Talk:Critical mass

message: 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?
:I did a calculation with MCNP5 two years ago using UF6 as a solid, which is about half as dense as H2O....  I found a phase diagram for UF6 that showed at what temperatures/pressures the compound was solid/gas.  If I recall correctly, the critical size was between 1-3 meters for a right cylinder in the solid state (~room temperature).—Preceding unsigned comment added by 72.81.90.108 (talk • contribs) 03:06, 8 July 2007
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
:Critically depends on mass and density, so that term is imperfect too. However, for a given mass critical density makes sense; similarly, for a given density, the critical mass is meaningful. --Patrick 15:21, 16 December 2005 (UTC)
thread_title:  Symbol for critical mass 

message: 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)
replies: text: As no-one has offered a third source, and I strongly suspect the 9kg is wrong, I'll delete Californium-251 from the table. Better no info than possible wrong info I think. -- Rwendland 14:08, 17 December 2005 (UTC)
thread_title: Californium-251 bare-sphere critical mass

message: 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)
replies: text: Alex, the meaning of this passage hinges on the words "supercritical" and "prompt critical". An assembly is prompt critical when only the neutrons resulting from fissioning nuclei are sufficient to sustain the chain reaction (supercriticality). The neutrons released after fissioning from the fission fragments (daughter nuclei) are not necessary to sustain the chain reaction. Yet I have to agree the final sentence is a bit mysterious!Wikkileaker (talk) 17:09, 23 January 2019 (UTC)
thread_title: Copyediting

message: 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)
thread_title:  Re: Criticality pathways tutorial 

message: 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)
thread_title:  Article name 

message: * Support.--Patrick 12:26, 18 December 2005 (UTC)
thread_title: Votes

message: Moved. WhiteNight T | @ | C 01:16, 28 December 2005 (UTC)
thread_title: Result

message: 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)
replies: text: Probably a bit late for your question, but anyway... To my understanding there are several effects that take place on different time-scales. Doppler broadening and thermal expansion of the fissile material kick in very rapidly, and limit the criticality excursion, after that the void-coefficient of the surrounding material may be important, in certain cases ( like the Chernobyl reactor ) increased formation of gas voids in the coolant increased the reaction rate. Finally the explosion itself will rip the fissile material apart. The main reason nuclear weapons release so much more energy than a reactor criticality accident is simply that they reach much higher levels of criticality. While even a k = 1.01 will cause a reactor to blow up, weapons typically achieve k exceeding 2 or 3, or even higher. Thus simply the time required for the explosion to overcome the inertia of the fissile material and tamper is enough to release a high yield, while in a reactor only a small amount of the fuel would fission before it is blown apart, since the chain reaction proceeds much more slowly. 85.230.199.154 03:11, 10 September 2007 (UTC)
replies: text: There are two types of fission reaction - fast fission, and moderated.  Reactors use moderated reactions, for the most part.  Critical mass is far lower for moderated reactions, because cross sections for lower energy neutrons are higher, but the timescale is longer (neutrons are moving slower and are reflected many times between fissile nucleus encounters, etc).  The timescale for doubling for moderated reactions is slow enough that for practical sized finite assemblies, the energy it can generate in the core before it dissassembles is only on the order of the same energy that conventional explosives generate.  It simply blows itself apart at (by atomic standards) very low energy output.  Depending on the details of the assembly, the yield may be negligible, or approach that of high explosives, or somewhere in between.  The dissassembly may be a steam explosion, or the fissile elements may melt or partly vaporize as well.  Georgewilliamherbert 20:48, 11 September 2007 (UTC)
thread_title:  Prompt supercritical assemblies 

message: 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)
thread_title: Diagram to add

message: 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)
thread_title: Critical Mass and Density

message: 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)
thread_title:  Overly technical 

message: 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)
thread_title:  concur 

message: 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)
thread_title:  Critical mass versus criticality accident images 

message: 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)
replies: text: Good point Patrick, however, an increase in temperature will cause a mass to expand-resulting in a larger volume/less dense mass-I think this is why (as I understand it). hope that kind of helps.--Read-write-services 23:03, 6 November 2007 (UTC)
replies: text: OK, I adapted it.--Patrick 01:23, 7 November 2007 (UTC)
thread_title:  Temperature 

message: 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)
thread_title:  Prompt critical 

message: I came across  this 'definition' from the the Swords of Armageddon work of Chuck Hansen 
:
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)
thread_title:  Definition of 'Critical mass' 

message: 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)
replies: text: Moderators are needed for low enriched material to reduce loss to U238.  In the case of mentioned accidents, it is more that you get reflection from such moderators. Well, which accidents do you mean?  The more famous accidents with HEU are, I believe, due to reflection. Also, HEU criticality accidents can get dangerous faster. Gah4 (talk) 02:35, 24 September 2019 (UTC)
thread_title:  Presence of moderating material 

message: 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)
thread_title:  Criticality experiment specifics 

message: 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)
thread_title:  initiator