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WikiProject Elements / Isotopes  (Rated Start-class, Low-importance)
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- someone has substituted in an old version of this template, which should be replaced with the updated one and coloured appropriately. Verisimilus T 23:22, 13 May 2007 (UTC)

Odd mass-number[edit]

Could this article state somewhere that isotopes with odd numbers of nucleons tend to be less stable, and thus more fissile, than those with even numbers because of nucleon pairing? The casual but curious reader is left wondering why the fissile isotopes tend to have an odd mass-number. I am not an expert myself, but I understand this is one the reasons. Another article on plutonium also does not mention this. Jimpd (talk) 10:53, 30 March 2011 (UTC)

It is not odd mass numbers, but odd neutron and proton numbers. Look through a list of stable nuclides, and you find very few (and only low numbers) that are odd-odd. So, for even Z, odd N are less stable (in general, not just for fission). U235 and Pu239 have a very large cross section (chance to absorb) for slow neutron fission. Similar to an electron filling an orbital almost full orbital, it really wants that neutron. Absorbing it supplies extra energy to the nucleus to get over the fission barrier. Gah4 (talk) 08:48, 9 January 2015 (UTC)

suprisingly little[edit]

considering this is the material that was used in the Little boy bomb, it carries quite a historical notoriety... the page seems quite small all things considered. surely there is someone who can elaborate?

Pstanton 04:42, 1 June 2007 (UTC)

Could you be more specific? A lot is in the linked related articles like Little Boy, Manhattan Project, Nuclear weapons design, Uranium, Uranium enrichment etc. --JWB 18:40, 1 June 2007 (UTC)

What is known about the 5% of the fission energy of 235 going off as neutrinos? That would be a large mass-conversion into neutrino's and produce humongous numbers, most of which would pass through the earth. Seems thermodynamically inefficent as there would be effectively (almost) 0% energy capture/remass conversion. Where does all that momentum go? My guess is a topologic displacement. Sounds symmetry breaking. (talk) 07:38, 25 November 2007 (UTC)

I corrected inexact data and the energy units conversions of amount of energy released in nuclear fission event. I attached a table with exact values too. —Preceding unsigned comment added by (talk) 11:46, 1 February 2009 (UTC)


The statement "It is the only fissile isotope found in any economic quantity in nature." needs a reference. I don't think its true, Thorium has fissile isotopes, although the techknology has not been developed, see Thorium fuel cycle, and Pulu (talk) 18:48, 6 November 2008 (UTC)

Thorium is fertile, not fissile. I.e. Th-232 can be bred to U-233, which is fissile.
—WWoods (talk) 21:33, 6 November 2008 (UTC)

Fission and Decay[edit]

The article is very confusing, as the two nuclear processes are not clearly distinguished. The half-life of uranium is about its rate of decay, which is quite different from its ability to undergo fission. Decay occurs, with a half-life of ~700 mYrs, irrespective of anything in the environment (like neutrons), and goes to thorium-234 and eventually to lead, with a mixture of alpha and beta emissions (no neutrons) as described in the Wiki article on decay chain. Fission has no half-life, its rate depends on how many neutrons are around, and its products and emissions are completely different from those of decay. Fission tends to get the attention, but decay is important to us as the heat generated from radioactive decay, mostly U-235, drives the convection currents within the earth and thus plate tectonics. I have enough physics to see the problem but not to fix it. Someone should? Kognos (talk) 23:01, 19 November 2013 (UTC)

No. Fission has a half-life like other decay methods. Accordingly, spontaneous fission occurs for U235. The usual way of describing a decaying nuclide is by half-life and fraction that decay that way, instead of half-life for each decay path. From [1] the fraction of spontaneous fission for U235 is 7.0e-9%, so pretty rare. For Pu240, it is 5.7e-6% which still sounds low, but it is enough to cause premature detonation for a plutonium bomb. Oh, and the primary decay path is alpha, to Th231. Gah4 (talk) 10:02, 9 January 2015 (UTC)

Table subject?[edit]

The table at the bottom of the page was unclear. It seems to be showing the amount of energy released by a nuclear reaction. However, it is not titled and does not specify the conditions of the reaction. The paragraph above talks about multiple masses and purities so I was unsure about the amount of U-235 used. I also question the accessibility of the data, the MeV is not a popularly used term and some terms should be made into links.2602:306:BCCD:49A0:E6CE:8FFF:FE0C:1F80 (talk) 09:53, 27 April 2014 (UTC)

I was wondering about the table, too. For one: Energy released when those prompt neutrons which don't (re)produce fission are captured seems not to count the energy of neutrons that do induce fission. Seems that, on average, the energy of fission-causing neutrons should increase the energy that comes out. But otherwise, the table should be only for U235, averaged over all fission reactions (different fission products). MeV is popular in nuclear physics, but the important part of the table is the fraction that goes out each path. That doesn't depend on the units. Gah4 (talk) 22:18, 30 September 2015 (UTC)

Parent isotopes[edit]

Infobox in lead section mentions 'Protactinium-235, Neptunium-235, Plutonium-239' as parent isotopes. I don't think Uranium-235 has parent isotopes. If so, ref should be given. (talk) 04:49, 6 August 2015 (UTC)

Seem to me that [1] should be a fine reference. Any nuclide that has U-235 as a decay product is a parent nuclide. Earlier in earth's history there would have been more of the heavier nuclides, and right now some of those are in very low concentration, but they are still parent nuclides. (In this context, nuclide is usually used instead of isotope.) Gah4 (talk) 17:49, 6 August 2015 (UTC)

  1. ^ a b "Interactive Chart of the Nuclides". Interactive Chart of the Nuclides. Brookhaven National Laboratory.