alt Hacker NewsI 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?
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.
Replies
"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
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.
A more fair and useful comparison would be to feed both LLMs with documentation about such niche knowledge in the contex, then ask.
Did you ask the question several times in fresh chat contexts to see if it sometimes gives the right answer ?
> For a human this would not be tricky at all.
I very much doubt that.
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.