ML13331A406
| ML13331A406 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 05/11/1990 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML13331A405 | List: |
| References | |
| NUDOCS 9005220346 | |
| Download: ML13331A406 (4) | |
Text
0 EUNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 ENCLOSURE SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO THE SAN ONOFRE FINAL THERMAL SHIELD MONITORING CRITERIA SOUTHERN CALIFORNIA EDISON COMPANY SAN ONOFRE UNIT 1 DOCKET NO. 50-206
1.0 INTRODUCTION
By letter dated January 17, 1990, the Southern California Edison Company, the licensee for the San Onofre Unit 1 nuclear power plant, submitted information regarding the proposed final acceptance criteria for the thermal shield monitoring program, as required by License Condition 3.M, "Cycle X Thermal Shield Monitoring Program" (Ref. 1).
Additional infor mation was submitted on March 19, 1990 (Ref. 2).
The purpose of the final thermal shield monitoring criteria is to assure that further deterioration of the thermal shield supports or the thermal shield integrity or position will be detected and appropriate action will be taken. The proposed final monitoring criteria are based on a data base collected after the plant resumed Cycle 10 full power operation, as provided by License Condition 3.M. The quantification of the monitoring criteria was a requirement of License Condition 3.M (Ref. 3), and was based on existing analytical results concerning thermal shield vibration frequency and amplitude, neutron and acoustical noise monitoring frequency, assurance of adequate detection signal margin and accounting for the core behavior versus burnup.
2.0 EVALUATION There are two systems for the thermal shield monitoring, i.e., excore neutron noise from the excore neutron detector signal and acoustical noise from pressure vessel mounted accelerometer signals. The purpose of the neutron noise is to identify any thermal shield position shifting or deterioration of its supports. The acoustical noise might be able to identify loose parts in the primary coolant system.
2.1 Neutron Noise Monitoring In this method neutron noise data are analyzed for power spectral densities (PSD), cross power spectral densities (CPSD) and coherence and phase functions 9R005220:346 90,0511 FDR DOCK 0!0002 0 6 P
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-2 to provide thermal shield vibration frequency and amplitude data for comparison with base line information. Such comparison can reveal (to the trained specialist) changes which might have taken place in the thermal shield. For this purpose, at least three neutron noise inputs from the excore neutron detectors are monitored for at least twenty minutes once a week as required by License Condition 3.M.
Seventeen sets of data were acquired in the power range from 85% to 92% in Cycle 10. These data formed the basis for neutron noise PSD levels for the final monitoring criteria for each of the excore detectors.
Upper and lower base line curves were generated by connecting the maximum and minimum envelop points of the base data, respectively. A second and third set of curves were then established at 80% and 200% above and below the upper and lower base lines, respectively. At 80% above (below) the base line at any frequency, the data will be analyzed for trends. At 200%
above (below) the base line at any frequency the NRC will be notified.
Within two weeks a report will be submitted to the NRC for staff review documenting and justifying future actions.
Burnup effects through the cycle will be accounted for in a linear manner as described in References 4 and 5.
The data base is considered adequate because:
(a) The frequency range of interest is in the range of 1-8 Hz which is estimated by analytical calculations and supported by thermal shield measurements in other plants. The corresponding period range is 0.125 to 1 second thus a 20 minute recording represents 540-3840 samples which form an adequate statistical base.
(b) The 17 samples span reactor operation of over 85% power level and a time period over 2 months. This is an adequate time period and power level for the core to have reached stable '
operating conditions.
(c) The estimated change of the thermal shield vibration frequency and amplitude due to a significantly more degraded condition is expected to be much greater than the monitoring and notification limits defined in these criteria, thus, the criteria are conserva tive.
(d) The data analyses are not limited to the PSDs but includes CPSDs, coherence and phase analyses which enhance diagnostic ability from the available signals.
For the reasons cited above we find the proposed final acceptance criteria and neutron noise monitoring program acceptable.
-3 2.2 Loose Parts Monitoring The loose parts (acoustical signal) monitoring complements the neutron noise monitoring. In this particular case, deterioration of the thermal shield support could result in some of the support bolts falling off (some bolts are known to be loose). Another possibility (albeit remote) is for the thermal shield to lose partial support and become wedged between the core skirt and the pressure vessel or lose partial support and have a fragment break off. Either a loose bolt or a loose fragment agitated by the coolant flow are likely to create a strong acoustical signal, identi fiable by the accelerometers which are mounted in the area of the vessel upper flange. This accelerometer location, near the upper flange, is not optimal for detecting impacts generated at the lower part of the vessel.
Due to access limitations, neither of the accelerometers could be located at or near the bottom of the pressure vessel. Additionally, calibrated impacts could not be generated near the bottom of the pressure vessel.
Therefore, there are no available signatures from the bottom of the vessel to the existing accelerometers. However, the anticipated part size and velocity is expected to create impacts of sufficient magnitude as to be identifiable by the existing accelerometers.
The accelerometer sensitivity has been tested and satisfies Regulatory Guide 1.133 requirements. Spectral characteristics at normal background levels have been recorded at 85% power level.
RMS voltage values are recorded twice a day for 5 minutes every day. These recordings are reviewed once a week by a cognizant engineer to identify deviations from the norm. The proposed final acceptance criteria provide for extended surveillance if the signal RMS value exceeds four times the average value, and NRC notification within two weeks if the count rate exceeds one per ten minutes over a two week period.
From the existing recordings of background noise we conclude that the four times peak over average value is conservative and is acceptable.,
3.0
SUMMARY
AND CONCLUSIONS The staff reviewed the submitted information regarding the proposed final acceptance criteria for neutron and acoustical noise thermal shield monitoring. Based on our estimation that (a) the prescribed limits are corservative, (b) they satisfy the provisions of License Condition 3.M, and (c) they satisfy the requirements of Regulatory Guide 1.133, we find them acceptable.
Principal Contributor: L. Lois Dated: May 11, 1990
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4.0 REFERENCES
- 1.
Letter from R. M. Rosenblum, Southern California Edison Company to USNRC, "Final Acceptance Criteria for the Thermal Shield Monitoring Program San Onofre Nuclear Generating Station, Unit 1," dated January 17, 1990.
- 2.
Letter from F. R. Nandy, Southern California Edison Company to the USNRC, "The Revised Final Acceptance Criteria for the Thermal Shield Monitoring Program, San Onofre Nuclear Generating Station, Unit 1," dated March 19, 1990.
- 3.
Letter from C. Trammell USNRC to Harold B. Ray Southern California Edison Company, "Interim Acceptance Criteria for the Thermal Shield Monitoring Program San Onofre Nuclear Generating Station, Unit No. 1," dated November 3, 1989.
- 4. F. J. Sweeny, J. March-Leuba and C. M. Smith, "Contribution of Fuel Vibrations to Ex-Core Neutron Noise During the First and Second Fuel Cycles of Sequoyah 1 Pressurized Water Reactor," Progress in Nuclear Energy, 1985, Vol. 15 pp 283-290.
- 5. NUREG/CR-2996, "Sensitivity of Detecting In-Core Vibration and Boiling in Pressurized Water Reactors Using Ex-Core Neutron Detectors," F. J. Sweeny, July 1984.