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PostPosted: Aug 05, 2014 5:03 am 
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LWR pressure vessels are constructed of pressure vessel steel (low alloy) with a corrosion resistant liner on the wetted portions of the inside surface.

I don't really understand why this is done. The idea seems to be to reduce cost by using a low cost pressure vessel steel and a thin, more expensive stainless steel liner. But fabricating and welding the liner, with pressure vessel complexities such as nozzles and control rod penetrations and whatnot, is very expensive.

Even a big pressure vessel like the ESBWR, weighs only about 1100 tonnes. If it were made of a really expensive superalloy like inconel 718, it would not need a liner and would weigh only 400 tonnes. It would have about $10 milion in materials cost. I'm pretty sure that a liner costs about the same in fabrication, weld overlay etc. In any case an ESBWR would probably cost more than 3000 million turnkey so a few million difference in the price seems insignificant. It would be much easier to forge such a lighter vessel, much taller ring forgings could be used for instance, reducing the number of welds as well..

So why not use a superalloy and get a lighter simpler vessel that costs about the same and has no risk of liner failure etc.? Why do all LWR designers use duplex construction?


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PostPosted: Aug 08, 2014 4:11 am 
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Canadians had the right idea on pressure vessel. They split it into a few hundred tubes. Easier to produce. Gives you more choice of moderator, coolant and fuel. A dozen easy to handle bundles can be pushed in a tube. You could replace some or all of the tubes when necessary.
If the Indians had not gone for vertical tubes in place of horizontal ones in existing reactors, they would have built a thorium fueled reactor by now. Just changed the fuel bundles in a reactor 'borrowed' for trials before it became due for re-tubing.
Suggestions for an MSR with liquid moderator and fuel in separated compartments have also been made in the forum.
SMSR idea from Moltex also uses tubes. It is perceived as a low cost solution.


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PostPosted: Aug 08, 2014 4:26 am 
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Is there any experience in forging large Inconel vessels?
I am pretty sure they would be the largest superalloy forgings ever attempted by a significant margin.


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PostPosted: Aug 08, 2014 7:04 am 
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jagdish wrote:
Canadians had the right idea on pressure vessel. They split it into a few hundred tubes. Easier to produce.


Yes, but there is also a major downside: the pressure vessel is now right in the middle of the core neutron flux. That limits choice of pressure vessel materials. Basically you need zirconium.


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PostPosted: Aug 08, 2014 7:27 am 
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E Ireland wrote:
Is there any experience in forging large Inconel vessels?
I am pretty sure they would be the largest superalloy forgings ever attempted by a significant margin.


Inconel 718 is easy to forge. Before final heat treatments its not as strong, so forging is easier. Remember, the sections are about 3x thinner than, say A533 grade B PWR pressure vessel steel. Grinding, milling and such are more difficult, but we don't need too much of that.

Realistically the most optimal way to go is to have two large forgings, the bottom head and first ring forging on top of that. Then all you have is 1 weld close to the core (and not too close actually, none in the beltline region). The top half of the pressure vessel could be field welded plate. It probably can't be transported in one vessel anyway - we're talking about a 27+ meter vessel.


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PostPosted: Aug 08, 2014 7:33 am 
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Well if you are building at coastal or large river sites you can transport the vessel in one piece on a barge....
But I take the point.

It appears that there is a lot of work on die casting Inconel 718, which might make it possible to stamp out vast numbers of reactor vessels with no forges at all.


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PostPosted: Aug 08, 2014 8:04 am 
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E Ireland wrote:
It appears that there is a lot of work on die casting Inconel 718, which might make it possible to stamp out vast numbers of reactor vessels with no forges at all.


That would be awesome. Inconel 718 has a relatively low melting point. Makes sense.

This would be especially suitable for really high volume production. Of course with 1550 MWe ESBWRs that's not really happing quickly. Can you imagine a factory die casting an entire ESBWR vessel a shift 8)


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PostPosted: Aug 08, 2014 4:22 pm 
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Well die cast Inconel 718 is apparently significantly weaken than forged Inconel 718, but it would still be stronger than the regular steel so its probably still a net saving.

And apparently there is work die casting Inconel turbine blades...... how much of a LWR can we die cast?


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PostPosted: Aug 09, 2014 1:27 am 
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Looks like you're right, at least in vacuum casting:

http://www.tms.org/superalloys/10.7449/ ... 27_636.pdf

Fatigue life is also worse and degrades more rapidly with fatigue cycles.

But, there are many ways to die cast... the "pore free casting process" with oxygen sparging looks most promising.

Still not sure about the economics of casting. It is used in automotive engine blocks because of high volume. With ESBWRs high volume means you'll provide enough castings to power the world in under a year with a single factory (!). For example if you make one a shift at 1000 shifts/year your factory makes 1.5 TWe worth of vessels a year!!

Good to keep dreaming I guess 8)


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PostPosted: Aug 09, 2014 3:29 pm 
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Perhaps we should see what happens with the original SBWR design?
The smaller vessel would be more amenable to die casting.

And 600MWe is hardly something to sniff at.


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PostPosted: Aug 10, 2014 7:08 am 
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If you recycle the Zirconium cladding from reprocessed used fuel, you could have pressure tubes by the hundred. You could produce them in automatic, radiation protected plants. The engineering is much lighter for tubes which are much more easily handled. Bigger vessels are best fabricated for lower pressures like calandria main drum or MSR vessels. Tubes can also be changed in part or the whole lot as required. They could be recycled after heat treatment.


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PostPosted: Aug 10, 2014 8:08 am 
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jagdish wrote:
If you recycle the Zirconium cladding from reprocessed used fuel, you could have pressure tubes by the hundred. You could produce them in automatic, radiation protected plants. The engineering is much lighter for tubes which are much more easily handled. Bigger vessels are best fabricated for lower pressures like calandria main drum or MSR vessels. Tubes can also be changed in part or the whole lot as required. They could be recycled after heat treatment.


Material from reprocessing is fiendishly expensive (10-100x inconel 718 price) and zirconium makes hydrogen in accidents. In718 has much better high temp capability.

CANDU calandria vessel is about 30 mm SS. We are talking about a 60 mm In718 vessel. Not that much more difficult.

In any case you will have big pressure vessels in your steam turbine casings. CANDUs also have enormous steam generator pressure vessels plus large steam turbine casings so it isn't fair to say they eliminate pressure vessels. Heck. The entire reactor building of a CANDU is a monstrous monolithic pressure vessel.


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PostPosted: Aug 10, 2014 8:32 am 
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The CANDU steam generator pressure vessels are a lot smaller though.
And building pressure vessels from concrete is cheap.

I wonder if anyone ever did a study on a PCSG for a CANDU.....


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PostPosted: Aug 10, 2014 8:53 am 
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E Ireland wrote:
The CANDU steam generator pressure vessels are a lot smaller though.
And building pressure vessels from concrete is cheap.

I wonder if anyone ever did a study on a PCSG for a CANDU.....


Concrete as a material is cheap... but so is steel. Making a 100 meter tall concrete structure that is aircraft crash and earthquake proof... more expensive. Concrete isn't leak proof so you need an expensive welded liner. In all it costs a pretty penny. Buildings of NPPs actually cost more than the whole reactor system!

Concrete is often used for shielding, so combining the pressure vessel and the rad shield should save money. Concrete does require cooling jackets and internal insulation for high temp application, that increases cost over steel/alloy with simple reflective insulation and no cooling jacket. For a steam generator it seems not that great. For an ESBWR vessel it might be interesting, if you can make a stainless steel welded sandwich in a factory, cooling jackets and all, then ship it to the reactor site, install it, fill with DUCTAL concrete, then post tension it. The cooling jacket becomes a safety feature (emergency cooling and in worst case, core catcher inside the vessel).

These things aren't simple. With steel/alloy pressure vessels, you can forge on nozzles and simply weld piping to that. This sort of design detail is much more difficult with prestressed concrete vessels. If you end up needing forgings for the sections with big nozzles, you lose much of the advantage. All that cooling jacket stuff doesn't make it any easier. This is pretty much a nightmare to me.


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PostPosted: Aug 10, 2014 10:47 am 
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Have you checked ASME Section III ?
As I recall, that's where all the PWR material and fabrication requirements come from.


Cyril R wrote:
Concrete isn't leak proof so you need an expensive welded liner.

Until now, none of the Candu reactor buildings have had a steel liner: It's always been an epoxy liner.

The EC-6 design includes a steel liner, but no EC-6's have been built yet (otherwise the design is similar to the C-6 units at Qinshan).


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