ML17340A656
| ML17340A656 | |
| Person / Time | |
|---|---|
| Site: | Turkey Point  |
| Issue date: | 01/15/1981 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML17340A653 | List: |
| References | |
| NUDOCS 8101300495 | |
| Download: ML17340A656 (12) | |
Text
UNITEDSTATES NUCLEAR REGULATORY COMMISSION
~y,5 REoo P
~4 0
~~i n0 i; C WASHINGTON, D. C. 20666 I g, SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 54 TO FACILITY OPERATING LICENSE NO.
DPR-41 FLORIDA POMER AND LIGHT COMPANY TURKEY POINT PLANT UNIT NO.
4 DOCKET NO. 50-251 INTRODUCTION By letter dated December 18, 1980 Florida Power and Light Company (the licensee) submitted the results of the most recent steam generator inspection at Turkey Point Unit 4 and requested authorization to operate Unit 4 for six equivalent months from the time of the inspection.
Equivalent operation is defined as operation with the reactor coolant greater than 350'F.
The implemented steam generator inspection and preventive plugging program are similar to those performed previously at this and other similarly degraded units and have been determined adequate by the NRC to support six (6) equivalent months of operation, subject to the licensee submittal for staff review, information concerning the tube wastage predicted to occur during the later half of the operating period which begins January 13, 1981 and extends for six equivalent months of operation.
This information is to be supplied by Fedrauary 28, 1981.
DISCUSSION The steam generator tube inspection performed at Turkey Point Unit 4 during November 1980 included programs to assess tube degradation associated with both the denting and wastage phenomena.
For denting, tube gauging was performed in all three steam generators using 0.650, 0.610, and 0.5 inches (diameter) eddy current probes.
The implemented gauging program was similar to those implemented previously at this and other similarly degraded units and included the gauging of all unplugged tubes within areas (tubelane, periphery,
- wedge, and patch plate regions of the hot leg, and tubelane region of the cold leg) where significant denting activity had been observed previously.
Significant denting, in this context, is considered to include (in addition to leakers) tubes restricting passage of a.610 inch probe (or less) and tubes at the periphery of the hot leg wedge location and on either side of the patch plate boundary which restrict passage.
of a.650 inch probe (or less),
since these tubes are the most likely candidates to develop inservice leaks.
In previous inspections of the tubelane region, finite element analysis had been used to determine the progression of significant tube restr iction activity for purposes of defining the boundary for the tubelane gaugi'ng inspection.
However, the 17.5% tube hoop strain contour which realistically bounded the significant tube restriction activity in the tubelane following the previous inspection is now predicted to cover most of the support plate.
Thus, the licensee elected to gauge the tubelane tubes within a boundary incorporating regions of previously observed activity (i.e. tubes restricting passage of 0.650 inch probes),
plus several rows of tubes beyond.
sxoxooo $ 41 g
1
Mith regard to the defined regions (discussed above) within which all tubes were gauged, if a restricted tube (tube restricting a 0.650'inch;probe) was found close to the inspection Boundary, the inspection was expanded in that area.
In addition, a sample population of tubes in the central bundle region, located outside these defined regions, was tested with a 0.700 inch. probe in the hot and cold legs, respectively, as part of the Regulatory Guide 1.83 eddy current inspection (to be discussed).
These latter inspections provide an early indication of any new deformation which exists away from the regions usually regarded as active (i.e. the tubelane, patch plate, edges and periphery).
Tubes restricted in previous sample inspections, but not adjacent to the areas of predominant activity were also incorporated into the inspection.
Measurements of the visible support plate flow slots in all steam generators were made to assess the condition of the support plates and to provide a gross measure of the continuation of denti'ng.
Eddy current inspection for wastage was conducted in accordance with Regulatory Guide 1.83 in all of the steam generators.
The U-bends of the unplugged tubes in rows three through five in steam generator C were also eddy current inspected.
The following tabulation summarizes the number of tubes included in the gauging and eddy current inspections:
A~BC L A~did L
~BB B L
~BC id L
~CB B L
~C'C id L Gauging 1184 U-Bend Rows 2-5 154 1159 117 1324
'] 61 79 R.
G. 1.83 397 INSPECTION RESULTS 317 383 527 268 461 The results of the gauging inspection in terms of the number of tubes restricting passage of a given size probe of 0.65 inch or less are summarized below:
SG A
~60
'610"
.540" Tubelane Hot Le Cold Le 29.
7 0
Periphery K Medge Hot Le Cold Le
~ 25 3
1 Patch Plate Hot Le SG B
.ERP'610"
.540" 32 4
0 0.
3 0
42.
8 0
3 0
0 15 5
1 The eddy current inspection performed in accordance with Regulatory Guide 1.83 identified 25 tubes that required plugging due to thinning indications.
The following summarizes the results of the Regulatory Guide 1.83 inspection:
I Size of Indication X Mall Penetration SG A SG C
.E5P 34 0
0 13
. 610" 5
1 3
1
.540" 1
0 0
'0 Tubes in the tubelane region that restrict a 0.650 inch probe or less are located adjacent to the areas fn which such restrictions have been observed during previous inspections.
In general, tube restriction activity observed in the periphery,
- wedge, and patch. plate areas appear s consistent with pr e-vious experience at this and other similarly degraded units.
In Steam Gener-ator B, four tube restrictions occurred away from the pattern of prior results.
Two tubes, one restricts a 0'.610 inch probe and the other restricts a 0.650 inch probe, were located adjacent to a tube which had been pulled in Septem-b'er 1975 as part of the earliest field investigations of denting.
The other two tubes, which restrict passage of 0'.650 inch probes, were located adjacent to the patch plate inspection area.
In Steam Generator C, one tube restrict$ ng a 0.650 inch probe was located adjacent to the patch plate inspect4on area.
The level of denting activity in the cold leg remains low compared to the hot leg activity.
Inlet Outlet Inlet Outlet Inlet Outlet
<20 20-29 30-39 40-49 50-59 60-69 54 40 20 1
1 1
76 174 71 0
0 0
50 30 21 0
1 0
125 133
=.
83 13 4
0 132 194 57 2
0 0
It should be noted from the above tables that the results for the cold leg side of Steam Generator A include four (4) indications which were picked up during gauging.
The results of the Regulatory Guide 1.83 inspection for wastage indicate an apparent increase in thinning activity relative to previous inspections, particularly in the cold leg side of steam generator B where 17 pluggable indications were found at the top of the tubesheet elevation.
The licensee reports an average increase tn thinning during the period April 1979 to
- I
November 1980 of 9.9X in steam generator B. and approximately 1% in steam generators A and C.
Data provided by phone on January 6, 1981 indicates that during the period May 1980 to November 1980 thinning increased by an average of 13$ for the 17 tubes in the cold leg of steam generator B which were found to contain pluggable fndf:catfons.
The 1 fcensee. helteyez that whf.le some additional thinning may have occurred since the previous tnspectf:ons, the overall increase is likely to be smaller than that indi'cated by the reported data.
Comparison of the affected tubes fn steam generator B with corresponding signals from previous inspections shows. the presence of an increased denting component.
The dent sfgnal influences the thtnntng signal, caustng slight phase rotation and overesti-mations of the depth of penetration.
Westinghouse is currently reevaluating the eddy current signals from both the November and May 1980 inspections in an attempt to determine a more realistic value of thinning increase during this period.
No eddy, current indications were identified in the U-bends of the unplugged tubes in Rows 3 through 5.
The measurerentz af the'ppqpt plate flog slots fndf;cated no deviations from anticipated conditions.
TUBE PLUGGING PROGRAM The plugging criteria implemented for dented tubes during the November 1980 steam generator inspection are the same as those implemented previously at this and other similarly degraded units.
These criteria include the:plugging of two tubes beyond any 0.540 inch restricted tube found in the tubelane region.
This latter criterion was based in the past upon finite element predictions regarding the progression of tube denting during the next operating interval.
Based upon their experience,
.the licensee believes that the finite element model no longer provides an appropriate basis for tube plugging.
Thus, they have discontinued the practice of updating the analyses with each succeeding inspection.
The criterion for plugging two tubes beyond 0.540 inch restricted tubes, which was previously developed using the finite element
- model, has been retained by the licensee since its application has generally been effective in reducing the frequency of tube leakage events resulting from denting.
The licensee notes that only one 0.540 inch restricted tube in the tubelane region, where this criterion is applicable, was found during this inspection.
Tubes: with greater than 405 through wall'ddy current indications were plugged.
Implementation of the plugging criteria resulted in 24, 37, and 44 tubes being plugged for denting and 3, 18, and 4 tubes befng plugged for wastage in steam generators A, B, and C, respectively.
Total steam generator tube plugging in all three steam generators is approximately 23.8K which i's conservatively bounded by the 25K tube pluggfng assumption ECCS analysis, approved on May 15, 1980.
1'
EVALUATION The November 1980 gauging and preventive plugging program at Turkey Point Unit 4 is similar to previous programs conducted in at this and other similarly degraded units.
This inspection included the gauging of all tubes within areas (tubelane, periphery,
- wedge, and patch plate regions) were significant denting activity, has been observed previously.
In addition, a sample population of tubes in the central bundle region were gauged as part of the Regulatory Guide 1.83 inspection for wastage.
Based upon our review of the gauging results, we find that the observed denting activity is generally consistent with previous experience at this and other similar,ly degraded units.
Although denting in four (4) tubes in Steam Generators B and one D ) tube in Steam Generator C occurred away from the pattern of prior results, we find significant tube restriction activity remains confined to areas immediately adjacent to previous activity.
The implemented gauging program was sufficient to adequately determine the condition of the steam generator from a denting standpoint.
The preventative plugging c~iteria implemented during this and previous inspections have proven successful in removing from service those tubes which are the most likely candidates to develop inservice leaks.
The inspection data and recent operating experience provide adequate justification for the implemented criterion of plugging two tubes beyond 0.540 inch restricted tubes in the tubelane.
We find that the implemented gauging program and preventive plugging criteria provide reasonable assurance that the vast majority of tubes most likely to develop inservice leaks have been identified and removed from service.
No forced shutdowns because of steam generator tube leakage occurred during the 4.75 months of operation since the previous inspection in Hay 1980.
The 0.3 gmp leqk rate limit in the license provides adequate assurance that even if through wall cracks and leaks occur, they will be detected and appropriate corrective action taken before excessive leakage can occur from tube degradation during normal operating, transient, or accident conditions.
With regard to the wastage phenomenon, the November 1980 eddy current inspection (in accordance with Regulatory Guide 1.83) and 40K plugging limit are similar to those implemented in previous inspections.
The results, however, indicate an apparent increase in thinning activity relative to previous inspections, particularly in the cold leg side of steam generator B where 17 pluggable indications were found.
A comparison of the November 1980 eddy current results with the previous May 1980 results indicates an apparent average increase of 13K in wall thinning penetration during this period for the 17 tubes in the cold leg of steam generator B containing the plugging indications.
The licensee believes that the increase in thinning is likely to be less than that indicated by the reported data due to the confound-ing effect of increased denting component on the eddy current signal.
They have not submitted adequate comfirmatory data to support this belief.
In view of data indicating a possibly high rate of wastage degradation in the Turkey Point Unit 4 steam generator, we have performed an eval-uation to determine if the implemented 4OX plugging limit provides ad-equate assurance that unacceptable wastage degradation of the tube walls will not occur during the six months preceeding the next steam generator inspection.
Me have used Regulatory Guide 1.121 (issued, for comment in August 1976) as guidance in evaluating the reported data.
Regulatory Guide 1.121 defines the basis for plugging degraded steam generator tubes.
The Regulatory Guide specifies, in part, that degraded steam gen-erator tubes should meet'he following criteria:.
(1)
A margin of safety against tube failure during postulated, accidents consistant with ASME code requirements (2)
A factor of safety against tube burst of at least 3 under normal operating conditions, and (3)
Loadings during normal operating conditions should not pro-duce a primary membrane stress in excess of the material yield stress at operating temperature.
I In meeting these criteria, the Regulatory Guide states that allowanre is to be made for future degradation.
On the basis of wastage degradation at the average rate of 13% (through wall) during the previous 4.75 equi-valent full power months of operation, we estimate that the above criteria can only be complied with during the first 3.5 months of operation from the time of the November 1980 inspection.
Based on information. supplied by the licensee in telephone conservations, we 'have performed simplified calculations that indicate that acceptable margins against. tube failure during postulated accidents will exist throughout a six month operating period.
- However, we estimate that during normal operating conditions the
'inimum margin to burst would be approximately 2.5 rather than 3 and the material yield strength would be exceeded by approximately 10K at the end of the,six month operating period.
Although compliance with Regulatory Guide Criteria pertaining to normal operating pressure cannot be shown to exist beyond 3.5 months, we believe that the unit can be safely operated for six months for the following rea-sons:
(1)
Margin with respect to t'ube failure under postulated accident conditions provides reasonable assurance against the occurrance of
. multiple tube failures and excessive leakage.
(2)
Hargin of approximately 2.5 with respect ot tube burst exists under normal operating conditions and provides assurance that multi-ple tube ruptures will not occur.
(3)
In degraded tubes where stresses exceed yield under normal operating conditions, the consequences would likely be limited, to small leaks of individual tubes.
The D.3 gpm primary to secondary
I r
leak rate limit in the Plant Licensee provides assurance that 4n the event that a leak occurs'n
- service, appropriate and timely corrective action will be taken.
From a denting standpoint, we find that the gauging inspection results, implemented plugging and existing leak rate limits adequately support six equilvalent months of operation from the time of the November 1980 inspec-tion.
By Febrauary 28, 1981, confirmatory data and analysis which demonstrates that unacceptable wastage degradation of the tube walls will not occur prior, to performing the next steam generator inspection will be submitted."
ENVIRONMENTAL CONSIDERATION We have determined that the amendment does not authorize a change in effluent types or total amounts nor an increase tn power level and will not result in any significant environmental impact.
Having made this deter-mination, we have further concluded that the.amendment involves an action which is insignificant from the standpoint of environmental impact and, pursuant to 10 CFR 651.5(d)(4), that an envi'ronmental impact statement or negative declaration and environmental impact appraisal need not be prepared in connection with the issuance of this amendment.
CONCLUSION We have concluded, based on the considerations discussed above, that:
(1) because the amendment does not involve a significant increase in the probability or consequences of accidents previously considered and does not involve a significant decrease in a safety margin, the amendment does not involve a significant hazards consideration, (2) there is reasonable assurance that the health and safety of the public wtll not be endangered by operation in the proposed
- manner, and (3) such activities will be conducted in com-pliance with the Commission's regulations and the issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public.
Pate:
January 15, 1981
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