ML19305B759
| ML19305B759 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 02/27/1980 |
| From: | Snaider R Office of Nuclear Reactor Regulation |
| To: | Eisenhut D Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML19305B760 | List: |
| References | |
| REF-GTECI-A-10, REF-GTECI-RV, TASK-A-10, TASK-OR NUDOCS 8003200262 | |
| Download: ML19305B759 (5) | |
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SUMMARY
Central Files NRCP3 m wn SEPB Reading NRR Reading H.-Denton E. Case D. Eisenhut R. Vollmer S. Hanauer J. Knight, DSS R. Mattson F. Schroeder S. Pa.vlicki R. Gamble L. Shao R. Denise M. Aycock G. Lainas P. Check OIF (5)
W. h siton J. Zudans N. Randall S. MacKay R. Mattu M. Bender, ACRS.
Ethrington, ACRS P. Shewmon, ACRS R. Snaider R. Johnson, 440 R. Klecker P. Boehnert M. Hum G. H. Neils, NSP H. T. Watanabe, GE J. B. Martin, NUTECH F.-Elam, TVA
- 3. Day, NSP A. Fife, NUTECH P. C. Riccardella, NUTECH OELD-l D. Crutchfield -
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i Fabruary 27, 1980 i
Generic Task No. A-10 i
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MEMDRANDUM FOR:
D. G. Eisenhut, Acting Director Division of Operating Reactors, NRR S. H. Hanauer, Director Unresolved Safety Issues Program FROM:
R. P. Snaider, Task Manager Generic Technical Activity A-10
SUBJECT:
SUMMARY
OF MEETING REGARDING BWR FEEDWATER.N0ZZLE LN-LINE LEAKAGE MONITORING SYSTEM On Thursday, February 21, 1980, a meeting was held between the NRC and representatives of NUTECH, Northern States Power Company (NSP) and other interested parties. A list of attendees is attached, as is a copy of the 4
transpamncies presented during the meeting.
The purpose of the meeting was to describe an on-line leakage detection system for BWR feedwater nozzles. Th'is system was designed by NUTECH and is in use at the Monticello Nuclear Generating Station. Cetection of leakage past the seals or welds of improved BWR feedwater spargers is considered extremely important, since it is such leakage, with resultant thermal cycling, that results in crack initiation on the metal surfaces of the nozzle bore and blend radius. As described in NUREG-0619, the staff's resolution of Generic Technical Activity A-10, General Electric (GE) analyses indicate that the GE triple sleeve, double piston ring sparger can no longer l
satisfactorily protect the nozzle when leakage past the secondary seal exceeds
.5 to 1.3 gpm, depending on the feedwater temperature of the plant in question.
The Monticello Nuclear Station is in a unique situation, in that the feedwater nozzle stress analysis showed that calculated primary membrane stresses in the nozzle safe ends were very close to those allowed by the ATME Code.
Therefore, removal of sufficient metal to provide the necessary seating sur-face for the double piston ring sparger was determined to be unsatisfactory.
Because of this, NSP decided. to install the GE-proposed alternative, a single sleeve, single piston ring design, after the stainless steel cladding was removed. Because the single piston ring seal is always subjected to a high (s23 psid) differential pressure during power operation, the staff has always assumed leakage would occur and has assigned, in NUPIG-0619, a relatively l
low " trigger" leakage value of 0.5 gpm as that value beyond which removal of the sparger for inspection would be necessary. However, in this determination e
. the staff has relied solely on mechanical, in-vessel leakage detection methods and in NUREG-0619 has only noted that industry investigations into on-line leakage detection methods have ' begun. This meeting afforded the staff its first look at "hard" data and, although a decision concerning the efficacy of such systems cannot be made until more data (particularly that of inspections) is in hand, the staff is encourhged by what was presented.
It was noted that the Monticello sparger sleeves were installed with a 13 mil diametral interferf; ace fit and that leakage is expected to occur when only a 4 mil fit remaine, as measured during shutdown. NSP and NUTECH advised the staff that they expect this 4 mil fit to open, due to thermal expansion, when the reacte r is taken to power. Therefore, they noted that the staff's position on leakage detemination in NUREG-0619 may not provide a positive detennination of leakage. The staff will consider this and, until the efficacy of on-line systems has been demonstrated and systems installed in all operating reactors, may tighten the leakage " trigger" requirements.
The NUTECH analysis of Monticello sparger effective lite (beyond which cracks could initiate in the nozzle surtace) took into account the fact that si,nce the Monticello spargers do not have top-mounted discharge nozzles, they are susceptible, at low feedwater flows ($_12". power), to backflow of hot reactor water. However, this unstable flow cycling and resultant stratification is not expected to result in sparger degradation during its anticipated life.
The analyses utilized plant-specific temoerature profiles and cyclic transiants, resulting in the determination that combined fatigue usage factors show ths useful life of the sparger will be'eight years, assuming design basis leakage.
NSP does intend to take action, in 1981, with regard to sparger replacement, although plans are hot yet firm enough to provide specific information con-cerning the replacement equipment.
Information was presented regarding the initial and advanced data analyses and their interpretations. Central to this discussicn was the topic of locai jet leakage, which was included in the advanced analysis. NUTECH concluded from the analysis that local leakage would normally be readily detected by the thermocouples in their present location. A special evaluation was required for the case in which there was high jet leakage at the top of the nozzle which was distributed in such a pattern as to avoid both sets of thennoccuples. This could still be detected by observing the bottom thermocouples (the most sensi '
tive ones) as power is increased during startup and decreased during shutdown.
Because the driving lead would increase with increasing power (and therefore feedwater flow), there would be a point at which the cold leakage flow would miss the lower thermocouples and the temperature would start to increase. At
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this point the leakage would be measured and extrapolated to rated conditions.
Since it is almost impossible that a jet could develop without being detected in its early stages, leakage by corrosion being a progressive mechanism, the licensee's procedures would be modified to require monitoring during startups and shutdowns when leakage is known to exist.
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. The physical installation at Monticello consists of three thermocouples at the top and three at the bottom of each nozzle. The best locations were determined by analytical predictions of thermocouple response to leakage compared to distance from the L cing; seal. The output of the thermocouples 1
is used with other parameters (feedwater temperature, feedwater flow, and reactor pressure) to determine leakage. The system has detected the not-
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unexpected leakage in 2 of the 4 Monticello feedwater nozzles and monitoring has shown the maximum leakage to be 0.74 gpm on nozzle D.
This would be considered to be above the NRC's present " trigger" value, but the staff is not yet convinced of the accuracy or efficacy of this system and is awaiting further data, in particular inspection results (if any) when the Monticello repair work is undertaken, tentatively in 1981. Meanwhile, the stress analysis shows substantial margin remains before crack initiation would be expected en the unclad nozzle.
As noted above, the staff is encouraged by the results of this discussion but desires more field data regarding the accuracy of the leakage rate determinations. Meanwhile, the staff will examine its leakage monitoring requirements with the intent of modifying the NUREG-0619 requirements if necessary.
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/2 Richard P. Snaider, Task Manager Generic Technical Activity.A-10 Attachments:
1.
Attendance List 2.
Transparent.ies cc:
See next page l
4 ATTENDANCE LIST R. Snaider, NRC, 00R, SEPB R. Johnson, NRC, DOR, EB G. H. Neils, NSP H. T. Watanabe, GE R. W. Klecker, NRC, DOR, EB J. B. Martin, NUTECH E. Elam, TVA B. Day, NSP A. Fife, NUTECH P. C. Riccardella, NUTECH P. Boehnert, ACRS.
M. Hum, NRC, 00R, EB e
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