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PostPosted: Jul 28, 2015 6:31 am 
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France had a lot more heavy industry in the 70s and 80s when the programme was in full swing.
As did America.


Well unfortunately, I think that the desindustrialization is also partly responsible of the problems on the EPR construction.

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And remember they were building dinky units - the average unit in the French programme is probably bigger than in the American programme - and even then most of the French units were only 900MWe PWRs - hardly big by modern standards.


There are twenty 1300 MWe power plants and four 1500 MWe power plants (the N4). Although it is true that the EPR vessel is different from the previous ones and for now there are only two companies which can do it : Japan Steel Work and Creusot Forge (Areva) if I am not mistaken.

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Actually many ferritic alloys are between moderate toughness and terrible,...


I won’t be able to argue about material and manufacturing methods since I know very little about it but I guess that they choosed ferritic steel for the PWR vessels for good reasons.
And for now it seems it was a good choice since they didn’t have a lot of problems with it (except corrosion problems at David Besse ?). For what I heard the 20 years lifetime extension of the PWR vessels seems plausible.
Also my teacher said us that the ferritic steels have a good weldability, maybe he wanted to say that the weldability is sufficient for what they want : for standard industrial applications it is maybe not good but here they do so many quality controls on the welds that it is maybe not a problem. He said that martensitic steels were rejected because of poor weldability and radiation embrittlement but ferritic steels are OK.

Concerning the nickel vessel I guess that you will have to heavily protect it from the neutron flux, but if it is doable OK it is maybe a superior alternative.

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you can do axial welds


Heretic ! :mrgreen:


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PostPosted: Jul 28, 2015 1:00 pm 
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I won’t be able to argue about material and manufacturing methods since I know very little about it but I guess that they choosed ferritic steel for the PWR vessels for good reasons. And for now it seems it was a good choice since they didn’t have a lot of problems with it (except corrosion problems at David Besse ?). For what I heard the 20 years lifetime extension of the PWR vessels seems plausible.
Also my teacher said us that the ferritic steels have a good weldability, maybe he wanted to say that the weldability is sufficient for what they want : for standard industrial applications it is maybe not good but here they do so many quality controls on the welds that it is maybe not a problem. He said that martensitic steels were rejected because of poor weldability and radiation embrittlement but ferritic steels are OK.


I guess we need to be more careful about the metallurgical definitions. If ferritic steel means low carbon mild steel like what they use in construction... then yes that's weldable and forgiving material. But ferritic stainless steel (for example) is difficult to weld and not as tough and ductile. Similarly high carbon ferritic steel isn't anywhere near as tough as low carbon (often called "mild") steel. Its always a tradeoff in metallurgy. Higher carbon means better strength but poorer toughness and more difficult welding.

Martensitic steels are difficult yes, though hybrid ferritic/martensitic are very popular in fossil steam plants. They are sort of half stainless (3-12% Cr typically) so they have acceptably small corrosion in clean steam (even very hot clean steam). Annoying thing is all the heat treatments you need.

Low alloy steel was probably chosen for PWRs/BWRs simply because its cheaper and stronger than austenitic stainless. Austenitic is a lot tougher, more ductile and more easy to weld though. Plus it has better radiation resistance. So I'm guessing cost and wall thickness reduction were considered more important. I don't get the cost part (as reasoned before) but wall thickness seems like a reasonable argument. If so, stronger alloys would be interesting especially if they are corrosion resistant so you wouldn't need that costly liner.

Even within the A5xx family there are tough but stronger pressure vessel steels such as 514 pressure vessel grade (517 spec). Not sure why these are not used. It may be a case of "we've always done it this way". Its ironic that the nuclear industry often gets itself into trouble with this apparent conservatism. Chloride stress corrosion cracking of 304 and 316 in dry storage being a good example. SCC resistant stainless steels (such as duplex grades) have been available for decades, but the nuclear industry only knows 304 and 316 even though they are well known to be unsuitable in chloride warm environments.


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PostPosted: Jul 28, 2015 1:04 pm 
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fab wrote:
Quote:
you can do axial welds


Heretic ! :mrgreen:


Well not really, its hard to see how a nuclear plant can be constructed without axial welds. Think about steel containment vessels and such. Steel forgings are overrated in that you still need the difficult circumferential weld. I can see how it makes a lot of sense for the vessel area around the core. Above the core, not much point in using forgings. It is much more strategic to use the limited forging capacity to make forgings for below the core parts.


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PostPosted: Jul 28, 2015 3:11 pm 
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Low alloy steel was probably chosen for PWRs/BWRs simply because its cheaper and stronger than austenitic stainless. Austenitic is a lot tougher, more ductile and more easy to weld though. Plus it has better radiation resistance. So I'm guessing cost and wall thickness reduction were considered more important. I don't get the cost part (as reasoned before) but wall thickness seems like a reasonable argument.


Yes that's exactly what my teacher said to us : austenitic steels are easier to weld and have better radiation resistance but the yield strength is too low, the vessel should have been 35 cm thick instead of 25 cm with ferritic and 15 cm with martensitic (if i remember correctly, but i think they are order of magnitude, not computed numbers). But martensitic steels have poor weldability and radiation resistance, so ferritic steels for the win.

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It may be a case of "we've always done it this way". Its ironic that the nuclear industry often gets itself into trouble with this apparent conservatism.


Although I also find the nuclear industry too conservative I can understand here the conservatism concerning the RPV : this component is too critical ( and you know how the requirements for an 80 years lifetime RPV are extremely high ), and I don't think we can replace a RPV, I always heard that the lifetime of the RPV is the limiting factor for the lifetime of the power plant. But I stay open mind, if the nickel alloys are sufficiently radiation resistant or if you can easily protect them with a neutron shield, they are to be considered.


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PostPosted: Jul 28, 2015 5:36 pm 
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Irradiation of the RPV is mostly a PWR issue. BWRs have about 5-15x lower vessel flux, so this isn't the limiting factor for a BWR. For example if PWR has a limit of 40 years then the limit with the same alloy in BWR would be 200-600 years or so. In BWR you don't have boric acid either. Likely for BWR the limit is the structural stuff. Concrete cracking, rebar corrosion, floors getting so strained as to be out of alignment for precision turbine components... stuff like that. On the other hand much of that is fixeable, though costly.

Although perhaps hydrogen embrittlement, or some unknown really long term ageing issue, will end up an being limiting, well before this. I doubt you'd want much more than 100 years lifetime anyway. I mean, would you be happy with being stuck with James Watt's Steam Engine? It would become obsolete, too silly and inefficient for modern times.


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PostPosted: Jul 29, 2015 9:35 am 
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For example if PWR has a limit of 40 years then the limit with the same alloy in BWR would be 200-600 years or so.


That is with the current materials, that's why you have to be sure to not add the neutron embrittlement risk with new alloys.

Quote:
I doubt you'd want much more than 100 years lifetime anyway.


Agree, 80 years should be the goal for a LWR. I have seen that Rubbia planned a lifetime of 200 years for his reactor concept (or maybe it was an error in the doc ?).


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PostPosted: Jul 29, 2015 9:52 am 
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I would be happy with Watt's steam engine if it burned a fuel that costs almost nothing with essentially zero impact on the environment.
The objective is the cheapest possible electricity and a longer reactor life simply increases the amount of electricity produced per dollar invested, even if that electricity is a century from now it is still worth something.

If you saw my posts in the APWR efficiency thread you will see that I am increasingly coming to the conclusion that the LWR reactor technology is sufficiently advanced to solve the world's energy problems indefinitely.


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PostPosted: Jul 29, 2015 12:22 pm 
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That is with the current materials, that's why you have to be sure to not add the neutron embrittlement risk with new alloys.


What I was saying is that BWR isn't vessel fluence limited like PWRs are.

However, the proposed alloys are not "new" at all. Inconel 718 was developed in the 1960's. It has been used in the nuclear industry for umpteen years. It has been used for a long time as nuclear fuel assembly spacers, so we know the irradiation performance.

Also we know that RPV alloys of today do embrittle, quite badly in fact. Belgians are starting to see the effects of cracked PWR vessels now. The standard alloys do not have a very good radiation resistance in fact.

If embrittlement were really considered seriously, PWRs would have employed replaceable neutron shield shrouds internal to the reactor vessel.


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PostPosted: Jul 29, 2015 3:08 pm 
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E Ireland wrote:
I would be happy with Watt's steam engine if it burned a fuel that costs almost nothing with essentially zero impact on the environment.


That's a fair point Ed.

It reminds me of Alvin Weinberg's argument of the "immortal energy system" (as a rebuttal to intergenerational concerns of waste storage and decommissioning, that the anti-nuclear people always bring up).

I suppose the most likely component to be out of date would be the steam system, as 50-100 years from now the steam systems must be a lot more efficient. Or maybe there will be a "new" way of converting thermal to electric, like the infrared nano antennas. But then again that should be pretty much a drop-in replacement (rotating parts don't last forever anyway so must be replaced at some point).


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PostPosted: Jul 29, 2015 4:38 pm 
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the proposed alloys are not "new" at all


The word "new" was just to mean that, as far as I know, these alloys were never chosen to be the main alloy of a commercial LWR RPV (there were used just for some parts of the vessel like the bottom and head penetrations).

Quote:
Also we know that RPV alloys of today do embrittle, quite badly in fact. Belgians are starting to see the effects of cracked PWR vessels now. The standard alloys do not have a very good radiation resistance in fact.


It seems that they will be able to increase the lifetime of the PWRs in USA and France for 20 years more. So it is not so bad. For the Belgians reactor I think I heard that it was due to the manufacturing methods of the vessels and that these problems are not present in the other vessels.


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PostPosted: Jul 29, 2015 5:03 pm 
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Ideally we would be looking for hundred year operating life.

60 is still a bit on a short side to get true intergenerational lifespan I think.
Having a plant that lasts a similar amount to the length of time it takes to decommission and clear the site after shutdown is a big political boon. Especially since once its cleared we can build a new reactor on the site.


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PostPosted: Jul 29, 2015 5:08 pm 
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fab wrote:
It seems that they will be able to increase the lifetime of the PWRs in USA and France for 20 years more. So it is not so bad. For the Belgians reactor I think I heard that it was due to the manufacturing methods of the vessels and that these problems are not present in the other vessels.


Yes, these vessels in Belgium were manufactured by RDM, the Rotterdam Drydock company in the 1970s. This Dutch shipbuilding company (now defunct) sought to expand its business beyond shipbuilding and entered the nuclear energy market, which was booming at the time. But not with much expertise, apparently.

This seems to be a very specialized business, and this capability to forge such large nuclear components is limited to a couple of countries like Japan, China and Russia. The only such facility in Western Europe is the Areva Le Creusot facility in Burgundy, France, as far as I know.


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PostPosted: Jul 30, 2015 6:59 am 
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'This seems to be a very specialized business, and this capability to forge such large nuclear components is limited to a couple of countries like Japan, China and Russia. The only such facility in Western Europe is the Areva Le Creusot facility in Burgundy, France, as far as I know.
Do you know what's happening with the EPR pressure vessel?


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PostPosted: Jul 30, 2015 10:50 am 
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Do you know what's happening with the EPR pressure vessel?


This is what I understood :

In the bottom of the vessel the proportion of carbon in the alloy is higher than in the rest of the vessel, so the toughness is lower in this area. This phenomenon of carbon segregation was known ( and maybe present in the previous vessels) but the surplus of carbon was higher here than expected. The main safety problem about a low value of toughness is that during a cold thermal shock the material will not be able to resist to the propagation of cracks and the vessel will break.

But here it is not a problem because the area where there is a surplus of carbon is in a place where the craks tend to be closed during a cold thermal shock ( the area is compressed and not teared apart ).

The older regulation for the vessel is the RCC-M regulation, and it seems that the EPR vessel is in accordance with this regulation but now we have new regulations for the vessel called ESPN. With this new regulation the value of toughness must be high everywhere so it seems that the EPR vessel is not in accordance with the new regulation.

The vessel was forged in 2005 and 2006 and at that time it was still legally possible to be in accordance with RCC-M and not ESPN. So the vessel is not illegal (if I understood well). But at that time, EDF asked AREVA to be in accordance with the new regulation and the safety authority wants more justifications about the safety of the vessel. The problem of the surplus of carbon was discovered in 2014.
So now there will be a new series of tests and discussion with the safety authority for the validation of the vessel.


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PostPosted: Jul 30, 2015 9:36 pm 
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Just imagine how difficult it is to do quality control and extremely tight chemical composition specification in a 250 mm thick pressure vessel - in a several hundred tonne vessel with complicated geometry and various parts more than 250 mm thick (near nozzles and such). These EPR problems aren't surprising, what is surprising is that so many PWRs have been built with few issues here.

With BWR you go down to some 180 mm with the A508 class. With inconel 718 vessel you can go down further to around 60 mm wall thickness and about 90 mm for bottom thickness, near nozzles etc. Almost sheet metal compared to PWR steel vessels, and there's no carbon in the 718 to begin with. You pay a few million $ more in material cost, but that's peanuts in total plant cost, plus will be easily regained in terms of easier quality control, welding and weight savings... high temperature capability (think invessel retention), and ultra high corrosion resistance of the 718 increase safety to boot.


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