Is there a way to lower critical mass?

[l0st]

Judging by https://en.wikipedia.org/wiki/Critical_mass , californium-252 has the lowest critical mass among known isotopes and isomeres. But it is still very high. Is there a way to reduce it significantly (down to microgramms) or, perhaps, a semi-stable isotope/isomere, which would have such a low critical mass initially?

What I am looking for, actually, is possibility (not interested in feasibility) to produce energy from fission in a similar way it is done in internal combustion engines. E.g. if an uncontrolled explosion caused by a chain reaction could be small enough to be contained and harvested for energy in some industrial setting (I imagine a giant turbine or a piston). For simplification, I assume it would be initiated by contact of small two half-spheres (not half-balls, e.g. empty inside) which would make material to get super-critical.

 

[l0st]

Oh, yes. Alternatively, would be interesting to know if there are ways to induce chain reaction with light, or some other easily achievable interaction.

 

[mfb]

A higher density leads to a smaller critical mass and neutron reflectors help a bit, but there is no way to get the density to values where the critical mass would be milligrams. And even if you could produce miniature explosions, a large fraction of the energy is emitted as gamma rays and doesn't help in an internal nuclear explosion engine. You also get ionizing radiation and activation everywhere.

Accelerator-driven systems don't have a minimal mass, but all the other issues with nuclear material still apply.

 

[Chincha40]

The lowest critical mass I know of is Americium 242m (m for meta stable which for this isotope has a half life longer than the non meta stable) in an optimally moderated aqueous solution (probably a AmO2 and D2O slurry in a specific ratio) and water reflected has a critical mass of just 20 grams. This is known as an aqueous homogeneous reactor and they have the lowest fissile material requirement of any reactor. In ordinary solid form its much higher. Here's a link to a page with the masses for several elements in aqueous solution and a solid lump: https://www.euronuclear.org/info/encyclopedia/criticalmass.htm

The problem with Am 242 is its massive neutron cross section (which gives it the low critical mass) makes it very rare in reactors and waste as that it is usually destroyed via fission or capture as it is produced. You would need a liquid fueled reactor to make it and extract it quickly enough to get anywhere near 20 grams. Perhaps a custom built liquid fueled breeder reactor running on Am241. There have been proposed reactors running on 20 or so grams of Am242m and have specified outputs of around 10Kw.
Also as stated above, the engine powered by nuclear explosions wouldn't be very efficient.

You might also be interested in the fission fragment reactor: https://en.wikipedia.org/wiki/Fission_fragment_reactor Check the PDF at the bottom off the wiki for a diagram I think.

 

[l0st]

in an optimally moderated aqueous solution (probably a AmO2 and D2O slurry in a specific ratio) and water reflected has a critical mass of just 20 grams. This is known as an aqueous homogeneous reactor and they have the lowest fissile material requirement of any reactor. In ordinary solid form its much higher. Here's a link to a page with the masses for several elements in aqueous solution and a solid lump: https://www.euronuclear.org/info/encyclopedia/criticalmass.htm

That is very interesting, and I have never heard about this before. What physical effect lowers critical mass in aqueous solutions?

The effect seems to be significant (from 8kg to 20g in Am242m).

 

[Chincha40]

That is very interesting, and I have never heard about this before. What physical effect lowers critical mass in aqueous solutions?

The effect seems to be significant (from 8kg to 20g in Am242m).

I don't know. I would assume it to be better homogenization with the moderators (water/heavy water obviously) and the fact that aqueous solutions are liquid making it much easier to get a spherical shape needed for criticality. Homogenized reactors (AHR, TRIGA) have always preformed spectacularly compared to heterogenized ones(PWR, BWR).
Take what I say with a grain of salt because I haven't gone to university yet (still in high school)to get a degree on it.

My question is why didn't they reflect it by Beryllium or Tungsten carbide? Both much more compact and effective than steel and water reflectors. So much so that you can build extremely high powered reactors with small size using beryllium, an exsample would be the HFIR https://en.wikipedia.org/wiki/High_Flux_Isotope_Reactor. And Tungsten has actually cause several criticality accidents because it reflect neutrons very well https://en.wikipedia.org/wiki/Demon_core.