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Discussions on session I
Int. J. Pres. Ves. & Piping 25 (1986) 139-151
Discussions on Session I (Chairman: K. E. Stahlkopf)
Paper 1 by Ahlstrand and Rajamiiki Chairman: Thank you, Mr Ahlstrand. We now have time for a couple of quick questions. Hutin (Electricitk de France, Paris)." In fact, I have two questions. You said that there is no indication of fatigue crack growth on the sample you took out. Did you try to perform fatigue crack growth analysis using the previous transients and cycles to compare the experimental evidence that there was no fatigue crack growth and what would have given what I would call classical fatigue calculations? My second question concerns dosimetry. Did you encounter some specific problem because of irradiation in performing the repair?
Ahlstrand (Imatran Voima O Y, Helsinki)." Regarding the first question, the only thing we did was to look at the fracture surfaces. We did not find any indication of crack growth or initiation even. So we did not analyze those mathematically any further than indicated in the paper. Concerning the second question, we did encounter radiation problems in two steam generators, where big defects existed and big repair procedures were required. We had to do decontamination. I have some slides I could show on this. I showed you already that we reduced the activity more than 100 times. This shows how we did it. First we used an alkaline procedure, then rinsed with water, then oxalic acid, followed by another water rinse. This procedure was then repeated. You can see from these figures that with oxalic acid we had good performance. The oxalic acid was the main effective decontaminating agent that we used. It helped very much. Finally, we rinsed with hydrogen peroxide to do some passivation of the tubes. 139 Int. J. Pres. Ves. & Piping (25) (1986)--© Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain
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Chairman: Thank you very much. Our next speaker is Dr Norm Edwards of N U T E C H Corporation, in San Jose, California, who will talk about 'Weld Overlay of BWR Flawed Piping'.
Paper 2 by Edwards Chairman: Thank you, Dr Edwards, for an excellent presentation. The floor is now open for questions. Iida (University of Tokyo, Japan): My question concerns Fig. 5 which shows the distribution of residual stress inside the pipe. Have you measured the results of residual stress just on the surface of the weld metal? I wonder if you have found any evidence of the decrease of residual stress on the inside surface of the weld?
Edwards (NUTECH Inc., San Jose, California)." One slide I showed deals with residual stress; in the written version there are many graphs of such data. The one I showed is representative and shows actual residual stresses on the inside of the pipe. The points identified in Fig. 5 with the number 'one' are in the heat affected zone of the original weld.
Iida: Yes, but I wonder if the residual stress inside the weld metal is also decreased on the compression side or not? I mean the stress at the center line of the weld metal.
Edwards: On the plots which you have the trend I showed, the last point identified as '1' is in the heat affected zone of the original weld. The analytical predictions, which I indicated, bound the test results and have a trend which is similar, but the compressive stress within the weld itself is not as compressive as in the heat affected zone and the area removed from the weld. However, the fact that the analytical predictions are still compressive and that these predictions bound the test results remote from the weld would lead me to believe in fact that we still have compressive stresses in the original weld.
Chairman. Do we have another question? Maurer (Commission of the European Communities, Brussels): As a consequence of this intergranular stress corrosion in reactor pipes, do you intend to modify for future plants the piping materials used? Edwards: The planned pipe replacement programs you will hear about;
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at least you will hear one description this morning. These give a great deal more attention to the selection of material and handling of that material. The replacements are usually done using the 316 N G (nuclear grade) material with emphasis being on the low carbon content. Just as important as materials selection, however, is the handling of that material--an issue which has received a great deal more consideration in replacement than was the case in the original construction.
Chairman: One more question. Schulz (GRS, Ki~ln): Do you have any experimental results on crack growth under typical environmental conditions on actual specimens of this type? Edwards." Yes. I am not intimately knowledgeable on the subject but there is a wealth of information on crack growth rates that have been determined from experiments sponsored by EPRI in the General Electric test facility and other sources.
Chairman: We do have environmental tests going on now with weld overlays of both small pipes and one full size (26-inch) pipe. Our biggest problem is getting cracks to grow at all. The compressive stress fields are indeed very large and as a result one must use stress much larger than would be seen under operating conditions to get the cracks to grow at all. So it is a very satisfactory method to halt the cracks. Hemsworth (National Installations Inspectorate (Nil), UK)." That was my question--are any of these weld repairs under test to predict performance?
Chairman." Yes, we do have one long-term test going on at nominal operating conditions. Actually it is at 110% of design stress. That is a full 26-inch diameter pipe. The tests are going on in fully oxygenated 200-ppb water which is typical of the BWR environment and, observing one of the cracks, we have been unable to make it grow. It has been under test for about 18 months now. We now predict that we will not see a crack move. Chairman." We will now hear about an alternate method of looking at what to do about cracked pipe in Boiling Water Reactors. From Dr William O'Donnell we will hear about the 'Applicability of Pipelocks as a Remedy for Intergranular Stress Corrosion Cracking in BWRs'-another approach to the same problem we just discussed.
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Paper 3 by O'Donnell and Hampton Chairman: That was a most interesting presentation, not only saying some things about the alternate methods to IHSI and repair welding but also raising some interesting points about them. I am sure there are some questions or statements from the audience on this paper. G. H. Neils (Northern States Power Company, Minneapolis, Minnesota): I have two questions: The first concerns your representation that the pipe clamp for a crack weld remedy satisfies the requirements of ASME Section III. To my knowledge, Section III does not allow the presence of an original defect in the pipe. I can understand how you made design of the pipe clamp, in and of itself, meet Section III but how do you address the issue of potential leakage which in the regulatory realm becomes just as important as structural integrity once it is discovered?
O'Donnell (0 'Donnell & Associates Inc., Pittsburgh, Pennsylvania): The pipelock does not seal the pipe at all. It is not intended to prevent leaks. If you have a leaking problem you put a weld patch, or seal basically, on the leak area. With respect to Section III of ASME Code, the pipelock picks up the load and takes it around the crack, so the pipe wall up to the point where it is carrying a load into the pipelock meets Section III. In other words we did the analysis including the pipe wall but the load does not go beyond where the two clamps are, on either side of the weld. The load passes up through the wall to the pipelock. So both the pipe wall and the pipelock are designed to Section III and meet the Code allowables for that condition. Neils: In other words you treat it as though it were a Section III flanged joint. Is that correct? Except, of course for the leakage question.
O'Donnell." Yes, precisely, it is like a flanged joint in Section III. /Veils: My second question regards'the use of mechanical stress improvement. In my observations there are many ways of creating corrosion sensitization besides that of merely creating sensitization from heat during welding, that is, the carbide precipitation in the heat affected zone. I think things like cold work and abnormal strain energy, surface condition and so forth have shown up, not frequently, but still in real circumstances of stress corrosion cracking in piping components in the absence of heat sensitization. How do you address the matter of the residual inside diameter surface tensile loads at or beyond yield in the
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area of the transition each side of the section which has been compressed circumferentially?
O'Donnell: The mechanical stress improvement process applies only compression at the weldment. In areas away from the weldment on the other side of the two clamps, Mr Neils is talking about the fact that the bending in the pipe wall is tension. That is correct. The tension over there, however, is much smaller than the tension from induction heating directly in the weldment, on the inside. We feel that the tension away from the weldment is not going to cause any sensitization of any material away from the weldment. I should mention also that the mechanical process leaves the circumference of the pipe smaller than it was when we started and by the amount of 1½ times yield approximately, 0.3% strain. This closes surface discontinuity and we feel that the residual compressive strain in addition to the residual compressive stress inhibits crack growth. With the thermal processes of repair you cannot apply a residual compressive strain at the weldment. Kiss (General Electric, San Jose, California): I have a number of concerns related to the presentation you have made on mechanical stress improvement. First, I do not believe you have completely presented differences between IHSI and mechanical stress improvement. My concerns are two: one is institutional and the second is technical. In the institutional sense before IHSI was implemented in the USA, even though it had been developed and used in Japan, we went through a very extensive qualification program to demonstrate that it indeed was a remedy to stress corrosion cracking. We demonstrated not only that it could be applied with no inherent problems related to the process but also, once done, it would indeed produce conditions to allow plants to operate for long periods of time without stress corrosion problems. This type of qualification has not been done on the mechanical stress improvement process. O'Donnell." No, that is not true. Are you asking a question or making a statement? Kiss: I am making a statement. You may comment on any statement, of course, but so far as I am concerned they have not been done. There may be something in progress. My statement is that such qualification tests have not been done to date. I have a second point. With respect to mechanical stress improvement as a potential remedy for an operational plant, my concern is somewhat
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similar to original concerns about IHSI, that, depending upon the precision of the UT inspection that is done, which is not 100%, you can indeed have undetected IGSCC in the weld. Now, when you apply the mechanical stress improvement process (MSIP), and cause the full cross section of the pipe to yield, you have a very high probability of opening up cracks or fissures and producing a condition that, in fact, is worse after the process is complete than that existing before you started. That would not be true on a new application, but there is an additional concern that I have where we used the type 316 N G material. That is consistent to what Mr Neils was saying, that is, we know that cold work can produce a susceptibility for IGSCC. The one thing that mechanical stress improvement does that the IHSI process does not do is produce a significant amount of cold work. Since the effect of cold work induced by the MSIP process is unknown, this represents a significant unknown that could only be qualified by extensive testing prior to use. I have one other point, that is that the mechanical stress improvement process you have described is final. The nice thing about IHSI is that if you do the process and monitor the temperatures, you have a very high degree of assurance as to what the thermal gradients were and reasonable assurance as to the residual stress picture. If the gradients have not been obtained, you can do it again. IHSI is strain controlled so the resulting residual stress on the inside diameter will be compressive; whereas the mechanical stress improvement process as presented will simply relieve the welding residual stresses that were present and will not generate significant residual compressive stresses on the inside diameter. In-service operational tensile stresses are added so that after the mechanical stress improvement process you may have little compression on the inside diameter. Again, the question arises as to how good this process is in the long term. This has to be addressed in the laboratory and the results made readily available for independent technical review. O'Donnell: Let me respond to all three points you have made. The third comment was about the residual compressive stress. The finite element analysis, the tests and the basic mechanics of the process show that you get very high compressive residual stresses at the end of the process. This is true in both the axial direction and in the hoop direction. We think that they are much more repeatable and reliable than they are with induction heating.
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Your second point concerned cold working. The difference between induction heating and mechanical stress improvement must be noted. Induction heating strains the material in tension at the inside surface. If you calculate the stresses elastically you get about 300 000 psi, so you are talking about something like ten times yield in tension. That is cold working the material in tension. We feel that this makes it more susceptible to stress corrosion cracking. We are cold working the material in compression and believe that this helps. So, I would say that if there is any cold working concern it definitely favors mechanical squeezing over induction heating. Your first concern had to do with acceptability and testing. We have run many, many tests. Maybe we have a competition between GE and Westinghouse arising here. Westinghouse is applying this process and had independently verified our results. When you squeeze the pipe by a known amount you will get very repeatable results. And, that is what we are getting. We have also done the magnesium chloride testing. So, it is our opinion that this process is more reliable than any thermal process. The first comment by Dr Kiss also concerned acceptability. You saw in my presentation, that we presented this to the Nuclear Regulatory Commission and they have given the utilities permission to use it on a 5059, which means you do not have to send in a report justifying the use of it from a safety viewpoint. (It is not a safety issue, so to speak.) The utilities currently are considering applying this process to their plants in the next couple of months. The other point that Ed Kiss raised was use of the process in old plants. I will tell you this, if I had an old plant and planned a stress improvement, I would want to do it in compression. I do not see how hoop compression can open any cracks anyway. This is my own opinion anyway.
Paper 4 by Frederick and Hernalsteen Chairman: I have looked with some interest at proposals for rotopeening and shotpeening as a way of solving problems particularly in the transition area. One thing that has concerned me is that both procedures lead to a very high degree of cold work on the surface. As a result of thermal expansion and contraction, sometimes in the past we have seen problems with this high cold worked layer opening or fissuring, then allowing, in essence, a crevice corrosion situation to take place where
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such opening has occurred. Have you considered this or done any longterm environmental tests involving thermal cycfing to perhaps allow the area cold worked by roto- or shotpeening?
Hernalsteen (Traetionel, Brussels): Do you mean that the surface layer could open up for the remaining part of the tube? Chairman: Yes. There have been situations where the highly cold worked surface formed by rotopeening, when subject to repeated thermal cycles has, in essence, broken in places thereby allowing crevice corrosion to take place. Have you tested the rotopeened surface under repeated thermal cycles to insure that this is not a problem in this material? Hernalsteen: No. No such long-term investigation with thermal cycling has been done. However, we believe that cold working is just as effective in producing the expected compressive stress. It is not a very strong cold working either (no detectable grain deformation). The residual compressive stress is in the range of about 600 MPa with a gentle curve down to zero at a depth of about 0.1 to 0-2 mm, in a progressive way. In none of the tests that we have made (not with long-term cycling) have we observed any tendency toward cracking but we cannot give the answer in the terms you have raised in your question. Hutin: I suppose that stress corrosion cracking involves phenomena which are related to some type of threshold in terms of stresses, K, and so forth. You said that on the outside of the pipe diameter you had quite low residual stresses so you would not expect any stress corrosion on the outside. Now, when you performed the corresponding tests, did you think about putting the pressure inside the tube because in fact, even if the residual tensile stresses are quite small like 5-6 ksi, if you add these stresses to the operating stress it may be a problem of a threshold being just barely exceeded. Did you think about putting a pressure inside during the tests'? Hernalsteen: Yes, this has been done and has been discussed. We measured the stresses induced by the shotpeening operation and added the operating stresses, including not only the thermal and pressure stresses but also the residual stresses as might result from the mill processing of the tube and from the shop rolling (at diameter transitions). (Just the processing of the straight tubes initiates circumferential stresses which might be rather high.) If we take the same kind of temperature and stress dependency (not necessarily a real threshold, as people are not sure that such a thing exists but a kind of power law dependence
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on stress) as has been assumed in the literature, we find that the cracking risk in the OD peened condition is significantly less than in the initial ID unpeened condition. Chairman." Now back to the BWR side. We heard this morning about weld overlays, pipelocks and compressive measures to handle stress corrosion cracking. Our next presentation will be by Mr Neils of Northern States Power Company, Minneapolis, who will talk about the 'Monticello Recirculation Piping Replacement Problems, Contingencies, and Evolving Work Scope'.
Paper 5 by Neils Chairman: We are now open for questions for Mr Neils. Neils (Northern States Power Company, Minneapolis, Minnesota) (comment added by author): I have one final comment. For a task of the scope described nothing is ever as simple as planned.
Chairman: You have applied IHSI to some joints, you have used nuclear grade materials; is any consideration being given by you to the use of hydrogen water chemistry or do you believe you are sufficiently covered by your present remedies for the remainder for the life of the reactor? Neils. For welds involved in this task, IHSI was applied to all except for two nozzle-to-fitting welds for the jet j u m p instrumentation nozzles. We did use the nuclear grade stainless steel, we did take extra special care in full construction practices, and believe that these together have reduced the probability of future stress corrosion cracking to an acceptable level of risk. However, we are contemplating the use of hydrogen addition chemistry. We are well along in those plans; the purpose or motivation behind that is to reduce the corrosion susceptibility of the vessel internal piping and vessel internal weldments rather than the recirculation piping itself.
Chairman: Thank you very much.
Paper 6 by Hunyadi Chairman." Thank you for your excellent presentation. We have questions from the audience. Vollmer (USNRC, Washington, DC): I was curious as to the depths of
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the cracks that you found in the tubes that were pulled that were not found by the eddy current testing.
Hunyadi (Swedish State Power Board, Vallingby): Usually leakage appears and we have to stop for leakage.
Vollmer: I thought that you indicated that you pulled 12 tubes and found cracks in 11 of the 12 that were not found by eddy current techniques. My question was what were the deepest of the cracks in the 11 that eddy current did not indicate? Hunyadi: They were about half-thickness or something of that size. We have learned that we can detect cracks at about 40-50% depth of wall thickness by eddy current. Then you see something. But for transition cracks these are much harder to detect. Some of the cracks were almost through the tubes before being detected. They are smaller and not so open so that the volume--what is missing or what eddy current is looking for--is very, very small. My answer once again: 40 or 50% deep cracks can be seen by eddy current, but the cracks in the transition region must be many in number for us to be able to see them. Neils: Could you tell us something of your initial plans for replacement. Specifically, is the containment equipment door large enough or must it be enlarged and is the capacity of your crane inside the containment adequate for handling this equipment?
Hunyadi: What you would like is to hear about the next paper--what we are doing to make the exchange. Three different options were studied before the final decision. If we wish to replace the complete steam generator as a unit we either have to open the side wall or open the top. (We could imagine the system as a jar of pickles where you just unscrew the top and pull out three pickles and put back three pickles, then put back the top again.)(Laughter)Why do we wish to take the complete steam generator? There are a number of advantages. ! mentioned one--you do not need to cut and weld inside and thereby you can reduce duties inside and thereby radiation exposure of the workmen since duties can be completed in a shorter time. After full discussion it was decided that we would open the containment in a 6 × 6 meter square opening. This piece of concrete will be put aside and, when the steam generator replacement is over, it will be put back, You believe that this is a joke but this is not. We have a containment which is reinforced with steel cables imbedded in grease. They can be removed and put back. Of course, there is a steel liner to the concrete. We did
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try to do this experimentally before we could be sure that we could put back the piece removed. It can be done---relax the containment, make the hole and when it is over the reinforcement can be put back and stress applied again. The crane has to be modified to enhance the lifting capacity. This means a modification of part of the crane. A third thing we wish to do before the removal is to partially decontaminate the steam generator before the pipe cuts will be done.
Chairman." T h a n k you very much for a most interesting presentation.
Paper 7 by Kussmaul, Blind et al. Chairman." T h a n k you, Dr Blind. Please stay for questions. O'Donnell: I am chairman of the A S M E Committee on Fatigue. I have a c o m m e n t on your last figure--Fig. 17. This is the one where you show crack initiation below the design curve. This has been typical o f our experience. We are finding crack initiation in the low cycle regime under the design curve. The one with 8 ppm being at one-fifth the design cycles is lower than most that we have seen. We typically do not get failures, actual failures, until you get cycles at about three times the design curve. So, I am not sure whether the crack propagation rates in your 8 ppm are faster than normal but, if they are about normal then you have a lot of crack propagation life even at the 8 ppm. This would not be too bad against the design curve.
Blind (MPA, Stuttgart)." F r o m m y side, in the tests you did one could see that crack initiation is very early with respect to design life. Has this been tested with ferritic or with austenitic material because here it is austenitic material?
O'Donnell." Yes, I was talking about failure experience and crack initiation with the austenitic steels. Varga (HSK, Swiss Federal Nuclear Safety Inspectorate): I wish to ask whether with the mechanical stress relieving which was done, did you have a check for the differences in strains, for instance by strain measurements during the tests? If yes, what was the difference?
Blind." Yes, we did measure strain. The cited 0.7% strain Was an equivalent stress calculated from strain measurements. We get this 0-7% strain in a number of cycles. Chairman." T h a n k you very much, Dr Blind. We will be hearing from you again later.
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Paper 8 by Kussmaul, Blind et al. Chairman." I have one question. You have a special machine that grinds the welded pipe inside and follows through with a television monitor in piping replacements. Have you used that same system for non-destructive examination? Blind (MPA, Stuttgart). Yes we do. I can show it on a slide. It can also be used to examine repair welding. The head can be changed as you like for weld root grinding and cladding operations and can provide a radiograph of 360 ° which is of much better quality than the normal procedure by irradiating twice through the wall of the pipe.
Kussmaul (MPA, Stuttgart):
What is the smallest diameter for which that type of equipment is being used?
Blind." 200 mm nominal diameter. Neils. It seems that in American BWRs or PWRs using austenitic piping, one of the problems we have perhaps caused ourselves is the practice of piping fabricators grinding the ID (inside diameter) surface of weld roots to eliminate imperfections in order to get nice clean radiographs for inspection of those welds. Unfortunately, the grinding caused work hardening and so forth in the counterbore area where the high residual weld stresses are. Could you comment on the ID grinding of welds in these materials.
Kussmaul (co-author with Blind): I would propose to ask the nuclear vendor, in this case Dr Bieselt possibly. Bieselt (Krqftwerk Union, Erlangen): What exactly was the question, I could not hear it well? Was the question addressing austenitic or ferritic material? Neils: My question concerned austenitic materials where we have caused ourselves some problems by grinding as stated above.
Bieselt." At first when we started grinding the weld root, we had the same fear that it could influence the inner surface. We conducted some investigations on this point but could not find any points that gave us any idea that a crack could be started because of the grinding. This related to a question about stabilized austenitic piping, did it not? Of course if you are concerned with 304 and 316 (not nuclear grade) I cannot answer. For these, the question remains because we have only
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investigated one steel, type 347 modified (nuclear grade), a stabilized grade. Neils: You have not observed the problem yet? Bieselt." We have no problems. We have ground surfaces and unground surfaces and the question of grinding was addressed mainly to ferritic steels to make sure that ultrasonic testing can find whether you have a crack or a geometrical discontinuity at a root which was improperly welded. To close this discussion, this was the reason we grind these pipes. For instance in one plant we have ground 1300 welds outside the containment. Is there any more discussion on this point? Blind." I completely agree with Dr Bieselt. Together we made a lot of corrosion tests with highly cold-worked stabilized austenite because of this question raised by Mr Neils and we could not find any difference in sensitization in regard to corrosion cracking.
Discussions on Session I (Chairman: K. E. Stahlkopf)
Paper 1 by Ahlstrand and Rajamiiki Chairman: Thank you, Mr Ahlstrand. We now have time for a couple of quick questions. Hutin (Electricitk de France, Paris)." In fact, I have two questions. You said that there is no indication of fatigue crack growth on the sample you took out. Did you try to perform fatigue crack growth analysis using the previous transients and cycles to compare the experimental evidence that there was no fatigue crack growth and what would have given what I would call classical fatigue calculations? My second question concerns dosimetry. Did you encounter some specific problem because of irradiation in performing the repair?
Ahlstrand (Imatran Voima O Y, Helsinki)." Regarding the first question, the only thing we did was to look at the fracture surfaces. We did not find any indication of crack growth or initiation even. So we did not analyze those mathematically any further than indicated in the paper. Concerning the second question, we did encounter radiation problems in two steam generators, where big defects existed and big repair procedures were required. We had to do decontamination. I have some slides I could show on this. I showed you already that we reduced the activity more than 100 times. This shows how we did it. First we used an alkaline procedure, then rinsed with water, then oxalic acid, followed by another water rinse. This procedure was then repeated. You can see from these figures that with oxalic acid we had good performance. The oxalic acid was the main effective decontaminating agent that we used. It helped very much. Finally, we rinsed with hydrogen peroxide to do some passivation of the tubes. 139 Int. J. Pres. Ves. & Piping (25) (1986)--© Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain
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Chairman: Thank you very much. Our next speaker is Dr Norm Edwards of N U T E C H Corporation, in San Jose, California, who will talk about 'Weld Overlay of BWR Flawed Piping'.
Paper 2 by Edwards Chairman: Thank you, Dr Edwards, for an excellent presentation. The floor is now open for questions. Iida (University of Tokyo, Japan): My question concerns Fig. 5 which shows the distribution of residual stress inside the pipe. Have you measured the results of residual stress just on the surface of the weld metal? I wonder if you have found any evidence of the decrease of residual stress on the inside surface of the weld?
Edwards (NUTECH Inc., San Jose, California)." One slide I showed deals with residual stress; in the written version there are many graphs of such data. The one I showed is representative and shows actual residual stresses on the inside of the pipe. The points identified in Fig. 5 with the number 'one' are in the heat affected zone of the original weld.
Iida: Yes, but I wonder if the residual stress inside the weld metal is also decreased on the compression side or not? I mean the stress at the center line of the weld metal.
Edwards: On the plots which you have the trend I showed, the last point identified as '1' is in the heat affected zone of the original weld. The analytical predictions, which I indicated, bound the test results and have a trend which is similar, but the compressive stress within the weld itself is not as compressive as in the heat affected zone and the area removed from the weld. However, the fact that the analytical predictions are still compressive and that these predictions bound the test results remote from the weld would lead me to believe in fact that we still have compressive stresses in the original weld.
Chairman. Do we have another question? Maurer (Commission of the European Communities, Brussels): As a consequence of this intergranular stress corrosion in reactor pipes, do you intend to modify for future plants the piping materials used? Edwards: The planned pipe replacement programs you will hear about;
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at least you will hear one description this morning. These give a great deal more attention to the selection of material and handling of that material. The replacements are usually done using the 316 N G (nuclear grade) material with emphasis being on the low carbon content. Just as important as materials selection, however, is the handling of that material--an issue which has received a great deal more consideration in replacement than was the case in the original construction.
Chairman: One more question. Schulz (GRS, Ki~ln): Do you have any experimental results on crack growth under typical environmental conditions on actual specimens of this type? Edwards." Yes. I am not intimately knowledgeable on the subject but there is a wealth of information on crack growth rates that have been determined from experiments sponsored by EPRI in the General Electric test facility and other sources.
Chairman: We do have environmental tests going on now with weld overlays of both small pipes and one full size (26-inch) pipe. Our biggest problem is getting cracks to grow at all. The compressive stress fields are indeed very large and as a result one must use stress much larger than would be seen under operating conditions to get the cracks to grow at all. So it is a very satisfactory method to halt the cracks. Hemsworth (National Installations Inspectorate (Nil), UK)." That was my question--are any of these weld repairs under test to predict performance?
Chairman." Yes, we do have one long-term test going on at nominal operating conditions. Actually it is at 110% of design stress. That is a full 26-inch diameter pipe. The tests are going on in fully oxygenated 200-ppb water which is typical of the BWR environment and, observing one of the cracks, we have been unable to make it grow. It has been under test for about 18 months now. We now predict that we will not see a crack move. Chairman." We will now hear about an alternate method of looking at what to do about cracked pipe in Boiling Water Reactors. From Dr William O'Donnell we will hear about the 'Applicability of Pipelocks as a Remedy for Intergranular Stress Corrosion Cracking in BWRs'-another approach to the same problem we just discussed.
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Paper 3 by O'Donnell and Hampton Chairman: That was a most interesting presentation, not only saying some things about the alternate methods to IHSI and repair welding but also raising some interesting points about them. I am sure there are some questions or statements from the audience on this paper. G. H. Neils (Northern States Power Company, Minneapolis, Minnesota): I have two questions: The first concerns your representation that the pipe clamp for a crack weld remedy satisfies the requirements of ASME Section III. To my knowledge, Section III does not allow the presence of an original defect in the pipe. I can understand how you made design of the pipe clamp, in and of itself, meet Section III but how do you address the issue of potential leakage which in the regulatory realm becomes just as important as structural integrity once it is discovered?
O'Donnell (0 'Donnell & Associates Inc., Pittsburgh, Pennsylvania): The pipelock does not seal the pipe at all. It is not intended to prevent leaks. If you have a leaking problem you put a weld patch, or seal basically, on the leak area. With respect to Section III of ASME Code, the pipelock picks up the load and takes it around the crack, so the pipe wall up to the point where it is carrying a load into the pipelock meets Section III. In other words we did the analysis including the pipe wall but the load does not go beyond where the two clamps are, on either side of the weld. The load passes up through the wall to the pipelock. So both the pipe wall and the pipelock are designed to Section III and meet the Code allowables for that condition. Neils: In other words you treat it as though it were a Section III flanged joint. Is that correct? Except, of course for the leakage question.
O'Donnell." Yes, precisely, it is like a flanged joint in Section III. /Veils: My second question regards'the use of mechanical stress improvement. In my observations there are many ways of creating corrosion sensitization besides that of merely creating sensitization from heat during welding, that is, the carbide precipitation in the heat affected zone. I think things like cold work and abnormal strain energy, surface condition and so forth have shown up, not frequently, but still in real circumstances of stress corrosion cracking in piping components in the absence of heat sensitization. How do you address the matter of the residual inside diameter surface tensile loads at or beyond yield in the
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area of the transition each side of the section which has been compressed circumferentially?
O'Donnell: The mechanical stress improvement process applies only compression at the weldment. In areas away from the weldment on the other side of the two clamps, Mr Neils is talking about the fact that the bending in the pipe wall is tension. That is correct. The tension over there, however, is much smaller than the tension from induction heating directly in the weldment, on the inside. We feel that the tension away from the weldment is not going to cause any sensitization of any material away from the weldment. I should mention also that the mechanical process leaves the circumference of the pipe smaller than it was when we started and by the amount of 1½ times yield approximately, 0.3% strain. This closes surface discontinuity and we feel that the residual compressive strain in addition to the residual compressive stress inhibits crack growth. With the thermal processes of repair you cannot apply a residual compressive strain at the weldment. Kiss (General Electric, San Jose, California): I have a number of concerns related to the presentation you have made on mechanical stress improvement. First, I do not believe you have completely presented differences between IHSI and mechanical stress improvement. My concerns are two: one is institutional and the second is technical. In the institutional sense before IHSI was implemented in the USA, even though it had been developed and used in Japan, we went through a very extensive qualification program to demonstrate that it indeed was a remedy to stress corrosion cracking. We demonstrated not only that it could be applied with no inherent problems related to the process but also, once done, it would indeed produce conditions to allow plants to operate for long periods of time without stress corrosion problems. This type of qualification has not been done on the mechanical stress improvement process. O'Donnell." No, that is not true. Are you asking a question or making a statement? Kiss: I am making a statement. You may comment on any statement, of course, but so far as I am concerned they have not been done. There may be something in progress. My statement is that such qualification tests have not been done to date. I have a second point. With respect to mechanical stress improvement as a potential remedy for an operational plant, my concern is somewhat
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similar to original concerns about IHSI, that, depending upon the precision of the UT inspection that is done, which is not 100%, you can indeed have undetected IGSCC in the weld. Now, when you apply the mechanical stress improvement process (MSIP), and cause the full cross section of the pipe to yield, you have a very high probability of opening up cracks or fissures and producing a condition that, in fact, is worse after the process is complete than that existing before you started. That would not be true on a new application, but there is an additional concern that I have where we used the type 316 N G material. That is consistent to what Mr Neils was saying, that is, we know that cold work can produce a susceptibility for IGSCC. The one thing that mechanical stress improvement does that the IHSI process does not do is produce a significant amount of cold work. Since the effect of cold work induced by the MSIP process is unknown, this represents a significant unknown that could only be qualified by extensive testing prior to use. I have one other point, that is that the mechanical stress improvement process you have described is final. The nice thing about IHSI is that if you do the process and monitor the temperatures, you have a very high degree of assurance as to what the thermal gradients were and reasonable assurance as to the residual stress picture. If the gradients have not been obtained, you can do it again. IHSI is strain controlled so the resulting residual stress on the inside diameter will be compressive; whereas the mechanical stress improvement process as presented will simply relieve the welding residual stresses that were present and will not generate significant residual compressive stresses on the inside diameter. In-service operational tensile stresses are added so that after the mechanical stress improvement process you may have little compression on the inside diameter. Again, the question arises as to how good this process is in the long term. This has to be addressed in the laboratory and the results made readily available for independent technical review. O'Donnell: Let me respond to all three points you have made. The third comment was about the residual compressive stress. The finite element analysis, the tests and the basic mechanics of the process show that you get very high compressive residual stresses at the end of the process. This is true in both the axial direction and in the hoop direction. We think that they are much more repeatable and reliable than they are with induction heating.
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Your second point concerned cold working. The difference between induction heating and mechanical stress improvement must be noted. Induction heating strains the material in tension at the inside surface. If you calculate the stresses elastically you get about 300 000 psi, so you are talking about something like ten times yield in tension. That is cold working the material in tension. We feel that this makes it more susceptible to stress corrosion cracking. We are cold working the material in compression and believe that this helps. So, I would say that if there is any cold working concern it definitely favors mechanical squeezing over induction heating. Your first concern had to do with acceptability and testing. We have run many, many tests. Maybe we have a competition between GE and Westinghouse arising here. Westinghouse is applying this process and had independently verified our results. When you squeeze the pipe by a known amount you will get very repeatable results. And, that is what we are getting. We have also done the magnesium chloride testing. So, it is our opinion that this process is more reliable than any thermal process. The first comment by Dr Kiss also concerned acceptability. You saw in my presentation, that we presented this to the Nuclear Regulatory Commission and they have given the utilities permission to use it on a 5059, which means you do not have to send in a report justifying the use of it from a safety viewpoint. (It is not a safety issue, so to speak.) The utilities currently are considering applying this process to their plants in the next couple of months. The other point that Ed Kiss raised was use of the process in old plants. I will tell you this, if I had an old plant and planned a stress improvement, I would want to do it in compression. I do not see how hoop compression can open any cracks anyway. This is my own opinion anyway.
Paper 4 by Frederick and Hernalsteen Chairman: I have looked with some interest at proposals for rotopeening and shotpeening as a way of solving problems particularly in the transition area. One thing that has concerned me is that both procedures lead to a very high degree of cold work on the surface. As a result of thermal expansion and contraction, sometimes in the past we have seen problems with this high cold worked layer opening or fissuring, then allowing, in essence, a crevice corrosion situation to take place where
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such opening has occurred. Have you considered this or done any longterm environmental tests involving thermal cycfing to perhaps allow the area cold worked by roto- or shotpeening?
Hernalsteen (Traetionel, Brussels): Do you mean that the surface layer could open up for the remaining part of the tube? Chairman: Yes. There have been situations where the highly cold worked surface formed by rotopeening, when subject to repeated thermal cycles has, in essence, broken in places thereby allowing crevice corrosion to take place. Have you tested the rotopeened surface under repeated thermal cycles to insure that this is not a problem in this material? Hernalsteen: No. No such long-term investigation with thermal cycling has been done. However, we believe that cold working is just as effective in producing the expected compressive stress. It is not a very strong cold working either (no detectable grain deformation). The residual compressive stress is in the range of about 600 MPa with a gentle curve down to zero at a depth of about 0.1 to 0-2 mm, in a progressive way. In none of the tests that we have made (not with long-term cycling) have we observed any tendency toward cracking but we cannot give the answer in the terms you have raised in your question. Hutin: I suppose that stress corrosion cracking involves phenomena which are related to some type of threshold in terms of stresses, K, and so forth. You said that on the outside of the pipe diameter you had quite low residual stresses so you would not expect any stress corrosion on the outside. Now, when you performed the corresponding tests, did you think about putting the pressure inside the tube because in fact, even if the residual tensile stresses are quite small like 5-6 ksi, if you add these stresses to the operating stress it may be a problem of a threshold being just barely exceeded. Did you think about putting a pressure inside during the tests'? Hernalsteen: Yes, this has been done and has been discussed. We measured the stresses induced by the shotpeening operation and added the operating stresses, including not only the thermal and pressure stresses but also the residual stresses as might result from the mill processing of the tube and from the shop rolling (at diameter transitions). (Just the processing of the straight tubes initiates circumferential stresses which might be rather high.) If we take the same kind of temperature and stress dependency (not necessarily a real threshold, as people are not sure that such a thing exists but a kind of power law dependence
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on stress) as has been assumed in the literature, we find that the cracking risk in the OD peened condition is significantly less than in the initial ID unpeened condition. Chairman." Now back to the BWR side. We heard this morning about weld overlays, pipelocks and compressive measures to handle stress corrosion cracking. Our next presentation will be by Mr Neils of Northern States Power Company, Minneapolis, who will talk about the 'Monticello Recirculation Piping Replacement Problems, Contingencies, and Evolving Work Scope'.
Paper 5 by Neils Chairman: We are now open for questions for Mr Neils. Neils (Northern States Power Company, Minneapolis, Minnesota) (comment added by author): I have one final comment. For a task of the scope described nothing is ever as simple as planned.
Chairman: You have applied IHSI to some joints, you have used nuclear grade materials; is any consideration being given by you to the use of hydrogen water chemistry or do you believe you are sufficiently covered by your present remedies for the remainder for the life of the reactor? Neils. For welds involved in this task, IHSI was applied to all except for two nozzle-to-fitting welds for the jet j u m p instrumentation nozzles. We did use the nuclear grade stainless steel, we did take extra special care in full construction practices, and believe that these together have reduced the probability of future stress corrosion cracking to an acceptable level of risk. However, we are contemplating the use of hydrogen addition chemistry. We are well along in those plans; the purpose or motivation behind that is to reduce the corrosion susceptibility of the vessel internal piping and vessel internal weldments rather than the recirculation piping itself.
Chairman: Thank you very much.
Paper 6 by Hunyadi Chairman." Thank you for your excellent presentation. We have questions from the audience. Vollmer (USNRC, Washington, DC): I was curious as to the depths of
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the cracks that you found in the tubes that were pulled that were not found by the eddy current testing.
Hunyadi (Swedish State Power Board, Vallingby): Usually leakage appears and we have to stop for leakage.
Vollmer: I thought that you indicated that you pulled 12 tubes and found cracks in 11 of the 12 that were not found by eddy current techniques. My question was what were the deepest of the cracks in the 11 that eddy current did not indicate? Hunyadi: They were about half-thickness or something of that size. We have learned that we can detect cracks at about 40-50% depth of wall thickness by eddy current. Then you see something. But for transition cracks these are much harder to detect. Some of the cracks were almost through the tubes before being detected. They are smaller and not so open so that the volume--what is missing or what eddy current is looking for--is very, very small. My answer once again: 40 or 50% deep cracks can be seen by eddy current, but the cracks in the transition region must be many in number for us to be able to see them. Neils: Could you tell us something of your initial plans for replacement. Specifically, is the containment equipment door large enough or must it be enlarged and is the capacity of your crane inside the containment adequate for handling this equipment?
Hunyadi: What you would like is to hear about the next paper--what we are doing to make the exchange. Three different options were studied before the final decision. If we wish to replace the complete steam generator as a unit we either have to open the side wall or open the top. (We could imagine the system as a jar of pickles where you just unscrew the top and pull out three pickles and put back three pickles, then put back the top again.)(Laughter)Why do we wish to take the complete steam generator? There are a number of advantages. ! mentioned one--you do not need to cut and weld inside and thereby you can reduce duties inside and thereby radiation exposure of the workmen since duties can be completed in a shorter time. After full discussion it was decided that we would open the containment in a 6 × 6 meter square opening. This piece of concrete will be put aside and, when the steam generator replacement is over, it will be put back, You believe that this is a joke but this is not. We have a containment which is reinforced with steel cables imbedded in grease. They can be removed and put back. Of course, there is a steel liner to the concrete. We did
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try to do this experimentally before we could be sure that we could put back the piece removed. It can be done---relax the containment, make the hole and when it is over the reinforcement can be put back and stress applied again. The crane has to be modified to enhance the lifting capacity. This means a modification of part of the crane. A third thing we wish to do before the removal is to partially decontaminate the steam generator before the pipe cuts will be done.
Chairman." T h a n k you very much for a most interesting presentation.
Paper 7 by Kussmaul, Blind et al. Chairman." T h a n k you, Dr Blind. Please stay for questions. O'Donnell: I am chairman of the A S M E Committee on Fatigue. I have a c o m m e n t on your last figure--Fig. 17. This is the one where you show crack initiation below the design curve. This has been typical o f our experience. We are finding crack initiation in the low cycle regime under the design curve. The one with 8 ppm being at one-fifth the design cycles is lower than most that we have seen. We typically do not get failures, actual failures, until you get cycles at about three times the design curve. So, I am not sure whether the crack propagation rates in your 8 ppm are faster than normal but, if they are about normal then you have a lot of crack propagation life even at the 8 ppm. This would not be too bad against the design curve.
Blind (MPA, Stuttgart)." F r o m m y side, in the tests you did one could see that crack initiation is very early with respect to design life. Has this been tested with ferritic or with austenitic material because here it is austenitic material?
O'Donnell." Yes, I was talking about failure experience and crack initiation with the austenitic steels. Varga (HSK, Swiss Federal Nuclear Safety Inspectorate): I wish to ask whether with the mechanical stress relieving which was done, did you have a check for the differences in strains, for instance by strain measurements during the tests? If yes, what was the difference?
Blind." Yes, we did measure strain. The cited 0.7% strain Was an equivalent stress calculated from strain measurements. We get this 0-7% strain in a number of cycles. Chairman." T h a n k you very much, Dr Blind. We will be hearing from you again later.
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Paper 8 by Kussmaul, Blind et al. Chairman." I have one question. You have a special machine that grinds the welded pipe inside and follows through with a television monitor in piping replacements. Have you used that same system for non-destructive examination? Blind (MPA, Stuttgart). Yes we do. I can show it on a slide. It can also be used to examine repair welding. The head can be changed as you like for weld root grinding and cladding operations and can provide a radiograph of 360 ° which is of much better quality than the normal procedure by irradiating twice through the wall of the pipe.
Kussmaul (MPA, Stuttgart):
What is the smallest diameter for which that type of equipment is being used?
Blind." 200 mm nominal diameter. Neils. It seems that in American BWRs or PWRs using austenitic piping, one of the problems we have perhaps caused ourselves is the practice of piping fabricators grinding the ID (inside diameter) surface of weld roots to eliminate imperfections in order to get nice clean radiographs for inspection of those welds. Unfortunately, the grinding caused work hardening and so forth in the counterbore area where the high residual weld stresses are. Could you comment on the ID grinding of welds in these materials.
Kussmaul (co-author with Blind): I would propose to ask the nuclear vendor, in this case Dr Bieselt possibly. Bieselt (Krqftwerk Union, Erlangen): What exactly was the question, I could not hear it well? Was the question addressing austenitic or ferritic material? Neils: My question concerned austenitic materials where we have caused ourselves some problems by grinding as stated above.
Bieselt." At first when we started grinding the weld root, we had the same fear that it could influence the inner surface. We conducted some investigations on this point but could not find any points that gave us any idea that a crack could be started because of the grinding. This related to a question about stabilized austenitic piping, did it not? Of course if you are concerned with 304 and 316 (not nuclear grade) I cannot answer. For these, the question remains because we have only
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investigated one steel, type 347 modified (nuclear grade), a stabilized grade. Neils: You have not observed the problem yet? Bieselt." We have no problems. We have ground surfaces and unground surfaces and the question of grinding was addressed mainly to ferritic steels to make sure that ultrasonic testing can find whether you have a crack or a geometrical discontinuity at a root which was improperly welded. To close this discussion, this was the reason we grind these pipes. For instance in one plant we have ground 1300 welds outside the containment. Is there any more discussion on this point? Blind." I completely agree with Dr Bieselt. Together we made a lot of corrosion tests with highly cold-worked stabilized austenite because of this question raised by Mr Neils and we could not find any difference in sensitization in regard to corrosion cracking.