ML20078B642
| ML20078B642 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 10/20/1994 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20078B640 | List: |
| References | |
| NUDOCS 9410260171 | |
| Download: ML20078B642 (3) | |
Text
-.- _.
. -. _ ~
j epa arc 4
UNITED STATES y-)W 3 i E
NUCLEAR REGULATORY COMMISSION 5 th 4 I WASHINGTON, D.C. 20555-0001
- SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO.189 TO FACILITY OPERATING LICENSE NO. DPR-77 AND AMENDMENT NO. im TO FACILITY OPERATING LICENSE NO. DPR-79 TENNESSEE VALLEY AUTHORITY SE0V0YAH NUCLEAR PLANT. UNITS 1 AND 2 DOCKET NOS. 50-327 AND 50-328
1.0 INTRODUCTION
By application dated September 8,1994, the Tennessee Valley Authority (the i
licensee) proposed an amendment to the Technical Specifications (TS) for Sequoyah Nuclear Plant (SQN) Units 1 and 2.
The requested changes would l
incorporate clarifications regarding the evaluation of steam generator tube l
defects by separating the portion of the steam generator tube starting at the end of the tube up to the start of the tube-to-tubesheet weld from the remainder of the tube for the purposes of sample selection and repair when
)
defects are found in this section of a steam generator tube.
j t
l Specifically, the licensee has proposed that a note be added following i
Surveillance Requirement (SR) 4.4.5.2.c.2 stating, " Tube degradation identified in the portion of the tube that is not a reactor coolant pressure boundary (tube end up to the start of the tube to-tubesheet weld) is excluded from the Result and Action Required in Table 4.4-2."
In addition, a similar statement would be added to SR 4.4.5.4.a.6, Plugging Limit Acceptance Criteria, to indicate that the criteria would not be applied to tube degradations found in this area. The Bases would also be changed to reflect this information, along with a statement that any indications that extend from this region will be detected during the scheduled tube inspections.
2.0 EVALVATION By way of background, during the SQN Unit 2 Cycle 6 refueling outage, the steam generator maintenance work included removal of the Combustion Engineering roll plugs from the Row 1 tubes in all four steam generators in order to return these tubes to service following U-bend heat treatment. The removal procedure included a helical scan of the inside diameter of the plug with a tungsten inert gas torch, followed by plug removal with a pulling tool, and visual examination with a remote TV camera.
The visual examinations of all tubes from which plugs were removed were acceptable.
However, inspection of the roll plugs that were removed from the first two steam generators (Nos. I and 4) revealed significant burn-through on some of these roll plugs. As a result, an unscheduled rotating pancake coil (RPC) eddy current exam was performed on the bottom 6 inches of the hot leg ENCLOSURE 3 l
9410260171 941020 (RR ADOCK 050003 7
d
\\
. tubes in steam grvators 1 and 4 to verify that no tube damage had resulted from the burn-thru,h on the roll plugs, before continuing the roll plug removal process on steam generators 2 and 3.
(Each Sequoyah steam generator tube has undergone a partial depth hardroll along with Westinghouse Explosive Tube Expansion (WEXTEX) expansion over the full depth of the tubesheet. The tube ends extend a minimum of 0.37 inches below the bottom of the tubesheet and 0.22 inches below the tubesheet cladding).
The RPr ams detected single and multiple axial indications in the tube ends ie below the bottom of the tubesheet in 39 tubes in steam generator that no. 1.
le indications were very short and appear to be both inside diameter and outside diameter in nature.
Since they are below the tube-to-tubesheet weld and the hardroll and the WEXTEX regions of the Row 1 tubes, the licensee has concluded that they do not affect the structural and leakage integrity of the primary-to-secondary pressure boundary. Thirty-three of the 39 tubes in steam 0 erator no. I will be returned to service, and 6 will be plugged for other a 1s.
No indications were detected in steam generator no. 4.
In additii
. amination of the Row 1 tube ends using the bobbin coil technique did not detect any degradation in steam generators 1 and 4 because the response from the tube ends greatly exceeds the amplitude of the indication, thus masking their presence. The licensee has been unable to determine the root cause of the indications, but believes taat they were probably caused by cold working the tube ends during installation or removal of the plugs. There is no apparent damage caused by loose parts.
Detection of the-indications and their evaluation has led the licensee to propose a change the TS to clarify that these indications should not be used for repair anu sample expansion criteria that would be applied to indications found in steam generator tubes that form the primary-to-secondary pressure boundary. This criteria is specified in Table 4.4-2.
The purpose of thir criteria is to require increased sampling and repair should cracks be found in a significant number of tubes, in order to ensure that the extent of the problem is determined and proper corrective actions employed.
The potential effect of the tube crack propagation on the tube-to-tubesheet weld was evaluated by examining the limiting case where the weld would no longer be effective because of the presence of cracking.
For this to be a concern, it would be necessary for the crack in the tube to propagate beyond the weld into the tubesheet region; i.e., into the start of the pressure-retaining section of the tube.
Should any crack propagate into this region it would be detected by bobbin coil examinations that are routinely performed each refueling outage to monitor steam generator tube integrity.
In addition, the licensee had committed to incorporate a special bobbin coil examination program for the 33 tubes in steam generator no. I to monitor the existing indications. The program will continue for each refueling outage until the tubes are plugged. This will make it possible means to detect any crack propagation into the pressure boundary area that could affect the structural integrity of the boundary.
Indications that are not detectable by bobbin coil in this region do not compromise steam generator integrity.
, The staff has reviewed the information provided by the licensee and agrees that the cracks found in the ends of the steam generator tubes below the tube-to-tubesheet weld should not be considered to be in the pressure boundary region, since leakage through any of the cracks would not propagate from the primary to the secondary system. Thus, the staff condludes that the technical j
specifications that require sample expansion and repair if indications are detected should not be applied to any indications found in this region of a tube. Also, the staff is satisfied that any present and future indications that extend to the start of the welded area will be detected, and that an adequate program has been implemented to monitor the present indications for any possible growth into the tube weld region.
Based on this analysis, the staff has determined that all safety concerns have been satisfied and that the proposed amendment is acceptable.
3.0 STATE CONSULTATION
l In accordance with the Commission's regulations, the Tennessee State official was notified of the proposed issuance of the amendment. The State official had no comments.
4.0 ENVIRONMENTAL CONSIDERATION
The amendment changes a requirement with respect to the surveillance requirements. The NRC staff has determined that the amendment involves no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission has previously issued a proposed finding that the amendment involves no significant hazards consideration, and there has been no public comment on such finding (59 FR 47962). Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendment.
5.0 CONCLUSION
The Commission has 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 the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor: David E. LaBarge Dated:
October 20, 1994
,__m