Is There Stress Corrosion Cracking in Pickering ‘A’?

Dr. Michael Moles
Transcript of CNSC Public Hearing
December 14, 2000

THE CHAIRPERSON: We will then move to the oral presentation by Michael Moles. That is presentation 00-H29.13A.

MR. MOLES: Good evening, Dr. Bishop, Commissioners and everyone else.

My name is Michael Moles. I was a long-time employee of Ontario Hydro Technologies and a reactor inspector. The question we have today is: Is there stress corrosion cracking in Pickering A?

What it really means is: Are Pickering A reactors safe? And in safe we are really referring to: Can we have a multiple LOCA?

The first question is: Is there stress corrosion cracking at Pickering A? I am going to try and do this in only semi-technically terms, because this is basically a technical presentation.

The most serious concern we have is the circumferential stress corrosion cracking in the feeder pipes, and we will get through all that in a few minutes.

First of all, a history of CANDU problems, many of which I have actually worked on.

The CANDU reactor is unique, as you all know, and it has certain generic problems. The history of CANDU, at least from an operating point, is a history of surprises. Something goes bang, and we can jump in and do something about it.

We have had a series of problems. There has been a string of pressure tube problems. there has been a number of steam generator problems, and now we seem to be looking at feeder pipe problems, all from the primary heat transport system.

The solution almost invariably is OHN or OPG, or OPN as it may be called now, has engineered the CANDU system out of its problems. They have changed pressure tubes. They have fixed up steam generator tubes, and so forth.

I will quickly run through first pressure tubes and steam generators and then on to feeder pipes.

The CANDU pressure tube reactor is a very interesting and novel design. I think it is quite a good one actually. It does have unique problems. It is a unique system. And invariably the reactors have the final say on the problems.

What has happened is that all the researchers I used to sit next to would predict that there would not be a problem with creep, but unfortunately they got the creep in the wrong direction, this kind of stuff.

One of the problems is that simply the scientific knowledge is limited, and there is very limited amount of in-reactor data.

Here is an example. Pressure tubes roll joint cracking. The reactor got it right; we didn’t.

Axial versus circumferential creep: the reactor got it right.

Deteriorating ingress, hydride blisters — and that was PG16 — the reactor got that one right.

Fracture toughness and some more recent ones, manufacturing defects. Bruce 2 NO6 is a very good example. Darlington Vibration, that was reactor. Debris fret marks, again that came from the reactor. Abnormal fuel support marks, we found that out from the reactor. Hard garter spring detection, which is a Point Lepreau problem, we found that out from the reactor.

Basically, the reactors win them all. So our target shooting is not that good.

With steam generators we have done slightly better, because other people have steam generators, even though we have different materials, particularly in Pickering A, and we do have some different operating conditions.

Again, the reactors have the final say.

Once one problem starts to appear, typically there is another bunch. Again we go to the same solutions: inspect, repair, replace, plug, control chemistry and operating conditions.

There is the score on steam generators, most of which you know. And it is not complete.

Feeder pipes seem to be the latest CANDU-specific component to start to show some problems. The only two that have really popped up so far are erosion/corrosion, which is universal, and the one axial SCC problem failure that we had in Point Lepreau.

Unfortunately, there has been very little inspection performed. The only inspection has been ultrasonic wall thickness measurements for erosion/corrosion, and this is not adequate at all to find any form of cracking.

We have very little knowledge on the state of material. To the best of my knowledge, nobody has done anything like strain gauge steam feeder pipes.

So we have to ask the question: Are there other potential failure mechanisms coming?

We can look at the feeder pipes over here. We can see we have these two over here: the erosion/corrosion, the axial stress corrosion cracking. These are another bunch of typical problems that can occur. After all, these are just carbon steel pipes. There is nothing particularly special about them, except they are an unusual material.

What are feeder pipes?

They are long carbon steel pipes that feed the PHT coolant into and out of the reactor, which essentially cools the fuel. The main problem, from an inspection point of view, is that they are very inaccessible. They are surrounded by end-fittings, and most of the internal portions or the long length of them can’t be touched at all from that side.

In practice, they are long, bent, somewhat dirty and corroded. Their radiation fields are very high. You can really only hang on to them for a few minutes.

Unfortunately, they are so close together that you can barely push your hand through. So there are a lot of physical constraints in inspecting feeder pipes.

This is pretty much what they look like. This is the face of the CANDU reactor over here, and these are all the feeder pipes coming off. There is one per pressure tube or fuel channel, so there are hundreds and hundreds of these things. These lengths here can rum up to 10 metres on a strip.

Typically, the access is very limited at the front and a few points along here unless you get into the feeder cabinet itself.

Along these zones here there is virtually no access at all.

This is what they look like close up. These end-fittings are two-and-a-half metres long, so you can literally only put your arm in and just catch the first bit down here near the graylocs.

The geometry is complex. There are several different sizes.

What happens if a feeder pipe fails?

If you get a “break before leak” situation, especially with a guillotine failure, then you can have some serious problems.

The first problem is you lose your coolant. The other problem is you potentially get whiplash. These feeder pipes are very long lengths with no supports whatsoever. If one of them goes under high pressure and high temperature, they can whip around and start smacking into other components. This is a relatively well-known problem in the engineering world.

This means that we can have potential multiple failures.

These feeders are typically bunched together in groups of nine. I was only at Point Lepreau, and I think it is nine at Point Lepreau. If you lose nine channels, you have a problem.

The other problem is that we don’t even know where they are going to fail, because there is an awful lot of places where they could fail.

So there is a number of serious failure mechanisms: manufacturing defects — we can forget that one for the moment; erosion/corrosion, probably not likely; corrosion fatigue, possible; stress corrosion cracking, very possible.

Just a few words about SCC.

It is not a well understood phenomenon. Nobody can predict whether you really have it or not. It kind of happens. It is a big problem in gas pipelines.

You need three things: you need a tensile stress; you need a material that can crack; and you need a suitable environment.

It is a very slow growth mechanism. It can take decades, and it can produce brittle failures, and very brittle failures with no warning.

It is not widely common in carbon steels, but it has happened in Point Lepreau feeders and it has also been seen in Pickering A calandrian vault. And it has been seen in a number of other things like gas pipelines. That is really the example right now.

This diagram comes from the NEB on gas pipelines. You need your three conditions: your environment; your tensile stress; and your pipe material.

We know from the fact that there has been one stress corrosion crack failure in Point Lepreau that you have to have your material susceptible and your environment must be susceptible.

You have these two factors already, so all you are really looking for is the stress.

These are the cracks. They are nasty little objects that wander all over the screen. They are typically multi-faceted and hard to find, and they can remain dormant for decades. You can discover that your material is full of nasty little cracks.

Can Pickering have SCC?

Well, the material and environment are suitable. The stress could easily come from bending. Those feeder pipes, if you remember the pictures, are very long; and as your reactor heats and cools during start-up and shut-down, you get thermal type stresses.

In the thermal portion of Ontario Hydro, as it used to be, they used to have this problem in a very serious fashion, because they were always what they called two-shifting, which is starting up in the morning and shutting down in the evening. There were all kinds of cracks in there.

If you have that kind of situation going on in Pickering, then you potentially have circumferential stress corrosion cracking and possible failure.

Quite frankly, there is nobody in this room or anywhere who can tell you whether we have SCC in there at the moment. Maybe it has and maybe it has not. It is not something that we can predict, like fatigue cracking, and so forth.

Unfortunately, very few inspections are being performed. There is high dose and very limited access, particularly for cracking. What has been done has been for erosion/corrosion, and that does not count.

To the best of my knowledge, there have been very few analyses done. I don’t think anybody has done any strain gauging. There is probably not much point in doing any stress analysis because we have so little idea as to what the actual stresses would be.

We didn’t find any SCC at Point Lepreau, but it was a relatively new reactor compared with Pickering A.

What we need to do is inspect; and if we find something, we need to repair it, weld it, replace it, whatever is the appropriate thing to do. But above all, we need to inspect.

I will give you a few conclusions.

First of all, the CANDU history indicates that we are going to get some more failures. This has been an historical fact. Quite frankly, the most likely candidate is the Pickering A feeder pipes. They have been totally neglected, just like pressure tubes were before PG16.

The most likely candidate is probably circumferential stress corrosion cracking. The problem with this is that it could lead to a LOCA and multiple channel failure.

Again the solution is engineering. We don’t want to go through a lot of R&D stuff. R&D is not helping us at all in stress corrosion cracking.

What we would like to recommend is we select the reactor with the most effective starts, whichever one of the Pickering A reactors it is. We can work that out. And effective here means that a brutal shutdown or start-up is likely to be more effective than a lot of gentle ones.

We use ultrasonic guided waves as a “go-no go” sorting tool. These we can fire down those long, long pipes, which are quite suitable for this kind of a technique, from the few accessible locations: the corners of the Graylocs and maybe from up in the feeder cabinet.

Then we need to develop a little scanner that runs along selected feeder pipes to fully analyze any damage. Hydro-Quebec has already developed a little scanner like this. It is actually for erosion/corrosion, but it is the kind of device that could be modified to look for cracking. This is a feasible technology.

When we find out what is actually in there, somebody can make an intelligent decision.

Just in case you are interested, these are my qualifications. I am grossly over-educated. I have spent almost 20 years in automated ultrasonics. I also spent three weeks in Point Lepreau and did quite a lot on feeder pipes, developing the technique, writing the procedure, and actually crew bossed one of the feeder pipe inspections.

Thank you very much.

THE CHAIRPERSON: Thank you, Mr. Moles.

We frequently have issues over feeder pipes brought before us.

Are there any questions?

Dr. Giroux.

MEMBER GIROUX: This is a very interesting presentation. I would like to hear reaction from OPG.

MR. STRICKERT: Mr. Pierre Charlebois, our Chief Nuclear Engineer, will speak to this.

MR. CHARLEBOIS: Thank you and good evening.

First of all, I would like to give you a bit of an overview of our life cycle program that we have in place for the feeder pipes that were referenced in the presentation.

We have under way, in fact, a program to inspect many of these feeder pipes with some of the tooling that is being discussed here. In fact, we are looking at making use of the GenTE(ph.) tooling itself for measuring the thickness and the conditions of the feeder pipes.

Maybe we need to put this into context.

We have currently 300,000 feeder operating years of experience. That is 28,000 feeder pipes for 15 years. We have had one failure.

The failure that occurred at Point Lepreau was an actual crack that formed in the feeder pipe. The conditions that led to that particular failure are a bit unique. The pipe supports were not in the correct location. The fuel channel itself was not locked into proper position.

The chemistry conditions on the heat transport side on the inside of the pipe were somewhat unusual at the time as well as a result of a separate incident that occurred. Therefore, it is entirely possible that the mechanism that took place was in fact caused by those particular conditions all acting together, as indicated by the presentation, causing the stress corrosion cracking.

In our particular plants we do not have the same support arrangement. In making sure that the supports are free to move, to make sure that the stresses are maintained below acceptable level, is something that we continuously pay attention to. In fact, we do adjustments on the reactor to move those supports and make sure they are in the proper position.

The last point I would like to talk about is we actually monitor the chemistry conditions of the system as well, to make sure that we don’t have a condition that could cause essentially attack of the material itself.

In a nutshell, we have a comprehensive program as we speak today at the Darlington plant. We are doing 100 per cent inspection of an elbow area where we see the most evidence of not stress corrosion cracking but erosion/corrosion.

Those in fact are being measured at the Darlington plant. This is an ongoing program to baseline and characterize the state of the feeder pipes, to make sure that we manage the life of the components appropriately.


MEMBER GIROUX: Essentially, if I understand correctly, you are sort of in agreement with Mr. Moles and acting on this with engineering solutions and inspections?

MR. CHARLEBOIS: Of course the discovery of I believe it was Channel S08 at Point Lepreau when the incident took place caused us to re-examine the scope and the completeness of our programs for feeder pipes. That has been expanded considerably in the industry over the years.

In fact, we have a considerable program in place already to assess the conditions of our feeder pipes.

MR. MOLES: May I comment on that, please?

THE CHAIRPERSON: Yes, go ahead. Then I want some comments from staff.

MR. MOLES: All the inspections being done at Darlington are irrelevant to stress corrosion cracking. They look exclusively for erosion/corrosion. You need an entirely different technique.

Second, anything — the water chemistry may or may not be important. With stress corrosion cracking, it crops up under a wide variety of conditions. It still does not mean that even if the chemistry is different at Point Lepreau from what it is in Pickering, that you are not going to get stress corrosion cracking.

THE CHAIRPERSON: I do want to look at the relationship to EA versus the operational maintenance and inspection aspects, but I do want some comment from staff.

MR. WADDINGTON: Thank you, Madam Chair.

First of all, the EA itself includes a nuclear accident, as we discussed before, which captures the accident. This is clearly a detail as part of the licensing review.

I am going to ask Mr. Jim Blyth to discuss the response in some detail; thank you.

Mr. Blyth.

MR. BLYTH: Thank you very much.

I have a couple of thoughts.

Certainly the accident that has been included in the EA bounds any credible consequences of a feeder failing. So I don’t believe that Dr. Moles’ presentation has any fact on what has been predicted or assessed in the EA.

OPG is right. There are over 300,000 feeder years of experience in Canada alone. There is another almost 5,000 feeders in overseas reactors. We have seen this once in quite a unique situation.

Certainly it is something that should be addressed going forward by inspection programs, having seen it once.

The other comment I would make is that steam generator tubes and certainly pressure tubes are rather exotic materials. Carbon steel is not quite as exotic. It is widely used in the nuclear industry in the process systems and all sorts of other industries.

There is a large body of experience with respect to this material, and stress corrosion cracking is a pretty rare occurrence.


MEMBER BARNES: I would like to get clarification from Mr. Charlebois.

Do I understand that you will be examining the feeder pipes in Pickering A for stress corrosion cracking in the near future; and if the answer is yes, could you give us some indication of the scope of that investigation?

MR. CHARLEBOIS: I would like to clarify that with my colleagues. There is a program to examine the conditions of the feeder pipes on the Pickering reactor, certainly to measure the thickness of the feeder pipes.

I do not have at my fingertips the scope of that particular program at the present time.

I think you should note that of all the reactors, in terms of erosion/corrosion, the Pickering reactors are the least susceptible to the mechanism. We have observed that in the past. We have measured that in the past.

Therefore the extent of the program on those reactors is of a lesser extent. It is more prominent on the CANDU 6 and the Bruce and Darlington design.

This is the erosion/corrosion program.

In terms of the SCC process, it is very difficult to actually measure stress corrosion cracking in carbon steel unless the mechanism is obviously active. The best remedy is to prevent the conditions from existing.

That is to make sure that the supports to the feeder pipes are in the proper position; that there is no interference between the feeder pipes as you saw in the picture; and that the chemistry conditions of the reactor are maintained within the specifications.

Those aspects are regular programs that we have in place to make sure that those positions are proper in all of our units, including the Pickering reactors.

MEMBER BARNES: I am just trying to correlate a point that Mr. Moles is making with your reply. He is focusing more on the SCC problem rather than erosion/corrosion, and you are saying you are doing erosion/corrosion at Darlington, and you have implied that there is some work going on at Pickering.

I am trying to establish whether you are in fact looking at the SCC problem in Pickering A?

MR. CHARLEBOIS: What we are doing at Pickering A is verifying in fact that the conditions of the supports and that the clearances between the feeders are such that the stresses during operation will not be excessive and therefore will not initiate the SCC process.

I am trying to explain that it is very, very difficult to detect the beginning of SCC in materials using today’s technology. It is a difficult process. You will detect it once a crack has started to form.

At Point Lepreau, for example, after they detected the one failure on S08 — which I might add, by the way, because of the nature of the carbon steel, which is a very ductile material, it was detected before it actually caused any loss of coolant.

They inspected, I believe, close to about 150 to 200 other channels to try to see if there was any evidence of crack formation or propagation, and they did not find any at that time.

THE CHAIRPERSON: Mr. Blyth, you wanted to make a comment?

MR. BLYTH: Yes. I think it is important to recognize that we have one isolated event of this occurring. It did leak before it broke — well, it did leak; it never did break. In fact, under very, very unusual circumstances, highly unusual circumstances that have not been replicated elsewhere.

I don’t believe it has been demonstrated that this mechanism is in fact active in these reactors. I believe the one incident can be explained.


Are there any other questions?

Thank you very much, Mr. Moles.

MR. MOLES: It was a pleasure.

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