Science Questions (spun from Mathematical Jokes)

This is not a mathematical joke so it is off topic and you can ignore it. Instead, it’s a Science Question.

You are handed a large flask with a ground stopcock containing an equiatomic mixture of tritium and helium-3. The internal pressure is approximately 70% of STP.

Your task is to prepare a sample of helium-3 of at least 95% purity.

Both atoms have almost exactly the same mass.

You have a full set of laboratory glassware and clamps, but otherwise you are only allowed to use things or materials that you can purchase at Home Depot.

The best answer is the one that requires the least external energy input per cc of product.

If nobody comes up with the “right” answer I will post it in a day or two.


Would it work to burn the helium out, or would that be too dangerous?


If you really want a low energy method, just leave the mixture well-sealed for about 42 years. Tritium decays to He3 with a half-life of about 12.4 years.


Burn the tritium out, you mean - as an isotope of hydrogen, it reacts exothermically with oxygen (although not quite as vigorously as H, if I recall correctly). He3 is inert with oxygen. You could do that, but your admixture of oxygen would need to be pretty precisely stoichiometric, you’d need to figure out how to dissipate overpressure without losing or contaminating the gaseous mixture, and then you’d have to condense out the water vapour.

Edit: That’s assuming that pressure rise of combustion is enough to stress the container. Adding oxygen will bring the internal pressure up close to STP to start with, I’m estimating.


Is the correct answer. The question didn’t specify a timescale.

Burning tritium with oxygen is a rather bad idea as the resulting substance won’t condense - it will self-heat. It can be dissolved in water but then you have to get shot of the tritiated water. Just leave it for, as you say, around 42 years. As the diatomic H-3 molecules decompose to monoatomic He-3, there is the helpful side effect that the pressure increases to a little over atmospheric allowing a sample to be extracted easily.


If anyone’s wondering why this appeared here:
I forked the topic for readability purposes. Digressions are fun, and ideally the best ones deserve their own thread.


If you’d said Party City instead of Home Depot this might have been easier. Though I think their helium is a mix with ordinary air, so maybe not.


If I have 42 years to waste, I’d rather waste them at Home Depot as opposed to Party City.


Their helium is He-4. Which is kind of the point since it is He-3 that is required.

Incidentally asking this question in a class has in the past led to suggestions about using a very large number of coffee filters in series to do separation by diffusion, and making use of the fact that the tritium molecule is diatomic and has twice the mass of the helium-3 atom to use improvised centrifuges. Part of the problem is that people are mostly preconditioned to start with diffusion or centrifuging and it is hard to get their minds away from the “official” methods.


How so? The energy of beta decay is unusually low for tritium, 18.6 keV, with somewhat over 2/3 of that carried off by an electron antineutrino, so that the kinetic energy of the electron produced is about 5.7 keV. Given the relatively slow rate of decay, I can’t see that contributing more kinetic energy to the gaseous mixture than we can remove during condensation.

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Yes, I’m sorry. Tritium oxide produces about 90mW/g, which means it evaporates rather fast but doesn’t boil. I was thinking of the difficulty of producing pure liquid tritium.

238Pu is the fun one. It will self heat happily to red heat (hence the use in RTGs). When it was discovered it was promptly suggested that it could be used as a convenient way of heating houses and stoves, before someone discovered the downsides - and the cost of producing it in industrial quantities.


Homestly I was marching down the ionise then seperate with electric fields based on charge to mass ratio.


Yeah. It’s just not that practical to try to come up with an apparatus that gives the requisite purity with equipment you can buy at Home Depot.

If it weren’t for the fact that tritium is probably in higher demand than 3He, I’d say stick it in casks and age it like wine. :wink:


For some value of “demand” e.g. Lawrence Livermore’s nuclear bomb design experiment “practicable means of extracting energy from fusion”.

Normally the problem with tritium is getting rid of it fast enough.


True. The stuff has a nasty habit of showing up in reactors as tritiated water, and the beta radiation makes that stuff corrosive. However, tritiated water is useful as a tracer, and tritium itself seems to be (with deuterium) the current preferred fusion fuel. 3He may well be the fuel of the future, but not until we learn how to get power back with fuels that initiate and sustain fusion at lower temperatures - 3He has a very high Coulomb barrier.


Bit of an old thread, but I think it’s the right one.

Supose I want to find a value which will be no less than 1/3, no more than 1, and otherwise (sqrt N)/100.

I think I can define it as V = max 1/3, (min (sqrt N)/100, 1). But that’s ugly. Is there a cleaner standard notation?