ML13316A062
| ML13316A062 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 06/25/1990 |
| From: | Morgan H SOUTHERN CALIFORNIA EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9007090291 | |
| Download: ML13316A062 (3) | |
Text
Southern California Edison Company P. 0.
BOX 128 SAN CLEMENTE, CALIFORNIA 92672 H. E. MORGAN TELEPHONE VICE PRESIDENT AND SITE MANAGER 714-368-9470 SAN ONOFRE June 25, 1990 U. S. Nuclear Regulatory Commission Document Control Desk Washington, D.C. 20555
Subject:
Docket No. 50-206 Special Report, License Condition 3.M(4)
Reactor Vessel Thermal Shield Monitoring San Onofre Nuclear Generating Station, Unit 1
References:
- 1)
Letter, H. E. Morgan, SCE to USNRC Document Control Desk, dated June 6, 1990.
- 2)
Letter, F. R. Nandy (SCE) to USNRC Document Control Desk, "TheRevised Final Acceptance Criteria for-the Thermal Shield Monitoring Program," dated March 19, 1990.
- 3)
Letter, H. B. Ray (SCE) to USNRC Document Control Desk, "Amendment Application No. 181," dated April 10, 1990.
Pursuant to License Condition 3.M, paragraph (4) a), SCE telephonically notified the NRC on June 12, 1990, that on June 11, 1990, San Onofre Unit 1 had exceeded the reporting requirements of the subject license condition.
This letter provides a written 14-day report, pursuant to license condition 3.M, paragraph (4) b), and provides the results of our.evaluation and our future plans and actions. Additionally, Reference 1 contained a typographical error which incorrectly indicated that the thermal shield support system repairs would be performed during the Cycle X refueling outage and should have indicated that the repairs would be performed during the Cycle XI refueling outage.
BACKGROUND OF THE THERMAL SHIELD MONITORING PROGRAM The neutron noise / loose-parts detection system is used to monitor the condition of the reactor vessel thermal shield throughout Cycle X. Details of this program have been previously described in References 1 and 2.
DC.
0000l
USNRC Document Control Desk June 25, 1990 Page 2 The neutron noise system utilizes eight excore neutron detectors which are located in four radial locations around the reactor vessel to measure vibratory motion of the thermal shield. Baseline trends of thermal shield motion were established at the beginning of the program. Based on these trends, criteria were established which are 200% above the baseline signals over frequency bands of 0.3 hertz to 19.0 hertz for ion chambers and 0.3 hertz to 13.5 hertz for fission chambers. These measurements are taken weekly for detectors which are below 80% above the baseline and twice per week for detectors which are between 80% and 200% above the baseline until the thermal shield support system is repaired.
As indicated in Reference 1, SCE has been trending two detectors (1208 A&B located ) between 7.5 and 9.5 hertz twice per week since December 1989, as the result of their exceeding a level which is 80% above the baseline. The results indicate an increase in the RMS value over these frequencies from December 1989 to February 1990. Since mid-February 1990, the RMS value has remained relatively constant.
A contributor to the general increase in signal level is a change in the scale factor between the source of the noise and the signal level which occurs as the result of decreasing boron concentration due to core burnup. As the boron concentration decreases, more neutrons are seen by the excore neutron detectors. SCE has not established revised acceptance criteria curves to account for these effects because the existing curves are conservative and it would not be practical to develop new curves since Unit 1 will be shutdown for refueling and thermal shield support system replacement on June 30, 1990.
INCREASED NEUTRON NOISE A surveillance performed on June 11, 1990 found that the signal from one of the eight detectors (1208 B) exceeded the 200% criteria only at a frequency of approximately 8.2 hertz. On June 12, 1990 the signal had dropped below the 200% criteria, and by June 19th had decreased to the levels observed before June 11th.
The signal increases observed from detector 1208 B on June lth, and May 23rd (previously reported by Reference 1) are believed to be random "spikes" which, when added to the slowly increasing trend in all of the signals due to the effects of core burnup (i.e., increasing moderator density coefficient and fuel depletion), resulted in exceeding the 200% criteria. This and the May 23rd instance of exceeding the criteria are not believed to be indicative of thermal shield support system degradation since appreciable changes in the condition of the support system would have shown up as a major increase in amplitude and/or a frequency shift. 'Additionally, the acoustical loose parts monitoring system data has been evaluated and no changes have been noted.
Based on this analysis of the neutron noise and loose parts monitoring data, we have concluded that no significant degradation of the thermal shield support system has occurred.
USNRC Document Control Desk June 25, 1990 Page 3 Due to the underlying increase in the signal trend, it is possible that there may be other instances of exceeding the 200% criteria before Unit 1 begins its Cycle XI refueling and thermal shield support repair outage.
SUMMARY
AND FUTURE ACTIONS Since last exceeding the 200% criteria on June 11, 1990, detector 1208 B and the other seven detectors have not subsequently exceeded the 200% criteria and have leveled out at approximately the.same amplitude which has been observed since February 1990. As a consequence, SCE has concluded that the high measurements found on May 23 and June 11, 1990 were of no safety significance and that the thermal shield remains stable.
SCE will continue to record neutron noise and evaluate thermal shield motion twice a week until the thermal shield supporting system is replaced in accordance with the plan described in Reference 3.
If you require any additional information, please so advise.
Sincerely, cc:
C. W. Caldwell (USNRC Senior Resident Inspector, Units 1, 2 and 3)
J. B. Martin (Regional Administrator, USNRC Region V)