[chiralSPO (OP)]

(I opted not to tack these disorganized musings on to the existing thread regarding plutonium oxide fuel...)

Could ⁴⁰K be a useful radioactive fuel for interstellar missions?

Potassium is widely distributed and highly abundant in the Earth’s crust (as well as throughout the inner solar system, I would bet). ⁴⁰K is a very minor component of all potassium (about 0.012 % per Wikipedia), but it has a half-life of about 1.25x10⁹ years, so it should be widely available. This also means that it can provide stable power output for a very long time. And because its atomic mass is so low, it has a very significant energy density. (there are 3 possible decay pathways, each with its own energy and probability, and I don't have all the numbers I need to calculate it, but I feel like it could be a lot of energy density, even if the power density is fairly low)

As I understand it, most commercially produced potassium metal is actually produced as a gas and then condensed and solidified (it is distilled out of high temp reactions of potassium salts with molten sodium or molten aluminum).

So we have the infrastructure for large-scale distillation of potassium, would it not be feasible to also use distillation (or other gas-phase techniques like centrifugation, chromatography, etc) to enrich the  ⁴⁰K from natural abundance of 0.012 % to something more like 5 %? (obviously the greater the enrichment, the higher the power density, but I don’t know what would be ideal for use as fuel, and what would be practical limitations on enrichment)
 
(prep mass spec is also an option, like the Manhattan project, but even though it allows use of stable K⁺ ions, I still suspect it would be the more difficult option)

The fact that one of the decomposition products is Ar might pose some engineering challenges, but could also potentially be put to use?

[Bored chemist]

The specific activity is about 32 Bq per gram
Each decay produces about 1.3 MeV
That's about 2* 10^-13 J

So the power density is about  2 *10^-12  Watts per gram.
That's about a billion times less than a compost heap.(Though it will last much longer).

Long half lives mean small power density

[alancalverd]

So the power density is about  2 *10^-12  Watts per gram.
That's about a billion times less than a compost heap.

I doff my cap, Sir.

[Eternal Student]

Hi.  I hope you are well.

I've not much to say other than what is on the other thread you've already mentioned.

Potassium is taken up by Biological organisms readily and used more widely than 238-Pu.  I'm not sue about the biological half-life for the stuff, it can probably be got rid off just as quickly (if the pilots have a clean supply of potassium salts to eat often - but that would mean they also need to drink more water etc.)  Anyway, for un-manned spaceships the risks of contamination for the pilots isn't an issue.

[alancalverd]

However if you are in no hurry to accelerate, a beta-emitter has a certain attraction. Imagine a rocket engine with a lump of K40 in the middle of the combustion chamber. After a while, the walls will become negatively charged and betas will be increasingly expelled from the nozzle, with speeds approaching c, so your rocket  will eventually reach a very high speed. This is the principle behind ion thrusters, which produce very little static thrust but with a very high exhaust velocity.

As with other means of transport, it is recommended to segregate the crew from the fuel.

[Bored chemist]

However if you are in no hurry to accelerate

Specifically, if you want a net thrust of zero.

You launch beta particles from the ship, and the ship becomes positively charged as a consequence.
After a while the charge will be so big (i.e when teh voltage reaches about 1.3 MV) that the betas will no longer have enough energy to escape from it, and all you will have will be a messy  X/ gamma ray source.