Talk:Uranium-235/Archive 1

Thorium
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 http://www.thoriumpower.com/. 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)

suprisingly little
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.209.101.236.168 (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 87.205.70.143 (talk) 11:46, 1 February 2009 (UTC)

Odd mass-number
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)

Fission and Decay
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 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)

Parent isotopes
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. 115.245.73.88 (talk) 04:49, 6 August 2015 (UTC)

Seem to me that 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)

Infobox needed
{ {infoboxneeded|infobox isotope}} - 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)

I need an atomic radius or diameter to continue research. Elijah Gardi (talk) 15:18, 30 November 2017 (UTC)

Table subject?
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)

It's not a kinetic energy of neutron. Fission of U-235 produces ~2.5 neutrons on average. In the thermal nuclear reactor statistically only one of those neutrons is captured by fissile nucleus causing next fission, the rest (~1.5 neutrons) is captured by same non-fissile nuclei (moderator's nuclei for instance). Such a capture causes few MeV of energy to be released. — Preceding unsigned comment added by 62.172.138.4 (talk) 18:27, 17 January 2018 (UTC)

Apparent contradiction re critical mass
I've come across an apparent contradiction. I'm neither a nuclear physicist nor a weapons designer, and I wonder if someone better qualified and/or with better access to sources than i would take a look at this.

This article says that the nominal spherical critical mass for an untampered U-235 nuclear weapon enriched to at least 85% is 56 kilograms. This is supported by a cited source which has undergone WP:Linkrot.

The Little Boy article says that that bomb contained over two critical masses of uranium, that it had a tungsten carbide tamper, and that it contained a total of just 64 kg of uranium mostly enriched to 89% but some only to 50%, with an average enrichment of 80%. Soime of that is supported by a cited dead tree source.

These assertions seem to me to contradict one another. Wtmitchell (talk) (earlier Boracay Bill) 07:20, 13 February 2018 (UTC)
 * Perhaps the presence or absence of a neutron reflecting tamper explains this (??). If so, perhaps the Nuclear weapon design article could use some clarification regarding this. Wtmitchell (talk) (earlier Boracay Bill) 02:37, 14 February 2018 (UTC)
 * I found a clarification in Critical mass: "[a tamper] can decrease the critical mass by a factor of four." Wtmitchell (talk) (earlier Boracay Bill) 02:55, 14 February 2018 (UTC)


 * There is a story from Manhattan project days, when they still didn't know the details of tamper, of ordering a big sphere (maybe a different shape) of gold. It turns out that U238 makes a fine tamper, but they didn't know that yet. The librarian had the box with gold on her desk, and would ask someone to move it to a different desk. Critical mass math isn't easy, as the mean-free-path is significant. Gah4 (talk) 19:06, 18 September 2018 (UTC)

abundance?
It seems from a recent edit, there is some question about the abundance of U235 in nature. From Abundance_of_elements_in_Earth's_crust, Uranium ranks 50th (out of 78) in abundance in the earth's crust. Elements that prefer oxides float on the molten earth, as it was cooling, and so are more abundant in the crust. If you reduce U abundance by a factor of 140, it is still above many popular elements. On the other hand, the Manhattan project lucked out in access to some very high concentration ores. Even without reprocessing, there seems to be plenty of it, and, barring politics, there is lots to reprocess. I wonder if the article should say more about its abundance. Gah4 (talk) 02:17, 4 June 2019 (UTC)


 * Continuing, just because it is promordial, doesn't mean that it is abundant enough for economic extraction. Gah4 (talk) 03:45, 5 June 2019 (UTC)

RBMK
Why is, specifically, the RBMK listed under Uses? The RBMK uses lower enrichment that just about any other power reactor design. Should all designs for operating power reactors be listed? Research and nuclear submarine reactors, too? (Submarines use somewhat higher enrichment than other uses.) Gah4 (talk) 22:17, 14 June 2019 (UTC)
 * Honestly, my guess is that the recently-aired HBO Chernobyl series is the reason. I see no reason to specifically mention the RBMK reactor since the first sentence talks about the usage of Uranium-235 as fuel for nuclear power plants in general. Aeluwas (talk) 22:09, 18 June 2019 (UTC)

probability vs. cross section?
The article says: since the probability for fission with slow neutrons is greater. Since the cross section is higher for slow neutrons, and also the absorption cross section of U-238, a given neutron is more likely to find and fission U-235. As far as I know, though once a neutron does find U-235, the probability of fission (vs. something else) is not higher. Maybe it just needs a little clarification. Gah4 (talk) 19:02, 4 May 2020 (UTC)

584.994 barns
Is the thermal neutron cross section really known to six digits? I suppose if you give an exact temperature for the neutrons it could be, but if it is just room temperature or average operating reactor temperature, there should more uncertainty. Gah4 (talk) 19:06, 4 May 2020 (UTC)
 * It looks like false precision to me, but I'm not qualified to comment on this and I see that figure in this source. Wtmitchell (talk) (earlier Boracay Bill) 19:39, 4 May 2020 (UTC)
 * This one says 582.6. I emailed them, partly because it is labeled in % instead of b, but also to ask about the uncertainty. Gah4 (talk) 20:14, 4 May 2020 (UTC)
 * OK, I should have read it more carefully, the uncertainty is 1.1 barns.
 * But then there this one which gives 584.32585508 barns with an uncertainty of 1.0216. Not so sure why eight decimal places when the uncertainty is about 1, though. I always like IAEA references, and as I understand from their page, this is the reference standard. Gah4 (talk) 21:08, 4 May 2020 (UTC)
 * But then there this one which gives 584.32585508 barns with an uncertainty of 1.0216. Not so sure why eight decimal places when the uncertainty is about 1, though. I always like IAEA references, and as I understand from their page, this is the reference standard. Gah4 (talk) 21:08, 4 May 2020 (UTC)