ML19098B396

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Submit Attachments, Remedial Program for U-Bend Cracking & Preventive Plugging Actions & Rationale to Address Question of Steam Generator Tube Integrity & Present Basis for Continued Operation of Unit 1
ML19098B396
Person / Time
Site: Surry  Dominion icon.png
Issue date: 01/03/1977
From: Stallings C
Virginia Electric & Power Co (VEPCO)
To: Reid R, Rusche B
Office of Nuclear Reactor Regulation
References
Download: ML19098B396 (41)
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Text

{{#Wiki_filter:* e IRGINIA. ELECTRIC.AND POWER COMPANY RIOHMOND, VIRGINIA 23261 January 3, 1977 Mr. Benard C. Rusche Director of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, D. C. 20555 Attn: Mr. Robert W. Reid, Chief Operating Reactors Branch 4

Dear Mr. Rusche:

Serial No. 260C/092276 PO&M/BRS:clw Docket Nos. 50-280 50-281 License Nos. DPR-32 DPR-37 The purpose of this submittal is to address the question of steam generator tube integrity and to present the basis for continued operation of Surry Unit No. 1. The information contained herein supplements and expands upon discussions presented in our previous submittals of October 19, 1976; October 25, 1976; and November 15, 1976. Three major problems regarding steam generator tubes have occurred in Surry steam generators: (1) tube wall thinning, primarily in the sludge adjacent to the secondary side of the tube sheet, (2) intergranular cracking at tube support plate locations where denting of the tubes is occurring, and (3) intergranular cracking at the apex of the U-bends in Row 1 only. Results of the eddy current test pro-gram per Regulatory Guide 1.83 indicate that tube wall thinning has essentially been arrested. This type of degradation, adjacent to the tube sheet, is readily detected by established eddy current techniques, and all tubes with wall degrada-tion exceeding the plugging limit have been plugged. Although the causal relationships involved with the cracking which is occur-ring in the dented areas at tube support plate locations and in the apex of the Row 1 U-bends are not fully understood from an analytical point of view, a great deal of knowledge and experience has been gained from field inspections, labora-tory examinations and tests, and analytical studies. This knowledge and experi-ence, when combined with good engineering judgement, permits development of a pre-ventive program to allow the continued safe operation of the Surry Unit No. 1 steam generators. This program consists of preventive tube plugging and the in-stallation of flow slot blocking devices in the top tube support plate of all three steam generators. contains the preventive program which will prevent U-bend tube failure, Attachment 2 contains the preventive program relative to the leaky dent 119

VIRGINIA ELECTRIC AND POWER COMPANY TO Mr. Benard.C. Rusche Page 2 problem and Attachment 3 contains answers to specific NRC questions. We feel the detailed technical data and other information included in the above attach-ments adequately supports our request for approval to operate Surry Unit No. 1 which is scheduled for restart January 8, 1977. Enclosure (40 copies) cc: Mr. Norman C. Moseley Very truly yours, Zi'. )) l 'y:Jl;Ut>;i '1 J,_/ J C. M. Stallings Vice President-Power Supply and Production Operations

t* *(, e . / Remedial Program for U-Bend Cracking As reported in the response to question No~ 8, a total of the 71 U-bends have been removed from Surry 1, Surry 2, and Turkey Point 4 steam generators. Of the 39 R,ow l U-bends examined in the laboratory, 19 were found to have cracks. No cracks were found in any of the 30 Row 2 and.2 Row 3 U-bends which were examined. As stated in previous submittals and in the response to question No. 9, the observations which have been made support the rationale that three factors mus~ be simultaneoµsly present, in sufficient magnitude, to permit initiation of the U-bend cracking. These are:

1. Significant plastic pre-straining of the metal
2. Tensile stresses
3. Dynamic straining in service due to continued flow slot hourglassing.

The fact that cracks have been found only. in the tightest radius, Row 1 U-bends is consistent with the f*act that these U-bends have the highest level of plastic pre-straining of any of the tube rows in the steam generator. Moreover, the longer term good operational experience in other units having equivalent U-bend configurations and stress conditions, but not experiencing flow slot hourglassing leading to dynamic straining of the U-bend material, is evidence that the presence of significant'plastic defo~tion in the tight, Row 1, tr-bends combined with pressure*and.residual stresses are not sufficient in themselves to cause failure. This conclusion is further substantiated by successful long term laboratory tes:ting of U-bend s~mples exposed to reference primarycool~nt. The additional factor believed necessary to initiate and propagate the U-bend cracking i~, the dynamic strain in the U-bend which results from continued flow slot hourglassing. I

As stated previously, results of laborato*ry examination of U-bend samples removed from affected steam generators have demonstrated that the U-bend cracking is confined to Row 1 tubes. A further measure of the susceptability of Row 1 tubes to the U-bend cracking phenomenon as compared to the remaining tube rows is provided by the equivalent strain calculations reported in the response to question No. 1. Although these equivalent strain calculations do.not account for the dynamic straining effects which are believed necessary, they do provide a measure of the total strain in the various U-bends due.to manufacturing and in-service hourglassing (U-bend leg displacement). Based upon the above, the corrective measure applied to the U-bend cracking phenomenon is to plug all of the Row 1 tubes in each steam generator. As an additional conservative_measure an insert of flow slot blocking device has been *designed, fabricated, and in_stalled in each of the flow slots in the top tube support plate of each Surry Unit No*. 1 steam generator. The flow slot insert device (see Figure 5-1) is a one piece assembly made of six blocks welded to a tube which spans the diameter of the tube support plate. The width of each block is custom machined to fit the smallest width of its respective flow slot. The insert assembly is installed through a 3 inch diameter hole in the steam generator shell andwrapper. Hooks on three of the blocks engage the support plate and prevent lifting of the insert assembly by secondary flow forces. A removeable plug in the wrapper maintains engagement of the hooks with the plate. -The length of the flow slot insert device, which is made of 304 SS, is approximately 120 inches. The function served by these blocking devi~es is to prevent further closure I or hourglassing.of these flow slots, thereby arresting the displacement of the legs of the U-bends, and preventing further dynamic straining of th~ tr-bend

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Therefore, installation of these devices will essentially eliminate one of the three factors, specifically the dynamic straining, believed necessary for initiation and propagation of U-bend cracks. A review of the safety related aspects of installing these flow slot inserts has resulted in the following conclusions:

1. The materials used are compatible with the steam generator materials and chemistry.
2. The inserts would not constitute a potential steam break missile hazard.
3. There is no significant effect on nonnal hydraulic perfonnance.
4. There is no effect on LOCA analysis results.
5. Restriction of movement in the flow slot will not result in exceeding stress limits on critical components such as the tubes and shell. (See response to question No. 4).
6. There will be no adverse effect on the operational tubes due to creation of additional "hard spots".

This is discussed below in more detail. The review concluded that there are no known safety concerns related to installation of and operation with these devices. An assessment of the affect of flow slot blocking devices has been accomplished using an elastic-plastic finite element model of a typical 51 Series tube support plate. The model simulates actual in-plane plate behavior when subjected to expansion loads. The plate was expanded through the use of a thermal expansion coefficient, and two analyses were performed. In the first analysis a plate was expanded by an.014 in/in expansion load. The flow deformations of this plate were then fixed to simulate the addition of blocking devices. The plate was then expanded to the equivalent of an.021 in/in expansion load. In the second analysis, the plate was expanded to the equivalent

I of an.021 in/in load without restraint on the flow slot deformations at any time. Strain intensity contour maps were obtained (Figures 5-2 and 5-3) and a comparison of plates with and without flow slot blocking devices was made. It is seen t:hat the flow slot ligament areas suffer the highest magnitude of tube hole ovality without ins~rts. With the installation of blocking devices, concentration of strain intensities are shifted away from the flow slot ligaments, to the peripheral wedge locations, without significantly changing the strain intensity field near the wedges.

Thus, the blocking devices accomplish the purpose of preventing further flow slot hourglassing, while reducing strain intensities near the flow slot ligaments, and'not significantly increasing strain intensities at the peripheral wedge locations.

VISIONS C I I I' I.,. I I. I.,. I. I. I.,. I I I ' I I ' ,-.I. l *l T - -~.. - f:u LL c, 'JO, Iv A /vi E I FLO,>> SLOT INSERT BLOCK 2 FLOW SLOT INS[RT HOOK BLOCK 3 Ft. OU,! SLOT INSERT BRIDGE 4 I" TUBING 5 BULLET(END PIECE) MAT "°" 14 LS MATERIAL 304 s/s ASTM A-276-73 00 304 5/5 ASTM A-240*74 304 s/s SEAMLESS ASTM A-213-75 SAME AS #3 ':,:..,/.1-J, 3 3 I 2 I .*.~ ..* -. ~~-,** 6 FLOW SLOT INSE:RT 0 45+/-2Y,' _L_ SLIP FIT FOR I /"Q D. TUBE J I,a+.oo T*C. -.03 t so~-03 . 1-*00 .62::i~ FLOW SLOT INSERT BRIDGC: SECT. AA TYP. for GENERATORS A f C <f> I I f c= ____ --*--- ___ J

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ATTACHMENT NO. 2 PREVENTIVE PLUGGING ACTIONS AND RATIONALE (LEAKY DENT) In order to accommodate the primary to secondary leakage events which the Surry plants have experienced, a preventive plugging program has been implemented. The basis for this program.is described more fully below, but in part, the expe-riences-of leakage at tube support plate elevations for the Surry and Turkey Point units have* been utilized in its formulation. Figure 1 gives a view of the specific location of leaking tubes for the si~, 51 Series-steam generators of the two Surry units. All the data are superimposed* to this one figure to give the areas of highest involvement that have evolved during the last 15 months of operation. A more explicit listing of the leakage events is given in Table 1 for the Surry units. It is noted for these 51 Series units, all of which are undergoing the dent-ing phenomena, that leakage has occurred primarily along the tube lanes. As oper-ating time accumulates, it is noted that tubes adjacent to the three o'clock and nine o'clock wedge regions are also becoming involved. To provide a more complete basis for the preve~tive plugging rationale, the. data obtained for the 44 Series steam generators of the Turkey Point units is given in Figure 2 and Table 2.

  • These data show the accumulation of leaking-dented tubes, superimposed in one figure for all six of the Turkey Point steam generators.

Like the Surry units, the majority of involved tubes lie adjacent to the tube lanes, . but a significant population also appears at the 1,1 o'clock wedge region. ~. In order, thereferc, to minimize the leakage events a preventive plugging pat-tern hasbeeil evolved to bound the problem areas-denoted by the Surry and Turkey Point data. This part of the pattern addresses the tube lane areas and the wedge regions. e e In addition, the program which was introduced in October 1976, to evaluate the Surry Unit No. 2 U-bend leakage event has been finalized. The data from this program which is described elsewhere in this submittal, shows a potential involve-ment of only the tubes in Row 1. Therefore, the preventive plugging program has been expanded to include all of the Row 1 tubes. The preventive plugging program, implemented at Surry Unit No. 1 is given in Figure 3, for each of the three steam generators. LOGIC FOR PREVENTIVE PLUGGING PATTERN (LEAKY DENT) To arrive at the preventive plugging pattern described in Figure 3, field data on leakage at dented regions of the tubes; support plate crush tests; and finite element analysis have been used. The pattern can be thought of a contain-ing two populations:

1.

Those tubes near the tube lane

2.

Those tubes near wedge locations. For population near the tube lane, field data on location of leaky dents and strain intensity profiles, obtained from finite element analysis* of tube support plates subjected to in-plane loads due to plate expansion, are used as a guide to determine the preventive plugging pattern. Results of the finite element analysis are shown in Figure 4. The leakage at dents in this region all fall in areas of high support pla~e strain intensities. Further, the region of high strain inten-sities does not change significantly with increased plate expansion and therefore the preventive plugging pattern remains unchanged. For population near the wedges, field data of leakage at dents and tube sup-port plate c~ush test results are used for this aspect of the preventive plugging pattern. The results of. the crush tests indicate that the tube population con-sidered for preventative plugging consists of two groups:

1.

Those at the outer perimeter of selected wedge areas e

2.
  • Those1tubes at the inner boundary between the wedge ar~a and that area where the circulation holes exist.

Currently, studies are being made to confirm these wedge area patterns by additional finite element analysis. Because of the accu*racy desired to properly determine wedge area plugging patterns, ear],ier support plate models are being refined. However, preliminary analyses, which already confirm the plugging pat-terns in the tube lane, appear to be converging to a solution which will confirm the peripheral plugging patterns when the analysis is completed. I e TABLE 1

SUMMARY

OF LEAKAGE AT DENTED REGIONS STATION SHUTDOWN DATE STEAM GENERATOR R-C-TSP Surry No. 1 9-26-75 A 2-31-7 A 2-47-6 A 2-63-7 12-10-75 A 2-48~ 3-10-}6 A 3-62-1 AC 3-63-2 4-1-76 C 1 4-27-76 B 2-47-3 7-15-76 A 2-73. B 7 C 3 8-13-76 B 2-45-4 C 3 9-24-76* C 2 10-17-76 B 6 C 1 C 2-42-3 C 12-86-3 C 12-88-3 C 13-88-3 Surry No. 2 1-14-76 C 1-62-. C 2-.48-2-2-76 C 2 3.:..3-76 C 3-63.... 3 4~22-76 A 3 I 9-15-76 C 11 C 11 C 12-,3-Q C 12 A 2 A 5-6 A 12-4-. A 12-5 A 37-76 12-22-76 A 4 A 12-4~- /

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TABLE 2

SUMMARY

OF LEAKAGE A~ DENTED REGIONS SHUTDOWN DATE 2-21-76 3-7-76 5-28-76 6-20-76 8-25-76 8-3-75 9-21-75 1-10-76 9-9-76 9-23-76 10-9-76 STEAM GENERATOR B B B A C A B B C B B B B B B C B B B B B R-C-TSP

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  • *PA't'rEIUi tW TUUES WHICH LEAKED AT NOZ2.LE ~

TUHE SUPPORT PLATES 1N DENTED. RECUDNS (TUIUtEV *rornr UNl1'S 3,4) FIGURE 2

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ROW la 2b 3b*, C 4b, C QUESTION 111 ANSWER Ill ATTACHMENT 3 Provide a tabulation summarv of the total strains in the circumfere~tial, longitudinal and radial direction at the U-bend apex for tubes at flow slot locations iri rows 1 to

4.

The summary should indicate the effects due to manu-facturing, service induced ovality (hourglassing), change in U-bend radiust operating thermal and pressure loads and accident loads.* Also compute the effective strains at the U-bend apex. The operating thermal and pressure straj,ns are trivial and can be neglected. Similarly, the strains due to SSE & other accident loads are quite small and are effectively zero when compared to those due to hour-glassing *. This can be shown.as follows for the steam line break accident condition. Maximum hoop strain during SLB = 20,700 30xl06 =.00069 in/in Where 20,700 *=largest hoop membrane stress occurring@ any location in vicinity of apex and 30 x 106 = Young's Modulus Our table will only include manufacturing and inservice'. (hourglassing) strains and a summation for each quantity requested. The summation is inclusive of operating and accident conditions.

SUMMARY

OF INSIDE SURFACE STRAINS AT THE u~BEND APEX - SURRY 1 LOADING CONDITION .EAXIAL EHOOP ERADIAL EEQUIVALENT Manufacturing .177 .001 -.040 .133 Hourglassing .004 .006 .* 000 Total .181 .007 -.040 .134. Manufacturing .112 .006

  • -.032

.085 Hourglassing .018 .034 -.014 Total .130 .040 -.046 .101 Manufacturing .082 .006 -.020 .066 Hour glassing .012 .032 -.014 Total .094 .038 -.034 .074 Manufacturing .064 .005 -.020 * .050 Hour glassing .010 .022 -.014 Total .074 .027 -.034 .063

a) hourglassing strains for row 1 were obtained using field data for tubes which have not developed indications in service (specifically RIC14 from Surry Unit No. 1, S/G "A", RIC3 from Surry Unit No. 2, S/G "A", and RIC3 from. Turkey Point Unit No. 4, S/G "4B") b) at full closure c) conservative estimate Qu;ESTION /12 ANSWER //2 Estimate the error band and specify the degree of confidence in the strain data provided in response to question 1, i.e. specify the tolerances in_the manufacturing strain. The tolerance in manufacturing strain based on 51 Series Steam Generator Engineering drawings_ are obtained as follows:

1. Axial strain:

Axial strain at the I.D. of the extradose is measured as where rt is the tube radius to the I.D. R is the bend radius at the middle surface R has a dimensional tolerance of +1 /32" = +.*03125" rt has a maximum value of rt= Max. Outside Tube Dia *. - Min. Wall Tliickness 2 =.880 - .045 =.395" 2* . and a minimum value of rt= Minimum Outside Tube Dia. - Max. Wall Thickness 2 =.8675 - .055 =.379" 2 Thus, the maximum axial strain for a row Ill tube, which will be most effected by these tolerances since it has the smallest radius is: Ea =

  • 395 max.

2.1875 - .03125 =.183 in/in . and the minimum axial strain is: = .379 2.1875 +.03123 =.171 in/in Therefcre: Ea=.177 +.006 in/in

I QUESTION/ANSWER #2 e e -~

2.

Hoop strain:

  • The maximum ovality for a 51 Series Steam Generator tube is specified as 10%.

This is equ~valent to a strain of e:1, =.007 .. max. The minimum strain is

1.

=.000

3.

Radial strain: Since the tube wall is always thinned at the extradose, the ratio of t~/t taken from mill data (Figure 2-1) will be used in the strain tolerance calculations. *From Figure 2-1 the tolerance on this quantity is less than +.01. =.95(tave) - tave = -.050 in/in t ave e:rmin =.97(tave) - taye = -.030 in/in tave The range of equivalent strain due to manufacturing processes can now be evaluated: Maximum Equivalent Strain: e: /2 1/2 equiv.max e: -3[(e:a e: )2 max hmi*n +(e:.<Lax - e: )2 + (e:h. -e:r. )2] Lil rmin min. min =.47[.1a32 +.2332 +.oso2J 112 +.141 in/in Minimum Equivalent Strain: e:equiv.min sv'Z c'(e:~ -e:h: * ; 2 +(e: e: ) 2 +(e:1, 3 n max ~n - .'ma

rmax, x

=.47 {.1642 +.2012 +.0372] 112 =.123 in/in 2 -e: ) ' ] rmax 1/2 Thus, the tolerance in_equivalent strain for tubes in Row 1 due to tube forming processes is given by: I e:equiv. - 0.032 +/- 0.009 in/in

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. AAJGLE ( o*~(:,/1.Gf.5, 0 )

QUESTJONlANSWER./12 Sin:iilar ~a~cuatil results: for tubes in Rows 2, 3, and 4-.ield the following* Row 2 Row 3 Row 4 e: e: e: E eq,p.v. Tolerance ma:x. a min. h min. ,-r min. + max. max. max. .min. 0.115 0.108 0.007 0.000 -0.04 -0.02 0.093 0.078 0.007 0.084 0.079 0.007 0.000 -0.03 -0.01 0.068 0.054 0.006 0.056 0.062 0.007 0.000 -0.03 -0.01 0.057 o.* 043 0.006 As shown, the equivalent-strains calculated for the different rows decrease (as the row number increases) due to the increasing bend radii, although the tolerances on tube dimensions remains the same. The significance of the calcualted equivalent strains given above and in* the response to Question 1 toward initiation of intergranular penetration is discussed in the answer to Question 9. QUESTION #3 .Indicate whether all tubes within the bundle are from the same heat and if possible provide information of what effect heat-to-heat variations would have on susceptibility to intergranular cracking at the U-bend apex of tubes in row 1 to 4. ANSWER #3 The tubing in the three Surry.Unit 1 steam generators were produced from a large number of heats, with hundreds of heats per bundle. There is no evidence in any of the examinations performed to date to suggest a heat to heat susceptibility to U-bend cracking. All cracks have been QUESTION f/4 .correlated with high levels of pre-strain augmented with service induced strains and are independent of heat i9entity. While it is conceivable that heat to heat or even within heat variations in composition, microstructure, grain size, etc., may result in more or less susceptibility to the cracking phenomenon, it is judged that these would he second order effects. Provide* a quantitive analysis of the* effect of flow slot. closure at the top and bottom support plate on thermal hydraulics flow' patt*erns, vortices and tube vibration, support plate and t*ubes, 1 in the dented regions, support plate wedges, subsequent wrapper deformations, and vessel shell integrity. ./

ANSWER 114 e The. effect of flow slot closure on thermal-hydraulic per-fo~nce includes a slight decrease in circulation ratio and liquid flow velocities with an increase in raw steam quality. The output capability of the steam generator would be negligibly affected. In light of the decrease in liquid flow velocities, flow induced tube vibrations are less likely *. With the closure*of the flow slots, the wrapper deformation rate should increase slightly, the average tube dentingtrate should increase negligibly, and the hour-glassing of the slot should tend to become less pronounced as the slot corners move inward.* Steam generator shell integrity will not be compromised by any support _plate expansion. Prelimiriary "crush" test at W have yielded* maximum plate-shell interaction loads of 60,000 pounds at each of the six block locations. The test does not consider the effect of increased plate stiffness due to tubes and *decreased stiffness of plates with reduced ligaments due to the corrosion process. It is felt that the second effect is much larger than the first and 60,000 pounds is a conservative number. The maximum shell stress intensities are a result of secondary bending stresses due to the local nature of their application, and have been conservatively calculated to be less than 75.0 ksi. They are conservative in that the loads applied to the mathematical model of the shell are conservatively larger than actual, due to the inability of Fourier expansions used to this point to truly represent the loads applied by the channel iron blocks. Nevertheless, the stress intensities calculated* in this conservative manner are less than these associated with the yield moment, that is: S<.1.5

  • Sy Where S = 75 Ksi Sy= 60 Ksi from tests performed at W for 533 Gr. A.

CL 1 material 1.5 Sy = 90 Ksi

  • t QUESTION fl5 ANSWER fl5 Date 07/72 12/74 11/75 10/76 e

Describe the anticipated extent of cracking of any support plate after the flow slot has closed." (Estimate the amomit of* operating time required for such closure.) Describe the subsequent effect on the tubes, wrapper shell. Based on finite.element analyses *of the plates subjected to in-plane loads, it is concluded that cracking of the support plate after full flow slot closure will b~ no more severe than prior to closure. This is based on dis-tribution of strain intensities, which increase very slightly at the plate periphery, but decrease almost every-where else. Nowhere do these strains increase significantly, not even at the plate "hard spots". Utilizing the results of the above plate analysis, and referring to the*response to qu~stion 4, it is concluded that the tube, wrapper, shell or any other component will see no significant change in applied tractions or deformations. The rate of flow slot closure for Surry Unit 1 may be approximated in a manner presented in Comment Number 7 in the November 15 submittal to the NRC on Surry Unit 2. The uppermost (7th) tube support plates of each steam generator of Surry Unit 1 have been inspected during the current outage and measurements of the flow slot openings ~ere made. Table 5-1 presents these results. This inspection was the first attempt to measure these upper plates since initial startup of the unit.

  • During the several regimes of plant operation, the following data are applicable:.

Total Cal. Months 29 40 51 Total EFPM 12.5 19.3 27.9 Event Initial Startup Transition P04-AVT Scheduled inspection-denting found Scheduled inspection-hourglassing found at . 7th support plate

'11 NOZZLE Opening at 7th Uppermost Plate SG A SG B SG C, 6 As discussed in the Surry Unit 2 referenced *submittal, the relationship between flow slot closure and operating time is estimated, assuming a linear rela~ion, from the time to transition to AVT water chemistry Taking _the smallest flow slot: opening (SG-A, Slot 6-1/2") from the as-built dimension of 2.75 inches, the displacement is found to be 2-1/4 inches. From the time of AVT transition, Surry 1 has operated for 21 calendar months (or 15.4 EFPM) which gives a flow slot closure rate of 0.11 in/mo ( or 0.15 in/EPFM). TABLE 5..,.1 11/1976 Measurements of Flow Slot Openings Surry Unit 1 .SLOT OPENING NUMBER

'> MANWAY 5

4 3 2 1 1/2"* 1" 1-1/16" 1-1/16" 1-1/8" 1-1/8" 1-3/4" 1-3/4" 1-3/4" 1-3/4" 1-13/16" 1-11/16" 1-9/16" l...;9/16" 1-5/8" 1-3/4" 1...:7 /8" 1-1/2"

  • 31 *U-bend samples were removed from this location.

While the flow slot closure rates calculated above would otherwise be relevant to future operation of Surry 1, there will, in fact, be no additional closure. *Each of the upper

  • tube support plates have been modified* with a flow slot blocking device which precludes further inward movement of the plate. Therefore, no additional hourglassing of the top tube support pl'B.tQ flow slots will be encountered during future operations of Surry 1.

QUESTION f/6 ANSWER f/6 e e When all flow slots completely close, the support plate will have a tendency to buckle but are restrained by the tubes. Provide an assessment of its effect (the tendency to buckle) on the tube integrity. Tubes have increased loading due to constraint placed on the plate, thereby preventing its buckling.

However, these tubes will experience only negligible increase in loading since any load increment necessary to maintain plate stability is shared by many tubes.

Hence, the effects on tubing integrity are minimal. In the unlikely event that the plate does buckle, tests have shown that Steam generator tubes withstand deformation imposed by support plate motion without violation on the pressure boundary. Tests were performed in which pressurized tubes were crushed in sharp edged support plate sections without impairment to tube structural integrity. /

e QUESTION #7 As originally designed the support plates did not restrain the tubes during the heat-up and cooldown axial thermal expansion of the tubes. With corrosion particle buildup in the annulus between each tube and the support plate, restraint to thermal expansion *is provided. Quantify the effect of such restraint upon the *tubes and the support plate.

  • ANSWER #7 Evaluation of the effect of tube fixity at the support plate due to corrosion buildup has been evaluated. The evaluation included an ASME Section III analysis of Design, Normal, Upset, and Test Conditions.

In addition~ a more recent evaluation has considered the effect on fatigue life of a tube fixed at the tube support plate closest to the tubesheet and fixed at the secondary face of the tubesheet. The tube was analyzed for a potential fatigue failure at the tubesheet when subjected to cyclic thermal and mechanical loads. The results of this investigation have shown that the steam generator tubing, at the tubesheet junction, is not susceptible to fatigue failure-under all anticipated transient conditions, including cold feedwater addition to a hot dry steam generator, even when considering a worst location tube, locked into the support plate closet to the tubesheet. The usage factor obta,ned from this analysis is less than 0.1. With regard to the effect of the tube-plate fixity on the plate, the maximum transverse plate deformations.are approximated by: 5@ top plate= fCtubes ~Ttubes C:Cwrap~er ~Twrapper) x ('1 ength { tubesheet - top plate]) = [7.85 (546.1 - 70) - 7.05 (581 - 70)] 10-6 X 353.5 * = '1.,.21." We judge that the plates can accommodate such deformations with minor* yielding.

e e ~ QUESTION #8 How many tubes have been examined at the U-bend apex, describe the methods of examination, the degree of confidence and the results. ANSWER #8 Laboratory Examinations A total of 71 U-bends have been removed from Surry 1, Surry 2, and Turkey Point 4 and have been examined by various non-destructive and destructive techniques. The apex of each bend was first examined by double wall radiography and selected bends were examined by eddy current techniques using a.540 inch diameter probe. If cracks were detected by both methods, only limited additional examinations were performed. If no cracks were detected, or there was a discrepancy between the two techniques, the bends were destructively examined by first sectioning transversely through the apex and metallograph-ically examining the cross section. This was followed. by cutting rings from the adjacent apex area, longitudinally slitting them, and bending them such as to place the ID surface in tension, as shown in Figure 1 and 2. This method of testing has the advantage of being able to examine a larger area than can be done metallographically, and has demonstrated the capability of detecting short, tight cracks that might be otherwise missed. After reverse bending, the ID surfaces were examined under a stereo microscope and if any fissures were detected, metallography was performed to determine the depth of penetration. A total of 19 Row 1 U-bends were found to be cracked. All but four were initially detected by radiography; these four each contained one very short (1/16 inches long) tight crack which was detected by the reverse bend test. In several cases, ~racks which were detected by radiography were not observed in the eddy current test as the signal was obscured by the background 11noise 11

  • There were no cracks found in Row 2 or Row 3 U-bends.

Summarizing, the destructive reverse bend tests provides the greatest assurance of detecting even minute cracks. The radiographic examination detected all cracks except those which were very short and tight. The eddy current examination had somewhat less sensitivity but generally agreed with the radiographic findings. The results of these tests are summarized on attached Table 1, 2, and,3.

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M,..,.("'1a NN(" 0 1 ~ "'"""O!'J . '!:i ~ ~~;~~a f:1"~~ ~ r** \\, ~ ( 4 TABLE 1 SURRY 1, S/G A (o_; in nnon;n,,, ~ COLUMN NO. i Exam G) G) G)*G) G) G) Q G) G) G (ii) Gi) G (ii) @ !Row X-'R I OK. --- OK CRK CRK CRK CRK CRK CRK CRK CRK r.RK CRK --- OK I* c:c.1 OK OK CRK CRK OK CRK CRK ~RK r.RK r.RK rov OK I MET OK OK -- CRK OK BEND / I TEST OK OK OK CRK CRK CRK --- --~ --- --- --- OK OK I IN. OVAL TY .074.068.075.13E. 2ll.256.361.275 ~139.279.248.221.142.069.05E OVAi ITV 8.4 7.7 8.6 15.5 '4.2 29.3 41.3 31.4 15.9 31.9 28. 2 25.. 16.2 7.9 6.4 I 2 X-R OK OK OK OK OK OK OK OK OK OK. OK I OK OK OK OK I I i ECL'l OK OK OK OK OK OK OK --- OK OK OK OK --- I I MET I --- --- OK OK OK OK OK OK --- --- ---- --- --- --- I BEND i OK OK OK* OK OK OK OK OK OK OK OK OK OK OK OK I TEST I OVAi IN..070.053.065.079.110 ITV .154.162

  • .162.141.10~.066.049.060 I

I I - I OVA4ITY I 8.0 6.0 7.4 9.0 12.6 --- 17.6 18.5 --- 18.3 16.1 12, 5 ! 7, 5 I 5.6 6.9 i I I 3 OK

X-R l I ECL'l

\\ I I I MET OK I I BEND OK fJ:'C:T . I IN. i I OVA ~ITV .050 I I I OVA... ITV 5.7 I I I I I CRK = Crack *Ovailty = (Diameter(max.).-.- Diameter (Min~)) .1 EC tests performed with standard U-bend-probe at 70-100 KHZ which did not have improved centering devites ~ 1-i I I I I l i I ! i

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'* -~ ].}:;*:-: .;t IROW I COLUMN NO. AM G) 0 G) G) G) G) Ci) G) G) 0 0 0 0 0 0 I I 1 X-R l OK OK OK OK CRK CRK CRK CRK CRK I I I l f CRK I EC OK OK OK --- --- CRK CRK I I MET OK OK.. OK CRK I I 1 --- CRK CRK --- --- BEND I OK OK OK OK t TEST 1 I I I In~ I OVt LITY .01.009.006.021.08~.120. .118.127 I 1

  • OVP _ITV 1.1 1.0 0.7 2.4 10.2 13.7 ---

13.5 14.5 I I I I 12 i i I I I i f i i I I ' j i I i ! I ! 3 I I I I I I I i I I I, i I ' I i J I I I I ' i X-R I I I ECLl II I . I I i I I I i I i II I t MET II i I 'I i I . I I, i I I I I I I i! I* I I I ! I li I t X-R II I I I I I ECLl I I i I I I I I I I ' I I I I MET I I i l \\ I i I i i ! I . i LlEC tests performed with standard U-bend probe at 70-100 !<HZ which did not have ! -improved centering devices. I CRK = Crack I 1 Crack was short, i.e.<l/16 inch and did not extend in length sample into adjacent bend~test . I

. *r TABLE 3 ~

  • I.

I' *. TURKEY POINT 4B (l.88. in.. openi_n~) I COLUMN NO. I ( I EXAM_@ ~-~ ~ @ @ ~ ~ @) @ 6)@ I i ROW I I I 1.. X-R !OK. OK OK

  • OK OK OK OK O.K OK OK OK OK OK OK I

ECll. OK OK OK OK OK OK OK OK I MET OK OK OK OK BEND =>=-.:, OK OK OK OK OK CRK OK OK CRK CRK 00.3 TEST OK OK ~§E IN.. * .034 .140 *.157 .14(.125.113 I ~~~ OVAL TY a .090 .159 .111 ~095.087 000 .. ~~~ 1-CD~CQ:, M1")M\\ OVAL TY a 3.9 10.4 16 17.9 18.2 16 14.3 12.9 12.7 10.9 9.9 r:.~N! -,.-:~2 OK I OK J. ~i:t I 2 X-R OK OK OK Ok OK OK OK OK OK OK OK OK OK 2 ECLl I I OK OK OK OK OK OK I I I I I MET OK OK OK OK I i I BEND OK OK OK OK OK OK OK OK OK OK OK OK I C I TEST I I. I OVAi IN.* I [TY 1.085.076 .079.099.096.103.091.085.097.082.090 ~069 I I I i !OVAL TY 9.7 8.7 9.0 11.3 11.0 11.8. 10.4 9.7 11.0 9.6 10.4 7. 9, I I j

  • I I ! 3 X,-R OK I

E:Cll. I. OK . f i I MET OK I BEND I I i TEST OK I i IN.*

  • 1 I

I

  • !OVAL TY

.044 I i I

  • I

!OVAL TY 5.0 I I I I I I I .*. I - . I I. I (

*Ovality is (diameter (max.) - diameter (min)) in inches I

i

  • CRK = Crack I

_ 1 **1/16 11 long axial cracks at I.D. extradose.

  • I fl.EC tests performed with standard U-bend probe at 70-100 KHZ which did hot have improved I

I

.*.., Q~ESTION #9 ANSWER #9

  • what ma.tude of service in.d1.1ced and/o.ota1 effective threshold strain is required to* initiate intergranular *cracking on either the extradose or i ntradose ID* surfa*ce at the U""bend apex, and how-is it affected by the change in the u.. beild radius and thus the pre... stra in or ova 1 i ty in rows 1 to 4?

The fact that intergranular stress-assisted penetration has been observed in Li.nits which have flow slot: 11 hour-glassing 1* developing during operation indicate that there may not be a threshold stress or strain required alone for initiation of attack, but in addition a strain rate range which is another important variable. The strain and corresponding strain rate derived from Von Karman bending stresses at the apex of the U-bend, brought on by the flow slot hour-glassing, decreases rapidly with increasing bend radius, for two reasons. First the leg displacement due to flow slot hourglassing is asymtotic to zero U~bends are increasingly more flexible. Thus, the Von Karman effect, which is the product of the two rapidly attenuates with increasing row number. With regard to the magnitude of the total equivalent strains, these are giv~n in the response to question #1 for the extradose at the apex. The values at the intradose would be smaller because both axial and radial strains would be considerable less. It is well to point out that all row #1 calculations are based on the tubes pulled at Turkey Point #4 and Surry 1 and 2. The equivalent strain of 0.135_in/1n r¢presents a*1o~er poun*c( on critical equivale~t ~train at this time for the following reasons:

1. All row #1 tubes with I.D. indications have equivalent strains greater than 0.135 in/in.
2.

Among the tubes with no indications the ones experiencing the largest leg deformations have equivalent strains of.135 in/in or less. In fact most tubes in the second category have had equivalent strains of.130 in/in.+ ~005 in/in. Again, it should be emphasized that the presence of significant plastic deform_ation in the tight U-bends combined with pressure and residual stresses are not sufficient in themselves to cause failure. This fs attested by both longer term operational experience in other units having equivalent U-bend configurations and stress conditions, as well as*successful long term laboratory testing of U-bend samples exposed to reference primary coolant. The necessary additional factor required to initiate and propagate the defects i_s the dynamic strain on the U-bend as a result of the flow slot hourglassing. Dynamic strain data on Inconel 600 _in primary coolant are not available ,although testing has been initiated by Westinghouse. However, the

  • effects observed with austenitic stainles*s* steels and high nickel*

alloys suggest that this dynamic straining effect is an important factor which is believed to be applicable to the U-bend failures.

QUESTION #10 ANSWER #10 e Since ovality and U-bend radius is strktly a geometrical° measure, what are the kinetical relationships between ovality or strain with total stresses at the U-bend apex.

  • For a.deformed tube, oval ity is defined as
  • ovality = Dmax - Dmin Do Where D

- maximum diameter after deformation max Dmax = minimum diameter after deformation D0 = nominal diameter before deformation From Westinghouse test data, it has been shown that tubes subjected to the same Von Karman Flattening phenomenon as those in the field have almost perfectly ellipitcal cross-sections. Figure 10-1 illustrates a tube with a 17 percent ovality which verifies that a flattened tube has an ellipical cross-section~ For a circular cross-section deformed into an ellipse, the nominal.

  • diameter before deformation is D -(D2

+ 02 )~ o - max min The curvature K0 for a perfectly round tube is simply the reciprocal of the radius. The change is strain at the.surface and along the hoop direction of an initially round tube deformed to an elliptical shape is given by t !le = 2 (K-K0 ) Where t = tube thickness K0 = l/R0 R0 = tube initial outer radius \\ At the intersection with the mtnor axis, the location of maximum inside su~face tensile strain, the change in strain is: !le= t (20min. - l) 2 D2max

  • R 0

\\

QUESTION/ANSWER #10 - Page Two Hence at this point, a.parametric relationship between ovality and change of strain is defined in tenns of Dmin and Dmax* In other words, given a Dmax and Dmin both, a unique ovality and change in strain can be determined. A graphical representation of this relationship is shown in Figure 10-2. The Dmin and Dmax values are from Westinghouse test data for 7/8 inch diameter tubes with bend radii similar to inservice tubes located in Rows 1 through 4, inclusive. Assuming the initial strain in a perfectly round tube is small, the hoop stresses associated with the Von-Karman Flattening effect can be determined from 6.e and the stress-strain curve for the tube material

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l QUESTION #11 Indicate the number of EFPD between the advent of tube denting and flow slot hourglassing. ANSWER -#11 The hour-glassing phenomenon is considered to occur coincidentally with denting. The hour-glassing results from the in-plane expansion of the tube support plate, which results from the accumulation of corrosion products _{predominantly magnetite) in the tube/tube support plate crevices. When sufficient magnetite is fanned by corrosion of the tube support plate to exert force on the inconel tubes, equal forces are also transmitted to the body of the support plate. The accumulation of the forces created in ~6700 tube holes in each support plate results in deformation of the support plate in.the vicinities of the flow openings, i.e., flow holes and flow slots. Surry Unit 1 , No data are available to suggest that a given degree of~denting must have occurred prior to the observation of,hour-glassing; however, it is of interest to note that hour-glassing has not been observed in plants where-denting has not affected the cold leg of the tube bundl~ or where the extent of involvement falls substantially short of 100% of the tube/tube support plate inter-sections. The dates of AVT transition, of the first inspections showing denting and hour-glassing and of the respective effective full power months of operation are listed below: TIME OF AVT TRANSITION DATE/EFPM* 12-74/12.5 TIME OF FIRST DENT INSPECTION DATE/EFPM* 11-75/19.3 TIME OF FIRST** HOURGLASS INSPECTION DATE/EFPM* 11-75/19.3 , *Total Effective Full Power Months From Initial Operation

    • Inspection was performed only at lower support plates.

QUESTION #12

  • Indicate the effects of frequent heat-up and* cooldown cycles on the support plate hourglassing and cracking due to thermal induced strains~

ANSWER #12 Since the number of support plate inspections is limited and measurements are not made continuously, the available data does not allow one to establish a correlation between heatup and cooldown cycles and support plate hourglassing and cracking. It is judged, however, that there is no relationship.}}

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