criteria for analysis of systems subjected to thermal stratification may be needed.-
We have no specific recommendations to make at this time with regard to ongoing work. We will continue to monitor operating experience associated with thermal stratification and the results of ongoing work to determine if further AEOD evaluation is warranted, l
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If you have any questions regarding the enclosed study, please contact Nelson Su of sqy staff. Dr. Su can be reached at X?4434 Please contact me if AEOD can provide any additional assistance.
' Original Signed by1 Thomas M. Novak, Director Division of Safety Programs, AEOD i
Enclosure:
As stated Distribution:
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