Energy From Thorium Discussion Forum

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 Post subject: MSR / LFTR Shielding?
PostPosted: Dec 27, 2014 3:43 pm 
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I'm curious about the level of gamma radiation that would be produced by a Thorium/U233/U232 reactor and how this compares to the levels found within conventional light-water nuclear reactors? I understand that in conventional reactors the water itself provides a substantial portion of the radiation shielding. What would be the design considerations in a (waterless) thorium MSR ? Would maintenance and inspection be less difficult or more difficult? Would the placement of personnel relative to the reactor be substantially different?

Thanks!


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PostPosted: Dec 27, 2014 4:18 pm 
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It depends greatly on the design of the reactor. The LFTR designs I am working on employ a thorium-bearing fluid as the "blanket" of the reactor that absorbs much of the neutrons and gammas before they reach the reactor vessel.


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PostPosted: Dec 27, 2014 7:33 pm 
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How about sheets of metallic thorium that could be later reprocessed in a single step of electrolysis? You could replace the thorium blanket before the fissile U-233 is enough for any substantial fission, say in 12-18months.


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PostPosted: Dec 28, 2014 2:24 am 
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Three things:
1) Thorium dissolves in fluoride salts so it will have to be clad. Most likely by graphite.
2) Thermal neutrons do not penetrate far into metallic thorium so in effect you only have a thin sheet of thorium in which the U233 will build up. It doesn't take too long for you to start getting fissions and the fission rate increases with time. This leads to a quadratic build up of fission gasses with time - you have to change out the metallic thorium before the gas pressure cracks the graphite.
3) You better mix in some U238 with the metallic thorium so you don't generate pure U233.

I decided to pursue other avenues at this point. I did not dig into them far enough to decide if they were insurmountable or solvable problems.


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PostPosted: Dec 28, 2014 9:48 pm 
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Kirk Sorensen wrote:
It depends greatly on the design of the reactor. The LFTR designs I am working on employ a thorium-bearing fluid as the "blanket" of the reactor that absorbs much of the neutrons and gammas before they reach the reactor vessel.



So the use of this "proliferation resistant" fuel would not create a set of serious design challenges related to appropriate shielding? You won't have the water or that thick steel pressure vessel. Certainly the thorium blanket will be a good thing relative to shielding. I am not knowledgeable in this area, but it seems like a question that people will ask.


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PostPosted: Dec 29, 2014 4:12 pm 
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most nuclear reactors of the world have a biological shield (1 -2 m concrete) around the reactor vessel to shield the rest of the world from Gamma and Neutrons.

Holger


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PostPosted: Dec 29, 2014 5:35 pm 
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davej wrote:
Kirk Sorensen wrote:
It depends greatly on the design of the reactor. The LFTR designs I am working on employ a thorium-bearing fluid as the "blanket" of the reactor that absorbs much of the neutrons and gammas before they reach the reactor vessel.



So the use of this "proliferation resistant" fuel would not create a set of serious design challenges related to appropriate shielding? You won't have the water or that thick steel pressure vessel. Certainly the thorium blanket will be a good thing relative to shielding. I am not knowledgeable in this area, but it seems like a question that people will ask.


All power reactors need shielding. Blankets are good for protecting reactor vessels from neutrons, but the rad levels are just too high for humans. Even a little fission in the blanket or little decay of isotopes quickly gets unacceptable to us humans. Not to mention the overconservative radiation exposure limits.

Various options exist for shielding. Water, concrete, even clean molten salt. It is a well known engineering detail of a nuclear powerplant, up to the point of even being a bit boring.


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PostPosted: Dec 30, 2014 5:50 am 
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There are proposal to use concrete with depleted uranium 'aggregate' as shielding for gamma rays as it will have massive amounts of mas in a small package.


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PostPosted: Dec 30, 2014 6:00 am 
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E Ireland wrote:
There are proposal to use concrete with depleted uranium 'aggregate' as shielding for gamma rays as it will have massive amounts of mas in a small package.


For a reactor shielding application though... leaking neutrons will make plutonium and fission some of that. Then you have radiocesium and -iodine in your shield. Isn't that an issue?


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PostPosted: Dec 30, 2014 6:23 am 
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Most of the neutrons won't even make it through the pressure vessel.
The neutrons are far less penetrating than gamma rays.
You can easily shield the handful of remaining neutrons by placing some boron loaded ceramic inside the gamma shield.


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PostPosted: Dec 30, 2014 8:41 am 
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E Ireland wrote:
Most of the neutrons won't even make it through the pressure vessel.
The neutrons are far less penetrating than gamma rays.
You can easily shield the handful of remaining neutrons by placing some boron loaded ceramic inside the gamma shield.


Yes, you could do that, but it gets more expensive this way. Compared to dirt cheap concrete. There would have to be large weight or volume related cost savings to offset this.


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PostPosted: Dec 30, 2014 1:24 pm 
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Cyril R wrote:
E Ireland wrote:
Most of the neutrons won't even make it through the pressure vessel.
The neutrons are far less penetrating than gamma rays.
You can easily shield the handful of remaining neutrons by placing some boron loaded ceramic inside the gamma shield.


Yes, you could do that, but it gets more expensive this way. Compared to dirt cheap concrete. There would have to be large weight or volume related cost savings to offset this.


I believe it cuts the thickness required for a given shielding by something like two-thirds.


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