[3eb7988a1663]

I am not a physicist but perhaps your question was leading more than you expected? I would take the question to pre-suppose I have an abundance of the stated material, ignoring practical realities of refinement. If I did have fully pure Pu-241, would that be a better fuel than U-235?

Or stated another way, "If you could run a generator on gasoline or jet fuel, which one would you choose and why?" I would answer jet fuel owing to slightly higher energy density and purity of the material - likely leading to a cleaner burn. Which would ignore that jet fuel is going to be a multiple of the gasoline price.

[onion2k]

If I did have fully pure Pu-241, would that be a better fuel than U-235?

Also not a physicist, but I assume from the fact that the OP is asking the LLM this question to trip it up, the point is that U-235 is better even if you have an abundance of both. It's scarcity of Pu-241 leads to the lack of data in training, not that it's actually better.

[adrian_b]

I am lazy to search for a more authoritative source, but Wikipedia says that Pu-241 has a greater neutron absorption cross section than Pu-239 and the same probability of fission after absorbing a neutron. It would also generate a slightly higher amount of energy per fission event, than Pu-239.

This means that it would actually be a better fuel than Pu-239 for a fission reactor and only its scarcity prevents its use.

The only disadvantage versus Pu-239 is its short half-life, of 14 1/3 years. This means that it cannot be stored for a long time, so it must be consumed as a fuel soon after it is produced, to avoid losses.

Pu-239 has a lower delayed neutron fraction than U-235, which makes the control of a Pu-239 fission reactor more difficult.

But according to:

https://www-nds.iaea.org/sgnucdat/a6.htm

Pu-241 has almost the same delayed neutron fraction as U-235 (0.016 vs. 0.0162), so that is not a serious disadvantage for it.

Both Pu-239 and Pu-241 produce more neutrons per fission event than U-235. This simplifies some things, by allowing a reactor to work with less fuel or less-enriched fuel, but it complicates the control, because there is a greater risk of instabilities.

The truth is that an LLM cannot say which is a preferable fuel between U-235, Pu-239 and Pu-241. It would be possible to design fission reactors that work fine for any of these 3. The best choice depends on economical factors, not on technical feasibility factors.

The only real reason why Pu-241 will never be used is that its production yield when irradiating uranium with neutrons is too low in comparison with Pu-239, so it would be too expensive.

[3eb7988a1663]

Again, really speaking out of my depth, but if there is a lack of plutonium training data, I would assume the LLM answer would be the far more commonly described U-235. To respond otherwise means there is some existing association with Pu-241 being better.

[Skwid]

That's 2.5% more neutrons, surely that must be better!

[teaearlgraycold]

It’s tough to write good questions for LLM evaluations. They’re so good at picking up subtleties they can pass a multiple choice test when given only the answers and not the questions.

[cyberax]

A higher delayed fraction of neutrons makes it easier to control the reactor. Without delayed neutrons you can only make a bomb.

[nullc]

post says delayed neutron fraction, I presume if were enriched with pu-241 the band between critical and prompt critical would be non-existent and you'll have made a bomb.

But I'm just riffing off the parent poster's text.

[tryagainian]

> Which would ignore that jet fuel is going to be a multiple of the gasoline price.

That doesn’t sound right. If my Duck Fu is any good, jet fuel is currently going due US$3.00 per gallon, avgas (leaded petrol) at $3.30, and gasoline at $2.88 gallon.

There’s nothing much special about jet fuel, it’s just kerosene, same as RP1 (Rocket Propellant), heater fuel, and lamp oil you can buy from the hardware store, with a touch of something to stop it gelling at low temperature if I understand correctly, but also jet fuel tanks are heated if I recall correctly.

I believe standard diesel fuel will also works in jet engines, but kerosene is cheaper.

I’m not in the US, and if I understand correctly their gasoline (petrol) price can vary greatly from state to state, California being the worst? Is that right?

[genewitch]

in 2012 i owned a car that could run 100 octane fuel, and that was $9 a gallon. a few more octane and you get Jet-A minus the additives.

according to jetfueltracker jetA is about $2 more than 87 octane right now and about $1 more than 93 octane. and still somehow cheaper than diesel.

I'm not used to seeing jet fuel this cheap, luckily there's none near me to waste money on.

[tryagainian]

> in 2012 i owned a car that could run 100 octane fuel, and that was $9 a gallon. a few more octane and you get Jet-A minus the additives.

So? Diesel and petrol (gasoline) are different fuels. Comparing them by RON is irrelevant.