Talk:Nuclear fission

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Corrupted page?[edit]

All the links in the references from the introduction is screwed up now and i cant seem to find a past version that works. Is there some bug on this page?--17:09, 30 March 2008 (UTC)Venny85 Ok, the code below seems to be some bad code.... Had it removed already.

template:Nuclear physics

well, nvm, sems like it was because of vandalism on the template

The Oklo reactor section at the end has some errors. As I recall, a professor at the University of Arkansas (not the discoverer) figured out why the natural reactor had operated in 1959, some thirteen years before the article claims. --Ascentury 19:24, 4 April 2007 (UTC)

Is this sentence in the main text correct?: "...however this process works better for heavier elements which have room in outer nuclear orbitals for the necessary extra neutrons." Aren't orbitals "populated" by electrons?. How ever though i need to check still. --Vrrp 13:13, 11 July 2006 (UTC)

In reading this page, two questions were left unanswered for me: a) what makes uranium and plutonium so special that they are easily used for fission? and b) what's a "thermal neutron"? The term is used without any introduction. --joe (joe at

  • Agree with b) --Chealer 17:10, 2004 Oct 1 (UTC)

b.) "Thermal neutrons" are at a lower energy level than the fast neutrons" released from the fission process and are more likely to be absorbed into a U235 atom to continue the fission reactions. "Fast neutrons" are moderated in nuclear reactors by the coolant (usually DI water) to slow them down for more fissions to take place...thus the term "thermal reactor."

a) There should be a sentence to explain that only a few nuclei have this property. This goes along with b) that it is (counterintuitively) easier to fission these nuclei with a slow than a fast neutron, which is why reactors require lower concentration of fissile material than bombs (which use fast neutrons). I did not see the term cross-section in the article.Avm1 (talk) 05:18, 19 November 2007 (UTC)

I think it looked much nicer the way it was, with the image on the right and the text flowing around it. What did you have against that? Mkweise 21:27 Mar 7, 2003 (UTC)fgjdlskdjglsjglsdjgslkjljdgglsdjglsd

Because to some people including me the width of the image is little too long. -- Taku 21:29 Mar 7, 2003 (UTC)

I wonder what's "isobars". I thought this was a meteorological term only. -- 14:22, 1 Oct 2004 (UTC)

  • It's atoms with the same number of nucleons, as the sentence describes. What confused me is that the sentence reads "giving several isobars" instead of for example "i.e. several isobars" so I was wondering if it was something else.--Chealer 16:26, 2004 Oct 28 (UTC)

Removed text:

Uranium's most common isotope is U-235 (there being 235 total protons and neutrons in the nucleus).

That is just plain wrong, see Uranium. Andrewa 16:00, 6 Nov 2004 (UTC)

Waste and Actinides[edit]

The article suggests that fission products with long halflives are the main reason why waste has to be stored for long periods of time. Whereas this is probably true in the intermediate term, I was of the understanding that the bulk of the long term radioactivity ( in excess of a few hundred years ) is down to the minor actinides. Some clarification could be in order 00:09, 2 March 2007 (UTC)

Short half life products decay fast. Really long ones aren't very radioactive. As far as I know, the actinides in reactor waste being discussed have intermediate (between U238 and Pu239) half lives. But actinides aren't fission products, they are otherwise generated in operating reactors. Gah4 (talk) 16:42, 12 November 2016 (UTC)

Criticality Issues[edit]

Here or somewhere else with a link, needs a much more detailed discussion of chain reactions, criticality, delayed neutron-fraction and prompt criticality.

Linuxlad 09:59, 26 Nov 2004 (UTC)

ALSO going Prompt-critical does NOT equate to a nuclear bomb. (Most nuclear reactors are designed against a range of postulated reactivity excursions, some of which take them (briefly) 'prompt-critical' - most don't even fail the fuel clad). Nuclear weapons, to be worthy of the name, are ramped through prompt critical at a rate of knots.

I know; the article really needs an explanation. The point is, it's not enough to be above critical mass, a weapon needs to be above prompt critical mass. There are brief explanations of critical/prompt critical in various places, but there should be one here too. Still, a link to prompt critical is essential. --Andrew 18:13, Apr 11, 2005 (UTC)

==Consider the physical characteristics of normal EE92Uranium ore. It is Supposed to have 72 EO92235 atoms for each 9923 EE92238 atoms, but let's round that off to 80 92U235 to 9908 92U238 or 8 92U235 per 992 92238 atoms. That means that inside each 10x10x10 cube of 92 Uranium there are 8 atoms of 92U235. And if we break the cube down further into 8 parts we get to a 5x5x5 = 125 atom cube with 1 92U235 atom in the center. So in that cube we have 1 92U235 atom immediately surrounded by 26 92U238 atoms plus 98 92U239 atoms removed at least one atom therefrom. And the system is noncritical. That's evidently because the fission of 1 atom of Eo92U235 is unable to provide enough activation energy to the system to to begin a cascading process of runaway activision of multiple fission of the EE92U238 atoms to sustain a nuclear fission process and the fission process fizzles out to the elimination of the remaining EO92U235 atoms from the ore. But what can we do? Well we can increase the amount of EO92U235 atoms in the mix and thus get more EE92U238 atoms in close proximity to adjacent EO92U235 atoms. And what should be our goal? Well if every EO92U235 atom is located in the center of a 3x3x3 =27 atom cube of 92 uranium, then the constituency of the Uranium becomes 1 EO92U235 atom per 26 atoms of EE92U238 or approximately 4 percent constituentcy. Which the amount the enrichment of 92Uranium ore that we need to cause the phenomenon of criticality. WFPMWFPM (talk) 02:58, 3 October 2008 (UTC).WFPMWFPM (talk) 03:00, 3 October 2008 (UTC)

Huge loss of content?[edit]

Take a look at the changes by on 09:58 3 Nov 2004. Much of the article was deleted, and I don't think anyone caught this. Should it be restored?

Looks like pure vandalism which was not noticed in time. IMHO, the deleted lines should be reconciled with all the edits to date. We can't just revert to 3 Nov. pstudier 00:31, 2004 Dec 8 (UTC)

Redundant Material[edit]

"So, if we separate the U-235 from the U-238 and discard the U-238 (producing enriched uranium), we promote a chain reaction. In fact, the probability of fission of U-235 by high speed neutrons may be great enough to make the use of a moderator unnecessary once the U-238 has been removed.

U-235 is present in natural uranium only to the extent of about one part in 140. Also, the relatively small difference in mass between the two isotopes makes isotope separation difficult. Nevertheless, the possibility of separating U-235 was recognized early on in the Manhattan Project as being of the greatest importance to their success."

This appears twice in the article. I wasn't sure exactly how to resolve the issue, but I wanted to bring it to light for those who have a better idea.

yow -- overhaul needed[edit]

The current article is so disorganized that it is difficult to see where to begin. I just added a discussion on delayed neutrons, then realized that one already existed -- it was just hard to find. I reverted my changes rather than make things even worse, but someone really needs to overhaul this article. Maybe that someone is me -- I'll try to get around to it in the next week or so, but no promises. zowie 17:16, 28 October 2005 (UTC)


In the second paragraph it is stated that neutrons are "generated" in a chain reaction. Not true. Neutrons are expelled, emitted, or ejected, they are not generated since they were present in the original U235. (talk) 23:44, 30 March 2011 (UTC)

It sounded like an acceptable jargon, (generated meaning generated free neutrons, outside the atoms), but I've changed it to emitted as neutrons can also be truly generated from protons. Materialscientist (talk) 23:52, 30 March 2011 (UTC)
The other choice would be to say free neutrons, which don't exist inside a U nucleus. Gah4 (talk) 16:44, 12 November 2016 (UTC)

Overhaul begun[edit]

I overhauled the introduction. I'm planning next to insert a "physics" section to describe critical and subcritical chain reactions, prompt criticality, fuel selection, delayed neutrons, and reactor poisons. That will probably subsume much of the structure in the sections below the "history" section. zowie 23:36, 7 November 2005 (UTC)

Overhaul in progress - split out reactor physics page?[edit]

I've been working my way through summarizing fission reactor physics, and got a comment from DV8 XL2 (Hi, DV8 XL2) that the article is getting quite long. I agree, but feel that the discussion of reactor physics is useful as there really isn't one elsewhere. What do people think about splitting the (still-in-progress) reactor physics section out into its own page, or about trying to merge it with nuclear reactor? zowie 18:35, 11 November 2005 (UTC)

  • Finish it up here zowie, and then deside. DV8 2XL 19:21, 11 November 2005 (UTC)

Overhaul finished. Reactor physics split out.[edit]

I believe the article reads much more smoothly now. I cut out a small amount of the historical discussion and merged the rest into the description of fission. We still probably need a small section on political importance (weapons + power) but maybe there's one somewhere else that we can just refer to? zowie 00:01, 19 November 2005 (UTC)

  • Well done. You have produced a very readable and informative article. The only problem is you may have set the bar too high for yourself. ;) DV8 2XL 00:28, 19 November 2005 (UTC)

Peer review / FAC preparation[edit]

I've requested peer review for this page as it seems to be sort of stable and I'd like to get it polished enough for FA status. (Also: I got rid of the political relevance to-do item on this talk page, since politics are mentioned in the intro and thoroughly discussed in some of the linked-to articles). zowie 03:58, 19 February 2006 (UTC)

The article is good, but if you leave away everything which concerning U and Pu fission than not much is there anymore. Fission is also possible with other particls than neutrons and nearly all other cores can be cleaved with high energy particles. And this is the major us in physiks. The only real use is for sure is U and Pu fission so the article is OK but it should mention the other possibilities too. 17:47, 19 February 2006 (UTC)
Thanks! That's a good point -- I'll add something by tomorrow night. zowie 18:10, 19 February 2006 (UTC)

Bomb energetics[edit]

The article notes that little boy had a yeild of 15,000 tons TNT with a 4 ton bomb (actually the yield was more like 13,000 tons but nevermind). That's still about 4,000 to 1, vs. TNT. The article says that modern weapons are "literally" thousands of times more energetic, but that's flat-out wrong. The most efficient bomb ever constructed was the Tsar Bomba, which had a yield of 50 MT and a weight of 26 tons. It would have had an identical weight but twice the yeild if a uranium-238 jacket instead of lead had been used (the lead was used as a substitute tamper to cut the yield of nasty fission products for the test). This gives a ratio 100,000,000/26 = about 4 million to 1 vs. TNT, which is at most 1000 times that for little boy. That's the best that has been done, and it has only been done at the increased efficiencies possible in a monster thermonuke. Modern smaller weapons (see the Wiki for W88) have efficiencies of about 475,000 tons/.36 tons = 1.3 million to 1, which is 1,300,000/4000 = 325 times that of little boy. I have therefore changed the statement to "hundreds of times," which is of the right order. Sbharris 20:25, 3 April 2006 (UTC)

fast fission[edit]

The phrase "fast fission" doesn't appear anywhere in the article, although it's an important type of fission. I think we need a mention and link to the article. Night Gyr 21:55, 19 April 2006 (UTC)

Article rating[edit]

I think this article is in very good shape, and probably meets the standards of Wikipedia:Good articles. However, I can't quite give it an A-class rating, because I think the overall structure of the article (what headings are used, etc.) could use some work, and there could be improvements in the references (more hard scientific literature). -- SCZenz 06:45, 6 June 2006 (UTC)


The section on fission bombs contains the phrase: Modern nuclear weapons are literally hundreds of times more energetic for their weight than the first atomic bombs, so that a modern single missile warhead bomb weighing less than 1/8th as much as Little Boy (see for example W88) has a yield of 475,000 tons of TNT, and could bring destruction to 10 times the city area. True, but the W88 is not a pure fission weapon — it is a hydrogen bomb. I'm not sure how one would want to re-work the sentence to make this more clear (I can't recall offhand if any of the "modern" nuclear weapons are not hydrogen bombs). Ivy King is the standard example of the max limits of a pure fission bomb, though it is not really a "modern" weapon (1952). Just a thought... --Fastfission 01:58, 20 July 2006 (UTC)


What is the source for the (very nice) quote from Frisch? Avm1 (talk) 05:18, 19 November 2007 (UTC)

User added erroneous comment.[edit]

While browsing this article I noticed the following comment added by user "all this is not korrekt this is fusion not fission". Maybe they were testing, or forgot to correct their edit. I guess they could have used the sandbox to test. Anyway, I deleted the statement. After further reading I see this should have been a major edit due to the deletion. OOPS, sorry, I marked it as a minor change. I also forgot to sign the edit. I'm kinda new at this and still learning.

ps. Since discovering Wikipedia, this has been my number one reference source on the Internet!

Bigwig77 (talk) 09:29, 12 December 2007 (UTC)

einstein was not austrian[edit]

always a fun issue because there usually is at least three countries who like to claim him as their citizen, but he is definitely not austrian!!! i think the guy you have in mind is the one who made einstein flee europe: hitler. anyways, einstein was born and raised in southern germany and later started his scientific carreer in switzerland and finally fled europe. —Preceding unsigned comment added by (talk) 09:01, 4 April 2008 (UTC)

Initial images[edit]

An induced nuclear fission event. A slow-moving neutron is absorbed by the nucleus of a uranium-235 atom, which in turn splits into fast-moving lighter elements (fission products) and free neutrons.
An induced nuclear fission event where a slow-moving neutron is absorbed by the nucleus of a uranium-235 atom, which in turn releases two fast-moving lighter elements (fission products), free neutrons and energy. Also shown is the capture of a neutron by uranium-238 to become uranium-239.

There are apparently two candidates for the first image in the article. One is a static diagram (SVG) showing a simple fission reaction, the other is an animated GIF that apparently is meant to show something of a fission chain reaction.

I think the static diagram is a better initial image. I'm not sure of the value of the animated one. (I don't think that animation necessarily improves something like this, where the emphasis is on understanding the concept.) In any case I don't think the first image should be of a chain reaction. (There is another chain reaction image further down in the article. The fission chain reaction is not the same thing as the single fission reaction.) I think the animated one takes too long to play (and the speed is very variable—it goes from excruciatingly slow to extremely fast very quickly), and doesn't really illustrate anything better than the static diagrams. I think it is hard to follow and unclear. The differences between the atoms are so small that you can't readily tell what is what without enlarging it. (What is the giant blue circle at the end supposed to represent? Is the atom hitting the viewer in the face?) And it represents fission as a little explosion, rather than a creation of fission products and neutrons. (The "energy" released by fission reactions is mostly kinetic energy of the component products, not "explosion" or photon energy, anyway.)

So I switched out the images. Comments appreciated. No unpleasantness meant towards the author of the animated image—I just don't think it's as good as the static one for this purpose, and I think it's a bit of a duck to use Tufte's term (contains too much irrelevant decorative elements, animation for its own sake, when static representation does something of a better job). -- (talk) 20:48, 10 July 2008 (UTC)

Non taken, however I must point out that the reason I made the animation in the first place was because the static diagram is inherently inaccurate as it gives one the impression all reactions are the same. Krypton and Barium are among the possible products, and the number of neutrons produced varies (can be 2 - 4). Moreover it has the added benefit of illustrating neutron capture and creation of uranium 239. represents fission as a little explosion rather than a creation of fission products and neutrons.... Not really, as the caption explained it's actually energy. (Reaction formulas usually include an amount of energy released in addition to the fission products and neutrons. (The static diagram symbolizes energy as an explosion as well.) I'm sorry it's hard for you to follow, would it help if I slowed it down? ...the giant blue circle at the end... is was a neutron coming toward the observer because these reactions happen in three dimensions.
I have to ask, since you mentioned having difficulty reading it as a thumbnail, did you click on it for an enlargement? If so I'm wondering how you missed the key? Take another look the old version... As to size, one can always increase or decrease the image size in the parameters, [[Image:xxxx|400 px (whatever you put for this value is the size)|]]. Anynobody(?) 05:51, 22 July 2008 (UTC)
The old key was terribly hard to read. The new animation is better than the old one though I think it is still not appropriate as the first image. I think it probably ought to go down under chain reaction. The problem with animation in general is that too fast and you miss it, too slow and you're talking about the reader spending 30 concentrated seconds staring at an image before they can make sense of it. -- (talk) 20:00, 25 July 2008 (UTC)
An additional problem with the new one is that it is still only an illustration of the chain reaction, not fission itself. I think it should go down in the chain reaction section. It's not a great first image, but it's fine to keep on the page, I think. I still thinking having the neutron come at the viewer is confusing and unnecessary. -- (talk) 20:02, 25 July 2008 (UTC)

Image incorrections?[edit]

I'm not a physics guru but I watched the lecture at berkley california online (Physics for future presidents, Very entertaining) and I thought that U-235 -> U-236 produced TWO stray neutrons, not three like the picture shows... —Preceding unsigned comment added by (talk) 23:25, 13 November 2008 (UTC)

Picture also looks misleading as it shows uranium fissioning into two large atoms+some neutrons. Is there any atom that would fission into two large parts rather than into one large and one or more small parts? (talk) 14:56, 20 February 2009 (UTC)

It can produce between 2 and 3 neutrons. On average there are only two fission products and they are of roughly equal size. So the image is correct. --Mr.98 (talk) 20:26, 8 June 2010 (UTC)
No, the fission products are always of unequal size. See "Fission product yields by mass for thermal neutron fission ..." chart halfway down the page.Shjacks45 (talk) 21:58, 4 November 2013 (UTC)


What is the name of the device which controls protons, in practice? And how does it work? What form is the energy used to iginite the fuel in practice? ~ R.T.G 09:21, 20 December 2008 (UTC)

"Protons"? Are you thinking of nuclear fusion?
—WWoods (talk) 04:16, 21 December 2008 (UTC)
No neutrons then, I don't see any engines here. Theories are great in theory but you would hardly use one neutron at a time and little ten molecule reactions to power a grid. I see no comparison of scale between what is practiced and what is theorised beyond bomb sized application. I may be unclear... What are the main components of the Meitner, Hahn and Strassmann apparatus? How does a nuclear reactor compare to that? Aside from eating uranium and producing electricity, what actually goes on? I have seen a picture of inside a nuclear reactor and it had several devices but how it feeds and produces is a mystery. Causing bombardment by neutrons is about as clear as causing lighting by radio waves. It's certainly a complex machine (first experiment pictured), is it a radio with a water heater or a laser with some other moving bit? Is it safe to touch? What was the effect they claimed proof of split atoms? Melted uranium or deposits or...? Having another check I don't see any description of the mechanics only fuel (I realise that may be restricted but interesting anyway). ~ R.T.G 09:15, 21 December 2008 (UTC)
It does say on Nuclear power that steam is used but again no mechanics or description of early experimentation processes (as components rather than fuel) or fuel management before it is waste (does it burn all the time? Do they put a constant ignition to it like electricity or something?) ~ R.T.G 09:21, 21 December 2008 (UTC)

Fission/fusion mistake[edit]

Under the history section of the article, the word "fusion" is used when it should be "fission"

There, the news on nuclear fusion was spread even further, which fostered many more experimental demonstrations.[1]

Jchubb19530 (talk) 20:44, 13 January 2009 (UTC)


  1. ^ Richard Rhodes. The Making of the Atomic Bomb, 267–270 (Simon and Schuster, 1986).


THis text: "Many believe that Ernest Rutherford became the first person to deliberately split the atom by bombarding nitrogen with naturally occurring alpha particles from radioactive material and observing a proton emitted with energy higher than the alpha particle.[3] In 1932 his students John Cockcroft and Ernest Walton, working under Rutherford's direction, attempted to split the nucleus by entirely artificial means, using a particle accelerator to bombard lithium with protons thereby producing two alpha particles. This did split the nucleus, but nevertheless was not quite the classical nuclear fission which is induced in heavy nuclei, because the daughter fragments are alpha particles — already well-known fragments of excited nuclei, and not considered to be a truly new phenomenon, even if two of them had been produced, and nothing else." Doesn't seem very useful or well-referenced, and is a weasel phrasing. If not Rutherford(s team) then whom? Midgley (talk) 16:49, 4 April 2009 (UTC)

Still needs work, I'll let soemone else have a go. Midgley (talk) 20:20, 4 April 2009 (UTC)

Needed changes to lead (de-emphasis of gamma yield needed)[edit]

As the article Effects of nuclear explosions makes clear, 5% of a typical weapon prompt yield energy is in the form of ionizing radiation, and since some good fraction of this is prompt neutrons, the actual burst of other kinds of ionizing radiations (alphas, betas, gammas) is less than 5%. Roughly the same must be true in nuclear fission-- at least 95% of the initial energy is carried off by the fission nuclei and neutrons, and other kinds of radiation are almost entirely produced later by decay of fission products. Thus, there is really no reason to emphasize "gamma rays" in the lead. The energy produced in a reactor is from the kinetic energy of fission products which is turned into heat when they are stopped. The amount of beta decay energy (much of it from rapid fission-product daugher beta-decay) can be estimated from the energy of neutrinos produced in a nuclear reactor, which is 5 to 6% of total reactor power, so the beta-only energy should be similar. But this is all from later product decay, not the initial fission. As for fission gamma yeild, I can find NO evidence that the average fission yields a "30 MeV gamma" which is what it says in the text (I'm going to [citation needed] tag this). In fact, in EMP calculations, where prompt nuclear weapon gamma-yield becomes an important number, it's typically assumed that 0.1% of a thermonuke yeild is gammas, and they aren't this "hot" (highly energetic). Since half the yield of modern thermonuclear weapons is from fission, we can perhaps upgrade that to 0.2% for a pure fission weapon or process, but that's far from the 15% suggested by a 30 MeV gamma from a 200 MeV process! Fission prompt gammas are more like 2 Mev, which would indeed give you 1% of the energy, so we have at least a factor of 10 discrepancy in both directions, and (again) no good reason to mention gammas in the lead at <1% energy, nor in the design of nuclear reactors, which certainly won't be getting much of their heat from absorbing gammas. Comments? SBHarris 20:33, 4 April 2009 (UTC)

Okay, I finally found a ref (added) which shows that in U-235 fission about 10 gammas are emitted, 7 MeV energy yield in prompt gammas, and 6.3 MeV yield in delated gammas associated with de-excitation after beta-decay. That's far less than 30 MeV and only half of it is prompt. The half that is prompt is only 3.5% of yield, which is what was estimated above. So we're finally getting the numbers right. SBHarris 13:35, 25 June 2009 (UTC)

Please edit this page.[edit]

The first sentence in the Physical Overview section reads:

Nuclear fission differs from other forms of radioactive decayut didnt fly in that it can be harnessed and controlled via a chain reaction: free neutrons released by each fission event can trigger yet more events, which in turn release more neutrons and cause more fissions.

" radioactive decayut didnt fly in that " Just a typo? (talk) 15:57, 6 April 2009 (UTC) CH

Montoya reference[edit]

This seems to have been added in some attempt to get the paper or site into the article. I cannot figure out what it means or what it adds. I've removed it and put it here, in case anybody else has any ideas.

Neutron and gamma rays emitted by fragments erase information about the fission process itself, making it difficult to study the reaction dynamics from the saddle point in the reaction coordinate to the scission point, where the fragments are released. Nevertheless there are a few fission events for which no neutron or gamma is emitted. These events are examples of so-called cold fission. [1]

Thanks. SBHarris 13:32, 25 June 2009 (UTC)


  1. ^ Cold fission has been studied by Modesto Montoya and others. See M. Montoya, "Mass and kinetic energy distribution in cold fission of 233U, 235U and 239Pu induced by thermal neutrons", Zeitschrift für Physik A Hadrons and Nuclei, Springer Berlin/Heidelberg, V. 319, No. 2, June 1984 (ISSN 0939-7922 (Print) 1431-5831 (Online)).

Strange way to understand something so relatively simple[edit]

Strange way to understand something so relatively simple and it seems rather intimating? But I will have to look at nuclear fusion before I point out that the 'Period Table' is a better resource (tool)to produce an understanding of both these type actions. —Preceding unsigned comment added by (talk) 08:28, 14 August 2009 (UTC)

Excuse me but you are thinking in a vacuum[edit]

Note that U235 itself decays in part by spontaneous fission (S.F.). Under [Uranium], U236 is a relatively stable isotope. The higher energy state of this isotope U236* decays by S.F. in microseconds to generate 2 large fragments and several neutrons. It should be noted that although the energy of all particles is ~202 MeV that net momentum is maintained, a gamma ray is emitted to main this symmetry if the particles' net momentum does not equal zero. Where is the reference for "Coulomb repulsion" in "~169 MeV appears as the kinetic energy of the daughter nuclei, which fly apart at about 3% of the speed of light, due to Coulomb repulsion". The sentence is contradictory, as it is clear that fission energy is converted to kinetic energy; does the sentence imply there is a coulombic component to the velocity that is not part of the nuclear force that is an order of magnitude greater at that distance? Is the "Coulomb repulsion" also responsible for the neutron velocity? Question: what happens to the electrons of the Uranium atom? Each nuclei will have one s electron 3 p electrons etc with the infall of outer electrons generating x-rays. In a vacuum were done, but if the fission fragment with ~80 MeV of energy and 90# mass hits a U238 what happens. Does the energy disappear? Is it imparted to the other U235 or U238 nuclei? Does it become a gamma ray? Does the Uranium atom simply elastically recoil from the fission product? [Neutron source] says 7-40 MeV energy induces neutron generation. Just as high energy (? MeV) photons will induce Spontaneous Fission in U236 when absorbed to produce the higher energy U236*, Gamma photons would also be expected to induce fission of U238 and U235. As 1.2+ MeV neutrons fission U238 and thermal are absorbed it seems a 1.2 MeV (gamma or xray) photon (or energy induced by collision) would fission U238. Shjacks45 (talk) 10:59, 17 February 2010 (UTC)

This is more stuff than I have time for, and some of your questions are going to have to be rephrased one at a time, if I'm even to understand them.
YOu ask: Where is the reference for "Coulomb repulsion" in "~169 MeV appears as the kinetic energy of the daughter nuclei, which fly apart at about 3% of the speed of light, due to Coulomb repulsion". The sentence is contradictory, as it is clear that fission energy is converted to kinetic energy; does the sentence imply there is a coulombic component to the velocity that is not part of the nuclear force that is an order of magnitude greater at that distance? Is the "Coulomb repulsion" also responsible for the neutron velocity? Fair question. Feynman says it's simple Coulombic repulsion in this lectures on physics. There's no contradiction about it. The nuclear force is only attractive (except for incredibly tiny distances where hadrons get too close to each other), and it is NOT larger than the Coulombic force at that distance (we're talking two fragments with 12 fm between their centers). That's the whole point of what drives fission-- the two fragments oscillate into semi-spherical droplets that finally are mores stable because they're closer to the curve of maximum binding energy; and then their charges drive them appart. Take an average fission product, which we'll pretend has an average Z of 46 and mass of 118 has a radius of about 6 fm (using the bimodal products doesn't change this calculation by more than a few %). Calculate the electric potential energy of repulsion of two such nuclei just touching each other (radius 12 fm = 12e-15 m) and you get energy in eV of (9e9*1.6e-19*(46)^2/(12e-15), which is 254 MeV. That's too close to be coincidence. It's doubtless a little less due to the nuclear force holding things back a bit, but there's your mechanism.

The question of whether simple Coulombic repulsion is responsible for fission neutron velocity/energy is more interesting, and the answer is no, but the mechanism is complicated. It's a sort of billiard-ball effect as the protons and neutrons jostle their way down into minimally low energy orbitals, among a plethora of neutrons. They're bound to "hit" (nuclear force repulsion) some neutrons as they go in, and those neutrons are likely to get knocked out, having no charge (protons are more likely to say bound simply because they are more rare in the new environment, with more orbital room for them close to the center). The struck neutrons overcome the thin nuclear force barrier holding them "on" and out they go. Their kinetic energy comes from the other nucleons, which in turn are getting it from nuclear forces pulling them into more stable (smaller) nuclei. You can also see this in much smaller reactions, such as the classic deuterium-tritium fusion. And "neutron knock-out" reactions (also called spallation) are well known when radiating nuclei with high energy protons. Neutrons are occasionally emitted as radioactive decay products from excited nuclei. What's YOUR explanation for this process? The nuclear force doesn't kick them out directly, but it does indirectly. However, the same mechanism is not going to work for large fission fragments. SBHarris 18:11, 9 April 2010 (UTC)

Fission has a tendency to occur in the isotope with extra neutron numbers between alpha emission (with fewer) and with stability or beta- emission (with more), and indicates that fission occurs when a nuclide is pretty nearly stable, but not quite. This might be the indication of some kind of structural disability condition, like the existence of a fracture plane.WFPM (talk) 16:50, 7 April 2011 (UTC)

As for the electrons, they pretty much get left behind. The fission fragment energy is so much higher than the ionization energy. Gah4 (talk) 23:27, 10 November 2016 (UTC)
In the usual drawing, the protons and neutrons are sitting quietly in the nucleus. This is not correct. They move at the fermi velocity of the appropriate nuclear shell, similar to the velocity (kinetic energy) of atomic electrons. As noted, the energy is redistributed during fission, but even so, the energy would still be plenty high enough. Gah4 (talk) 23:27, 10 November 2016 (UTC)

Edit request from, 25 October 2010[edit]

{{edit semi-protected}} ya i just wanted to edit this article cuz u said it needed cleanup woth a little picture of a broom. so if you could let me clean it up before the end of this class period thtd b cool. (talk) 19:09, 25 October 2010 (UTC)

Not done: The Nuclear fission article is protected from editing from IP and new users. If you wish to edit this article yourself, you need to create an account and wait 4 days and contribute 10 edits to other articles. If you have a specfic suggestion for this page and don't want to wait 4 days and 10 edits, feel free to re-use the {{edit semi-protected}} template making a detailed proposal for your edit such as: "Please change X to Y", or "Please add Z after A." Thanks, Stickee (talk) 22:02, 25 October 2010 (UTC)

definition of fission[edit]

Can you clarify "lack of a defined product"? I don't understand that.

Also, the definition I added (comparable size) had sources, so maybe that definition is used, at least in nuclear chemistry. Cluster decay also distinguishes cluster decay from fission according to the mass of the products, so maybe you need to look at that article too. —JerryFriedman (Talk) 21:24, 3 April 2011 (UTC)

Nuclear fission can produce any two (or even three) products so long as energy is conserved and the atomic mass A and charge Z add up to the parent. The products produced from binary fission show up as two bell-curves, not a single simple set of nuclear products that you can write down (except to pick out one common example at random). The products are not defined-- there are hundreds of them. Adding in ternary fission (3 charged products) they can be elements of any atomic number, so long as it's less than Z of the parent, and other constraints are met.
Fission product yields by mass for thermal neutron fission of U-235, Pu-239, a combination of the two typical of current nuclear power reactors, and U-233 used in the thorium cycle.
Cluster decay produces products as large as neon-20 or larger (well within the range of ternary fission) but when it does so, that's the ONLY product. Since cluster decay and alpha decay are types of radioactive decay, not chaotic nuclear reactions induced by nuclear oscillations from capture of an outside particle, they are far more predictable (incidentally, spontaneous fission when it occurs as a sort of radioactive decay doesn't behave like the induced fission from neutrons, and it is also more predictable in product yield). Radioactive decay and nuclear reactions are two fundamentally different physical processes. For example, 90% of ternary fissions produce alphas at 16 MeV, which is gigantically larger than you'll ever see in alpha decay.

If you want to know more, I suggest you do a Google Scholar search on "ternary fission." It's an important source of helium and tritium gas in nuclear reactors. [1]

SBHarris 22:08, 3 April 2011 (UTC)

Thanks for the detailed answer!
The context of this was that two people in a usenet discussion suggested that all radioactive decay (or all except beta decay) was a form of spontaneous fission, and one of them quoted the version of this article at the time. True, that person is confused about a number of things, but I was trying to edit the article so it would show people with that idea how "fission" excludes alpha decay and the like.
In my opinion, the article doesn't do that yet. The main distinction that needs to be discussed is between alpha and cluster decay on the one hand and spontaneous fission on the other. (Maybe also the distinction between spallation and induced fission?) I don't think the word "defined" is the clearest—people will think of defined words and maybe defined muscles, which are different. Maybe "predictable", as you used above. Or maybe something like, "The fission of a given kind of nucleus can produce many combinations of daughter nuclei, typically having masses in a ratio somewhere near 60:40 [ref]. This characteristic distinguishes fission from alpha decay and cluster decay, in which the products are always the same." (If this is true of spontaneous fission as well as induced.)
Also, the definition I put in—that fission is defined by having similar product masses—had two reliable-looking sources. I think that definition should be in the article unless there's something wrong with the sources, and if a definition in terms of the spectrum of products is also in the article, a source would be good. —JerryFriedman (Talk) 18:01, 6 April 2011 (UTC)

I've done a lot more reading in the recent literature, and as usual, the plain and simple truth is rarely plain and never simple. Says Wilde. I've taken your advice to heart and taken out the word "defined" and replaced it with synonyms. We both know what we're trying to say and if you've got a better idea of the fact that fission doesn't produce the same products each time, but all the other processes do (more or less-- even if they might have a few parallel channels), please fix it as best you can.

This really is the best way *I* can think of, to define fission. The products do overlap with cluster decay, yet cluster decay doesn't produce a spray of prompt neutrons, and fission usually does. (However, very occasionlly you see none). Most probably the number goes from mean of 2.3 or so, with U and Pu, to more than 4 for a spontaneous fission (SF) isotope like Cf-252, and Cf-252 can produce up to 8 neutrons per fission; alas it's not fissile.

Mechanistically, fission starts with a nucleus at the energy point where it's almost ready to fall apart, like a brick building collapsing. That can be from neutron absorbtion, or for very heavy elements, it's that way in the ground state! When it does fall apart, the energy released makes it look like a watermellon with an M-80 in the middle of it. Most of the time you get two large halves, but sometimes 3 chunks, and often a lot of bits.

BY CONTRAST, cluster and alpha decays go through a much larger energy barrier, so usually only one species of emitted particle is far in front of the pack, and nearly always the decay happens that way, and the species that gets out is the same each time. In the chaotic conditions of fission, anything can happen, and does. The higher the barrier to a process (and the thinner the barrier is in physical width, and the lighter the escapee) the more tunnelling plays a role. By the time you get to fission of a fully energized nucleus, the barrier is so low and broad that in lot of ways it's a semi-classical process, like liquid drop splitting. There's even a phenomenon where a droplet in the "bridge" (or neck) between two fragments finds stumps withdrawing on either side, leaving it hanging out with no place to go. Those neutrons (making up 1% of the total, perhaps) are called "scission neutrons"). The rest "boil off" the fragments, in what I assume is a process that relies a lot on tunnelling. The mix of classical and quantum and chaotic effects make nuclear physics, especially as regards fission physics, such a nasty unclean science, almost like biology. There are some desperate papers out there trying to explain the results. See [2]

One other thing: spontaneous fission hardly ever happens in U and Pu, which have large energy barriers to it. In those isotopes, the binding energy of the incoming neutron products 6-8 MeV (and 1-2 from pairing for odd fissile isotopes). Very large nuclei like Cf-252 are not fissile (no odd neutron) but 3% of the time do undergo SF. Cf-252 is a great commercial neutron source for this reason. When a nucleus is naturally this close to falling apart, it already looks like excited U-236* or Pu-240* after neutron capture. The product mass fraction ratio for Cf-252 is 4:3, though, not 3:2. And the energy is only 185 MeV, not the 205-210 MeV of induced U and Pu fission. So you see how close it is to excited Pu and U, subtracting the maximal 9 MeV the neutron brings in for them. In those circumstances, again you get no single products, but a spray of different ones. [3]. SBHarris 20:23, 6 April 2011 (UTC)

When you tri-partition 1000 atoms of EE92U238 you can get 8 125 atoms (5x5x5) subsections in each octant subsection, and it is noted that the normal constituency of the OE92U235 is 7 per 1000 or about 1 atom per subsection. So the U235 atom will be in contact with only 26 of the 125 atoms of each subsection and the rest will be 1 atom removed. When the fuel is improved to approx 4% there will instead be 1 U235 atom per 26 U238 atoms and each of most of the U238 atoms will be in contact with a U238 atom. Doesn't this indicate the necessity of direct contact with a U235 atom for a U238 atom to be capable of being fussioned?WFPM (talk) 22:04, 6 April 2011 (UTC)
No. The fission cross section of U-238 was measured directly with cyclotron-produced neutrons (from accelerated deuterons) long ago. And of course in thermonuclear weapons you can even use depleted uranium for the jacket, and it works just fine.SBHarris 22:53, 6 April 2011 (UTC)
That is why they make tables for 14MeV neutron fission for U235 and U238. A fair fraction of the yield of thermonuclear bombs is the U238 tamper and casing. Gah4 (talk) 07:52, 14 November 2016 (UTC)

I'm not saying you can't do it otherwise with fast neutrons. I'm talking about the possible shielding effects of the nearby U238 atoms on the ability of the removed atoms in the case of the natural EE92U238. Mighten that then explain the reduced activity of fission in the normal ores of Uranium.WFPM (talk) 23:15, 6 April 2011 (UTC)

But normal uranium at 0.71% U-235 fissions just as well as you'd expect from the U-235 content. In fact not only the first CP-1 reactor ever, and the 3 giant Hanford reactors that produced the plutonium for the first plutonium atom bombs, all used natural uranium with no enrichment. Enrichment allows use of light water as the exclusive moderator, whereas with no enrichment, you must use either heavy water (CANDU) or a carbon/graphite moderator, and only enough water for cooling/heat extraction. The Hanford production design, which threw away power and operated at low temps, was still 250 MW heat output, carbon moderation, but COOLED by pipes of running water. Carbon moderators are dangerous, even though some of of the world's power reactors (see RBMK) still use them. Zero cheers for the USSR, which gave us a carbon-fired no-containment Chernobyl. SBHarris 23:51, 6 April 2011 (UTC)

Well I'm no expert and was only trying to have a geometric concept of the situation. And in Rhodes book Fermi was always monitoring the radiation "density" build up and I got the concept that the carbon (and water) was necessary to retain and thus build up the density of thermal neutrons against the tendency of the U238 to gobble them up.WFPM (talk) 02:28, 7 April 2011 (UTC) But since we need fast (or faster) neutrons for U238 fission, maybe they also don't get slowed down quite as much.

No, U-238 doesn't fission from its own neutrons even if they're never slowed. They simply don't have enough umph. Fermi was watching the pile for density of neutrons, and yes the U-238 was part of the absorber problem. But not from U-238 fission, just capture to U-239 which decays to Pu-239. With a highly enriched U-235 system they certainly could have made a smaller reactor. Which is one reason submarines use highly enriched U-235-- it gives them the possiblity of a minimum size reactor (along with lots of fuel reserve). SBHarris 03:36, 7 April 2011 (UTC)
It does, but just not very often. The probability is a lot less than one, but also greater than zero. Gah4 (talk) 17:02, 12 November 2016 (UTC)

So you're saying that the U238 has to be impacted by the U235 (or other) fast fission neutrons and I'm saying that distance is a factor and then you're saying that that can be overcome by the moderator materials, so that somehow the reduced U235 content (or other) emissions can still reach all the U238 atoms, and without reducing their fissioning ability, due to their capture and 93Pu239 emission producing capability? I guess the question then becomes as to what minimum amount of initial fast neutron emission is required? Because something initially has to get it started, and I thought it was the U235 content. And you're fouling up my geometric concept.WFPM (talk) 09:59, 7 April 2011 (UTC)

Please listen. Although U-235 in a reactor produces "fast neutrons" most of them are NOT fast enough to fission U-238. Okay? U-238 is fissioned by REALLY FAST neutrons (14 MeV) such as come from hydrogen fusion in an H-bomb. Otherwise, U-238 simply absorbs neutrons and never fissions until it decays to Pu-239, which takes hours. SBHarris 19:19, 7 April 2011 (UTC)

Okay! I'm listening, and apologizing and the EE92U238 doesn't fission until it becomes OE93Pu239, except in the bomb. Note that that change is from a 3Z-38 atom to a 3Z-40 atom with reduced stability. And thanks for your attention.WFPM (talk) 19:53, 7 April 2011 (UTC)

U-238 doesn't fission even in a bomb, unless it's an H-bomb. Which is why they leave out as much U-238 as they can, if the fission core is uranium. SBHarris 21:45, 7 April 2011 (UTC)
I don't know the exact fraction, but a fair amount of the U238 tamper in a fission bomb does fission. It adds some to the yield at low cost, and with neutrons that would otherwise escape. It also reflects back some neutrons. Gah4 (talk) 17:02, 12 November 2016 (UTC)
The fraction is small, but there is a lot of it, so it is significant enough. Gah4 (talk) 07:52, 14 November 2016 (UTC)

I've been trying to get LANL to modify their "Map of the nuclides" charts // to get formatted so that they will have the data organized between elements such as to indicate the stability trend lines. But I haven't had any luck. Don't you think a chart like JWB's skew 1 nuclide chart (with the trend lines) would be appropriate for this discussion? I worked with him to get it almost completed.WFPM (talk) 23:49, 7 April 2011 (UTC) See User:JWB/Nuclide chart with skew 1

See Table of nuclides for a table of tables of nuclides on WP. There are half a dozen different charts there, along with a list of nuclides if you don't like charts. A full chart is at Table of nuclides (complete). SBHarris 03:40, 8 April 2011 (UTC)

They're nice charts. but, unfortunately, they have the wrong format. The correct format diagonally lines up the EE's and the OE's along one diagonal line, and the OO's and EO's along another, so that the formula A=3Z-an even number makes an advancing continuous line, until the unstability factor causes it to drop down by 2 extra neutrons. Mine's on the wall in my image, but I didn't want to discuss it due to OR. And the general accumulation advancement tendency is 3 nucleons per time, which I guess would be 1 more deuteron plus an additional extra neutron. Of course, I guess that nature made some of them all, but the stability factor eventually results in the developments of the stability trend lines. And each box has to have the Log second halflifetime value for comparative analysis.WFPM (talk) 14:28, 8 April 2011 (UTC)

EE, vs EO's[edit]

The article doesn't mention that the principal "fissionable" elements are EO's with an even number of protons and an odd number of neutrons. Evidently the EE elements in that mass area are sufficiently stable against disturbance such as to not permit a cascading of the fission phenomena.WFPM (talk) 16:03, 4 April 2011 (UTC)

The article does mention that most fissile elements have an add neutron number (see energetics section), and says why this is so (pairing effects from a new neutron give an extra 1-2 MeV of energy which doesn't have to be supplied as kinetic energy, thus slow neutrons work). The rule for whether an element is fissionable (fast neutrons) is in spontaneous fission. It doesn't really require odd neutrons, just Z^2/A >47, and a noted odd-even fissionable (but non fissile) is Np-237 with odd-protons of 93 and even neutrons. This element fissions with its own fast fission neutrons and can be used to make fission bomb, but it does not fission with its own slow neutrons, so cannot be used in a moderated nuclear reactor (i.e., it's not a nuclear fuel but can be a nuclear weapon). EE fissionable elments are possible as well, like Cf-252. Cf-251 is another EO fissile material that could be used to make bombs-- it's really the odd neutron number that determines slow-neutron fissionability. Odd-Odd nuclides in the fissionable range of Z^2/A >47 would be fissile also, but they have such short half lives to beta decay (energetically highly likely since it converts them to even-even nuclides) that they have half lives too short to be useful. SBHarris 18:00, 4 April 2011 (UTC)

I would agree with you except that I would say that it is the unbalance caused by the odd extra neutron that permits thermal neutron fission in EO isotopes, which is nit picking I know. And it is noted that the primary EE/OE nuclear stability line in this nuclear area has the formula A = 3Z - 38 and that the stability decreases as the value goes to 40 and 42 and that with OE93Np237 and EE98Cf252 you have A = 3Z - 42. So you're getting away from the "stability?" needed for fission as you go to these more massive elements.WFPM (talk) 23:44, 6 April 2011 (UTC)

Yes I didn't mention the difference between thermal and fast neutron stability. And the EE's have to have fast neutrons. That's what Neils Bohr figured out "in a flash" as reported in Richard Rhodes' book. So why don't we go to EE90Th232 - EO92U233 development for nuclear power and get away from all this hassle about nuclear weapons propagation.WFPM (talk) 18:28, 4 April 2011 (UTC)

We could, but it wouldn't help that much in proliferation-- only in the greater availability of fuel, since the Earth has a lot more thorium than U-235. U-233 is fissionable AND fissile (you can make a bomb from U-233 and it has been done in Operation Teapot). SBHarris 19:52, 4 April 2011 (UTC)

Thanks for the info and sorry about that. But the U233 system is liquid fueled and more chemical and efficient, and less likely for weapons, I hope.WFPM (talk) 01:59, 5 April 2011 (UTC)

  • Simpler is whether there is a short lived metastable state reacheable by energy of neutron absorption. Some S.F. in ground state seems to be a common thread. Always wondered if fission of U234, U236, and U238 by fast neutrons was by absorbtion or by energy transfer. isotopes of uranium
  • Is there such a thing as a moderated bomb? I've always assumed that the entire life of the chain reaction in a bomb is fast neutrons. Cross section of 235 and 238 approach each other as neutron energy increases.
  • "Average" 15% (5-25) of reactor energy comes from U238. Unlikely all from Plutonium conversion.
  • Best arguement for U233 is lower yield of problem long-lived radioisotopes, better waste profile
  • Uranium and Thorium are usually a by product in mining other metals like Rare Earths or Molybdenum. Uranium ores exist, concentration by Uranium's redox characteristics. Thorium is more common but ppm in Granite isn't appealing.
  • Technically Fermi's Chicago Pile just shows that you can make a reactor, or a bomb, using unenriched Uranium if you have a large enough critical mass. Note that Paki bomb maker posted Uranium Deuteride as a low enrichment option: D, Li-7, Be-9, B-11, all multiply neutrons via (n,2n) reaction.
  • Early Manhatten project Plutonium production (before Chicago Pile) was via Cyclotron accelerated Deuterons.

Shjacks45 (talk) 13:09, 5 November 2013 (UTC)

Sameera Moussa's Equation[edit]

Did Sameera Moussa really made an equation for atomic fission for cheap elements like copper? if so, what is its mathematical formula and derivation? Waiting for answer — Preceding unsigned comment added by (talk) 15:32, 7 July 2011 (UTC)

There seems to be a rumor on the Web that she found a way to use cheaper metals as nuclear fuels. That seems impossible—metals such as copper have less nuclear energy in them than what they would turn into by fission. —JerryFriedman (Talk) 14:43, 8 August 2011 (UTC)

It is not a mere rumor because it is stated in many sites on the web. I think the only way to be sure is to read and study her research papers
(Where to find them by the way hers and Mostafa Musharafa and the other scientists stated in the facebook page??!!!
couldn't find any trace of information especially for Musharafa or Moussa.)

Random symantic errors in article[edit]

1. 235U is "Fissile" but it DOES NOT fission, it decays by alpha emission. It has a high cross section for absorbing (fusion with) a thermal neutron which then yields a high energy metastable state of 236U: Uranium-236 "U-235 that absorbs a thermal neutron may go one of two ways. About 82% of the time, it will fission. About 18% of the time it will not fission, instead emitting gamma radiation and yielding U-236."236U ground state half life is 2.348 x10^7 years. 2. Higher energy fast neutrons have a smaller cross section of reaction but cause the fission of both 235U and 238U; (5-25%)average 15% of Reactor fissions come from 238U fission from fast neutrons. U238 is an absorber of thermal neutrons, and a significant fission percentage is due to Plutonium produced by U238 neutron absorption. 3. Bombs made using Nuclear fission are primarily fast neutron devices. Shjacks45 (talk) 10:50, 8 August 2011 (UTC)

There is some spontaneous fission of U-235 but the half life is 10^19 years, so it's not important from the engineering or weapons sense. [4] SBHarris 18:02, 8 August 2011 (UTC)

Factors initiating fission - aneutronic fission[edit]

It should be specified in the article if there are reported results of initiating fission without neutron flux like with other particles (protons, deuterons, alpha particles, etc) or by applying an alternating electric field to uranium an uranium compounds (a Debye Falkenhagen-type effect)?-- (talk) 20:51, 28 October 2013 (UTC)

Seems that there is an article about aneutronic fusion, but not aneutronic fission.-- (talk) 21:05, 28 October 2013 (UTC)

See isotopes of Uranium. Note that the result of Uranium-235 absorbing a Neutron is the excited state of Uranium-236 (U236m) of a couple of MeV above ground state, in 18% of the Thermal Neutron absorbtion the U236m emits a Gamma and remains U236; 82% of the time it fissions. Uranium-238 also has a metastable state of ~2.5 MeV so it could presumably fission from Gamma rays or absorption of consecutive x-rays. Under Cyclotron is noted Japanese cyclotron that hurls Uranium atoms to 30 or 40 MeV (conditions that ignite the Uranium tamper of thermonuclear weapons); runs at 500 microAmp and 1 Amp = 10^18 ions per second(electrons). Note under neutron source that Deuterium splits on input of ~1.5MeV and that ion sources accelerating Deuterium 20,000volts+ (Deuterium is 4000x massive as electron; energy = mass times velocity squared). Fission of Deuterium via acceleration into a target. Fission yes, but not a chain reaction. And energy input isn't covering the deficit from creating the reaction.
Also note that aneutronic fusion posits irradiating Lithium-7 or Boron-11 with a Proton flux, the intermediate result is Berylium-8 or Carbon-12 which shed energy by emitting Helium-4 nuclei (Alpha particles) and Gamma rays. Note that in "fission", a Neutron fuses with a Uranium-235 atom forming an intermediate Uranium-235* (metastable high energy state) of which 18% emits a Gamma and stays U236 and the rest shed energy by dividing into a smaller and a larger atomic fragment along with several neutrons. So both fission and fusion essentially operate by identical methods.

Shjacks45 (talk) 11:41, 5 November 2013 (UTC)

Chicago Pile[edit]

Pile was built with Uranium Oxide. Oxygen is similar in mass to Carbon, can moderate; Oxygen-16 is a stable isotope like Carbon-12. includes "Fermi's own words" Shjacks45 (talk) 12:04, 5 November 2013 (UTC)


X = a + b; such that sum of the binding energy of the products being a maximum. 90/140 > 50/180 or 30/200 for U233. — Preceding unsigned comment added by ASG141 (talkcontribs) 04:05, 7 April 2014 (UTC)

Spontaneous Fission article[edit]

There is a separate article on Spontaneous_fission which mentions Georgy Flyorov and Konstantin Petrzhak for the 1940 discovery (on the Russian side) of spontaneous fission, induced by cosmic rays?. Can we combine that article into this one?

First, you forgot to sign your article. I suppose there is cosmic ray induced fission, but there is also spontaneous (no inducing particle) fission. Gah4 (talk) 07:35, 14 November 2016 (UTC)

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Assessment comment[edit]

The comment(s) below were originally left at Talk:Nuclear fission/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.

where does the original atom from a fission reaction come from????

Last edited at 15:00, 24 September 2008 (UTC). Substituted at 01:34, 30 April 2016 (UTC)

mixed heavy/light fission-fusion[edit]

Iron is stable so iron fission is moronic. Mixed heavy/light fission-fusion is fine. You create a plasma mixture of very heavy and very light atoms. The best option. An old fission-fusion idea that works perfectly!!! — Preceding unsigned comment added by 2A02:587:410A:1B00:4407:84CF:A6E7:7F6B (talk) 01:42, 27 June 2016 (UTC)

So-called neutron bombs (enhanced radiation weapons) have been constructed[edit]

Is there a reference for actual construction? I know that they have been discussed, and maybe designed, but have any been built? Gah4 (talk) 23:33, 10 November 2016 (UTC)

Assuming that the cross section for fast-neutron fission of 235U was the same as for slow neutron fission, they determined that a pure 235U bomb could have a critical mass of only[edit]

Assuming that the cross section for fast-neutron fission of 235U was the same as for slow neutron fission, they determined that a pure 235U bomb could have a critical mass of only They would have known pretty early that wasn't true. The fission cross section for U235 is large, as the deBroglie wavelength of slow neutrons is large. Faster neutrons have shorter wavelength, and necessarily smaller cross section. But exactly what was known, and when, I will have to look up. Gah4 (talk) 07:05, 12 November 2016 (UTC)

I don't follow this. If this were true, the fission cross sections for U235 and U238 would be the same, and they are not. Hawkeye7 (talk) 11:03, 12 November 2016 (UTC)
The fast neutron cross sections are about the same. Neutron_cross_section has the graph for U235. For slow neutrons, you think of them as a de Broglie wave, with wavelength much larger than the nucleus. I like to think of it as the U235 sucks in the neutron wave. It isn't like a little ball, but a big fuzzy cloud. (A little unscientific, but it gets the idea across.) The wavelength for 2MeV neutrons is much smaller, so they are more particle like. The cross sections are about what you would expect hitting a ball with another ball, the sizes of neutron and U nucleus. One barn is about the physical cross section area of the U nucleus. (Cut a sphere in half and measure the area.) The fission cross section table for slow and fast neutrons, and for U235 and U238 (among others) are on Neutron_cross_section. The graph for U235, other than some resonances, goes down with the wavelength. It was the realization, that, even with the smaller cross section, that a sphere of pure U235 would explode that led to the bomb project. Gah4 (talk) 16:36, 12 November 2016 (UTC)
I know the history, which is this: Bohr and Wheeler postulated in 1939 that U235 was the major source of fission, but this was not initially accepted by everyone, particularly, and most notably, not by Fermi. So in 1940 Dunning, Booth and von Grosse obtained samples of pure U234, U235 and U238, and measured some of the cross-sections. Armed with that data, Peierls and Frisch then made their famous calculation of the critical mass of a sphere of pure U235.
The reactions are:
+ 6.5 MeV
+ 4.8 MeV
But the respective fission barriers per Bohr and Wheeler's theory are 5.7 MeV and 6.4 Mev (actually, looking at their paper again, they substituted in the value for U238), so in the former case there is enough energy to fission no matter how negligible the kinetic energy of the captured neutron is, whereas the latter requires a neutron with at least 1.6 MeV ie a "fast" neutron. Hawkeye7 (talk) 20:30, 13 November 2016 (UTC)
I am suspecting that this might have been the back of the envelope calculation, before doing the more accurate calculation. Cross section affects mean free path, which determines how big a sphere you need to keep enough neutrons inside. But pretty soon, they would have known that the fast neutron cross section is smaller. Gah4 (talk) 22:14, 13 November 2016 (UTC)
I finally found my copy of The Los Alamos Primer[1] which tells pretty much what was known in April 1943. It also includes the text of the Frisch-Peierls Memorandum which seems to be from early 1940. At that point, it seems they already knew they needed high enriched uranium, and were using 10 barns for the cross section. The cross section determines the mean free path, which determines the size needed to keep enough neutrons inside. But I am also not sure when they knew about the large cross section for 235
. Gah4 (talk) 03:06, 14 November 2016 (UTC)
My mistake. At the time that Frisch and Peierls wrote their famous memorandum in early 1940, the cross-sections had not yet measured for uranium-235. So they used a figure of 10-23 cm2 (10 barns) for neutron capture derived theoretically from the Bohr-Wheeler model. Hawkeye7 (talk) 06:45, 14 November 2016 (UTC)
OK, this is what I was trying to figure out. At that point they didn't yet know about the large thermal neutron cross section for U235, so assuming that the fast neutron cross section was the same as the (wrong) assumption for slow neutrons made sense. Anyone interested in making this more obvious in the article? Gah4 (talk) 07:32, 14 November 2016 (UTC)


I didn't know about {{nuclide}} before, though it seems that one is now supposed to use {{nuclide2}} instead. I was recently working on some tables that can use this, and sub/superscripts didn't do it right. Thanks to User:Hawkeye7 for showing it to me,when I didn't even ask. Gah4 (talk) 21:00, 13 November 2016 (UTC)

  1. ^ Server, Robert (1992). The Los Alamos Primer (1 ed.). Berkeley, California: University of California Press. ISBN 9780520075764.