Energy From Thorium Discussion Forum

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PostPosted: Apr 06, 2015 4:59 am 
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In the present lot of (LWR) reactors there is an imbalance between fertile and fissile fuels. To begin with, fissile is enriched for the LWR fuel, leaving most of mined uranium as Depleted Uranium. Then the burning of fissile and a small part of fertile results in most of this fuel getting 'spent' for the LWR.
The only way to finish off the surplus fertile is to have a breeder reactor which will burn proportionately more of fertile. A couple of fast (breeder) power reactors are working in Russia and more are under construction in Russia, China and India.
The best design of reactor for the purpose of burning off the used LWR fuel would be a fast MSR.
http://www.forbes.com/sites/kirksorense ... -digester/
A thorium fueled reactor is
a. More efficient in burning fertile. Even a thermal spectrum reactor could be a breeder. Shippingport experiment proved it.
b. Generally avoids the problem itself by using thorium fertile.
Most reactor designers have not taken the thorium choice. First thorium fueled reactor is likely to come up in India or China. The pity is that the Waste Digester choice is also not being taken.


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PostPosted: Apr 07, 2015 8:31 am 
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jagdish wrote:
The only way to finish off the surplus fertile is to have a breeder reactor which will burn proportionately more of fertile. A couple of fast (breeder) power reactors are working in Russia and more are under construction in Russia, China and India.
The best design of reactor for the purpose of burning off the used LWR fuel would be a fast MSR.


(Technically,) what is the effect of fast neutrons compared to thermal neutrons in the reactor?

Thermal neutrons are much more likely to cause fission of U-235, but are also capable of converting U-238 into various forms of Pu, but also of causing the Pu to fission. After a while, because of the resulting fission products, the spent fuel has to be removed and has a lot of Pu - as well as unused U235 and untouched U-238. Removing the fission products during operation means the reactor can continue for longer, breeding and burning more Pu, but U-235 must still be added to maintain a source of neutrons.

How does this change with fast neutrons? They are less lilely to cause a U-235 to fission, so are therefore more available for converting U-238 into Pu-239? Doesn't the reactor then run out of neutrons? Can fast neutrons be "reflected" from the reactor wall?

In a thorium reactor, thermal neutrons are capable of converting Thorium into U-233 - hence breeding - either in a seperate fluid or in the fuel itself (starting fuel being LEU20 + Th) . What is the advantage of a fast spectrum?


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PostPosted: Apr 07, 2015 9:28 am 
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The biggest advantage of the fast spectrum is that t he neutrons have a non-zero probability of directly fissioning the 238U, rather than simply being absorbed.


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PostPosted: Apr 08, 2015 6:54 pm 
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The cross sections for fission and absorption are all different at different energy. They also vary differently for various neuclides. The neutrons produced by fission also vary.


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PostPosted: Apr 09, 2015 3:31 am 
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jagdish wrote:
The cross sections for fission and absorption are all different at different energy. They also vary differently for various neuclides. The neutrons produced by fission also vary.


In other words, we need a fast neutron reactor to breed fuel because the neutronics simulation software demonstates this. But this isn't the case for thorium fuel, which can be converted into U233 with thermal neutrons.

Do we also need a fast reactor to destroy long lived actinides? Isn't the main requirement to effectively separate out fission products and put the pure actinides back into the fuel salt. With perfect separation, there'd be no long lived waste


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PostPosted: Apr 09, 2015 5:42 am 
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Some of the even numbered plutonium and other isotopes don't have very big neutron cross sections at thermal energies.

So things like 242Pu will just tend to pile up in the reactor, as it won't decay and it won't fission much in a thermal spectrum.


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PostPosted: Apr 09, 2015 7:12 am 
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E Ireland wrote:
Some of the even numbered plutonium and other isotopes don't have very big neutron cross sections at thermal energies.

So things like 242Pu will just tend to pile up in the reactor, as it won't decay and it won't fission much in a thermal spectrum.


Right - so if you had a thermal spectrum, single fluid, 235U powered MSR, with perfect FP removal - eventually, after a few decades, you'd end up with too much 240Pu and 242Pu for the reactor.

Probably an order of magnitude less than with a PWR, but nevertheless, needing a fast spectrum reactor to fission it.


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PostPosted: Apr 09, 2015 8:14 am 
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A fast spectrum reactor with a tiny fraction of the thermal output of the DMSR - even if it is a pure LEU fuelled one (not sure why you would use pure LEU fuel though, since the Thorium reduces your uranium input and doesn't cause much of a problem really).


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PostPosted: Apr 09, 2015 8:57 am 
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It only helps for burn up over 45MWd/t, which is very much possible.
It really pays in creation of 233U, the superior fissile for reactors.


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PostPosted: Apr 20, 2015 2:15 am 
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E Ireland wrote:
A fast spectrum reactor with a tiny fraction of the thermal output of the DMSR - even if it is a pure LEU fuelled one (not sure why you would use pure LEU fuel though, since the Thorium reduces your uranium input and doesn't cause much of a problem really).


So, we could have a future powered by MSRs, starting off with 235U, but moving towards Th/U233, providing most of the world's power.

Then a handful of fast reactors - e.g PRISM design - to get rid of excess even-numbered Pu. (The only issue is The Pu transport. Obviously it's no good for bomb making - but needs to be shipped accident free.)

"Waste" needs to be put away safely for 500 years or so.


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PostPosted: Apr 20, 2015 7:45 am 
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Th boosted DMSRs would still require continuous LEU input, just much less than would otherwise be the case.


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PostPosted: Apr 20, 2015 8:28 am 
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E Ireland wrote:
Th boosted DMSRs would still require continuous LEU input, just much less than would otherwise be the case.


Once generating U233, why does it need U235 to be added?


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PostPosted: Apr 20, 2015 8:46 am 
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alexterrell wrote:
E Ireland wrote:
Th boosted DMSRs would still require continuous LEU input, just much less than would otherwise be the case.


Once generating U233, why does it need U235 to be added?
If you don't, it soon STOPS being a DMSR.

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DRJ : Engineer - NAVSEA : (Retired)


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PostPosted: Apr 20, 2015 9:44 am 
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Reaching a breeding ratio of 1.0 with a single fluid reactor is very challenging.

A DMSR doesn't even try, instead trading away a little bit of breeding ratio (and thus requiring LEU makeup) to gain major benefits in the design of the reactor and in the processing apparatus.
The reference DMSR design only needs to change the salt every 30 years and has no salt processing system, the only material removed is removed through the off gas system.


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PostPosted: Apr 20, 2015 11:16 am 
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Right - so Xenon and a bit else out, as enriched as possible/allowed Uranium in. Fresh Thorium in as well?

After 30 years it's clogged up with other fission products and 240 and 242Pu.


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