Energy From Thorium Discussion Forum

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PostPosted: Jul 10, 2015 3:15 pm 
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Sure, a proven benign and ductile alloy has a bit more neutron absorptions than an unproven ODS material that has only shown to be exceedingly difficult to work with. that's not really a fair comparison, is it? In any case, FeCr is only going to be marginally better neutronically than FeCr with 15% Ni.

Metallurgy isn't a free lunch. We can't have cake and eat it. ODS looks good on paper, it sucks when you actually have to make stuff. That's why the fast reactor people keep getting back to good old austentic stainless steel.

As for extrusion, ODS alloys are powder metallurgy based... no extrustion, I'm pretty sure. You'd be "baking" fuel elements and then hope to God that the dispersoids are even after welding.


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PostPosted: Jul 10, 2015 3:22 pm 
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E Ireland wrote:
Do we need that many welds in a fuel assembly?


Oh, about a hundred thousand welds in a large reactor system? Or more. ESBWR must be pushing 150000 core welds if its not substantially more than that.


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PostPosted: Jul 10, 2015 3:51 pm 
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Based on Nudat Cross sections SAE316 has comparable neutronic performance to the AGR's original alloy (~3.11b)
However if there was a way to get rid of the Manganese content the neutron cross section would drop by 6-7% just for that.

Have you ever worked with 'Duplex' type stainless steels? With DepMo Molybdenum is potentially a low cross-section (for these sorts of materials) additive for steels.


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PostPosted: Jul 10, 2015 4:03 pm 
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Duplex is ferritic and austenitic, co-existing in the alloy in about equal proportions. This gets you the strength and stress corrosion resistance of ferritics, but nearly as good weldability (with some precautions) and ductility as austenitics. Its becoming very popular because its stronger than austenitic stainless but it has better strength and corrosion and similar cost. Too bad its only generally considered suitable for low temperature, because of embrittlement above 300C. But maybe this isn't a problem in fuel cladding application. (Zircalloy is actually pretty badly embrittled after PWR levels of burnup.)

Have to be careful with molybdenum - CO2 will oxidize it, and molybdenum has brittle low melting oxides (MoO3 I think it was). Ok for minor element usage though.

I'd be fine with multiplex SiC cladding, brazed at both ends with high temperature brazing (aluminium something, can't remember).


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PostPosted: Jul 11, 2015 10:48 am 
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Here's an idea which would work, and would use only today's technology: TRISO fuel. Not the graphite elements - just the tristructural particles.

Imagine if the fuel channel actually holds a concentric perforated sleeve (Zr alloy) which is just a strainer for the fuel particles, so it has nil stress and can be made of anything (it can even be allowed to corrode to a rather large extent without issues). Gas comes in around the edges of the fuel channel, between the sleeve and the channel wall. It then moves in radially through the fuel particles bed, into a central perforated tube. That tube exits at the channel end.

With this idea you can have very long fuel channels without the high pressure drop. The actual heated flow length could be under 0.1 meter, if needed.

Not actually my idea this - it comes from the supercritical water cooled reactor (see earlier discussions, ref. Tsiklauri et al). But you can get enormous power density this way - without the graphite the TRISO particles have enormous surface area... should be quite cheap too without the graphite manufacturing step in normal TRISO.

This arrangement would be neutronically efficient, since there's only ultra thin gauge zirconium alloy in the fuel channel, rest being UO2, C, SiC. In fact this would be so thin you could use stainless steel and don't care about it.

It also greatly reduces the normal heat loss, since you have cold inlet gas flowing over the inside of the fuel channel.

As discussed before the perforations would probably be large slots with a fine mesh strainer "sock" over it, to avoid plugging.


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PostPosted: Jul 11, 2015 7:20 pm 
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Beryllium cladding, that won't work either. Its the typical, physicists triumphing over engineers, that. It happens far too often in nuclear engineering.


I was also surprised when I saw the beryllium cladding, but it seems it was not just an idea on a paper, they made tests and research on this cladding and they solved some problems ( addition of calcium in order to resist to corrosion in wet CO2, elaboration of satisfying methods of welding and manufacturing, gains on ductility and creep resistance with a better purity of the metal, etc). In the paper they were confident that this would work but I don't know what happened later. Maybe the creation of helium and tritium under irradiation makes the cladding too brittle after too little burn up. Or maybe they just stopped the researches since the PWR was chosen to be the main technology. The back up solution that they imagined was the development of zirconium alloys resistant to the corrosion in hot CO2.

For EL-4 the first claddings was in stainless steel.


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PostPosted: Jul 11, 2015 8:45 pm 
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that's correct, beryllium suffers severe radiation embrittlement. That's not surprising, since it fissions! (the n,2n reaction annihilates it completely).

There's been many fancy ideas that were really just fancies of physicists in a time when nuclear efficiency was the be all end all of nuclear power. Because, after all, we'd run out of uranium 10 years from now! Oh no!

In the end the only high temperature cladding that has worked is stainless steel (plus TRISO, in a more broad sense, since it serves the role of cladding). Even today, the vast majority of non TRISO high temperature fuel belongs to the stainless category. Such a wonderful material. Can we have some love for the kitchen sink please?


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PostPosted: Jul 11, 2015 8:53 pm 
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SO how would the TRISO particles be contained? I'm a little confused by this concept.

As to almost all high temperature fuel that has actually been irradiated being clad with stainless steel - that might have somethign to do with the fact that almost all high temperature reactor fuel irradiated has been AGR fuel.....

EDIT:
We have to be careful with TRISO fuels in contact with superhot carbon dioxide as it will attack the exposed pyrolytic carbon.
This requires methane be added to the coolant to suppress this effect, but you have to be very careful as excess methane can start to cause carbon deposition on the elements.


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PostPosted: Jul 11, 2015 10:54 pm 
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Czech A1 CO2-cooled, heavy water-moderated reactor had natural uranium metal wire with magnesium/beryllium coating, so heat transport out of the fuel should have been better than even the skinniest oxide rods. It still ended in tears though.


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PostPosted: Jul 12, 2015 12:22 am 
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The TRISO would be packed in an annular 'sock' made of some metal or composite. Could be SiC composite, it serves only to hold the particles and prevent them from dislodging. The coolant moves radially through the annular particle bed, then axially out the channel through the inner part of the 'sock' that is a perforated tube. I think all of this can be made out of triplex SiC, even the wire mesh could be made of bare woven SiC fiber. Then we have an all ceramic fuel "bundle".

You're right, most likely the pyrolytic carbon will be replaced by another SiC coating, to avoid oxidation. To tell you the truth I'm not really sure why pyc is used when you already have SiC, even in regular TRISO.


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PostPosted: Jul 12, 2015 8:46 am 
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I think the pyrocarbon is there as a final barrier against SiC cracking and also to improve contact with the graphite based matrix material.
It sounds an intriguing concept but I worry about keeping the particles in place against the rather significant pressure drop between inlet and outlet.


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PostPosted: Jul 12, 2015 9:36 am 
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There's other coatings that have been succesfully applied, such as ZrC. But probably just swapping the pyc with another SiC layer will do fine, if its really there for graphite matrix adhesion, which we don't have...

The concept is like a hollow fiber filter. Except that there is just one big fiber per channel, and coolant moves inward into the "fiber". So you end up with very low pressure drop. You actually get both (mostly) radial and (some) axial flow through the particle bed. These forces should be quite small as long as the bed is shallow in the radial direction. I'm thinking 0.1 meter bed depth, and full channel bed length.

Image


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PostPosted: Jul 12, 2015 9:47 am 
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It turns out the outer layer of pyrolytic carbon is important for the fuel element's survivability.
It shrinks when under irradiation (as graphite moderator blocks do in the LFTR I imagine), this prestresses the silicon carbide (which is the primary barrier layer) so that it is able to resist attempts by the fuel kernal to swell.

Whilst TRISO particles may make it very difficult to get to reasonable heavy metal densities - how about simply having stacked donut-shaped discs of fuel material with an appropriate cladding, which are then stacked on top of each other using spacers.
Coolant then flows inwards from the outside like in the concept above, but thanks to the thin nature of the plates you get an enormous heated surface area exposed to the coolant, which means the peak cladding temperature can be made approximately equal to the temperature of the coolant - which would allow zircalloy which solves the neutronics issues even if we use lots and lots of it.


Last edited by E Ireland on Jul 12, 2015 9:52 am, edited 1 time in total.

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PostPosted: Jul 12, 2015 9:49 am 
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I have to ask though, why not consider helium as your coolant? I doubt it will make much cost difference, certainly if you have a canned circulator. You'd appreciate the better heat transfer properties of helium over CO2.


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PostPosted: Jul 12, 2015 9:53 am 
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I've been thinking about helium for a while - but I was worried about the availability of suitably powerful canned circulators.
If those exist there seems little reason not to - but that would give away any chance of any kind of pressure suppression in the containment that might be problematic.


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