ML20216B838
| ML20216B838 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 04/14/1998 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20216B834 | List: |
| References | |
| NUDOCS 9805180467 | |
| Download: ML20216B838 (8) | |
Text
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etaugk UNITED STATES gy y
g NUCLEAR REGULATORY COMMISSION
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4' WA8HINGToN, D.C. 20666 4 001
- ,+,o SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT N0. 220 TO FACILITY OPERATING LICENSE NO. NPF-3
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TOLED0 EDIS0N COMPANY CENTERIOR SERVICE COMPANY AND THE CLEVELAND ELECTRIC ILLUMINATING COMPANY DAVIS-BESSE NVCLEAR POWER STATION. UNIT N0. 1 DOCKET N0. 50-346 l.
INTRODUCTION By letter dated February 26, 1998, as supplemented by letter dated March 20, 1998, the Toledo Edison Company, Centerior Service Company, and the Cleveland
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Electric Illuminating Company (the licensees) submitted a request for changes to the Davis-Besse Nuclear Power Station, Unit No.1, Technical Specifications (TSs).
The proposed amendment would revise TS Section 3/4.4.5, " Reactor Coolant System - Steam Generators," TS Section 3/4.4.6.2, " Reactor Coolant System - Operational Leakage," and the associated bases.
The purpose of the amendment is to implement alternate repair criteria in the TSs for steam generator tubes that have degraded roll joints inside of the upper tubesheet.
The alternate repair criteria would allow new roll joints to be installed below the degraded roll joints in the upper tubesheet.
The alternate repair criteria were based on a qualification program documented in Framatome (formerly Babcock and Wilcox) Topical Report, BAW-2303P (proprietary), "0TSG Repair Roll Qualification Report," Revision 3. which was a part of the submittal.
This topical report was approved by the NRC staff in the safety evaluation dated November 21, 1997, for Oconee Nuclear Station, Voits 1, 2 and 3.
DBNPS has two model 177FA Once-Through Steam Generators (OTSGs) that were l
manufactured by Babcock and Wilcox. The OTSG tubes were fabricated from Inconel Alloy 600 material and were restrained by roll expansion joints in the upper and lower tubesheets.
The original tube-to-tubesheet rolls were expanded by a hard roll process and are about 1-2 inches in axial length extended into the tuoesheet.
The tubesheet is about 24 inches thick and a tube seal weld is provided at the primary face of the tubesheet to prevent leakage from the primary to secondary systems.
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9805180467 980414 PDR ADOCK 05000346 P
. 2.
BACKGROUND General Design Criterion (GDC) 14 of Appendix A to 10 CFR Part 50 requires
-that the reactor coolant pressure boundary be designed, fabricated, erected, and tested so as to have an extremely low probability of abnormal leakage, of-rapidly propagating failure, and of gross rupture. A significant portion of the reactor coolant pressure boundary is maintained by steam generator tubes which have experienced various levels of degradation.
Regulatory Guide (RG) 1.121 provides guidance on an acceptable method for establishing the limiting safe conditions of tube degradation.
In addition, the plant TSs require periodic inspections of steam generator tubes.
TSs also require.those tubes with defects,in excess of the repair limits (for example, 40 percent through-wall) be repaired or. removed from service.
The joint between the tube and tubesheet is an interference fit constructed by roll expanding the tube into the bore of the tubesheet, followed by a seal weld at the primary face of the tubesheet. The tube-to-tubesheet roll joint provides sufficient strength to maintain adequate structural and pressure boundary integrity.
The industry experience has shown that defects have developed in the tube-to-tubesheet roll jointe as a result of various degradation processes.
In general, tubes with <egraded roll joints are either plugged or repaired by sleeving. However, the NRC has accepted alternate repair criteria allowing repaired tubes with degraded roll joints to remain in service provided that the repaired tubes can maintain adequate structural and leakage integrity under loadings from normal operation, anticipated operational occurrence, and postulated accident conditions.
RG 1.121 recommends that the margin of safety against tube rupture under normal operating conditions should be equal to or greater than 3 at any tube location where defects have been detected.
For postulated accidents, RG 1.121 recommends that the margin of safety against tube rupture be consistent with the margin of safety determined-by the stress limits specified in NB-3225 of Section III of the Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers (ASME).
Normal structural loads imposed on the tube-to-tubesheet roll primarily are derived from the differential pressure between the primary and secondary sides of the tubes.
Loadings from a postulated main steem line break event can be significant.
In addition, cyclic loading from transients (for example, startup or shutdown) should also be considered in the J
qualification of the roll joints.
1 3.
EVALUATION 3.1 Qualification Program On the basis of the qualification program, the licensees established that a i
1-inch roll length for the new roll joints would carrf all structural loads and minimtze potential leakage. The qualification program consisted of (1) preparing the mockup, (2) establishing tube loads for the qualification tests, and (3) performing verification tests and analyses.
. The mockup consisted of_ a perforated metal block inserted with eight steam generator tubes.that simulates the tube-to-tubesheet configuration in the field.: The tubes were expanded into the mockup tubesheet using an expanding tool that had the same critical dimensions as the tool used in the field.
To determine the strength of the roll joints, the licensees applied loads to the sample tubes to simulate or exceed normal, thermal and pressure cyclug transient, and postulated accident conditions.
In accordance with RG 1.121, the test pressure applied to the sample tubes exceeded 3 times normal operating pressure and 1.43 times main steam line break pressure.
To obtain conserutive leakage results, the sample tubes were severed 360 degrees
~through the tube wall in the roll joints.
In the qualification program, the licensees also considered the impact of tubesheet bowing on the roll joints because the tubesheet bore diameter can change during certain operating conditions.
The combined effects of primary to secondary pressure differential and thermal loads may cause the tubesheet to bow in' one direction or the other which can lead to tubesheet bore dilation or shrinkage. When the tubesheet bore is dilated, the contact stress between the roll joint and the tubesheet would decrease, reducir.g the pullout-resistance of the roll joint.
Considering the bowing effect, the Topical Report, BAW-2303, Revision 3, specified an exclusion zone in the tubesheet where the reroll joint would not be' installed.
3.2 Structural and Leakage Integrity Based on the results of the qualification testing, the licensees determined a roll length _of 1 inch is necessary to ensure adequate margins of structural and leakage integrity. With regard to the structural. integrity, the licensees demonstrated by their ultimate load testing (testing to simulate accident conditions) that the tube with the new roll would not be pulled out from the upper tubesheet under the worst possible combination of loadings. Also, no motion of the tubes relative to the simulated tubesheet were observed during the thermal and fatigue cycling tests.
With regard to the leakage integrity, the qualification tests showed that if each of the tubes (about 15,500 tubes) in a steam generator was rerolled in J
the upper tubesheet and had a 100 percent through-wall flaw in the reroll, the total leakage from all flaws would be minimal. As a defense-in-depth measure, the licensees proposed to implement-a primary-to-secondary leakage limit of 150 gallons per day (gpd) per steam generhtor in the plant TSs.
The 150 gpd requirement is more limiting and conservative than the current TS'11mit of 1440 gpd and is consistent with the staff's position regarding alternate tube repair criteria.
3.3 Field Installation and Inspection The licensees propose to apply a single repair method to install one roll
-(teroll) in the tubes that have degradation in the original roll region.
The
' repaired roll is typically installed using a manipulator and a tool head, monitored by a control system that tracks the position and monitors the torque of the roll expander.
The roll expander'is 1-inch long but the actual roll t
. will have a 1/4-inch taper on each end.
The torque is automatically controlled during the rerolling and is recalibrated after installation of a certain number of rerolls to ensure the minimum torque is maintained to produce proper fit.
After the installation, as provided in revised TS 4.4.5.4 and TS 4.4.5.9, the licensees will inspect all rerolls using eddy current techniques to ensure proper diametral expansion and that the reroll regions are free of degradation. Any reroll not satisfying the acceptance criteria will be either plugged or repaired with a method other than rerolling.
For future inservice inspections, the licensees will inspect all rerolled tubes during steam generator inspection activities.
If degradation is found in the reroll region, the affected tube will be plugged or repaired by means other than rerolling because only one reroll per tube is allowed by the proposed amendment (the licensees did not propose a method for multiple rerolls on the same tube in this amendment).
3.4 Proposed Technical Specification Changes Surveillance Requirement (SR) 4.4.5.2.a will be revised by removing the parenthetical statement:
(subsequent to the baseline inspection)
SR 4.4.5.3.a will be revised by removing the first sentence which states:
The baseline inspection shall be performed to coincide with the first scheduled refueling outage but no later than April 30, 1980.
The above statements in SR 4.4.5.2.a and SR 4.4.5.3.a are no longer necessary because the baseline inspection has been performed and continued reference is no longer necessary.
Since these changes are administrative they are acceptable.
SR 4.4.5.2.a.l. will be revised as shown:
All tubes or tube sleeves that previously had detectable wall penetrations (>20%) that have not been plugged or repaired by repair roll or sleeving in the affected area.
(Tubes repaired by sleeving or repair roll remain available for random selection.)
Tae proposed revision to this SR adds " repair roll" as a type of repaired tube that is not required to be included in the tubes selected for the first sample of a steam generator inservice inspection.
This revision also adds the statement that tubes with a repair roll will still remain available for inspection on a random basis.' The required inspection for repair roll repairs
.(repair roll region) is addressed in a proposed new SR 4.4.5.9.
The new reference to " repair roll" is acceptable based on the evaluation presented below.
The parenthetical statement on availability of repaired tubes for random selection is acceptable because it is conservative and provides a restriction consistent with current staff guidance.
. The licensees propose to revise SR 4.4.5.3.c.1 to add the following paragraph:
If the leak is determined to be from a repair roll joint, rather than selecting a random sample, inspect 100% of the repair roll joints in the affected steam generator.
If the results of this inspection fall into the C-3 category, perform additional inspections of the new roll areas
'in the unaffected steam generator.
This proposed revision provides guidance for escalating the inspection scope when the inspection is required by SR 4.4.5.3.c.1 and repair roll joints are identified as the cause of the primary-to-secondary leak.
Since this revision is consistent with current staff positions on additional SG inspections, it is acceptable.
The proposed revisions of SRs 4.4.5.4.a.4, 4.4.5.4.a.6, and 4.4.5.4.a.7 are to include the use of " repair roll" as a repair method.
These are administrative changes consistent with the addition of " repair roll" as an acceptable technique, and are therefore acceptable, i
The following paragraph is proposed to be added to SR 4.4.5.4.a.7.
i The repair roll process will only be used to repair tubes with defects in the upper tubesheet area.
The repair roll process will be performed only once per steam generator tube using a 1 inch (sic] reroll length.
The new roll area must be free of degradation in order for the repair to be considered acceptable.
The repair roll process used is described in the Topical Report BAW-2303P, Revision 3.
j This proposed modification defines the applicable usage of the reroll repair process.
The reroll repair is limited to the upper tubesheet and to one reroll repair per steam generator tube.
The specification also provides a clear definition of length of the reroll and acceptance criteria of rerolled tubes.
Additionally, this change identifies the previously NRC-approved.
Topical Report BAW-2303P, Revision 3, as the topical report that describes the repair roll process to be used.
The staff finds this modification acceptable as the process described in the topical report will provide reasonable assurance of steam generator tube integrity.
SR 4.4.5.4.a.9 will be revised to add the following sentence:
The previously existing tube and tube roll, above the new roll area in the upper tube sheet, can be excluded from future periodic inspection requirements because it is no longer part of the pressure boundary once the repair roll is installed.
This requirement clarifies the scope of tube inspection.
Since the reroll regions are the new pressure boundary, the original roll areas need not be inspected. Therefore, the staff finds this specification acceptable, l
- -SR 4.4.5.4.b will be revised as follows:
~ The steam generator shall' be determined OPERABLE after completing the l
corresponding actions (plug or repair by ' repair roll-or sleeving in the j
affected areas all tubes exceeding the repair limit and all tubes containing through-wall cracks)- required by Table 4.4-2.
L This proposed modification adds " repair roll" as a repair method to repair by plugging and sleeving when declaring the SGs operable.
Since this safety evaluation finds acceptable the repair by rolling under the specified j
conditions, the above revision to SR 4.4.5.4.b is acceptable.-
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SR 4.4.5.5.b will be revised to include " repair roll" as a' repair method, similar to plugging or. sleeving, to be reported to the NRC.
Since this safety j
evaluation finds acceptable the repair by rolling under the specified i
conditions, this revision is acceptable.
Proposed SR 4.4.5.9 will state the following:
When steam generator tube inspection is performed as per Section '4.4.5.2, an additional but totally separ:n.e inspection shall be
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performed on special interest tubes that have been repaired by the repair roll process. This inspection shall be performed on 100% of the
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tubes that have been repaired by the repair roll process. The inspection shall be limited to the repair roll joint and the roll transitions 'of the repair roll.
Defective or degraded tubes found in the repair roll region as a result of the inspection need not be included in determining the Inspection Results Category for the general steam generator inspection.
This surveillance requirement clarifies the licensees' intent that all reroll 1
regions of the repaired tubes will be inspected during each scheduled inservice SG inspection. The staff finds this requirement acceptable because
.it provides a comprehensive monitoring of potential degradation in the rerolled regions of the repaired tubes.
Table 4.4-2, " Steam Generator Tube Inspection," will be revised to include
" repair rolling" as a repair method.
Since this safety evaluation finds acceptable the repair by rolling under the specified conditions, this revision is acceptable.
Limiting Condition of Operation (LCO) 3.4.6.2.c, " Operational Leakage," will be revised to state:
150 GPD primary-to-secondary leakage through the tubes of any one steam generator, This. change reduces the present limit of 1440 gpd total primary-to-secondary leakage through the steam generators to 150 gpd primary-to-secondary leakage through the tubes of any one steam generator. The proposed leakage limit of 150 gpd for one steam generator is more limiting and conservative than the current limit.
Therefore, this change is acceptable.
4 6 SR 4.4.6.2.1.e will be added to state:
An evaluation of secondary water radiochemistry for determination of primary to ' secondary leakage through the steam generators at least once per 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> during steady state operations.
This new SR will reflect.a more accurate method for detecting steam generator primary-to-secondary leakage in a<:cordance with the more restrictive LC0 limit. Therefore, it is acceptable.
Bases Section 3/4.4.5, " Steam Generators," will be revised to include references to the repair roll method of repair and to a leakage limitation of 150 gpd through any one steam generator, instead of I gallon per minute (1440
,gpd) total leakage through the steam generators.
The revision will provide reference'to the repair roll method of repair and to the steam generator primary-to-secondary leakage limit of 150 gpd through any one steam generator.
These revisions are consistent with the proposed TS changes, and are therefore l
acceptable, j
Three paragraphs will be added to Bases section 3/4.4.5, to provide a description of he repair roll process.
The descriptions.are consistent with the TS change' ad provide clarifying information, and are therefore acceptable.
Bases Section 3/4.4.6.2, " Operational Leakage," will be revised to include a reference to the new leakage limitation of 150 gpd through any one steam generator instead of 1 gallon per minute (1440 gpd) total primary-to-secondary
- leakage limit. Also, the phrase " consistent with the assumptions" used in the third paragraph will be removed.
The revised paragraph will state that although the leakage limit is 150 gpd primary-to-secondary leakage through the tubes of any one steam generator, the accident analysis assumed 1 gallon per-minute.(1440 gpd).
The revised sentence will read "A 1 GPM total primary to secondary leakage limit is used in the analysis of these accidents." These
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revisions are consistent with the approved changes above, and are therefore i
acceptable.
4.
SUMMARY
The licensees have proposed to implement an alternate repair method using reroll to repair tubes having indications in the original' roll regions of the upper tubesheet.
The technical basis for the proposed reroll method is documented in Topical Report, BAW-2303P., Revision 3.
The staff has deterniined that (1) the licensees' alternate repair criteria using reroll were established on the be. sis of the qualification' tests that used specimens simulating the actual tube-to-tubesheet joint configuration of the steam generators; (2) the loads for structural and leakage tests were specified and applied in accordance with RG 1.121; and (3) the proposed changes to plant TSs satisfied the regulatory requirements and technical basis.
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. On.the basis of the submitted information, the staff concludes that the proposed l reroll repair for degraded roll joints in the steam generators at Davis-Besse is acceptable because the licensees have demonstrated through an acceptable qualification program that the reroll satisfies GDC 14 of
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Appendix A to 10 CFR Part 50 and RG 1.121.
4 5.: STATE CONSULTATION q
l In accordance with the Commission's regulations, the Ohio State official was notified of the proposed issuance of the amendment. The State official had no comments.
6.
ENVIRONMENTAL CONSIDERATION This amendment changes a requirement with respect to installation or use of a l
facility component located within the restricted area as defined in 10 CFR l
'Part 20 or changes a surveillance requirement.
The staff has determined that
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the amendment involves no significant increase in the amounts, and no significant change in the types, of any effluent 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 findix that the amendment involves no significant hazards consideration,
- id there has been no public comment on such finding (63 FR 11460).
The supplemental information submitted-by the licensees did not affect the proposed findings. 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.
7.
CONCLUSION The staff 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 this amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor:
A. Keim Date: April 14, 1998 i
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