# How many elements can undergo nuclear fission? **URL:** \ **Category:** Factual Questions\ **Created:** [December 10, 2012, 3:45pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606 "2012-12-10T15:45:18Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [December 10, 2012, 3:45pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/1 "2012-12-10T15:45:18Z")
…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? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 10, 2012, 3:49pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/2 "2012-12-10T15:49:57Z")
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. ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [December 10, 2012, 3:50pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/3 "2012-12-10T15:50:50Z")
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** ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [December 10, 2012, 3:56pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/4 "2012-12-10T15:56:50Z")
Last time I studied physics was in high school and I barely passed. How about all of them, nukes, reactors, labs, theoretically all. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [December 10, 2012, 4:21pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/5 "2012-12-10T16:21:14Z")
There’s also a difference between [fissionable and fissile](http://en.wikipedia.org/wiki/Fissile) isotopes. There are isotopes of Thorium & Neptunium that could be used to generate energy on a commercial scale. ---
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [December 10, 2012, 4:45pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/6 "2012-12-10T16:45:47Z")
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. ---
**Author:** ![Alessan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alessan/32/465_2.png) [@Alessan](https://boards.straightdope.com/u/Alessan)\ **Post date:** [December 10, 2012, 5:10pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/7 "2012-12-10T17:10:54Z")
Theoretically, anything heavier than iron. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [December 10, 2012, 6:31pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/8 "2012-12-10T18:31:44Z")
Why is iron special? ---
**Author:** ![slalexan](https://avatars.discourse-cdn.com/v4/letter/s/e47c2d/32.png) [@slalexan](https://boards.straightdope.com/u/slalexan)\ **Post date:** [December 10, 2012, 6:35pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/9 "2012-12-10T18:35:45Z")
If I remember correctly, Iron is the last thing stars produce from fusion before they die. ---
**Author:** ![Alessan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alessan/32/465_2.png) [@Alessan](https://boards.straightdope.com/u/Alessan)\ **Post date:** [December 10, 2012, 6:36pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/10 "2012-12-10T18:36:29Z")
> [@Leo\_Bloom](#): > > Why is iron special? 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. ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [December 10, 2012, 6:54pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/11 "2012-12-10T18:54:26Z")
> [@Alessan](#): > > 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. ---
**Author:** ![Baron\_Greenback](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/baron_greenback/32/48_2.png) [@Baron\_Greenback](https://boards.straightdope.com/u/Baron_Greenback)\ **Post date:** [December 10, 2012, 7:02pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/12 "2012-12-10T19:02:50Z")
> [@dracoi](#): > > Nitpick: stars DO fuse things heavier than iron. I hope I’m remembering this right, but lots of iron is a decay product of a heavier nickel isotope fused in a star. ---
**Author:** ![Michael63129](https://avatars.discourse-cdn.com/v4/letter/m/3da27b/32.png) [@Michael63129](https://boards.straightdope.com/u/Michael63129)\ **Post date:** [December 10, 2012, 7:09pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/13 "2012-12-10T19:09:01Z")
> [@dracoi](#): > > 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. Stars produce those heavier elements when they go [supernova](http://curious.astro.cornell.edu/question.php?number=345); otherwise, fusion only goes through iron, and even in supernova, the process isn’t fusion: > [@](#): > > In a supernova explosion, neutron capture reactions take place (this is not fusion), leading to the formation of heavy elements. This is the reason why it is said that most of the stuff that we see around us come from stars and supernovae (the heavy elements part). If you go into technical details, then there are two processes of neutron capture called rapid process (r-process) and the slow process (s-process), and these lead to formation of different elements. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 10, 2012, 11:57pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/14 "2012-12-10T23:57:45Z")
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. ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [December 11, 2012, 1:43am UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/15 "2012-12-11T01:43:12Z")
> [@Michael63129](#): > > Stars produce those heavier elements when they go [supernova](http://curious.astro.cornell.edu/question.php?number=345); otherwise, fusion only goes through iron, and even in supernova, the process isn’t fusion: Interesting. Thanks for the correction. I had never heard it phrased as neutron capture before. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [December 11, 2012, 4:27pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/16 "2012-12-11T16:27:38Z")
> [@Chronos](#): > > … so you certainly can’t get any energy out of it by fissing or fusing it… 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?” ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [December 11, 2012, 6:40pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/17 "2012-12-11T18:40:18Z")
> [@Leo\_Bloom](#): > > do physicists use “fissing” often as a light-hearted save-a-syllable word among themselves? Nope. I’ve certainly never seen or heard it used. Always “fission”/“fuse” and “fissioning”/“fusing”. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 11, 2012, 10:56pm UTC](https://boards.straightdope.com/t/how-many-elements-can-undergo-nuclear-fission/643606/18 "2012-12-11T22:56:52Z")
I don’t think “fissing” is actually a word. But I do think it _should_ be.