Energy From Thorium Discussion Forum

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PostPosted: Oct 23, 2014 5:17 pm 
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E Ireland wrote:
Oxide has insufficient density to make these reactors really attractive and metal is useless as it swells up horribly after a tiny amount of burnup.

Nitride is a nice option if you can get the nitrogen enrichment to work.


Metal can be made to work. Leave a big gap between the cladding and fuel, fill the gap with molten lead-bismuth eutectic for good thermal contact. The metal can swell a lot. Uranium metal has extremely high density so you can leave a big gap and still end up with better HM density than uranium nitride.


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PostPosted: Oct 23, 2014 6:12 pm 
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Having molten lead bismuth bonded metal fuel in a water reactor sounds like a terrible idea, especially as any cladding failures will almost inevitably drop the entire fuel rods FP inventory into the coolant. (The bonding material will likely be blasted into the fuel plenum during a failure and put pressurised water and steam into intimate contact with the fuel itself which will be spongelike).


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PostPosted: Oct 24, 2014 12:21 am 
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Reduced moderation could be an interesting idea for application to MSR/LFTR. A boiler/calandria cofiguration with limited moderation by water in the tubes and liquid fuel outside could do it. It would be really economical if it can be made to work in a stable state. Conversion ratio could be moderate in initial design and improved in later versions to breeding. There has been a successful light water breeder and could be possible in a reduced moderation MSR.


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PostPosted: Oct 24, 2014 2:34 am 
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E Ireland wrote:
Having molten lead bismuth bonded metal fuel in a water reactor sounds like a terrible idea, especially as any cladding failures will almost inevitably drop the entire fuel rods FP inventory into the coolant. (The bonding material will likely be blasted into the fuel plenum during a failure and put pressurised water and steam into intimate contact with the fuel itself which will be spongelike).


What made you think ultra hot uranium mono nitride won't react? Oxide is more stable.

In actual fact even UO2 reacts quite vigorously with hot pressurized water. It wants to be UO3 or U3O8 and in the process releases FPs. It also swells upon this oxidation reaction.

Molten lead should be mitigating in cladding leaks. It doesn't react with the fuel or coolant and it reduces chemical contact area between the two. Molten lead or Pb-Bi also works well with UO2 fuel, allowing a much higher power density with a bigger gap space, for the same peak fuel temperature.

Cladding leaks aren't as dramatic as you'd think. Because the cladding is ductile, leaks are small if they occur in normal operation. During a serious accident with loss of cooling the cladding can burst, but at that point you're in a big mess already. The main issue I think would be the extra hydrogen that would be produced, especially for pressure suppression containments. However there is no nitrogen (noncondensable) unlike with nitrogen fuel.

It would not be difficult to ensure the top of the fuel rods is the weakest (thinnest) so rupture would occur there which prevents most of the bond metal from being expelled.


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PostPosted: Oct 24, 2014 3:36 am 
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Nitrogen, while noncondensible, is likely preferable to large quantities of hydrogen being produced inside the fuel assembly after a rupture.

The gains are not really that large.
The Uranium Density in UN is 13.5 or so, even metal can only manage 19 without any gaps at all.


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PostPosted: Oct 24, 2014 3:51 am 
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E Ireland wrote:
Nitrogen, while noncondensible, is likely preferable to large quantities of hydrogen being produced inside the fuel assembly after a rupture.


If the containment is inerted, hydrogen or nitrogen doesn't matter much. Hydrogen can be recombined passively to be made condensable. Nitrogen can't.

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The gains are not really that large.
The Uranium Density in UN is 13.5 or so, even metal can only manage 19 without any gaps at all.


This is substantial, since UO2 is at 10 or so theoretical, if UN is a major improvement on it. Going to metal would be like, again as much improvement over UN as UN is over UO2.

I expect UN to be compatible with poor metal eutectics, so the metal bond can be applied in all fuels to great advantage.


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PostPosted: Oct 24, 2014 4:44 am 
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Corrosion in lead cooled reactors has been an issue. Does the lead-bismuth bond inside the fuel rod minimize this because the liquid metal is static, rather than being pumped through at speed? I suppose the LWR would be much cooler than a metal cooled reactor as well.


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PostPosted: Oct 24, 2014 5:22 am 
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jon wrote:
Corrosion in lead cooled reactors has been an issue. Does the lead-bismuth bond inside the fuel rod minimize this because the liquid metal is static, rather than being pumped through at speed? I suppose the LWR would be much cooler than a metal cooled reactor as well.


Yes, it is static and quite cool. Also zircalloy resists Pb-Bi well. The Pb-Bi actually protects the cladding from fission products that would otherwise slam into it and attack it.


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PostPosted: Oct 27, 2014 9:53 am 
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I looked up the dissolution of UN in nitric acid and apparently it does better with the Purex fuel cycle than oxide fuels do.
Which is one less roadblock to adoption.

Another problem with metal bonding is I don't know if any research has been done about the effects of lead and bismuth nitrates being present in PUREX extraction solutions.


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PostPosted: Oct 27, 2014 10:20 am 
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E Ireland wrote:
I looked up the dissolution of UN in nitric acid and apparently it does better with the Purex fuel cycle than oxide fuels do.
Which is one less roadblock to adoption.



But fuel reconstitution with nitride fuel is totally unproven! Metal fuel benefits from the extensive metal fuel fast reactor work. Metal fuel is more amenable to pyroprocessing.

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Another problem with metal bonding is I don't know if any research has been done about the effects of lead and bismuth nitrates being present in PUREX extraction solutions.[/


The lead and bismuth would first be recovered. Lead and bismuth are poor metals so very different chemically than the stuff you want - actinides.


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PostPosted: Oct 27, 2014 10:44 am 
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Pyroprocessing does not appear to be going anywhere fast.

The whole benefit of the RBWR approach, as you said, is that it has the minimum of development work and the project can progress in the mean time - requiring your unproven fuel technology use undeveloped reprocessing technology weakens that argument.
We need innovations that can all proceed independently.
Mixed Nitride fuel pellets have been fabricated before - indeed they have been fabricated for the last 20 years for fast reactor fuel irradiation tests.

And how would you recover all the lead-bismuth reliably? By the end of fuel life it will be dispersed inside the fuel pellets as well as around them - it is very difficult to draw out except by dissolving the fuel. Which means your raffinate is contaminated with lead and bismuth nitrates - while they are chemically very different to actinides they might still cause nasty third phase problems later on.

Fluorides are different to the things you want but they also cause issues for PUREX extractions.


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PostPosted: Oct 27, 2014 1:01 pm 
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E Ireland wrote:
Pyroprocessing does not appear to be going anywhere fast.


As opposed to what? PUREX? PUREX isn't going anywhere either.

PUREX is proven, yes - proven to cost a furtune and ten fortunes to clean up the plant and manage the wastes. No private investor interest.

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The whole benefit of the RBWR approach, as you said, is that it has the minimum of development work and the project can progress in the mean time - requiring your unproven fuel technology use undeveloped reprocessing technology weakens that argument.
We need innovations that can all proceed independently.


If that is the approach then stick to zircalloy and MOx. Don't bother with nitride at all.

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Mixed Nitride fuel pellets have been fabricated before - indeed they have been fabricated for the last 20 years for fast reactor fuel irradiation tests.


Do you have a source for this?

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And how would you recover all the lead-bismuth reliably? By the end of fuel life it will be dispersed inside the fuel pellets as well as around them - it is very difficult to draw out except by dissolving the fuel.


It is rather easy to remove actually, by vacuum distillation at modest temperature. This also removes most of the cesium so you avoid big issues in later processing steps.

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Which means your raffinate is contaminated with lead and bismuth nitrates - while they are chemically very different to actinides they might still cause nasty third phase problems later on.


I rather doubt this. Tin and cadmium are significant yield FPs. If this were a problem, it would be an issue to be dealt with or without LM bond, because of the poor metal FPs. Poor metals do not have strong nitrate bonds so likely they will go with the noble metal FPs. These guys aren't trouble-makers.


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PostPosted: Oct 27, 2014 5:37 pm 
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Cyril R wrote:
As opposed to what? PUREX? PUREX isn't going anywhere either.
PUREX is proven, yes - proven to cost a furtune and ten fortunes to clean up the plant and get rid of the wastes. No private investor interest.

If we are going to go by private.investment interest then you might as well forget nuclear and embrace our glorious shale gas overlords.
PUREX is a proven process performed every day in industrial quantities at at least three civil facilities (Sellafield, La Hague and Mayak).

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If that is the approach then stick to zircalloy and MOx. Don't bother with nitride at all.

Why? Nitride fuel is not required for any of the other innovations and requires no other innovations- if it works fine but if it doesn't it doesn't cause any other problems.
As for Mixed Nitride fuel testing - here is one paper that references them. "High temperature behavior of irradiated mixed nitride fuel during heating tests" if the link is broken. Published by Isamu Sato et.al.
Apparently lots of runs in Joyo.

Quote:
It is rather easy to remove actually, by vacuum distillation at modest temperature. This also removes most of the cesium so you avoid big issues in later processing steps.

This sort of thing is a can of worms. Pyroprocessing has never worked on an industrial scale.
And if the lead and bismuth has absolutely no effect on the process chemistry it would be better to just dissolve the bond entirely into the raffinate - it avoids a complex and expensive handling step which requires lots of specialised equipment.


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PostPosted: Oct 27, 2014 6:10 pm 
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A little vacuum still? Complicated?

liquid-liquid aqeous exchange of mobile volatile cesium/FP and actinide compounds in big vessels, with criticality concerns everwhere, is that simple?


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PostPosted: Oct 27, 2014 10:05 pm 
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Considering we have been doing it for decades on massive scales its not really that complex in terms of making it work reliably.
Insanely radioactive materials evaporating into an artificial vacuum sounds like the kind of thing engineers have nightmares over.


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