…and how many are usable or have been so used.
U-235
Pu
U-238, I believe undergos fission in three stage nuclear devices.
Any else?
First of all, are you just counting the ones that fission exothermically? Because anything above hydrogen can be forced to fission, if you whack it hard enough.
Second, are we counting alpha decay as a special case of fission? It sort of is, since an alpha particle is an atomic nucleus.
I think the answers will be different depending on if we’re talking about nuclear weapon conditions, power plant conditions, or laboratory conditions (eg, getting a few atoms of some element to fission by shooting high velocity neutrons at them.)
ETA: Hi Chronos
Last time I studied physics was in high school and I barely passed. How about all of them, nukes, reactors, labs, theoretically all.
There’s also a difference between fissionable and fissile isotopes.
There are isotopes of Thorium & Neptunium that could be used to generate energy on a commercial scale.
If the OP doesn’t mind can we re-phrase the question into something a little more concrete.
For what elements are the most common isotope of that element exothermically fissionable.
And for that matter can we ask the reverse,
For what elements are the most common isotope of that element exothermically fusionable.
Theoretically, anything heavier than iron.
Why is iron special?
If I remember correctly, Iron is the last thing stars produce from fusion before they die.
IIRC, it’s the one element on which neither fusion nor fission can be used to produce more energy than it takes to create the reaction. That’s why stars can’t fuse iron into anything heavier. A star with an “dead” iron core eventually collapses into a supernova.
Nitpick: stars DO fuse things heavier than iron. Virtually every heavier element we observe is believed to have been created through stellar fusion.
The catch is that this fusion doesn’t produce net energy, so it’s not helping to prolong the star’s life.
I hope I’m remembering this right, but lots of iron is a decay product of a heavier nickel isotope fused in a star.
Stars produce those heavier elements when they go supernova; otherwise, fusion only goes through iron, and even in supernova, the process isn’t fusion:
Iron-56 is the peak (or valley, depending on how you look at it) of the binding-energy-per-nucleon curve, so you certainly can’t get any energy out of it by fissing or fusing it, but that doesn’t mean it’s the only nucleide for which that’s true. For something a little lighter than iron, the binding energy per nucleon might not be quite as large as for iron, but it’ll still be larger than that of an element twice its size. Likewise, for something a little heavier than iron, the binding energy per nucleon will be larger than that of an element half its size.
Interesting. Thanks for the correction. I had never heard it phrased as neutron capture before.
I thank you all for these answers. I’ve wondered about why iron figures so highly in the odd stellar fusion threads I’ve stumbled into. (Cf. my “where does stuff come from” thread-starter…)
Chronos, do physicists use “fissing” often as a light-hearted save-a-syllable word among themselves?
The way some musicians will simply say “horn” or “fiddle?”
Nope. I’ve certainly never seen or heard it used. Always “fission”/“fuse” and “fissioning”/“fusing”.
I don’t think “fissing” is actually a word. But I do think it should be.