ML13316B373
| ML13316B373 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 05/15/1989 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML13316B371 | List: |
| References | |
| NUDOCS 8905220211 | |
| Download: ML13316B373 (4) | |
Text
0RE UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 SAFETY EVALUATION BY THE OFFICE-OF NUCLEAR REACTOR REGULATION RELATED TO.AMENDMENT-NO.127TO PROVISIONAL.OPERATING LICENSE NO. DPR-13 SOUTHERN CALIFORNIA EDISON-COMPANY SAN-D1EGO GAS-AND ELECTRIC COMPANY SAN ONOFRE NUCLEAR GENERATING-STATION, UNIT NO.-1 DOCKET NO. 50-206
1.0 INTRODUCTION
By letter dated February 17, 1989, as supplemented March 21 and 23, and May 3 and 8, 1989, Southern California Edison Company (SCE or the licensee) requested a change to Provisional Operating License No. DPR-13 for operation of San Onofre Nuclear Generating Station, Unit No. 1, located in San Diego County, California.
2.0 DISCUSSION In response to an alert from the reactor vendor that reactor vessel thermal shield fasteners at another facility had been found degraded, SCE inspected the thermal shield for SONGS-1 on January 3-4, 1989. The thermal shield at San Onofre Unit 1 surrounds the reactor core barrel.
It is 2P" thick and about 10 feet in height, and weighs 48,000 lbs. It is supported at the bottom by six support blocks and thirty bolts which attach it to the core barrel. Support at the top is provided by six flexures and 4 limiter keys. Five of the six flexures have been known to have been broken since 1978.
The licensee described the inspection results and presented video tape recordings at a January 27, 1989 meeting. During the last refueling outage visual examinations were performed with the internals installed using a high resolution underwater television camera system. Although the core was loaded during the inspection, selected fuel assemblies were shuffled to provide access.
The licensee confirmed that five of six thermal shield flexure fixtures are broken as detected in a previous inspection. Three out of thirty thermal shield support block bolts are broken. These are the 7/8-inch top bolts in support blocks at the 00 and 2400 locations. No other signifi cant degradation was observed visually. Ultrasonic testing was not performed.
8905220211 890515 S
PDR ADOCK 05000206 P
PD~C
-2 The licensee has evaluated operation with the thermal shield in the condition observed and considered three cases which it characterizes as:
worst expected, worst credible, and worst conceivable.
The worst expected case involves the degradation of bolts at the third support block, and the sixth flexure remaining intact. The worst credible case assumes that all support blocks degrade (all bolts broken) and the last flexure breaks. The worst conceivable case involves the thermal shield dropping or moving downward eleven inches to rest on the core barrel radial support keys.
In addition to the analyses presented by the licensee, the licensee proposes to inspect the thermal shield during a June 1990 mid-cycle outage using the same equipment and methods used during the January 1989 inspec tion. The licensee also proposes to use two monitoring methods while in operation to detect any further degradation of the thermal shield:
neutron noise monitoring and loose-parts monitoring. The licensee proposes to shut down if it determines through the monitoring program that the sixth flexure has failed.
3.0 EVALUATION The video tapes recorded during the visual examination demonstrate that the thermal shield is still in its original position at this time. The tapes do not show any evidence of motion of the thermal shield. The tapes show that three bolts (two in one block; one in another) are protruding sufficiently far beyond allowable tolerances that it is reasonable to assume that they have drifted inward from vibration and are broken.
Because the inspection was only visual it cannot be known if these are the only broken or cracked bolts. Licensee's vibration analysis concludes that the 2400 and 00 blocks are degraded and that the 300* block would probably degrade during cycle X.
Furthermore, the licensee states and the inspection does demonstrate that the thermal shield is not in the "worst credible" condition or close to it because the last flexure is still intact and no support block wear or thermal shield motion has occurred. Support block wear would be expected if all of the bolts and dowel pins in a block were failed. No evidence exists to suggest that any individual support block assembly has progressed to this condition.
The vibration analysis predicts that a third support block will probably degrade during operation in cycle X, but that no damage will occur to the thermal shield. The analysis performed by the licensee used a simplified model consisting of beam elements and springs to represent linear and rotational stiffnesses of the system. The staff reviewed the pertinent information provided by the licensee and concluded that there are serious flaws in the methodology, modeling and in the evaluation of stresses which would result from the impactive loads on the support blocks induced by the vibratory motion of the thermal shield. Because the analysis was found to be unacceptable by the staff, the license has been requested to
-3 perform a mid-cycle inspection and to improve the proposed monitoring program whereby any further degradation of the shield supporting elements could be quickly detected and appropriate action taken by operating personnel.
At the meeting with the licensee on May 1, 1989, the NRC staff presented its requirements for an inspection of the thermal shield at the mid-cycle outage and changes to the proposed license conditions on thermal shield monitoring. The licensee agreed with the staff position and confirmed this agreement in its letters to NRC dated May 3 and 8, 1989.
In evaluating the safety issues regarding the thermal shield the staff took under consideration the following sequence of events which must take place prior to the situation which may cause a safety concern. The scenario which would cause a concern is that the shielo could drop to the bottom of the reactor vessel and therefore obstruct the flow of coolant to the core. In order that such a situation could exist the shield must be deprived of its supporting elements and the following stages of further degradation would have to occur:
(1) Failure of the sixth flexure (2) Failure of the all bolts at each support block (3) Shearing off the support blocks which hold the shield in the present position, and (4) Failure of the lower core radial supports The staff criteria require that the licensee provide an adequate neutron noise monitoring program which will detect any further degradation of any of the above elements and that the plant will be shut down immediately after failure of the remaining flexure, thus precluding any further deterioration of the reactor internals. The staff believes that such an arrangement coupled with the mid-cycle inspection provides adequate assurance of safety.
The noise signal from the ex-core power range neutron flux detectors will be recorded periodically and analyzed to monitor internal vibrations of the thermal shield. Four accelerometers mounted on the reactor vessel flange will monitor acoustical noise in order to detect the possible appearance of loose parts in the lower dome of the vessel.
The neutron noise analysis will probably not be effective in detecting gradual degradation of the fasteners, but failure of the last flexure would allow a large beam-mode oscillation at a much lower frequency which would be detectable. The plant would be shut down for repairs should this occur, as discussed above.
The three bolts which were found to be broken will likely drift out all the way and become loose parts at some point in cycle X. These parts will most likely fall to the bottom of the pressure vessel because of their weight and settle in a location of low flow velocity. Detection of loose parts such as these under these circumstances would not be likely
-4 due to the arrangement of the accelerometers which are mounted on the reactor vessel flange. In the unlikely event that the loose parts are lifted up against the flow distribution or core support plates, no adverse impact is expected, and these impacts may be detectable since these locations communicate more directly with the accelero meters.
The licensee and consultant (Westinghouse) have analyzed the changes in reattor coolant flow to the core in the event the shield should tilt or drop 11" to the core barrel radial supports. The changes in flow and flow distribution would be minor and within the design parameters, and are therefore acceptable.
We conclude that operation in cycle X as proposed is acceptable.
4.0 ENVIRONMENTAL CONSIDERATION
Pursuant to 10 CFR 51.21, 51.32, and 51.35, an environmental assessment and finding of no significant impact have been prepared and published in the Federal Register on May 11, 1989 (54 FR 20459). Accordingly, based upon the environmental assessment, the Commission has determined that the issuance of this amendment will not have a significant effect on the quality of the human environment.
5.0 CONCLUSION
We have concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations and (3) the issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributors: R. Lipinski L. Lois C. Trammell M. Hum Dated:
May 15, 1989