Energy From Thorium Discussion Forum

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PostPosted: Jan 31, 2015 11:52 am 
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The recent announcements from Thorcon and Terrestrial has got me thinking about the benefits of Thorium and Uranium MSRs. Up till now I've been sold on Thorium - but to what extent are the benefits captured in Uranium MSRs?

Trying to separate out, the benefits of a well designed MSR are:
1. Negative void coefficient prevents the reactor from getting too hot.
2. A high operating temperature ~900C - meaning higher electrical efficiency and potentially process energy.
3. Operation at ambient pressure = no explosive forces
4. No metal fuels which can burn in air
5. The ability to put in a freeze plug to further prevent overheating (with the above points it's walk away safe)
6. Radioactive materials are bound in salts with high boiling points - even an external explosion can't spread radiation far.
7. The easy ability to remove noble gas poisons which means the reaction can go longer and burn up more fuel without creating more unwanted actinides.
8. The not so easy ability to process the fuel on an ongoing basis to remove fission products (poisons). Plutonium, U233 and other really nasty stuff can be put back in ad-infinitum.
9. Can be designed for burner or breeder, and to burn up all our long lived nuclear waste (if it can be extracted from spent solid fuel).

I'm sold on that. MSRs are the future. Are all of the above benefits available for both U235 and Thorium?

Benefits of Thorium in a MSR over and above U235:
1. Thorium is about 3 orders of magnitude more abundant than U235.
2. The Thorium fuel cycle produces negligible transuranic waste (though given (8) above, is that such a major benefit)
3. The fuel cycle is less conducive to making nuclear weapons
4. No complicated enrichment is needed

Am I missing any?


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PostPosted: Jan 31, 2015 12:54 pm 
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Here's a few more:

Two-fluid thorium reactors have a much easier time keeping neutron flux off the reactor vessel wall.

Thorium-breeders can keep reactivity nearly constant as the reactor operates.

Thorium breeders can eliminate the need for enrichment, the simplest vector for the diversion of nuclear material.

Thorium fuel cycle simplifies chemical processing since a fuel salt containing only uranium and neptunium can be "cleansed" by fluorination. Plutonium doesn't come out in regular fluorination, but would come out in a reductive extraction step that is undesirable because it is intended to remove fission products.

Also, I would challenge that 900C is a feasible operating temperature for any MSR based on Hastelloy-N materials. Likely the maximum temperature will need to be below 700C.


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PostPosted: Jan 31, 2015 2:06 pm 
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Kirk Sorensen wrote:
Two-fluid thorium reactors have a much easier time keeping neutron flux off the reactor vessel wall.


Not really an advantage, it is a disadvantage. You're throwing out a non-problem (all reactors manage this today) for a very serious barrier fluence issue. You cut reactor vessel fluence from a 1-2% or so value to 0.1% but in return you have a two fluid barrier vessel which has to take 50% of the core flux. That's one step forward, ten steps back.

Quote:
Thorium-breeders can keep reactivity nearly constant as the reactor operates.


Not a major advantage if you already have online fuelling a la MSR.

Quote:
Thorium breeders can eliminate the need for enrichment, the simplest vector for the diversion of nuclear material.


Indeed there is no need for enrichment, you're providing separable HEU right in the blanket, which is relatively clean and accessible. Again, one step forward, ten back.

Quote:
Thorium fuel cycle simplifies chemical processing since a fuel salt containing only uranium and neptunium can be "cleansed" by fluorination. Plutonium doesn't come out in regular fluorination, but would come out in a reductive extraction step that is undesirable because it is intended to remove fission products.


Debatable. Thorium breeding requires online fuel processing at each reactor site. Converters can do the reprocessing in a single centralized facility. So its say 100 tiny online processors handling fresh nasty fission products, or 1 central facility handling old cooled down fission products.

Quote:
Also, I would challenge that 900C is a feasible operating temperature for any MSR based on Hastelloy-N materials. Likely the maximum temperature will need to be below 700C.


Agree. Even with superalloys, lower temp = better performance. 650C is a good compromise, not much reduced thermal efficiency, slight increase in pump power, easy on materials.


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PostPosted: Jan 31, 2015 2:08 pm 
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alexterrell wrote:
The recent announcements from Thorcon and Terrestrial has got me thinking about the benefits of Thorium and Uranium MSRs. Up till now I've been sold on Thorium - but to what extent are the benefits captured in Uranium MSRs?

Trying to separate out, the benefits of a well designed MSR are:
1. Negative void coefficient prevents the reactor from getting too hot.
2. A high operating temperature ~900C - meaning higher electrical efficiency and potentially process energy.
3. Operation at ambient pressure = no explosive forces
4. No metal fuels which can burn in air
5. The ability to put in a freeze plug to further prevent overheating (with the above points it's walk away safe)
6. Radioactive materials are bound in salts with high boiling points - even an external explosion can't spread radiation far.
7. The easy ability to remove noble gas poisons which means the reaction can go longer and burn up more fuel without creating more unwanted actinides.
8. The not so easy ability to process the fuel on an ongoing basis to remove fission products (poisons). Plutonium, U233 and other really nasty stuff can be put back in ad-infinitum.
9. Can be designed for burner or breeder, and to burn up all our long lived nuclear waste (if it can be extracted from spent solid fuel).

I'm sold on that. MSRs are the future. Are all of the above benefits available for both U235 and Thorium?

Benefits of Thorium in a MSR over and above U235:
1. Thorium is about 3 orders of magnitude more abundant than U235.
2. The Thorium fuel cycle produces negligible transuranic waste (though given (8) above, is that such a major benefit)
3. The fuel cycle is less conducive to making nuclear weapons
4. No complicated enrichment is needed

Am I missing any?


See https://en.wikipedia.org/wiki/Liquid_fl ... um_reactor for a comprehensive listing of pro/con and other features


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PostPosted: Jan 31, 2015 2:55 pm 
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Thanks Cyril & Kirk. Remember to keep disagreements clean :)

Quote:
Thorium fuel cycle simplifies chemical processing since a fuel salt containing only uranium and neptunium can be "cleansed" by fluorination. Plutonium doesn't come out in regular fluorination, but would come out in a reductive extraction step that is undesirable because it is intended to remove fission products.


Debatable. Thorium breeding requires online fuel processing at each reactor site. Converters can do the reprocessing in a single centralized facility. So its say 100 tiny online processors handling fresh nasty fission products, or 1 central facility handling old cooled down fission products


This struck me as a major difference. If you can cleanse only with flourination, lots of small sites (at the 250MW level) may not be a problem.


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PostPosted: Jan 31, 2015 3:08 pm 
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Fluorination isn't enough. It won't remove most FPs. It is good for transporting HEU around in clean form and make regulators and politicians and powers-that-be nervous. It can't remove lanthanides. It can't remove plutonium (eventually it will fluorinate, but not before your container has fluorinated altogether).


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PostPosted: Jan 31, 2015 6:49 pm 
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Thorium as mined is a single isotope of atomic mass 232 and it cannot be enriched. However these are just words as it needs fissile, either enriched uranium or Pu239 or U233 obtained by irradiating U239 or Th232 and chemical separation (reprocessing).
An important benefit of thorium is U233 fissile created by its irradiation in a neutron fluence which leads to some of other benefits mentioned.
Use of thorium fuels as solid in the Shippingport experiment lead to an ISO-breeder also technically claimed as breeder.
U233 is a good fissile for reactors and doubtful for weapons. Even existing reactors can produce more power from the same amount of uranium by using thorium as part or whole of fertile fuel as per an Indian document.


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PostPosted: Feb 01, 2015 6:48 am 
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Cyril R wrote:
Fluorination isn't enough. It won't remove most FPs. It is good for transporting HEU around in clean form and make regulators and politicians and powers-that-be nervous. It can't remove lanthanides. It can't remove plutonium (eventually it will fluorinate, but not before your container has fluorinated altogether).


In steady state, there should be no plutonium - Th and U233 shouldn't lead to Plutonium.

However, given the need in a thorium reactor for a starter charge of U235 or Plutonium, some means of removing Plutonium is needed.


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PostPosted: Feb 01, 2015 10:41 am 
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Actually you do get some plutonium (or neptunium) eventually even in a reactor that is fed only thorium.
Ignoring the short lived decay intermediate isotopes each neutron capture results in the following. Where fission occurs I show the capture
probability - otherwise it is 100%.
Th232 -> U233 (10%) -> U234 -> U235 (18%) -> U236 -> Np237 -> Pu238

So roughly 2% of the fuel converts to plutonium over the long haul. For a GWe-yr that means around 18kg Pu.
If it is left in the reactor the Pu has a large cross-section so it will build towards a modest inventory and then stay at that level until the plutonium is removed. If you are reprocessing and returning the plutonium (americium, curium) to the reactor then they do burn down and your transuranic flow to the waste stream is set by the imperfect recovery in your separation scheme.


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PostPosted: Feb 01, 2015 11:04 am 
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The more I learn about the availability of uranium the less I think its much of a benefit that thorium is more abundant.
We are at the point where the entire landmass of the world is underlain by Ghawar sized oil fields in energy terms.

We have so much uranium we are never going to need it all.


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PostPosted: Feb 01, 2015 11:10 am 
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Plutonium or no in the fuel salt isn't terribly important, its the fact that you have FPs and an aside is that many of those stick with plutonium (at least some of the bad poisons namely lanthanides).

That means if you want to breed, with this reactor which is thermal, it needs online trifluorides removal. Not simple like fluorination!! and it needs to be done online with fresh fission products all over the place.

To tell you the truth, I'm not comfortable even with converter reactors "simple" removal of radioactive offgas with 3 minute half lives and nil heat capacity. If we could keep the nasty gas in the reactor somehow then that is a major advantage in my mind, well worth loading up on some more makeup LEU fuel.


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PostPosted: Feb 01, 2015 6:04 pm 
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The availability of fertile material is irrelevant. Both u-238 and thorium are available in abundance with no trouble.
Cyril is right that in a thermal machine we are starved for neutrons and the Th/U233 route gives us more neutrons than the U/Pu route in the thermal spectrum. It is also true that separating plutonium is harder than separating uranium. (It is made even more difficult due to worries about proliferation).


IN the fast spectrum the U/Pu route looks most promising but it does mean one has to take on the challenging task of separating plutonium from fission products. It doesn't have to be rapid and continuous but it is still a challenge.


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PostPosted: Feb 02, 2015 3:02 am 
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Lars wrote:
Th232 -> U233 (10%) -> U234 -> U235 (18%) -> U236 -> Np237 -> Pu238

So roughly 2% of the fuel converts to plutonium over the long haul. For a GWe-yr that means around 18kg Pu.
If it is left in the reactor the Pu has a large cross-section so it will build towards a modest inventory and then stay at that level until the plutonium is removed. If you are reprocessing and returning the plutonium (americium, curium) to the reactor then they do burn down and your transuranic flow to the waste stream is set by the imperfect recovery in your separation scheme.


Though aren't the problems Pu239 for weapons and Pu240 because it makes geological disposal a political problem with it's 6400 year half life?

For disposal terms, can't Pu238 be treated as a fission product?

But then I see this,
http://en.wikipedia.org/wiki/Reactor-gr ... sahara.svg

Pu238 -> Pu239 (91%) -> Pu240 (36%).

So the Pu238 doesn't stick around - and both thorium and uranium reactors need to separate Pu from fission products.

The Thorcon proposal is to do this centrally, about a decade after the first reactor starts producing electricity. (As I commented on the Thorcon topic - that makes for an interesting business model).


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PostPosted: Feb 02, 2015 10:37 am 
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Some 238Pu will fission, the rest will come 239, most of which will fission.
But you will still get build up of significant amounts of 240Pu, 241Pu and 242Pu and their respective daughters.
Especially when a large reactor might expect to fission over a hundred tonnes of material during its life.


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PostPosted: Feb 02, 2015 12:48 pm 
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E Ireland wrote:
Some 238Pu will fission, the rest will come 239, most of which will fission.


This assumes that you leave it to its own devices. As would be the case in a converter reactor in deep burn mode, such as IMSR or Thorcon. Not for an online processor thermal thorium breeder. That just sends the Pu, whatever the isotope, to the trifluoride waste bucket unless you attempt difficult online hot reprocessing and separation of the lanthanides from transuranics.


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