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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.

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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.

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

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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.

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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.

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

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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.

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Am i the only one feeling my soul being emptied every time i read another "brutal shorthand" or "honest take"?
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It's genuinely depressing.
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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.
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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?
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"aesthetically tacky" and "succinct" overlap a lot. But actually I suspect there is indirect overloading of certain concepts. If you train it to "be honest" a billion times, I'm not surprised that "honest caveat:" is an indirect affectation.
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A more fair and useful comparison would be to feed both LLMs with documentation about such niche knowledge in the contex, then ask.
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Did you ask the question several times in fresh chat contexts to see if it sometimes gives the right answer ?
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Nah, n=1 is enough to give evidence that something is entirely broken, of course.

/s

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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.
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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.

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

I very much doubt that.

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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.
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