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Article merged: See old talk-page talk:Critical size
Is there a common or standardized symbol for the critical mass of an element? For example, μU-233 if "μ" was the appropriate symbol and U-233 was the element in question.
Honestly I am not sure there is any symbol
Does anybody know the critical mass for a gas-core "nuclear lightbulb" reactor?
The critical mass is certainly dependant on the material, rather than what device it is being used in. I have no idea what the critical mass of uranium hexaflouride is (the NLB propellant), but Im guessing it's irrelevant as the mixture is heated to a plasma when used and the resulting increase in pressure generated will allow a higher rate of fission. So unless you deliberately compress the gas too much during transport you shouldn't get into any problems with it going fissile.
Also, I think critical mass is a but of a bad name, it should really be critical density
The article says Californium-251 bare-sphere critical mass is 9kg, based on an incidental comment in this article. But the Nuclear Weapons FAQ (section 6.2.4.4) estimates it to be 1.94kg. Does anyone know of another source for this info? - Rwendland 11:24, 2 December 2005 (UTC)
The paragraph:
The realisation that a supercritical assembly is not necessarily prompt critical is attributed to Enrico Fermi, and made the construction of a nuclear reactor using a fission chain reaction possible. Any prompt critical assembly will explode if not rapidly brought below prompt criticality.
seems to need some editing for clarity, however, I don't know the original meaning and I am not a nuclear physicist so, rather than introducing new errors of terminology, if someone else would like to fix it ... Alex.tan 03:16, 9 December 2005 (UTC)
This passage is just out of place here both in content and style. Also I suspect that it was not originally written for this article, but comes from someplace else unattributed. In my opinion it should be deleted outright. DV8 2XL 19:39, 16 December 2005 (UTC)
I think this physical concept is the primary "critical mass" and the page should be at critical mass. How do others feel about that? -- Yath 05:07, 18 December 2005 (UTC)
Wikimedia is being too finicky to move the page over the current redirect for some reason, so I listed it at Wikipedia:Requested moves. -- Yath 20:37, 21 December 2005 (UTC)
The first paragraph in the section titled, "Criticality via additional mass" is not accurate; this is why I added the second paragraph and three enumerated points regarding thermal feedback. The statements are not quoted from any particular source. I did not wish to delete the previous entry regarding the marbles.
The term, "critical mass", was invented in the 20th century (OED dates it 1941, Femto) specifically to describe the concept of the amount of mass required for a sustained nuclear chain reaction. It has had primarily that (or no other) meaning for many decades. If you use the term with anyone, if they happen to know what it means, they think of nuclear reactions first and foremost.
The mere fact that it has other meanings as well is not enough to make a disambiguation page the primary page for the term. Since the physical concept is the primary meaning (by far), it belongs at the main page, with a link to the disambiguation for other meanings of the term. For example, there are about 9 articles about various concepts associated with the word "ruby", but since the gemstone is the main meaning, it is at ruby rather than a disambiguation page. (This is in response to 70.51.178.2 and Tedernst: your reasoning is incorrect.)
I hope more people will vote for this page move, since giving the primary meaning of a term the proper prominence has a definite effect on readers' understanding of the term and its more modern derivatives. -- Yath 19:45, 22 December 2005 (UTC)
What exactly are those other uses of equal importance? All I see on the disambiguation page are things or groups that evidently borrowed the concept. A dictionary might give equal weight to various uses of words in describing the world, but an encyclopedia should give priority to the basic definitions. Femto 20:49, 22 December 2005 (UTC)
It seems to me that the physical meaning is indeed the primary one. Not only because I had never heard of the other meanings (which is a personal thing), but more importantly, all other meanings are clearly derived from the physical meaning. -- Jitse Niesen ( talk) 21:56, 22 December 2005 (UTC)
All of the discussion and support votes assert that the physical concept is primary, either because it came first, or just because it obviously is more important. Do we have any evidence that it's vastly more important? I do not have any evidence either way, which is why I ask. We should not be giving supremacy to one concept over another because "it's obvious" to some people that this is the way it should be. Tedernst | talk 17:58, 23 December 2005 (UTC)
Moved. WhiteNight T | @ | C 01:16, 28 December 2005 (UTC)
In discussions on the A-bomb, there is generally an argument being made that one has to use high explosives to get the prompt supercritical mass for explosion, otherwise the excursion will be terminated before a huge amount of energy is released. Indeed, when one looks at cases like the SL-1 reactor, which became prompt supercritical, there wasn't an atomic explosion (though there was a steam explosion, which had the good effects of removing the moderator).
The time constants for neutron propagation and fission look very small compared to the time constants for, say, vaporizing the moderator water. So what exactly are the mechanism that prevent a "simple" prompt supercritical mass from exploding?
(We have exchanged ideas but none seem very convincing.) David.Monniaux 21:06, 30 May 2006 (UTC)
by-- Light current 02:06, 13 September 2006 (UTC)
THe critical mass of pure U235 is about 50kg so how do you get this business of only 100ml to make a bomb? Density (solid) at 20.7 °C 5.09 g/cm3. So 100cc would weigh 509g -- far short of the 50kg required even if Pakistan could make 100% pure U235. Yes?-- Light current 01:02, 13 September 2006 (UTC)
How have you come to the conclusion of 50Kg. I guess this figure is for 80% enriched version. nids (♂) 01:13, 13 September 2006 (UTC)
Im not an expert, but I dont think it works like that. You need critical mass to get fission (by definition). After that the thing blows apart and complete fission of all the material may not occur (unless its designed very well), But youve still made a bomb! I think it is false logic to assume that since only 0.65 kg fissioned, this is equal to the critical mass of 235. Any way if you want to continue this discussion, it should be moved to an appropriate page as i thikn we have now answered your questions.-- Light current 01:48, 13 September 2006 (UTC)
Even i am not an expert and so i asked the experienced editors to give a better figure. But 50 Kg. is absolutely wrong. That would mean that we havent done anything in last 60 years. Moreover, there has to be a big difference between 80% enriched versions and 99.9% ones. nids (♂) 01:56, 13 September 2006 (UTC)
I would also like to know that what purity for enriched Uranium has been acheived. And what will be the minimum amount required in your view, if we do compress it as much as it is legitimate. nids (♂) 08:14, 13 September 2006 (UTC)
Please correct this statement on the Enriched Uranium page, if this is wrong.
“ | The critical mass for 85 % of highly enriched uranium is about 50 kilograms. | ” |
nids (♂) 08:20, 13 September 2006 (UTC)
I have made a diagram. However I can't edit to the page because of some sort of idiotic spam system that has gone into place and cannot differentiate between subdomains and domains. In any case if they ever fix it, someone can perhaps add this image to the second section. (Perhaps I am being less than charitable re: the spam link system but I am frustrated because it does not seem to be something I can override even though I am an admin, and the appeal system seems quite slow.) -- Fastfission 04:34, 11 December 2006 (UTC)
The paragraph beginning with: "The critical mass is inversely proportional to the square of the density: if the density is 1% more and the mass 2% less, then the volume is 3% less and the diameter 1% less." is not well written. It's fair enough to point out that density is a factor in critical mass, but perhaps someone could rewrite it? Volantares 17:33, 8 March 2007 (UTC)
for those who aren't completely acquainted with this subject, this article is a little confusing, because it is very technical. Statue2 12:11, 28 August 2007 (UTC)
This is a fairly wonky bit of writing, and an intro with pictures describing the principles in plain, but technically correct, language would be welcome. -- 75.73.1.89 ( talk) 17:53, 6 November 2013 (UTC)
The image used is about a criticality accident-that maybe would be better off on the criticality accident page (there is a picture of Godiva-difficult to understand the point of the article there as well). Anyway, perhaps a subcritical bare sphere of plutonium, then a critical sphere (with or without surrounding reflective hemispheres), as well as a supercritical mass would show the concept better? I feel that using an accident recreation seems to show a "negative view" rather than an encyclopedic description of critical mass. What do you think?-- Read-write-services 23:43, 7 October 2007 (UTC)
Is criticality really dependent on temperature itself (seems unlikely) or just in the sense that density decreases with temperature?-- Patrick 13:16, 6 November 2007 (UTC)
Is there a simple and approximately correct formula for the bare critical mass of a material that is a homogeneous mixture (an alloy, say) of other materials that have bare critical masses? I assume this would depend on the relative densities, but are there any other significant complicating factors (e.g. differences in neutron energy spectra)? NPguy ( talk) 00:40, 9 May 2008 (UTC)
I'm only a humble engineer without knowledge on this subject. However, reading the beginning of the article...
"A numerical measure of a critical mass is dependent on the neutron multiplication factor, k, where:
k = f − l
where f is the average number of neutrons released per fission event and l is the average number of neutrons lost by either leaving the system or being captured in a non-fission event. When k = 1 the mass is critical."
...doesn't have a right ring to me.
How about k = f / l instead? —Preceding unsigned comment added by 201.210.220.78 ( talk) 19:35, 18 July 2008 (UTC)
I know prompt critical has its own article and it is also linked in the last sentence of this one. However I think either the link should be made more prominent or some core information from that article should be included here as well, as it is not so obvious to the casual reader of this one that a critical mass doesn't simply equal a nuclear explosion. -- 77.21.99.8 ( talk) 11:49, 6 February 2012 (UTC)
I came across this 'definition' from the the Swords of Armageddon work of Chuck Hansen
“ | The minimum mass of a fissionable material that will just maintain a fission chain reaction under precisely specified conditions. These conditions include the particular material and its purity, the amount and type of tamper or neutron reflector used, the density or degree of compression of the fissionable material, and its physical shape and geometry. The critical mass of a bare, untamped and unreflected sphere of oralloy (uranium enriched to contain 93.5% or more of the uranium-235 isotope) is 112 lbs. (51 kg). A critical mass is the quantity of fissionable material within which the average number of neutrons remains constant over time, i.e., the number of neutrons lost by leakage and capture by nuclei is equal to the number of neutrons spawned by nuclei division. A neutron inserted into it will always exist, either as the original neutron or as a replacement neutron spawned by a fission reaction. At Los Alamos, an early measurement of criticality was the "Christy Crit," which was the amount of fissionable material required to sustain a critical mass when it was immediately surrounded by an effectively infinite tuballoy tamper. A critical mass is also sometimes called a "crit." | ” |
Source @ Swords of Armageddon: The Development of U.S. Nuclear Weapons Volume 1- Glossary, Weapons Physics, Postwar Fission Weapons Development , page 1-11, PDF- 5.66 Mb via http://www.usna.edu/ 220 of Borg 02:55, 28 August 2015 (UTC)
Hello NPguy. Let's discuss this issue here, rather than in edit comments. I added the following paragraph, which you then deleted:
Due to spontaneous fission a supercritical mass will undergo a chain reaction. For example, a spherical critical mass of pure uranium-235 will have a mass of 52 kg and will experience around 15 spontaneous fission events per second (see Spontaneous fission rates). The probability that one such event will cause a chain reaction depends on how much the mass exceeds the critical mass. If there is uranium-238 present, the rate of spontaneous fission will be much higher.
Let me start by asking, if what I say is correct, do you think it would be a known fact that can be found in some book or article somewhere?
Eric Kvaalen ( talk) 07:42, 25 September 2017 (UTC)
From Spontaneous fission:
Nu- clide |
Half-life (yrs) |
Fission
prob. per decay (%) |
Neutrons per | Spontaneous half-life (yrs) |
Z2/A | |
---|---|---|---|---|---|---|
Fission | Gram-sec | |||||
235 U |
7.04·108 | 2.0·10−7 | 1.86 | 0.0003 | 3.5·1017 | 36.0 |
238 U |
4.47·109 | 5.4·10−5 | 2.07 | 0.0136 | 8.4·1015 | 35.6 |
239 Pu |
24100 | 4.4·10−10 | 2.16 | 0.022 | 5.5·1015 | 37.0 |
240 Pu |
6569 | 5.0·10−6 | 2.21 | 920 | 1.16·1011 | 36.8 |
250 Cm |
[2] | 8300~74 | 3.31 | 1.6·1010 | 1.12·104 | 36.9 |
252 Cf |
[3] | 2.64683.09 | 3.73 | 2.3·1012 | 85.7 | 38.1 |
52000 g /(235g/mole) * (6e23 atoms/mole) / 3.5e17y / 365 d/y / 86400 s/d = 12/s. Close enough to 15 for me. Gah4 ( talk) 14:09, 18 December 2021 (UTC)
References
When I do a CTRL-F on this article I see only two instances of the word "moderator", and neither has to do with the fact that the presence of a neutron-moderating material close to a near-critical assembly can send it 'over the edge' into supercritical territory. This is abundantly clear from the literature, in particular I have in mind the book Atomic Accidents by James Mahaffey. Wikkileaker ( talk) 17:13, 23 January 2019 (UTC)
Due to spontaneous fission a supercritical mass will undergo a chain reaction. For example, a spherical critical mass of pure uranium-235 (235U) with a mass of about 52 kilograms (115 lb) would experience around 15 spontaneous fission events per second. citation needed
Further down this talk page there is an old version of this text with a link to /info/en/?search=Spontaneous_fission#Spontaneous_fission_rates -- multiplying the 0.0003 neutrons per (gramme Neutron) by 52000 gramme gives ~15.6 neutrons per second, which seems to roughly match the figure quoted. Though since it's given 1.86 neutrons/fission, perhaps that figure should have been 8.4 fission events per second?
I believe that's how that table should be interpreted -- though, I don't have access to the source material and there is no explanation in the surrounding article text. Also am not sure how you would quote such a calculation under Wikipedia's referencing system? Modelmat ( talk) 09:27, 23 October 2021 (UTC)
p.s. Is essentially none of the textual material in this article sourced? Of the 13 references, only two are attached to text, the rest to a table, and that's just for a minor note. There's not even a list of reference books which some articles have. Modelmat ( talk) 09:27, 23 October 2021 (UTC)
This may not be the right place but what I'm wondering about is what specifically the early criticality experiments sought to find out, especially the ones that led to the now-infamous criticality accidents of Louis Slotin and Harry K. Daghlian.
In a "normal" quantitative science experiment, what you do is keep all parameters constant except one, which you vary in order to obtain different results. These results then make it possible to arrive at formulas or algorithms to describe the interrelations between the different parameters.
For example, a simple experiment about the electrical conductivity of liquid solutions would be to dip two electrodes into a sample solution, apply a current and measure the voltage. In a series of experiments you could then vary the distance between the electrodes, or the concentration of the solution, or the current, or the voltage etc, and even do multiple series for different electrolytes to compare those to each other, and arrive at the specific conductivity of given electrolytes.
Now it is clear that in the criticality experiments, they measured neutron flux. But the variable parameter in the Daghlian experiment seems to have been the "number of reflector blocks" which seems oddly undspecific; the results would seem to be applicable only to his sepcific experimental set-up. And in the Slotin experiment, the shape of the slot between the two hemispheres is variable but there doesn't even appear to be any measurement of some distance that would allow you to calculate its geometry. Cancun ( talk) 12:07, 23 November 2021 (UTC)
The section seems to indicate that the reaction is started by spontaneous fission. In real bombs, they put in a Po+Be initiator, where the two mix and generate neutrons. The goal is to get the SF rate low enough not to start too early, and then a high probability when it is time to go off. Gah4 ( talk) 14:13, 18 December 2021 (UTC)
Not my area, but I found this surprising
"If there is uranium-238 (238U) present, the rate of spontaneous fission will be much higher."
I'd expect the reverse, not least because there is more 238 than 235 around.
But if it isn't a literal, might it deserve a reference or a bit more explanation somewhere? Midgley ( talk) 17:25, 5 April 2024 (UTC)
![]() | This ![]() It is of interest to the following WikiProjects: | ||||||||||
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![]() | The contents of the Critical size page were merged into Critical mass on 11/22/14. For the contribution history and old versions of the redirected page, please see its history; for the discussion at that location, see its talk page. |
Article merged: See old talk-page talk:Critical size
Is there a common or standardized symbol for the critical mass of an element? For example, μU-233 if "μ" was the appropriate symbol and U-233 was the element in question.
Honestly I am not sure there is any symbol
Does anybody know the critical mass for a gas-core "nuclear lightbulb" reactor?
The critical mass is certainly dependant on the material, rather than what device it is being used in. I have no idea what the critical mass of uranium hexaflouride is (the NLB propellant), but Im guessing it's irrelevant as the mixture is heated to a plasma when used and the resulting increase in pressure generated will allow a higher rate of fission. So unless you deliberately compress the gas too much during transport you shouldn't get into any problems with it going fissile.
Also, I think critical mass is a but of a bad name, it should really be critical density
The article says Californium-251 bare-sphere critical mass is 9kg, based on an incidental comment in this article. But the Nuclear Weapons FAQ (section 6.2.4.4) estimates it to be 1.94kg. Does anyone know of another source for this info? - Rwendland 11:24, 2 December 2005 (UTC)
The paragraph:
The realisation that a supercritical assembly is not necessarily prompt critical is attributed to Enrico Fermi, and made the construction of a nuclear reactor using a fission chain reaction possible. Any prompt critical assembly will explode if not rapidly brought below prompt criticality.
seems to need some editing for clarity, however, I don't know the original meaning and I am not a nuclear physicist so, rather than introducing new errors of terminology, if someone else would like to fix it ... Alex.tan 03:16, 9 December 2005 (UTC)
This passage is just out of place here both in content and style. Also I suspect that it was not originally written for this article, but comes from someplace else unattributed. In my opinion it should be deleted outright. DV8 2XL 19:39, 16 December 2005 (UTC)
I think this physical concept is the primary "critical mass" and the page should be at critical mass. How do others feel about that? -- Yath 05:07, 18 December 2005 (UTC)
Wikimedia is being too finicky to move the page over the current redirect for some reason, so I listed it at Wikipedia:Requested moves. -- Yath 20:37, 21 December 2005 (UTC)
The first paragraph in the section titled, "Criticality via additional mass" is not accurate; this is why I added the second paragraph and three enumerated points regarding thermal feedback. The statements are not quoted from any particular source. I did not wish to delete the previous entry regarding the marbles.
The term, "critical mass", was invented in the 20th century (OED dates it 1941, Femto) specifically to describe the concept of the amount of mass required for a sustained nuclear chain reaction. It has had primarily that (or no other) meaning for many decades. If you use the term with anyone, if they happen to know what it means, they think of nuclear reactions first and foremost.
The mere fact that it has other meanings as well is not enough to make a disambiguation page the primary page for the term. Since the physical concept is the primary meaning (by far), it belongs at the main page, with a link to the disambiguation for other meanings of the term. For example, there are about 9 articles about various concepts associated with the word "ruby", but since the gemstone is the main meaning, it is at ruby rather than a disambiguation page. (This is in response to 70.51.178.2 and Tedernst: your reasoning is incorrect.)
I hope more people will vote for this page move, since giving the primary meaning of a term the proper prominence has a definite effect on readers' understanding of the term and its more modern derivatives. -- Yath 19:45, 22 December 2005 (UTC)
What exactly are those other uses of equal importance? All I see on the disambiguation page are things or groups that evidently borrowed the concept. A dictionary might give equal weight to various uses of words in describing the world, but an encyclopedia should give priority to the basic definitions. Femto 20:49, 22 December 2005 (UTC)
It seems to me that the physical meaning is indeed the primary one. Not only because I had never heard of the other meanings (which is a personal thing), but more importantly, all other meanings are clearly derived from the physical meaning. -- Jitse Niesen ( talk) 21:56, 22 December 2005 (UTC)
All of the discussion and support votes assert that the physical concept is primary, either because it came first, or just because it obviously is more important. Do we have any evidence that it's vastly more important? I do not have any evidence either way, which is why I ask. We should not be giving supremacy to one concept over another because "it's obvious" to some people that this is the way it should be. Tedernst | talk 17:58, 23 December 2005 (UTC)
Moved. WhiteNight T | @ | C 01:16, 28 December 2005 (UTC)
In discussions on the A-bomb, there is generally an argument being made that one has to use high explosives to get the prompt supercritical mass for explosion, otherwise the excursion will be terminated before a huge amount of energy is released. Indeed, when one looks at cases like the SL-1 reactor, which became prompt supercritical, there wasn't an atomic explosion (though there was a steam explosion, which had the good effects of removing the moderator).
The time constants for neutron propagation and fission look very small compared to the time constants for, say, vaporizing the moderator water. So what exactly are the mechanism that prevent a "simple" prompt supercritical mass from exploding?
(We have exchanged ideas but none seem very convincing.) David.Monniaux 21:06, 30 May 2006 (UTC)
by-- Light current 02:06, 13 September 2006 (UTC)
THe critical mass of pure U235 is about 50kg so how do you get this business of only 100ml to make a bomb? Density (solid) at 20.7 °C 5.09 g/cm3. So 100cc would weigh 509g -- far short of the 50kg required even if Pakistan could make 100% pure U235. Yes?-- Light current 01:02, 13 September 2006 (UTC)
How have you come to the conclusion of 50Kg. I guess this figure is for 80% enriched version. nids (♂) 01:13, 13 September 2006 (UTC)
Im not an expert, but I dont think it works like that. You need critical mass to get fission (by definition). After that the thing blows apart and complete fission of all the material may not occur (unless its designed very well), But youve still made a bomb! I think it is false logic to assume that since only 0.65 kg fissioned, this is equal to the critical mass of 235. Any way if you want to continue this discussion, it should be moved to an appropriate page as i thikn we have now answered your questions.-- Light current 01:48, 13 September 2006 (UTC)
Even i am not an expert and so i asked the experienced editors to give a better figure. But 50 Kg. is absolutely wrong. That would mean that we havent done anything in last 60 years. Moreover, there has to be a big difference between 80% enriched versions and 99.9% ones. nids (♂) 01:56, 13 September 2006 (UTC)
I would also like to know that what purity for enriched Uranium has been acheived. And what will be the minimum amount required in your view, if we do compress it as much as it is legitimate. nids (♂) 08:14, 13 September 2006 (UTC)
Please correct this statement on the Enriched Uranium page, if this is wrong.
“ | The critical mass for 85 % of highly enriched uranium is about 50 kilograms. | ” |
nids (♂) 08:20, 13 September 2006 (UTC)
I have made a diagram. However I can't edit to the page because of some sort of idiotic spam system that has gone into place and cannot differentiate between subdomains and domains. In any case if they ever fix it, someone can perhaps add this image to the second section. (Perhaps I am being less than charitable re: the spam link system but I am frustrated because it does not seem to be something I can override even though I am an admin, and the appeal system seems quite slow.) -- Fastfission 04:34, 11 December 2006 (UTC)
The paragraph beginning with: "The critical mass is inversely proportional to the square of the density: if the density is 1% more and the mass 2% less, then the volume is 3% less and the diameter 1% less." is not well written. It's fair enough to point out that density is a factor in critical mass, but perhaps someone could rewrite it? Volantares 17:33, 8 March 2007 (UTC)
for those who aren't completely acquainted with this subject, this article is a little confusing, because it is very technical. Statue2 12:11, 28 August 2007 (UTC)
This is a fairly wonky bit of writing, and an intro with pictures describing the principles in plain, but technically correct, language would be welcome. -- 75.73.1.89 ( talk) 17:53, 6 November 2013 (UTC)
The image used is about a criticality accident-that maybe would be better off on the criticality accident page (there is a picture of Godiva-difficult to understand the point of the article there as well). Anyway, perhaps a subcritical bare sphere of plutonium, then a critical sphere (with or without surrounding reflective hemispheres), as well as a supercritical mass would show the concept better? I feel that using an accident recreation seems to show a "negative view" rather than an encyclopedic description of critical mass. What do you think?-- Read-write-services 23:43, 7 October 2007 (UTC)
Is criticality really dependent on temperature itself (seems unlikely) or just in the sense that density decreases with temperature?-- Patrick 13:16, 6 November 2007 (UTC)
Is there a simple and approximately correct formula for the bare critical mass of a material that is a homogeneous mixture (an alloy, say) of other materials that have bare critical masses? I assume this would depend on the relative densities, but are there any other significant complicating factors (e.g. differences in neutron energy spectra)? NPguy ( talk) 00:40, 9 May 2008 (UTC)
I'm only a humble engineer without knowledge on this subject. However, reading the beginning of the article...
"A numerical measure of a critical mass is dependent on the neutron multiplication factor, k, where:
k = f − l
where f is the average number of neutrons released per fission event and l is the average number of neutrons lost by either leaving the system or being captured in a non-fission event. When k = 1 the mass is critical."
...doesn't have a right ring to me.
How about k = f / l instead? —Preceding unsigned comment added by 201.210.220.78 ( talk) 19:35, 18 July 2008 (UTC)
I know prompt critical has its own article and it is also linked in the last sentence of this one. However I think either the link should be made more prominent or some core information from that article should be included here as well, as it is not so obvious to the casual reader of this one that a critical mass doesn't simply equal a nuclear explosion. -- 77.21.99.8 ( talk) 11:49, 6 February 2012 (UTC)
I came across this 'definition' from the the Swords of Armageddon work of Chuck Hansen
“ | The minimum mass of a fissionable material that will just maintain a fission chain reaction under precisely specified conditions. These conditions include the particular material and its purity, the amount and type of tamper or neutron reflector used, the density or degree of compression of the fissionable material, and its physical shape and geometry. The critical mass of a bare, untamped and unreflected sphere of oralloy (uranium enriched to contain 93.5% or more of the uranium-235 isotope) is 112 lbs. (51 kg). A critical mass is the quantity of fissionable material within which the average number of neutrons remains constant over time, i.e., the number of neutrons lost by leakage and capture by nuclei is equal to the number of neutrons spawned by nuclei division. A neutron inserted into it will always exist, either as the original neutron or as a replacement neutron spawned by a fission reaction. At Los Alamos, an early measurement of criticality was the "Christy Crit," which was the amount of fissionable material required to sustain a critical mass when it was immediately surrounded by an effectively infinite tuballoy tamper. A critical mass is also sometimes called a "crit." | ” |
Source @ Swords of Armageddon: The Development of U.S. Nuclear Weapons Volume 1- Glossary, Weapons Physics, Postwar Fission Weapons Development , page 1-11, PDF- 5.66 Mb via http://www.usna.edu/ 220 of Borg 02:55, 28 August 2015 (UTC)
Hello NPguy. Let's discuss this issue here, rather than in edit comments. I added the following paragraph, which you then deleted:
Due to spontaneous fission a supercritical mass will undergo a chain reaction. For example, a spherical critical mass of pure uranium-235 will have a mass of 52 kg and will experience around 15 spontaneous fission events per second (see Spontaneous fission rates). The probability that one such event will cause a chain reaction depends on how much the mass exceeds the critical mass. If there is uranium-238 present, the rate of spontaneous fission will be much higher.
Let me start by asking, if what I say is correct, do you think it would be a known fact that can be found in some book or article somewhere?
Eric Kvaalen ( talk) 07:42, 25 September 2017 (UTC)
From Spontaneous fission:
Nu- clide |
Half-life (yrs) |
Fission
prob. per decay (%) |
Neutrons per | Spontaneous half-life (yrs) |
Z2/A | |
---|---|---|---|---|---|---|
Fission | Gram-sec | |||||
235 U |
7.04·108 | 2.0·10−7 | 1.86 | 0.0003 | 3.5·1017 | 36.0 |
238 U |
4.47·109 | 5.4·10−5 | 2.07 | 0.0136 | 8.4·1015 | 35.6 |
239 Pu |
24100 | 4.4·10−10 | 2.16 | 0.022 | 5.5·1015 | 37.0 |
240 Pu |
6569 | 5.0·10−6 | 2.21 | 920 | 1.16·1011 | 36.8 |
250 Cm |
[2] | 8300~74 | 3.31 | 1.6·1010 | 1.12·104 | 36.9 |
252 Cf |
[3] | 2.64683.09 | 3.73 | 2.3·1012 | 85.7 | 38.1 |
52000 g /(235g/mole) * (6e23 atoms/mole) / 3.5e17y / 365 d/y / 86400 s/d = 12/s. Close enough to 15 for me. Gah4 ( talk) 14:09, 18 December 2021 (UTC)
References
When I do a CTRL-F on this article I see only two instances of the word "moderator", and neither has to do with the fact that the presence of a neutron-moderating material close to a near-critical assembly can send it 'over the edge' into supercritical territory. This is abundantly clear from the literature, in particular I have in mind the book Atomic Accidents by James Mahaffey. Wikkileaker ( talk) 17:13, 23 January 2019 (UTC)
Due to spontaneous fission a supercritical mass will undergo a chain reaction. For example, a spherical critical mass of pure uranium-235 (235U) with a mass of about 52 kilograms (115 lb) would experience around 15 spontaneous fission events per second. citation needed
Further down this talk page there is an old version of this text with a link to /info/en/?search=Spontaneous_fission#Spontaneous_fission_rates -- multiplying the 0.0003 neutrons per (gramme Neutron) by 52000 gramme gives ~15.6 neutrons per second, which seems to roughly match the figure quoted. Though since it's given 1.86 neutrons/fission, perhaps that figure should have been 8.4 fission events per second?
I believe that's how that table should be interpreted -- though, I don't have access to the source material and there is no explanation in the surrounding article text. Also am not sure how you would quote such a calculation under Wikipedia's referencing system? Modelmat ( talk) 09:27, 23 October 2021 (UTC)
p.s. Is essentially none of the textual material in this article sourced? Of the 13 references, only two are attached to text, the rest to a table, and that's just for a minor note. There's not even a list of reference books which some articles have. Modelmat ( talk) 09:27, 23 October 2021 (UTC)
This may not be the right place but what I'm wondering about is what specifically the early criticality experiments sought to find out, especially the ones that led to the now-infamous criticality accidents of Louis Slotin and Harry K. Daghlian.
In a "normal" quantitative science experiment, what you do is keep all parameters constant except one, which you vary in order to obtain different results. These results then make it possible to arrive at formulas or algorithms to describe the interrelations between the different parameters.
For example, a simple experiment about the electrical conductivity of liquid solutions would be to dip two electrodes into a sample solution, apply a current and measure the voltage. In a series of experiments you could then vary the distance between the electrodes, or the concentration of the solution, or the current, or the voltage etc, and even do multiple series for different electrolytes to compare those to each other, and arrive at the specific conductivity of given electrolytes.
Now it is clear that in the criticality experiments, they measured neutron flux. But the variable parameter in the Daghlian experiment seems to have been the "number of reflector blocks" which seems oddly undspecific; the results would seem to be applicable only to his sepcific experimental set-up. And in the Slotin experiment, the shape of the slot between the two hemispheres is variable but there doesn't even appear to be any measurement of some distance that would allow you to calculate its geometry. Cancun ( talk) 12:07, 23 November 2021 (UTC)
The section seems to indicate that the reaction is started by spontaneous fission. In real bombs, they put in a Po+Be initiator, where the two mix and generate neutrons. The goal is to get the SF rate low enough not to start too early, and then a high probability when it is time to go off. Gah4 ( talk) 14:13, 18 December 2021 (UTC)
Not my area, but I found this surprising
"If there is uranium-238 (238U) present, the rate of spontaneous fission will be much higher."
I'd expect the reverse, not least because there is more 238 than 235 around.
But if it isn't a literal, might it deserve a reference or a bit more explanation somewhere? Midgley ( talk) 17:25, 5 April 2024 (UTC)