ML13330B268
| ML13330B268 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 03/22/1988 |
| From: | Medford M SOUTHERN CALIFORNIA EDISON CO. |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| Shared Package | |
| ML13330B269 | List: |
| References | |
| TAC-67463, NUDOCS 8803250035 | |
| Download: ML13330B268 (10) | |
Text
Southern California Edison Company P. 0.
BOX 800 2244 WALNUT GROVE AVENUE ROSEMEAD, CALIFORNIA 91770 M. 0. MEDFORD TELEPHONE MANAGER OF NUCLEAR ENGINEERING (818) 302-1749 AND LICENSING March 22, 1988 U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555 Gentlemen:
Subject:
Docket No. 50-206 Revision to Proposed Change No. 182 San Onofre Nuclear Generating Station Unit 1
Reference:
Letter, Kenneth P. Baskin, SCE to U. S. Nuclear Regulatory Commission, Amendment Application No. 149, March 10, 1988 The referenced letter provided a request to revise San Onofre Nuclear Generating Station, Unit 1 (SONGS 1) Technical Specification 4.16, "Inservice Inspection of Steam Generator Tubing," to include a modified tube plugging limit for imperfections located in the tube roll expansion region of the steam generator tube. This application was based upon a change approved for use at the Connecticut Yankee - Haddam Neck Plant. Based upon a comparison of the analyses performed to support that change, SCE concluded that, due to design similarities, such a change was justified for use at SONGS 1. In response to questions raised by the NRC Staff, subsequent to the submittal of the referenced letter, it has been identified that a revision to the SONGS 1 change is necessary to facilitate NRC review. Accordingly, the appropriately revised Proposed Change No. 182, Revision 1 is enclosed for your review and approval.
If you have any questions, please let me know.
Very truly yours, cc: J. B. Martin, Regional Administrator, NRC Region V F. R. Huey, NRC Senior Resident Inspector, San Onofre Units 1, 2 and 3
- 3. 0. Ward, California Department of Health Services A00 8803250035 880322 PDR.
ADOCK 05000206
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REVISION 1 DESCRIPTION AND SIGNIFICANT HAZARDS CONSIDERATION ANALYSIS OF PROPOSED CHANGE NO. 182 TO THE TECHNICAL SPECIFICATIONS PROVISIONAL OPERATING LICENSE NO. DPR-13 This is a request to revise Section 4.16, "INSERVICE INSPECTION OF STEAM GENERATOR TUBING," Appendix A Technical Specifications for San Onofre Nuclear Generating Station Unit 1 (SONGS 1).
DESCRIPTION OF CHANGE Technical Specification 4.16 requires periodic inspections of the steam generator tubing to assure its integrity for continued plant operation. The specification delineates the scope of each inspection, frequency, corrective actions and reporting requirements. Included in the corrective actions is a specification that all tubes with imperfections that exceed the plugging limit shall be repaired or plugged.
Proposed Change No. 182 would revise Technical Specification 4.16 to include a modified tube plugging limit for tubes with imperfections in the mechanically expanded portion of the tube within the tubesheet and update the technical specification bases for this criteria.
The modified tube plugging limit would allow tubes with imperfections below the uppermost one inch of a sound tube roll expansion to remain in service, provided that there are no imperfections in this region. The revision includes definitions of the tube roll expansion, sound roll modification of the tube plugging limit, revision of the inspection expansion and reporting criteria and a requirement to report the results of inspections of all tubes in service with defects below the uppermost one inch of sound roll.
Revisions are also proposed to the basis for Specification 4.16.
The revisions to Specification 4.16, as constituted in Attachment 2, are proposed to be consistent with those changes approved for use at the Connecticut Yankee (CY) Haddam Neck Plant, modified, as necessary, to accommodate the differences between the Haddam Neck and SONGS 1 technical specification format and other reasons, as discussed below. The details of each change and the basis for that change is discussed below.
- 1. The definition of degraded tube is modified to be consistent with that of the CY specification, with the exception that the words "greater than or equal to" are added consistent with the Standard Technical Specification (STS) definition of degraded tube. This change does not,constitute a significant revision to the approved CY version.
- 2. Consistent with the change identified in Item 1 above, Part A.4 is revised to include a requirement that every inspection include all degraded tubes, currently in service in the generator(s) to be inspected. This represents no change to the existing SONGS 1 requirement that all tubes with previously detected imperfections greater than 20% of the nominal tube thickness shall be inspected. The definition of
-2 degraded tube will ensure that, in addition to those tubes with imperfections above the tube roll expansion that are greater than or equal to 20% of the nominal tube thickness, all non-plugged tubes with imperfections in the region one-half inch below the uppermost one inch of a sound roll shall also be inspected. This is not a requirement imposed by the CY specification, but SCE has determined it necessary to ensure that all degraded tubes, including those considered to be degraded by the revised definition, be inspected.
- 3. Consistent with Item 2 above, the terms "greater than 20%" and "detected" are revised in Parts B.1, B.2, B.3, D.2, E.2.a, E.3.a and E.4.a, to assure that the definition of degraded tube is applied when considering both inspection expansion and reporting criteria. These changes are consistent with the CY specification. The CY specification did not require specific revisions to accomplish this result, as their existing Table 4.10.1-2 accomplished this, when the revised definition of degraded tube was considered.
- 4. The definition for "imperfection" is revised for consistency with the definition in the CY specification.
- 5. The definition for "defect" is revised for consistency with the definition in the CY specification.
- 6.
The definition of "plugging limit" is revised for consistency with the CY specification to specify a special plugging criteria for the tube in the region of the tube roll expansion.
- 7. The terms "tube roll expansion" and "sound roll" are defined, such that their use elsewhere in the technical specification is understood. These definitions are not included in the CY specification, but SCE considers them important for clarity.
- 8. Part E.5 is added to require reporting of steam generator inspection results for tubes with defects below the uppermost one inch of sound roll.
This will ensure that the NRC is cognizant of the application of the modified plugging limit. There is no corresponding item in the CY specification, but it is SCE's understanding that the NRC desires a report of this data.
- 9. The revision to the basis is consistent with the body of the specification. The basis revision is not the same as that included in the CY specification primarily due to the fact that SCE has chosen to describe that the one inch of required sound roll contains an allowance of.25 inch for eddy current measurement uncertainty.
Accordingly, for the reasons discussed above, it is concluded that compliance with the technical specifications as proposed in Attachment 2 will ensure that the assumptions and conclusions presented in the following information are maintained.
-3 BACKGROUND Introduction The Westinghouse-Series 27 steam generators at San Onofre Nuclear Generating Station, Unit 1 (SONGS 1) were fabricated with a partial depth roll expansion in the lower end of the tube above the tube to tubesheet weld. For SONGS 1 this partial depth roll expansion is two inches, measured upward from the bottom of the steam generator tube. The depth of this expansion is based upon the prevalent Westinghouse steam generator design practice at the date of manufacture. The partial depth expansion, along with the tube to tubesheet weld, forms both a structural and pressure boundary for the steam generator tube. One of the structural design considerations is tube pullout under normal or postulated post-accident leading conditions. A modified tube plugging criteria has been developed for use in determining whether or not repairing or plugging of partial depth hardroll expanded tubes is necessary for degradation which has been detected in the mechanically expanded portion of the tube, within the tubesheet. Existing SONGS 1 steam generator tube plugging and repair criteria apply throughout the tube length but do not take into account the reinforcing effect of the tubesheet on the external surface of the tube.
The elastic preload and friction between the tube and the tubesheet due to the mechanical hardroll provide the force required to resist pullout, previously constrained by both the hardroll and the tube to tubesheet weld. The presence of the tubesheet acts to constrain the tube and complement its integrity in that region by essentially precluding tube deformation beyond its expanded outside diameter, i.e., the resistance to both tube rupture and collapse is significantly strengthened by the tubesheet. In addition, the proximity of the tubesheet significantly affects the leak behavior of through wall tube crack in this region.
The primary criterion included in the modified tube plugging limit developed for roll expanded tubes is designated the F* criterion. The F* criterion represents a length of continuous roll expansion in the tubesheet such that tube pullout would not occur during either normal operation or postulated accident condition loadings. The length is measured from the bottom of the transition between the roll expansion and the unexpanded tube.
Discussion As neither tube burst or collapse can occur within the tubesheet region, steam generator tube integrity has been assessed on the basis of both tube pullout (the equivalent of a steam generator tube rupture) and primary to secondary leakage considerations.
In order to evaluate the F* criterion concept for indications within the tubesheet, it was postulated that a circumferential severance of a tube could occur, which is contrary to existing plant experience. Based on plant operation and laboratory experience, Westinghouse has determined cracking in
-4 this region of the tube, the configuration of any cracks, should they occur, is axial.
For axial or nearly axial indications in the tubesheet region, the tube end remains structurally intact minimizing any potential for-tube pullout. Implicit in assuming a circumferential severance to occur in the development of the F* criterion is the consideration.that degradation of any extent or orientation within the tubesheet below the distance F* is demonstrated to be acceptable during normal and postulated accident conditions.
An evaluation consisting of analysis and testing programs aimed at qualifying and verifying the residual radial preload of Westinghouse steam generator tubes hardrolled into the tubesheet has been conducted to determine the length of sound hardroll engagement necessary to resist tube pullout forces during normal and faulted condition loadings. The required engagement length of sound roll, F* distance, to preclude tube pullout under SONGS 1 normal operation loading conditions was calculated by Westinghouse, with a safety factor of 3.0, to be 0.75 inches. The calculation consisted of a verification, by Westinghouse, of the applicability of the Westinghouse CY calculations.to SONGS 1. This engagement distance is measured from the bottom of the tube roll transition between the roll expansion and the unexpanded portion of the tube and does not include an allowance for eddy current measurement uncertainty. The required engagement length of roll expansion to preclude tube pullout under postulated accident conditions was also calculated and, consistent with that calculated for CY, is determined to have a calculated safety factor of 1.428 or more. These conclusions are consistent with the NRC staff findings for CY as documented in NRC letters dated July 30, 1986 and September 27, 1987. A detailed comparison of the differences between SONGS 1 and CY, and the effects of these differences on the F* calculations, is provided as Attachment 4.
In the event of a circumferential break below the tube roll expansion, the stress levels in the tube above this elevation would not be any larger than for a tube without a break. Thus the stress levels would not be in excess of allowable limits for normal and postulated accident conditions. Based on the determination of required engagement length and acceptable stress levels, the probability of the steam generator tube rupture accident, previously evaluated, is not increased by the use of the modified plugging limit. Also use of the modified tube plugging limit could not result in the consequences of a hypothetical steam generator tube rupture accident more severe than the previously analyzed case of a double ended guillotine break of the tube.
The engagement length determination method was derived from preload testing and was verified as conservative by both tube pullout and hydraulic proof (pressure) testing, performed for CY. Specifically, the F* criterion was calculated from a derived preload force and a conservative static coefficient of friction for tube to tubesheet contact. Both the tube pullout and hydraulic proof testing conducted on rolled joints provided support for the derived preload force. Also, in assessing the F* criterion for CY, it was verified that the radial preload resulting from the roll is sufficient to significantly restrict leakage during normal operating and postulated accident condition loadings.
-5 Use of the F* criterion does not change the operating conditions or function of the steam generator or any other component in the plant. The tube rupture accident bounds any condition related to tube integrity or degradation.
Therefore, the use of the modified tube plugging limit does not provide a mechanism for any accident not previously analyzed.
The factors of safety used in the development of the F* criterion outlined above are consistent with the safety factors found in Section III of the ASME Boiler and Pressure Vessel Code and the bases for plugging degraded steam generator tubes found in NRC Regulatory Guide 1.121.
Thus, the use of the F*
criterion does not involve a reduction in the design margin of safety.
Conclusions On the basis of this discussion, it is determined that tubes with any tube degradation within the tubesheet region below the F* pullout criterion (the uppermost 0.75 inches of the tube roll expansion) can be left in service. The specified distance does not include an allowance for eddy current uncertainty in locating the elevation of an indication. Tubes with degradation located a distance of less than 0.75 inches below the bottom of the transition between the roll expansion and the unexpanded portion of the tube should be removed from service by plugging in accordance with plant technical specification requirements.
The criteria defined in this evaluation have been demonstrated to result in tube integrity considerations commensurate with Reg. Guide 1.121 criteria both analytically (tube preload) and empirically (tube pullout and proof testing and leakage testing).
The margin of safety which, in part, is provided by the safety factors included in the Section III of the ASME Code and NRC Regulatory Guide 1.121 is not reduced.
EXISTING TECHNICAL SPECIFICATION See Attachment 1 PROPOSED TECHNICAL SPECIFICATION See Attachment 2 SIGNIFICANT HAZARDS CONSIDERATION ANALYSIS As required by 10 CFR 50.91(a)(1), this analysis is provided to demonstrate that a proposed license amendment to implement a modified tube plugging limit for the San Onofre Nuclear Generating Station Unit 1 (SONGS 1) steam generators represents a no significant hazards consideration. In accordance with the three factor test of 10 CFR 50.92(c), implementation of the proposed license amendment was analyzed using the following standards and found not to:
- 1) involve a significant increase in the probability or consequences for an accident previously evaluated; or 2) create the possibility of a new or different kind of accident from any accident previously evaluated; or 3) involve a significant reduction in a margin of safety.
-6 This change is proposed due to eddy current indications of tube degradation in the mechanical roll expanded portion of the tubes within the tubesheet in the steam generators at SONGS 1. These steam generators were fabricated with a partial depth mechanical roll at the bottom of the tube. It has been determined through interpretation of eddy current examinations that the tube degradation occurring in the SONGS 1 steam generators is of the type associated with primary water stress corrosion cracking (PWSCC). It can be shown that tube plugging or repair is not required in many cases to maintain tube bundle integrity. Using the existing Technical Specification tube plugging limit, many of the tubes with these indications would have to be removed from service. The proposed amendment would preclude occupational radiation exposure that would otherwise be incurred by plant workers involved in tube plugging operations. The proposed amendment would minimize the loss of margin in the reactor coolant flow through the steam generator in LOCA analyses. The proposed amendment would avoid loss of margin in reactor coolant system flow and therefore assist in assuring that minimum flow rates are maintained in excess of that required for operation at full power.
Reduction in the amount of tube plugging required can reduce the length of plant outages and reduce the time that the steam generator is open to the containment environment during an outage.
The proposed change addresses the action required when degradation has been detected in the mechanically expanded hardroll portion of steam generator tubes within the steam generator tubesheet. Existing tube repair or plugging criteria, i.e., current applications of NRC Regulatory Guide 1.121, do not take into account the effect of the tubesheet on the external surface of the tube. The presence of the tubesheet will enhance the integrity of degraded tubes in that region by precluding tube deformation beyond the expanded outside diameter. Additionally, a portion of the expansion at the top end of the roll expansion is sufficient to preclude pullout of the tube during normal operation and postulated accident condition loadings, if a tube were postulated to sever circumferentially during plant operations, in the portion of the tube-covered by the proposed amendment. Finally, the tube roll expansion of the tube into the tubesheet provides a barrier to significant leakage for through wall cracking of the tube in the expanded region.
The proposed change designates a portion of the tube for which tube degradation does not necessitate remedial action, except as dictated for compliance with tube leakage limits as set forth in the SONGS 1 Technical Specifications. As noted above, the area subject to this change is in the mechanically expanded portion of the tube within the tubesheet of the steam generators. The length of tube roll expansion required to resist pullout for all postulated conditions has been determined to be 0.75 inches. For the purposes of the evaluation of this distance, the unexpanded portion of the tube was assumed not to provide resistance to pullout or leakage. Since the uppermost.75 inches of tube roll expansion is sufficient to preclude pullout of the tube, use of the F* criterion does not depend on any determination of the condition of tube degradation in the portion of the tube below the uppermost one inch of tube roll expansion.
-7 The proposed amendment would modify Technical Specifications 4.16, "INSERVICE INSPECTION OF STEAM GENERATOR TUBING," which provides tube inspection requirements and acceptance criteria to determine the level of degradation for which the tube may remain in service. The proposed amendment would add definitions required for the modified tube plugging limit, prescribe the portion of the tube subject to the criteria and require reporting of the results of inspections of those tubes in service, with defects below the prescribed portion of the tube. The proposed Technical Specification defines the portion to be the uppermost one inch of a tube roll expansion versus the 0.75 inches justified above, as an eddy current measurement error of 0.25 inches is included in the proposed specifications.
As stated in Attachment 3 to this document, the eddy current test techniques, probe and contractor are the same as qualified for use at CY to implement a similar criterion for their Westinghouse Series 27 steam generators. As such, the basis for determining the eddy current measurement uncertainty for ascertaining the vertical location of any PWSCC cracks in the steam generator tube is identical.
Therefore, it is concluded that the use of + 0.1 inches is an accuracy demonstrated by both engineering and laboratory analyses.
Notwithstanding this conclusion, the proposed SONGS 1 Technical Specification includes an allowance of 0.25 inches for eddy current measurement uncertainty for a total distance of one inch to be applied in the field when determining the acceptability of the location an eddy current testing identification of PWSCC in the region of the tube roll expansion.
Analysis Conformance of the proposed amendments to the standards for a determination of no significant hazard as defined in 10 CFR 50.92 (three factor test) is shown in the following:
- 1. Will operation of the facility in accordance with this proposed change involve a significant increase in the probability or consequences of an accident previously evaluated?
RESPONSE: NO The supporting background information above regarding the subject criteria demonstrates that the presence of the tubesheet will enhance the tube integrity in the region of the hardroll by precluding tube deformation beyond its initial expanded outside diameter. The resistance to both tube rupture and tube collapse is strengthened by the presence of the tubesheet in that region. The result of the hardroll of the tube into the tubesheet is an interference fit between the tube and the tubesheet. Tube rupture cannot occur because the contact between the tube and tubesheet does not permit sufficient movement of tube material. In a similar manner, the tubesheet does not permit sufficient movement of tube material to permit collapse of the tube during postulated accident loadings.
-8 Additionally, through analysis and testing performed for CY, by Westinghouse, the steam generator vendor, it has been demonstrated that one inch of sound tube roll expansion is sufficient to preclude pullout of the tube from the tubesheet. Even with the conservative assumption that a tube could completely sever circumferentially below the uppermost one inch of tube roll expansion, test results demonstrate that pullout of the tube is precluded under normal and postulated accident condition loadings. This assumption is conservative, as PWSCC observed in this region of operating steam generators has been typified as short and axially oriented.
Relative to expected leakage, one inch of sound tube roll expansion is sufficient to preclude significant leakage from tube degradation located below this region. The existing Technical Specification leakage rate requirements and accident analysis assumptions remain unchanged in the unlikely event significant leakage from this region does occur. Any leakage out of the tube from within the tubesheet at any elevation in the tubesheet is fully bounded by the existing steam generator tube rupture analysis for SONGS 1. The proposed modified tube plugging limit does not adversely impact any other previously evaluated design basis accident. Therefore, it is concluded that operation of the facility in accordance with this proposed change will not involve a significant increase in the probability or consequences of an accident previously evaluated.
- 2. Will operation of the facility in accordance with this proposed change create the possibility of a new or different kind of accident from any accident previously evaluated?
RESPONSE: NO Use of the criteria does not provide a mechanism to result in an accident outside of the region of the tubesheet expansion. Any hypothetical accident as a result of any tube degradation in the expanded portion of the tube would be the same as the existing steam generator tube rupture accident analysis. Therefore, it is concluded that operation of the facility in accordance with this proposed change does not create the possibility of a new or different kind of accident from any accident previously evaluated.
- 3. Will operation of the facility in accordance with this proposed change involve a significant reduction in a margin of safety?
RESPONSE: NO The use of the modified tube plugging limit has been demonstrated to maintain the integrity of the tube bundle commensurate with the requirements of NRC Regulatory Guide 1.121 for indications in the steam generator tubes and the primary to secondary pressure boundary under normal and postulated accident conditions. Acceptable tube
-9 degradation is any degradation in the tube more than one inch below the bottom of the transition between the roll expansion and unexpanded portion of the tube. The safety factors used in the determination of the required tube roll expansion are consistent with the safety factors in Section III of ASME Boiler and Pressure Vessel Code used in steam generator design. This distance has been verified by testing to be greater than the length of roll expansion required to preclude significant leakage during normal and postulated accident conditions.
Implementation of the modified tube plugging limit will decrease the number of tubes which must be taken out of service with tube plugs.
Plugs reduce the RCS flow margin, thus implementation of the modified plugging criteria will maintain the margin of flow that would otherwise be reduced in the event of increased plugging.
Based on the above, it is concluded that the proposed change does not result in a significant reduction in a margin with respect to plant safety or the bases of the plant technical specifications.
Therefore, it is concluded that operation of the facility in accordance with this proposed change does not involve a significant reduction in a margin of safety.
SAFETY AND SIGNIFICANT HAZARDS CONSIDERATION DETERMINATION Based on the preceding analysis, it is concluded that operation of San Onofre Nuclear Generating Station, Unit 1 in accordance with Proposed Change No. 182, does not involve a significant hazards consideration as defined by 10 CFR 50.92; and (2) there is reasonable assurance that the health and safety of the public will not be endangered by the proposed change. - Existing Specifications -
Proposed Specifications - SONGS 1 Eddy Current Test Program for Tubesheet Roll Region Flaw Identification - Comparison of SONGS 1 and Connecticut Yankee (CY)
Steam Generator Operating Conditions and Effect on F*
LAB:9360F