[credit_guy]

I just tested it with a slightly tricky question

  > If you could run a nuclear reactor with U-235 as fuel or Pu-241 (both mixed with 95% U-238), which one would you choose and why? 

For a human this would not be tricky at all. For an LLM it could be, because this question certainly does not exist in any sort of training, because Pu-241 does not exist in pure form, it only exist as a minor component of reactor-grade plutonium, where Pu-239 would dominate, with Pu-240 coming second and Pu-241 coming third.

In any case, LongCat-2.0. gave a very well reason but incorrect answer that Pu-241 is preferable.

I then tested on Qwen 3.7 Plus, and it correctly answered that U-235 is preferable because of its much higher delayed neutron fraction. I then went to Gemini Flash, which answered the same, with much more confidence, and with much stronger arguments, and the speed of the answer was much higher.

Overall I rate Gemini Flash the best, Qwen 3.7 Plus an acceptable second, and LongCat-2.0 an ok'ish third, if you have nothing better.

[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.

[mlmonkey]

"For a human this would not be tricky at all."

Which humans have you been hanging out with? :-D

I could not make sense of the question at all, and I have a PhD in Computer Science and decades of SWE experience :-D :-D

[croes]

> For a human this would not be tricky at all.

I very much doubt that.

[credit_guy]

What I meant is: for a human who has the context, like someone who works in the field, or took some courses, or read some materials about this. A human who just has a vague idea about fissile isotopes and needs to google the properties of U-235 or Pu-241 would do no better; they would probably do much worse.

[bel8]

A more fair and useful comparison would be to feed both LLMs with documentation about such niche knowledge in the contex, then ask.

[icepush]

Did you ask the question several times in fresh chat contexts to see if it sometimes gives the right answer ?

[zythyx]

Nah, n=1 is enough to give evidence that something is entirely broken, of course.

/s

[3eb7988a1663]

Well, when we had deterministic tools, it would only take a single example of a calculator claiming 1+1=4 for me to throw it in the trash.

[CamperBob2]

That's like saying, "It would only take a single example of a table saw cutting someone's thumb off for me to switch back to hand saws."

A noble sentiment, perhaps. But while the table saw user might lose a digit every now and then, you'll get flattened. Determinism is vastly overrated.

[IshKebab]

And if you can come up with a deterministic tool that can do everything LLMs can then that would be amazing! Until then, we have to accept the non-determinism.

[paintbox]

For comparison allow me to add chatGPT 5.5:

"Choose U-235 if the goal is safe, boring, practical electricity generation. Choose Pu-241 only if the goal is specifically to consume/recycle plutonium in a reactor designed and licensed for that fuel.

In brutal shorthand: Pu-241 is a better “fissile isotope” in some nuclear-physics ways, but U-235 is a much better reactor fuel in the real world."

If only I knew anything about nuclear reactors. But it sounds to me that the answer is also correct.

[thecopy]

Am i the only one feeling my soul being emptied every time i read another "brutal shorthand" or "honest take"?

[dbuxton]

It's genuinely depressing.

[smallmancontrov]

Yeah, but the default is extreme yap mode. I prefer "in a sentence" or "rows, not paragraphs." "Brutal" sounds like bait from an influencer trying to sell me a $400 course about how 4AM workouts will make me a millionaire.

[jdiff]

I don't even know how we got here. This isn't that deeply represented in the training data. Is this what RLHF hath wrought? A new dialect of English based on corporatespeak and influencers, two heavy-hitting bullshitters?

[ChucklsTheBeard]

Remember how, for SEO purposes, every food blog has to bs a multi-page story that everyone scrolls past to get to the 10 line recipe?

That's training data too; that's how we got here.