ML20087B773
| ML20087B773 | |
| Person / Time | |
|---|---|
| Site: | Robinson  |
| Issue date: | 06/25/1971 |
| From: | Weiss S AFFILIATION NOT ASSIGNED |
| To: | Seidle W US ATOMIC ENERGY COMMISSION (AEC) |
| Shared Package | |
| ML20087B746 | List: |
| References | |
| NUDOCS 8403090362 | |
| Download: ML20087B773 (29) | |
Text
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- Dr. Stanley Weiss _
4841 North Oakland Milwaukee, Wisconsin 532 Mr. William C. - Seidle
. Senior Reactor Inspector
' U. S. Atomic Energy Commission Division of Compliance-Region 11 230 Peachtree Street, N. W.
Suite 818 -
. Atlanta, Georgia 30303
Dear Mr. Seidle:
In reference to Report L. DC 89, " Inspection of Leaks and Cracks in the (A) Steam Generator at H. B. Robinson II," June 23, 1971, I would like to offer the following opinions and recommendations: (1) Westinghouse is primarily placing the blame for this leakage
; and cracking problem on the inadequacy and reliability pro-blem's associated with the explosion bonding cladding process.
There is justification in placing high suspicion on this process. Experience has indicated that inconsistent bond integrity has been obtained on large, relatively complicated parts. . For example, bond adhesion problems might be expected to locali7.e in the central and outer edge portions of a component such as the steam generator. Although the basic principles and po-i- tential of this process have merit, the application of the process to various components often presents considerable practical difficulties. The process dates back to the late 1950's and early 1960's 4 and, is generally based on the Cowan, G. R. , Douglass, J. J. , and Holtzman, A. : H.,' U. S. . Patent No. 3,137,937 issued June , 3,1964 and assigned to E. I. .du Pont de Nemours and Co. ' A-typical cladding arrangement, as well as the basic operating principles are illustrated in Figure 1. After the explosive is detonated, .the prime inetcl is accelerated very rapidly!to a t: by the_ detonation pressure. As the deto .
-nation highfront velocity (D) (Vy'm) oves across the plate, an angle isLe
.between its undeflected and deflected' portions. When the deflected portion of the prime metal collides with the backer plate, t_hc__ region of high pressure collision moves scross the plates at 1;igh speed.' This velocity 6'c)' equals the detonation 8403090362 71 ;
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! 2 velocity for the illustrated situation. Ahead of the collision region in each plate, high pressures cause surfaces of both- _
metals to flow plastically into the space between the plates. The jet which forms acts to remove surface films of oxides and of other materials normally detrimental to bonding. At the same time, the clean metal surfaces are subjected to high pressures in the collision region causing plastic deformation. The metals supposedly come into interatomic contact with each other, establishing a metallurgical bond. Furthermore, difficulties have been experienced in the past in arriving at destructive testing techniques which result in obtaining reliable bond strength information. Based on this failure and past experiences of manu-facturers of large vessels, it is recommended that past-and intended future application of this process be thoroughly reviewed. (2) It is the writers opinion that the following factors contributed to this failure: (a) Marginal clad bond integrity resulting from the explosive cladding process. (b) liigh residual stresses resulting from welding of the divider plate to the clad tube sheet. These stresses probably further deteriorated the clad bonded interface and initiated separation. (c) Operating and applied stresses then may have led to propagation of the clad bond separation resulting in failure nnd leakage. (3) Undoubtedly horic acid solutions and residues have, by capillary action penetrated up the sides of the tubing and along the propagating cladding cracks. At ordinary temperatures, 750F, g the horic acid solution is caly slightly acidic, reflecting the weak ionization of the borate ion. The corrosivity of the solution at' chis temperature toward ordinary system materials, e. g. , stainless steels,inconel'is insignificant and caa be disregarded. However, the existing con.lition # can present problems because of the contact which has been made with the underlaying forged steel tube sheet and the potential of higher temperatures. Also, arcated boric acid colutions resulted in signincr.at!y higher ratec R
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i - of corrosion as compared to deaercated boric acid solutions. lt is recommended that efforts be undertaken to analyze the potential hazards of this condition and the difficulties associated with eliminating the condition. (4) It is recommended that the stress analysis currently being performed by Westinghouse be submitted, as soon as possible, for review by an independent, outside consultant (e. g. , Parameter, Inc. ). (5) It is the opinion of the writer that generic aspects of this problem exist. Evidence of this has already occurred at the Robinson 11 site where two out of three steam generators already exhibit the problem. Furthermore, as a result of our conversations, it was indicated that the problem ~is imminent at other sites. (6) The following observations opinions and recommendations are offered regarding the Westinghouse proposed icpair procedure: (a) The removal of defective areas should be per-formed in a careful and pla'nned manner so as to
. enable a rigorous fracture analysis.
(b) Although the repair completion was estimated as one month, it is believed tha' this estimate is overly optimistic. The man hours required and working conditions imposed will probably result in a multiplication factor of 11/2 to 2 times the estimate. (c) No mention was made in the procedure of methods or attempts for removing the trapped boric acid solutions. (d) Detailed "in-process" and " final" inspection pro-cedures were not described. Obviously far more than just the code requirements will be necessary for this repair. Mention was made of a secondary hydrotest as a check for leaks. (e) Westinghouse is arranging to subcontract the localized stress relieving operation (Cooper Heat was men-tioned). They will attempt to accomplish a localized j heat treatment of the' deposited rep'a ir cladding by i, designing and building equigment capable of heating the area of interest to 1000 F. Westinghouse claims wide experience in accomplishing this type of weld stress relieve. This localized stress relief will result in high thermal gradients and stresses. Efforts T
l' . m o o n 1 4 should be made to determine beforehand the effects this operation will have on the surrounding hardware _ (e. g. , remaining explosion bonded interfaces, metallurgical effects in the thermal gradicat region, etc.). Ver ' truly yours,
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AT LANT A, GEORGI A 30303 TmaqWM i CO INQUIRY REPORT NO. 71-14 Carolina Power and Light Company Docket No. 50-261 License No. DPR-23 H. B. Robinson No. 2 CLADDING SEPARATION IN STEA'4 GENERATOR NOS.1 AND 3 (SUPPLEMENTAL INFOR'<ATION TO INQUIRY REPORT NOS. 71-11,,71-12 AND 71-13) Prepared By: / uf M D. C. Kirkpatrick, Reactor Inspector J/ Dat'eNM ' Tnis report advances the highlights of a special inspection made to the subject facility on June 18, 1971, by D. C. Kirkpatrick and Dr. S. Weiss, AEC Consultant. A report of the visit also vill be made by Dr. Weiss and will be attached to the forthcoming CO Report No. 50-261/71-08. On May 27, shortly prior to the reactor shutdown due to turbine vibration, lov level radictetivity was detected for the first time in water samples from the secondary side of the stean cenerators. Tne primary side domes of all thrce steam generators were entered and exEminations were made which included visual cbcervations, Ical tests and ultrasonic tests. On June 18, the consultant and the inspector entered the No.1 steem generator to make visual observations and to take photographs. In both the No.1 and No. 3 steam generators, the one-fourth-inch inconel tube sheet cladding has been pulled loose from its base metal in the area ovcr the divider which separates the two halves of the lover dome. Tne separations extend laterally from both sides of the divider to the edjacent rows of seven-eighths-inch 0.D. inconel tubes. A sectional view of the failure is shown in Exhibit A (provided by Westinghouse). Tne separation extends along the top of the divider to within six inches of each cnd of the divider. The separation area is shown in Exhibit B. The cladding is cracked along the edge of the separation area. Tais crack
' extends to the bottom surfaca cf the cladding in the cold leg ; ides of both stecn generators. The cladding is also broken away frcu the tubes which are next to the divider so that the crach is continuous, proceeding across the web between two tutes, arcund the tube fillet veld, across the next veb, around the next fillet nnd so on foc25 to 30 tubes. (See Exhibit B.) In zy *
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p e' Qi CO Inquiry Rpt. No. 71-14 some cases the crack breaks through the vall of the tube rather than throu6h the fillet veld. The crack tends to be small in the web between the tubes and vide in the fillet. One crack was measured to be about .025-inch vide. The crack is not visible on the hot leg side of either of the two damaged steam generators, probably because the tubes are farther from the divider on that side. Two leaks emanate from the crack in the No.1 steam generator cladding at the rate of about one drop per minute with 800 psi of water pressure en the secondary side. Small amounts of rust were observed at numerous locations along the No.1 steam ge,nerator crack. No damage was observed il the No. 2 steam generator. A discussion on the problem was attended by representatives of Compliance, CP&L and Westinghouse on June 18. Westinghouse attributed the failure to inadequate bonding strength between the cladding and the tube sheet. The cladding had been applied by the
" explosive" method. Samples of explosively applied cladding, tested by Westinghouse, proved to be inhomogeneous in bond strength, varying from 30,000 psi to 60,000 psi. It was pcstulated that the failures started during the 3106 psi hydrostatic test and have spread since then due to expansion and contraction during thermal cycling. The calculated stress in the cladding to tube sheet bond was 20,000 psi over the divider during the hydrostatic test.
The divider to cladding fillet velds have not been stress relieved. It was stated that Westinghouse had stress relieved similar velds . at their plant and thermal stresses had broken the cladding loose during the treatment. Since the steam generators are under varranty, Westinghouse is responsible for the repair effort, with CP&L providing radiation control. The main repair steps include:
- 1. RemcVing the cladding over the divider, out to a line between the first and second rows of tubes on either side of the divider. (See Exhibit B.)
- 2. Removing enough of the top of the divider to provide access to the space above the divider for revelding the cladding.
- 3. Grinding off the first row of tubes on cach side of the divider flush
. With the tube sheet and plugging these tubes.
- h. Replacing the cladding by veld overlay which vill also cover over the ends of the' plugged tubes.
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Report on f Inspectiory of I.caks and Cracks in the (A) Steam Gsnerator at 11.11. Itobinson 11 Report No. DC 89 June 23,1971 Prepareci for: U. S. Atomic Energy Commission Division of Compliance A BC Contract AT (11-1)-1658 Subcontract No.10 by: Dr. Stanley Weiss Materials Department s, University of Wisconsin-Milwaukee ~ through: Parameter. luc. Consulting lingineers lilm Grove, Wisconsin
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NOTICE This report was prepared as an account of work sponsored by the United States Govern-ment. Neither the United States nor the United States Atomic Energy Commission, nor any of their eniployees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, complete-ness or usefulness of any information, a ppa ra t us, product or process disclosed, or represents that its use would not infringe privately-owned rights.
v Distril ution: O -O Cegies: AEC Regulatory Organization i Division of Compliance Technical Programs - Washington (2) L. Kornblith, Jr. D. L. Pomeroy Region II, Atlanta W. C. Seidle (3) AEC Contracts Division Chicago Operations Office (1) 11 . N. Miller, T. Katisch PARAMETER, Irac . (1) Dr. S. Weiss (1) l t-i 5 F M,'S'7Y ( #Tf7-7?"'g*.* pKM( j49 R ^*i'Q,7%lC[f j[]/ ,' Q r.1*f c' '
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- l. Discussions and Observations Prior to Inspection A visit was made to the H. B. Robinson 11 site by Mr. Donald Kirkpatrick of Region 11 and the writer on June 18, 1971 to review and inspect the reported leaks and cracks occurring in the A steam generator.
Prior to the inspection, discussions were held with Messrs. G. P. Beatty and R. Ilessac of Carolina Power and 1.ight. We were informed that we would be allowed into the steam generator for only a limited time (approximately 12-15 minutes) because of existing conditions and their safety requirements. The following in-formation was presented to us during these inital discussions: (1) The inconel cladding overlay was bonded to the base plate by the explosion bonding process. (2) Continuous leaks in the form of droplets were observed in the A generator in two locations, approximately in the region of the 40th and 44 tubes, as counted from the north direction towards the south, in the row of tubes adjacent to the divider plate in the cold leg of the generator. The C stearn generator was reported to exhibit dampness but no droplet formation, whereas, the B generator was reported as dry with no apparent leaks. l (3) Ultrasonic testing performed by Westinghouse indicated that separation had occurred at the bond interface between the eindding and the 22 inch thick tube sheet nuiterial between j the rows of tubes inunediately adjacent to the divider plate 3
g, _ . s_ in the hot and c - - 1 cgs of the A generator. The cond sepa-C ration is believed to be extensive so as to encompass at least -
- 80. tubes along these first rows. Ultrasonic testing further indicates that bond separation between the cladding and base .
plate ceases approximately at the mid-point of each of the first rows of tubes adjacent to the divider plate. (4) Cracks were reported in the fillet welds extending through the cladding between the tubes containing the cracked fillets in the cold leg of the A generator. Approximately 20 to 30 tube fillets were reported as visually cracked. (5) It was reported that the C generator was dry boiled whereas the A and B generators were not. (6) Preliminary reports indicate that Point Beach may be ex-periencing a similar problem although the intensity of the problem is not yet fully known. II. Inspection of the Steam Generator Mr. Kirkpatrick and the writer were " suited-up" and proceeded to enter indi-vidually and inspect the cold Icg of the A steam generatoF. The inspection was i performed visually. using a low power magnifying lens. Mr. Kirkpatrick attempted to photograph the conditions we observed with a 35 mm. camera using a close-up i-lens. . ~ The following conditions Orc observed during this-inspection: (See Figs. 1,2 Ed i h (1) Severe cracks were present. in the fillet welds of approxi Li
.mately 30 tubes.- The highest-intensity nf fillet cracking was
. apparent -in the region ~ included in and surrounding the 35th t < d
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to '45th tubes (r0 ""'"" * '"" " "'" ' ""r"O " These cracks were opened approximately . 030 to . 050 inches and were approximately semi-circular on the side of the fillet welds facing the flow divider plate. (2) Cracking in the 1/4 inch thick cladding was extensive in the region described above. These cracks were linear in the north-south direction (parallel to the divider plate) and were w located approximately in line with the center of the first row of tubes. These cracks were tighter than those ob-served in the fillets and appeared to range in the vicinity of . 005 to . 010 inches wide and extended the entire length of the cladding between the cracked tubes. (3) Rusting and carrosion products were observed in the immed-inte vicinity of the fillet wcld cracks progressing into the i cladding cracks. This evidence shows that galvanic action and electrochemical corrosion has already occurred between the tube sheet material and the austenitic materials involved, 1 through the borated solution which acted as an electrolyte. (4) The writer visually observed a contoured region of the cladding, adjacent to the divider plate and in the vicinity of the 40th to 50th tubes that showed definite signs of having been separated from the tube sheet. (5) Linear crack indications in the fillet weld joining the flow div: der plate 10 the tube sheet cladding were ohnerved. These
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O O cracks appeared to be very tight, were located in the toe of the weld adjacent to the cladding and, were aligned in the direction of the weld. (6) The fillets of the tube welds appeared to be undersized in various locations. A throat size of 1. 5 T is required for this joint where T is nominally . 050 inches, the tube wall thickness. (7) Inspection of the fillet weld joining the opposite end of the flow divider plate to the steam geneiator wall indicated no apparent defects. (8) The divider plate is located significantly closer to the first row of tubes in the cold leg of the A generator than to the first row of tubes in the hot leg of this generator. III. Post Innpection, Discumions, and Observations in the ! ate afternoon a tnceting was held between Mr. Kirkpatrick, myself and personnel from CP and L, Westinghouse and lihasco as indicated by the attached meeting list. The personnel shown on this list had participated dming the morning and early afternoon of June 18 in a closed meetir;r to review the cause of the problem and proposed repair procedures. Pertineni facts brought out by our later discussions with them were an follows: (1) During the initial manufacturing of these steam generators all processing was completed with the exception of the fillet weld joining the flow divider plate to the tube sheet. Prior to comph: ting this joint, the unit wa> strees relicved to code requir ement. Th final weh! joinio;. Ihr divider phne to the
s Page 8 (^} t U,m tube sheet was then performed without any subsequent stress y , 1 relief. ' (2) Westinghouse has not arrived at final conclusions with regards to the cause of the overall problem. They will attempt to per-form a failure analysis based on samples removed during the intended repair. At this time they believe the problem is primarily related to deficiencies in the bond strength associated with the explosive cladding process (commercial process generally used is "Detaclad"). They believe that stress relief, operating stress conditions, and stress vibration considerations were not primary factors in causing this failure. (3) Westinghouse has eliminated the explosion cladding process based on numerous diffien1 ties which they have experienced with obtaining consistent bond strengths. They have reverted back to conventional arc fusion cladding techniques. (4) Westinghouse reported that the worst stress condition which they believe the A generator had been subjected to was a one-time hydrotest subjecting it to 3106 pmnds on the chamber side. They believe this resulted in a 40,000 psi stress in the divider plate and consequently a.20,000 psi stress on the cladding. Although they have experienced widely varying bond strengths ( in the explosion clad tube sheet (reportedly ranging from 30,000 to 50,000 psi), they a re not able at t his time to determine. 4
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whether or not the hydr' ust performed initiate I tInc failure. s (5) Westinghouse stated that no serious problems are anticipated related to boric acid attack resulting from the leakage which had occurred. They furthermore indicated that stress corrosion and hydrogen cracking problems will not arise. (6) When asked about'the adequacy of their stress analysis of this vessel Westinghouse replied that they are currently undertaking a thorough re-study of this subject. (7) When questioned with regards to the anticipated life of a repaired vessel, Westinghouse replied that they believe the repair will eliminate the defective areas and perform in a service manner so that reoccurrence of the problem is precluded. It wac admitted, however, that a more complete answer to this question would he forthcoming when the failure analysis and factua! reasons for the cause of the problem have been determined. (S) Westinghouse personnel stated they do not have sufficient infor-marion at this time to determine the generic nature of the problem; however, it is their belief at this time that it is not widespread. No data or information was presented at this ilme to substan:iate this opinion. (9) Ultrasonic inspection by iVestinghouse of the hot leg of the A f generator has confirmed that separation has cecurred between the cladding and tube sheet. Although there is no visual con-firmation of cracking in this location, a is suspected that a crackiag condition does e> irt. i
(10) A repair l rocedure i proposed by Westinghouse was described
. to tis. The foll(Oog repair considerations wer<Qscussed:
(a) A minimum amount of material, including the fillet welds, be removed from the divider plate. (b) Defectively bonded cladding will be removed by arc and grinding methods. (c) Attempts will be made to utilize the removed material as the basis for a failure analysis study to implement a more thorough understanding of the cause of the basic problem. (d) The tube sheet will be re-clad using shicided metal arc (coated electrodes) techniques and the cracked tubes will be ground back and permanently blocked off by scal-welding. (c) The repaired region will be locally stress relieved at 1000 F prier to accomplishing .m final weld between the flow divider plate and tlic tube sheet. (f) The final divider plate to tube sheet weld would then be performed wit hout any suln:equent st ress relieving. (g) Inspection procedures planned were to perform penetrant testing to thecode requirements. When asked if that was the only non-destructive resting to be performed, it was stated that,supplemcntary ultrasonic testing will be used
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ENCLOSURE 3 WESTINGHOUSE STEAM GENERATOR PROBLEM trrnzn ArrLICABLE REAurvR FACILITIES Reactors No, of Units Ginna 2 Turkey Point-3 3 Turkey Point 4 3 Indian Point 2 4 - Indian Poin*: 3 4 Robinson 2 3
- Point Beach 1 2 Point Beach 2 2 i
Surry 1 3 l Salem 1 1 i Diablo 1 1 k 0FFICE > . . . _ . . . :
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[u ' MTE > .. . ... - , _ _ . . . . _ feem ABC-Ste (Rev.9-53) ABCM 9340
- u s covenwurnt enem crace. 1s7 s 416-4e e -
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1 4 ENCLOSURE 4 ? WESTINGHOUSE STEAM GENERATOR PROBLEM INFO S
- 1. Ginna - No inspections of the steam generators have
'l been performed. There has been no indication of primary to secondary steam generator leakage to date.
- 2. Point Beach 1 - No inspections of the steam genera-tors have been performed. A p*imary to secondary
< steam generator leak rate of 80 gallons per day has l been experienced. The cause of the leakage has not 1 yet been determined. 3 Point Beach 2 - Westinghouse personnel have performed . visual and ultrasonic inspections of the cladding. on both steam generators. No defects were detected. The preoperational hydrostatic test has been performed.
. 4. Indian Point 2 Westinghouse personnel'have performed
, . visual and ultrasonic inspections .of the:eladding on all four steam generators. Cladding failures, sini-lar to that experienced at Robinson, have been experienced on each of the four stesa generators.
5 d 0FFICE > .
. 6
{ m SM> .. .. Pasun ABC-Ste (Rev.9 53) ABCM 0240 .
- u a covspNWNT WRWTWQ OMCf 1973 418 465
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- 5. Turkey Point 3 - Westinghouse personnel have performed visual and ultrasonic inspections of the cladding on all three steam generators. Cladding failures, simi-lar to that experienced at Robinson, have been detected i
on two of the three steam generators. i i 6. Turkey Point 4 - Westinghouse personnel have performed visual'and ultrasonic inspections of the cladding on j i all three steam generators. No defects were detected. The preoperational hydrostatic test has not been 1 . performed. W
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Ensun ABC-Ste (Rev.9-53) ABCM GNO , - ou e aovanwusur ervaa oarco s org' 4:s 4ee -
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