Let's Talk About Exotic Fissiles

[LostCosmonaut]

Trash can sized nuclear reactors!

Quote:

A theoretical exercise published in 2006 showed that the smallest possible thermal fission reactor would be a spherical aqueous homogenous one powered by a solution of Am-242m(NO3)3 in water. Its mass would be 4.95 kg, with 0.7 kg of Am-242m nuclear fuel, and diameter 19 cm. Power output would be a few kilowatts. Possible applications are space program and portable high-intensity neutron source. The small size would make it easily shielded.

 

Sourced from:

http://www.world-nuclear.org/info/nuclear-fuel-cycle/power-reactors/small-nuclear-power-reactors/

 

I doubt superheavy elements such as Americium will be a viable fuel source in the near future, but it's still an interesting topic to discuss. As that snippet mentioned, I think one of the best applications would be in space, where mass is at a premium (though I doubt the savings in mass would be enough to offset the massive materials costs).

[Collimatrix]

The biggest issue with any exotic fissile isotopes is that the only primordial actinides on Earth are uranium and thorium.  You would have to transmute these elements into the desired isotopes by a rather tortured, multi-step process that probably wouldn't be that efficient because you'd be getting a bunch of other crap from side reactions.  I suppose a dedicated reactor cycle could get the efficiency up, don't know how much it's been looked at.

 

I'm a fan of Neptunium, personally.  Very low spontaneous fission rate, which is good for high burnup in bombs.

[Collimatrix]

I take it back; neptunium has an enormous critical mass.  Not at all ideal for bombs.

 

It's not really an exotic fissile radioisotope, but I'm curious why uranium carbide isn't used more, like the fuel in the Soviet NTR.  Greater density and higher melting point seems like significant advantages.

[Collimatrix]

Does ²³³U count as exotic anymore, or has Kirk Sorensen ruined it for fissile isotope hipsters?

 

'Cause apparently the nuclear lightbulb was supposed to use that stuff.

[LostCosmonaut]

It doesn't seem that exotic to me, but I have read a lot about nuclear related stuff.

 

Interestingly, the open-cycle gas core NTR I read a paper about at work (unclassified, but can't seem to find it on the real internet) was planned to use Pu-239.

[Collimatrix]

It provides decent thrust at a high I_SP, and it fumigates hippies while doing so.  What's not to love?

 

(and if you say "all the highly radioactive waste it leaves shoots into the atmosphere as high temperature gasses and aerosols," well, the government of Kerbin finds your attitude to be highly counterrevolutionary.)

 

I'm a bit curious about where they thought they were going to get this ²³³U.  Was there anywhere making the stuff at the time?

 

 

NTRs are probably an exotic enough application they they might deserve exotic fissile fuels.  What are the typical approaches for increasing power density in naval reactors?  Um, that you're allowed to talk about, of course.

[LostCosmonaut]

Increasing enrichment and therefore loading of 235 per unit volume, mostly.

 

Burnable poisons are pretty helpful if you're cranking up the power density, lets you keep shutdown margin early in life, and as the fuel burns up so will the poisons, so your power curve stays relatively flat for a while.

[Collimatrix]

OK, so increasing power density is simply a matter of increasing the amount of fissioning stuff per cubic furlong.  Makes sense.

 

Would this argue for using something with a highish critical mass, like the aforementioned neptunium?  That way, for a given amount of energy produced, the percent loss of fissile material would be lower and the compensatory mechanisms could be easier to design.

 

(sorry if this is babby's first reactor design 101)