Energy From Thorium Discussion Forum

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 Post subject: Compact Uranium Storage
PostPosted: Jan 06, 2014 7:31 am 
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Well, I like to write hard science fiction as a hobby, mostly for the catharsis, and I've got a slight issue.

My ships are using dusty bed fission fragment rockets which require large quantities of high boiling fissile material, this pretty much means I have to use Uranium as Plutonium compounds will tend to sublime away at the temperatures required (within a couple of hundred Kelvin of Uranium dioxide's melting point).

Due to the shear amount of fuel required I need to store large quantities weapons grade uranium as compactly, and as lightly, as possible without causing criticality issues.
Anyone got any suggestions?
I can convert the material to oxide easily enough so I just need a compact, lightweight and stable way of storing such a huge amount of weapons grade material without it going boom at the slightest provocation.


On a related note, thanks to the shear amount of material required to support a fleet of such ships, use of 235U is impractical and I am thus forced to breed large amounts of 233U in surface reactors, as in this specific application, uranium is far superior to Plutonium.

Does anyone know what the highest possible breeding ratio for a Thorium-Uranium cycle reactor is?


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PostPosted: Jan 06, 2014 11:28 pm 
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An enriched boron or boron carbide honeycomb structure filled with thin uranium metal rods?


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PostPosted: Jan 07, 2014 6:45 am 
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E Ireland wrote:
Well, I like to write hard science fiction as a hobby, mostly for the catharsis, and I've got a slight issue.

My ships are using dusty bed fission fragment rockets which require large quantities of high boiling fissile material, this pretty much means I have to use Uranium as Plutonium compounds will tend to sublime away at the temperatures required (within a couple of hundred Kelvin of Uranium dioxide's melting point).

Due to the shear amount of fuel required I need to store large quantities weapons grade uranium as compactly, and as lightly, as possible without causing criticality issues.
Anyone got any suggestions?
I can convert the material to oxide easily enough so I just need a compact, lightweight and stable way of storing such a huge amount of weapons grade material without it going boom at the slightest provocation.


On a related note, thanks to the shear amount of material required to support a fleet of such ships, use of 235U is impractical and I am thus forced to breed large amounts of 233U in surface reactors, as in this specific application, uranium
is far superior to Plutonium.

Does anyone know what the highest possible breeding ratio for a Thorium-Uranium cycle reactor is?

Wash-1097 gives the best idea.


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PostPosted: Jan 07, 2014 10:33 am 
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Apparently the highest feasible breeding ratio for Thorium cycle is 1.4.
That is compared to 1.7 for Uranium-Plutonium.

How about a composite cycle?
Fissioning ~4t of uranium-pu cycle fuel produces 2.8t of 'excess' plutonium based fissile.
2.8t of Pu fissile fissioning produces 3.9t of uranium based fissile.

3.9t of fissile from 6.8t of fissioned material.
Thats a conversion ratio of ~1.57t

Since both DU and Thorium will be even cheaper in the future than now it the higher the conversion ratio the better. (As buildnig the reactors is the expensive part).

Also: would it be possible to easily convert uranium boride into uranium nitride or oxide?
Does anyone know if uranium boride dissolves in a salt for electroreduction or similar?


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PostPosted: Jan 07, 2014 10:38 am 
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Why not use metal fuel?


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PostPosted: Jan 07, 2014 10:42 am 
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In the reactor?

The uranium nitride particles in the reactor are running close to 3000K, the vapour pressure of uranium metal at that sort of temperature is non-zero which will lead to major losses of uranium vapour from the reactor's thrust nozzle (the reactor itself is in vacuum) - it would also be a reactor operating as a mist of uranium droplets rather than particles which is... problematic.

I am concerned about relying on macroscopic structures to maintain subcriticality in the fuel bunkers because kinetic impacts, either from micrometeroids or enemy weapons fire could trash them and drive the pile of hundreds of tonnes of fissile materials supercritical.

Which would be very very bad.

EDIT:

As an example of just how insanely hot the reactor is running, it can produce something like a thermal gigawatt per kilogramme of material in the core, which gives you a lifetime of something like 23.4 hours for a fuel particle injected into the core.

Which is a half life of 16 hours.


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PostPosted: Jan 07, 2014 1:30 pm 
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What made you think uranium nitride fuel won't decompose at such ridiculous conditions? 3000K, high vacuum, likely even uranium mononitride will decompose. The more because of the high radiation environment (uranium mononitride is not ionic ,but covalent).

In nuclear reactors, uranium mononitride can be used with reasonable temperatures (say 1500K), and nitrogen losses form a cover gas nitrogen overpressure.


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PostPosted: Jan 07, 2014 1:42 pm 
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Here's a paper that indicated UN will decompose at 3000K (10000/K of 3.33, decomposition above 0 atm).

I think if you go for UN fuel, most of it will decompose, leaving residual stoichometric UN and some weird molten uranium phase (perhaps saturated in nitrogen).

I'll note that thorium metal is a lot less volatile than uranium metal (and especially plutonium where the melting point is a major problem).


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PostPosted: Jan 07, 2014 1:47 pm 
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Theoretical studies seem to indicate that the particles' uranium nitride should decompose sufficiently slowly that it is a non issue compared to the particles disintegrating as they fission away (apologies, operating temperature under normal conditions is roughly 2800K or lower when the beam is actually operating properly).
They are also sufficiently small that a large fraction of fission products will escape the particles and can be funnelled using a magnetic field into a relativistic jet, which is the aim of the entire reactor.

Thorium isn't fissile and is thus no use in a reactor that is only barely able to function with weapons grade material.
This reactor essentially has a conversion ratio of ~0.

EDIT:

Inputed material is almost entirely 233U, and on the power output scales this reactor is dealing with there are three fissile isotopes before any plutonium is manufactured.
They are: 233U, 235U and apparently 238Np (which has a half life measured in days).


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PostPosted: Jan 07, 2014 3:31 pm 
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hundreds of tonnes of pure U233 is going to be impossible to store without serious risks. There's a big criticality risk even with loads of neutron poison around (not very effective for fast neutron absorption), in anything like a compact geometry.


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PostPosted: Jan 07, 2014 3:43 pm 
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E Ireland wrote:
I am concerned about relying on macroscopic structures to maintain subcriticality in the fuel bunkers because kinetic impacts, either from micrometeroids or enemy weapons fire could trash them and drive the pile of hundreds of tonnes of fissile materials supercritical.

Which would be very very bad.

Boron composites can be very strong, and the fuel container really only needs to withstand a minor to moderate impact / attack. It doesn't really matter if the fuel detonates in an event sufficiently catastrophic that your crew wouldn't survive anyways. The threat of a major explosion could be a sort of attack deterrence, kind of like detonating warp cores in Star Trek.


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PostPosted: Jan 07, 2014 4:31 pm 
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The rocket is a lattice mast structure several hunderd metres long so the volume doesn't necessarily have to be ultra compact.

The mass is the important thing within reason.
As to "major events", a small piece of metal slamming into the composite at several tens of kilometres of second will shatter or puncture any reasonable composites, which is hardly a 'minor' event.

What stable isotope has the largest cross section per atomic mass unit?
I am imagining it is probably boron-10.


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PostPosted: Jan 07, 2014 5:21 pm 
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E Ireland wrote:
The rocket is a lattice mast structure several hunderd metres long so the volume doesn't necessarily have to be ultra compact.

The mass is the important thing within reason.
As to "major events", a small piece of metal slamming into the composite at several tens of kilometres of second will shatter or puncture any reasonable composites, which is hardly a 'minor' event.

What stable isotope has the largest cross section per atomic mass unit?
I am imagining it is probably boron-10.


Probably helium-3, but that's academic, considering the low density.

More importantly, neither He-3 nor B-10 has any meaningful cross section at fast neutron energies. Well under 1 barn.

at 1 MeV, the fission cross section for U233 for example is about 10x the (n,a) capture cross section of B-10.


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PostPosted: Jan 07, 2014 6:49 pm 
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What about Uranium Borohydride?
The hydrogen would moderate the neutrons so the boron can absorb them.

It would also reduce the probability of a catastrophic detonation as the failed tests of Uranium hydride bombs showed that thermal spectrum explosions are impractical.


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PostPosted: Jan 07, 2014 8:28 pm 
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E Ireland wrote:
The uranium nitride particles in the reactor are running close to 3000K..........
Try 4000K.....


Attachments:
UTVR-MHD abstract, J Prop & Pow, Jan 93 p98.jpg
UTVR-MHD abstract, J Prop & Pow, Jan 93 p98.jpg [ 219.44 KiB | Viewed 1429 times ]
UTVR-MHD process schematic, J Prop & Pow, Jan 93 p98.jpg
UTVR-MHD process schematic, J Prop & Pow, Jan 93 p98.jpg [ 117.54 KiB | Viewed 1429 times ]
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