Physics questions 34?

Question

1) U-235 releases an average of 2.5 neutrons per fission while PU-239 releases and average of 2.7 neutrons per fission. Which of these elements would you expect to have a smaller critical mass?
2) Fermi's original reactor was just barely critical because the natural uranium that he used contained less than 1% of the fissionable isotope U-235 (half life 713 mil. years.) What if, in 1942, the earth had been 9 billion yrs old intead of 4.5 billion years old? Would Fermi have been able to make a reactor go critical with natural uranium?
3) If uranium was split into three segments of equal size, instead of two, would more or less energy be released? Why?
4) Why is there, unlike fission fuel, no limit, to the amount of fusion fuel that can be safely stored in one locality?

Answer 1

1) If the capture cross-sections were the same (big if), the plutonium would have a smaller critical mass.

2) Natural uranium has about 0.7% U235. If you add another 4.5 billion years, that's like 6 or 7 half-lives, so you'd have about 1% of the U235 you have now (ie about 0.007%). That's not NEARLY enough.

3) Binding energy per nucleon maxes out at Iron-56. So by splitting U-235 into 3 equal pieces, you get closer to that optimum point than you do by splitting into 2. So you would get a bit more energy.

4) Fusion doesn't happen unless you have a LOT of energy to drive nuclei together. It can't get kicked off just by thermal neutrons from a spontaneous reaction.

Answer 2

I know the answer to number 4. Fission fuel has unstable nucleii and spontaneously decays every so often. If there's enough of it together it can start a chain reaction when the high energy particles from the decay bombard other nucleii and produce more high energy particles.
Fusion fuel has typically has stable nucleii and doesn't spontaneously decay. Also, a nucleus that is lighter than iron, which defines fusion fuel, doesn't release energy when broken apart. So even if a nucleus is hit with a high energy particle and breaks apart, no energy is released and the reaction kind of fizzles out.