Thread by @GBruhaug

Micro-reactors are quite the popular topic right now, so let's talk about how you make a REALLY micro-reactor using the best (thermal) nuclear fuel we know of, Americium! Specifically, the isotope Am-24m. 🧵1/

Americium was discovered in 1944 by Glenn Seaborg using a cyclotron, but is a pretty common transuranic (i.e. element after uranium) to encounter. It is a common isotope in spent nuclear fuel (~100g/ton), although most of it is Am-241 and Am-243. 2/

You may even have Americium in your home in the form of a smoke detector! It is a powerful alpha radiation emitter, which makes it a good source of ionization for smoke detectors. Also potentially a good RTG fuel, with ESA looking heavily into this. 3/

All isotopes of americium can fission, but the rarer Am-242m is unique because its thermal fission cross section (chance of fission) is ~7000 barns! That means it is ~12X higher than U-235 (and U-233) and~9X higher than Pu-239! It also emits ~1.2X more neutrons per fission! 4/

This means it can get stunningly small critical masses! Some studies have come up with reactors that only needed <8 grams of Am-242m! This can allow for extremely small reactors for space or potentially medical applications! 5/

It should be noted that this huge advantage is only present for THERMAL reactors though, where we slow the neutrons down in something like water. For fast fission, Am-242m has little advantage over conventional fuels and thus your sci-fi micro nukes won't be using it! 6/

There are plenty of other sci-fi applications though! Am-242m has been investigated for ultra tiny reactors for neutron therapy at hospitals, ~4 MW/kg nuclear rockets, and ultra small aqueous power reactors. 7/

There were also proposals to make better gas core and fission fragment rockets, and even revisiting the nuclear piston engine concept using Am-242m! With 70% savings in mass for the rocket and high specific power nuclear piston engines, it may be worth another look. 8/12

Of course the big question is how do we make this rare isotope? Am-241 is common portion of spent nuclear fuel as mentioned above and we can use *careful* neutron capture to make Am-242m. Typically the schemes proposed rely on fast neutron capture, to lower the loss 242. 9/12

One proposal actually used special rods placed into a typical LWR power reactor! The rods would have Am-241 behind a thermal neutron absorber (Gd in this case). They showed that ~1.1 kg/GWe-yr of 8.65% Am-242m can be made and you get >7 kg/GWe-yr of >80% Pu-238 as well! 10/12

The other proposal I found was for an Am-241 breeding blanket in a fast reactor. It was able to create >120 kg of Am-242m using ~10 tons of Am-241, which is quite a lot! The Am-242m would be of high purity though, and thus immediately usable. 11/12

Who knows if Am-242m will ever be used for as a nuclear fuel, but it sure looks like something interesting to consider for speciality applications! Maybe in a fast reactor powered future we could even get the supply large enough (and price low enough) to consider! 12/12

Sources and more reading:
en.wikipedia.org/wiki/Americium
inis.iaea.org/collection/NCL…
researchgate.net/publication/23…
tandfonline.com/doi/abs/10.131…
tandfonline.com/doi/abs/10.131…
tandfonline.com/doi/abs/10.131…
tandfonline.com/doi/abs/10.131…
digital.library.unt.edu/ark:/67531/met…
deepblue.lib.umich.edu/bitstream/hand…
sciencedirect.com/science/articl…