ML20149N090
| ML20149N090 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 02/25/1988 |
| From: | Andrews R OMAHA PUBLIC POWER DISTRICT |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| LIC-88-091, LIC-88-91, NUDOCS 8803010182 | |
| Download: ML20149N090 (4) | |
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g Omaha Public Power District 1623 Harney Omaha, Nebraska 68102 402/536-4000 February 25, 1988 LIC-88-091 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555
References:
1.
Docket No. 50-285 2.
Letter from 0 PPD (W. C. Jones) to NRC (R. A. Clark) dated April 4, 1984 (LIC-84-090) 3.
Letter from 0 PPD (R. L. Andrews) to NRC (A. C. Thadani) dated August 28, 1986 (LIC-86-421) 4.
NRC Safety Evaluation, from NRC (W. A. Paulson) to OPPD (R. L. Andrews) dated February 12, 1987 5.
Letter from OPPD (R. L. Andrews) to NRC Document Control Desk dated October 13, 1987 (LIC-87-673)
Gentlemen:
SUBJECT:
Threshold Levels for the Fort Calhoun Internals Vibration Monitoring System In 1984, the Omaha Public Power District (0 PPD) committed to perform an in-spection of the Fort Calhoun Station reactor vessel thermal shield during the 1987 outage (Reference 2).
Based upon comprehensive reseanh and analysis of the thermal shield degradation phenomena, OPPD replaced the 1987 inspection commitment with a commitment to conduct an ongoing thermal shield monitoring arogram capable of detecting precursors to internals degradation (Reference 3).
The NRC's Safety Evaluation (Reference 4) concluded that the inspection deferral was acceptable contingent upon OPPD's commitment to submit addition-al analyses of the Internal Vibration Monitoring (IVM) System.
Additional information was submitted to the NRC in October of 1987 (Reference 5) and a commitment was made to supply the NRC with IVM threshold levels which will be used to indicate the initiation of the slow degradation process of the Corn Support Barrel / Thermal Shield support structure.
The IVM monitoring program and threshold values for the Fort Caltoun Station have been developed.
The attached flow chart displays the steps to be taken for the IVM threshold level program. The methodology used to accomplish the IVM threshold levels are as follows:
1.
Acquire, reduce, and evaluate 100% power IVM data at the beginning, end, and at three month intervals for each fuel cycle.
Calculate the inphase (O') RMS amplitudes of the 0-4 Hz, 4-6 Hz, 6-10 Hz, and 10-15 Hz fre-g0fg quency ranges.
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LIC-88-091 Documert Control Desk Page 2 2.
Compare the RMS amplitude for the 6-10 Hz range for the present data to that of the previous quarterly data.
i a
3.
If there is a definite 12.5 Hz in-phase peak in the PSDs, and the in-crease in the RMS amplitude for the 6-10 Hz range is less than 25%, con-tinue to monitor at three month intervals.
If both of these conditions are met, the thermal shield is adequately supported.
(The 25% increase is based on an expected three month increase associated with fuel burnup
.i of 15% plus a potential 10% IVM system amplitude uncertainty).
1 4.
If there is a greater than 25% but less than 100% increase in the RMS amplitude for the 6-10 Hz in-phase range from the previous quarterly data, begin acquiring, reducing, and evaluating data at one month inter-i vals. The only action to be taken at this time is increased monitoring for trending purposes. No inspection is recommended at this time.
l 1
5.
If there is no definite 12.5 Hz in-phase (0') peak in the PSDs, and if the increase in the RMS amplitude of the 6-10 Hz range is greater than 100%, the changes in the neutron noise data indicate a loss of effective-t ness of the positioning pins. Begin to monitor IVM data at one month in-F tervals for trending purposes. An inspection of the thermal shield sup-port structure is recommended. The inspection should occur within the t
next six months.
l 6.
If six successive months of monitoring indicate a loss of effectiveness j
of positioning pins and if this is corroborated by evaluation of Loose Part Monitoring (LPM) data, it will be recommended that the plant be shut i
down and an inspection of the thermal shield support structure be perform-l 1
ed as soon as practicable.
l The technical approach utilized was based on the failure mechanism analysis of the St. Lucie 1 thermal shield.
It was found that degradation in the ther-mal shield support system was manifested as frequency peak shifts in the 4
spectra of the excore detector noise signals.
For Fort Calbun, as was also e
i the case for St. Lucie Unit 1, there are significant reductions in frequency of the COS 20 mode of vibration of the thermal shield and core support barrel 1
i system when simulated damage of the thernal shield support system is intro-i duced into the analysis.
This C0S 20 mode, which results in in-phase relationships between cross core pairs of detectors, provides the requisite identifier of the condition of the thermal shield support system which can then be tracked via the IVM system.
For the nominal, as-designed condition of the thermal shield support system, the frequency is 12.5 Hz. With the postulated loss of effectiveness of all positioning pins, the power assoc-iated with the 12.5 Hz peak will shift to a peak at 7.9 Hz.
With the post-i ulated addition of support lug / support pin wear, the power originally associated with the 12.5 Hz peak will shift further to a peak at 5.4 Hz.
1 i
l l
LIC-88-091 Document Control Desk Page 3 The present IVM and loose Parts Monitoring (LPM) programs at Fort Calhoun ac-l quire date on a monthly basis and evaluate it each quarter.
The collection of IVM data on a monthly basis was committed to in Reference 3.
The IVM threshold analyses have shown that the present method of monthly IVM and LPM data acquisition ano quarterly evaluation can be changed to a program of quarterly data acquisition and quarterly evaluation until early signs of degradation are discovered.
The degradation process can be detected in the early states by analyzing IVM data. OPPD believes the analysis indicates that the degradation process can be detected with quarterly data acquisition while still assuring 0 PPD's need to prevent degradation of the Fort Calhoun thermal shield and thermal shield support system. OPPD, therefore, would like to replace the commitment for monthly data acquisition made in Reference 3 with a commitment for quarterly data acquisition.
Should early signs of degradation be discovered, appropriate actions shall be taken. OPPD seeks NRC concurrence with this commitment change.
If you have any questions, please contact us.
Sin re i
i R. L. Andrews Division Manager Nuclear Production i
RLA/sa l
Attachment c:
LeBoeuf, Lamb, Leiby and MacRae 1333 New Hampshire Avenue, N.W.
Washington, DC 20036 8
R. D. Martin, Regional Administrator j
P. H. Harrell, NRC Senior Resident Inspector i
I 4
INTERN ALS VIBRATION MONITORING FORT CALHOUN STATION IVM THRESHOLD LEVEL FLOWCHART i
l MONITOR IVM DATA ON A QUARTERLY BASIS ir IS THERE A 12.5 HERTZ (HZ) PEAK IN THE IN PHASE PSD PLOT AND AN INCREASE IN THE 6-10 HZ RANGE LESS THAN 25%7 YES NO u
ir CONTINUE TO MONITOR ON IS THE INCREASE IN THE 610 HZ A QUARTERLY BASIS RANGE LESS THAN 100%
YES NO 3
1 I BEGIN IVM MONITORING AT BEGIN IVM MONITORING AT ONE MONTH INTERVALS ONE MONTH INTERVALS AND PLAN FOR AN INSPECTION OF THE THERMAL SHIELD SUPPORT MECHANISM IN APPROXIMATELY SIX MONTHS L
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