Talk:Prompt criticality

message: It seems that the "game" at Prompt Critical Video Game has nothing to do with nuclear physics, and just uses a cool-sounding word. Does anyone object to deleting it?
68.57.72.229 (talk) 19:01, 25 January 2009 (UTC)
thread_title:  Game? 

message: The statement "and the increase in the reaction will be extremely rapid and uncontrollable, causing an explosion within a few milliseconds" is also untrue, as the reaction is not uncontrollable, just rather difficult to control (it can be controlled using careful design, material properties, etc) and an explosion is only a potential result, not an assured one (again, ref. GODIVA)  —Preceding unsigned comment added by 199.46.196.231 (talk) 16:06, 13 July 2010 (UTC)
replies: text: Prompt critical means k=1, no exponential growth.  Mostly it is used as the dividing line between prompt subcritical and prompt supercritical. The nice thing about delayed critical is that mechanical systems (control rods) are fast enough to keep it there. It would be difficult, but maybe not impossible, to keep a system exactly at prompt critical.  Even more, though, what is important is the reactivity of the whole system. Some parts will be above, and some below, keeping the average where you want it.  (For both delayed and prompt.)  In theory, you can have a "merely critical prompt critical assembly", practically, it is hard to say. Gah4 (talk) 17:43, 24 August 2018 (UTC)
thread_title:  Disputes 

message: I did some major rewriting to try to keep the article on-topic and make it easier to understand for those not already familiar with the topic. I may have lost some relevant bits in the process so please do add them if you feel they should remain. I will try to split the article into sections as well. J.Ring 22:45, 8 September 2006 (UTC)
replies: text: I will review over the weekend.  Thanks for the cleanup efforts!  Georgewilliamherbert 00:27, 9 September 2006 (UTC)

text: I added this without the Chicago_Pile-1, and clarified the exponential decrease.--Patrick 14:31, 24 October 2007 (UTC)
thread_title: Some rewriting

message: I noticed that the two unnumbered references are broken.  One is a 404 and the PDF link redirects to a list of links at the Department of Energy Technical Standards Program and not to a file.  Also, in general, there aren't very many inline references in this article for the amount of information presented.  I don't know enough about the subject to work on it though.Xblkx (talk) 18:51, 31 May 2012 (UTC)
thread_title: References

message: "Many reactor designs succeed in making prompt criticality practically impossible. Some pressurized water reactors, for example, do not contain enough fuel of high enough enrichment to make a prompt critical assembly with the materials in the core." 
I agree that you cannot take out the core put it into a big lump and make a critical assembly. However, this fact is irrelevant when talking about reactor safety. If control rods / chemical shims are not used then there can be a very large and rapid increase in power. The difference between delayed and prompt criticality is about the ability to control the nuclear reaction not whether dangerous power surges can occur.
"Such reactors can still overheat and even melt if the ability to cool them is lost (a loss-of-coolant accident)"
The main safety feature of a PWR is that you hopefully have a negative void coefficient.
"but they are unlikely to explode"
Prompt criticality is not the only thing that can make reactors explode - Fukushima. The paragraph seems to suggest that if you can get prompt criticality it could cause a 'nuclear explosion'.  Plunk502 (talk) 11:28, 29 January 2013 (UTC)
thread_title:  PWR and prompt criticality 

message: "an assembly is prompt critical if for each nuclear fission event, one or more of the immediate or prompt neutrons released causes an additional fission event. This causes a rapid, exponential increase in the number of fission events."
I think this isn't strictly true, because if, for each nuclear fission event, exactly one of the prompt neutrons released causes an additional fission event, then the rate of fission events simply stays constant, doesn't it?
So it should read "if for each nuclear fission event, more than one of the immediate or prompt neutrons released causes an additional fission event" --Cancun771 (talk) 08:47, 15 April 2013 (UTC)
replies: text: Yes, but you are riding the knife edge to end all knife edges.  If the drunkard's walk of the amount of reactivity going on starts to increase, you'll have a run-away situation going to saturation in less then a microsecond.SkoreKeep (talk) 01:36, 27 August 2015 (UTC)
replies: text: It does seem that critical is often enough, maybe even the WP:COMMONAME, for supercritical. Depending on exactly how big k is, the exponential growth can be pretty slow, or very fast. It is commonly believe to be 2 for bombs, I suspect partly because the calculation is easier. In a moderated reactor, the mean free time is pretty long, so growth can be pretty slow. Gah4 (talk) 03:54, 24 September 2019 (UTC)
thread_title: Erm

message: There doesn't seem to be much discussion for moderators, and their contribution to fission rate, and growth rate. For a bomb, you need unmoderated prompt criticality. With enough moderator, and especially with a temperature sensitive moderator, you should be able to achieve steady state prompt criticality. Gah4 (talk) 18:22, 3 October 2016 (UTC)
replies: text: I was wondering the same thing -- it seems there are three different tiers of supercriticality: delayed-supercritical, moderated/thermal-supercritical, and unmoderated/fast-supercritical. Each of these has a typical time scale, ranging from seconds-minutes (reactor startup - delayed supercritical) down to milliseconds (reactor accident - thermal supercritical) and then down to nanoseconds (nuclear bomb - fast supercritical). The latter two both get classified as "prompt criticality" since they're too fast for a mechanical system to react to, but besides this the behaviour should be quite different. --Nanite (talk) 17:43, 23 September 2019 (UTC)
replies: text: There are two relevant factors that determine the characteristics of criticality: (1) slow vs. fast neutrons (related to the presence of moderator materials) and (2) prompt vs. delayed neutrons.  Prompt neutrons are those released immediately in the fission event.  Delayed neutrons are those that result from the subsequent decay of fission products.  A small fraction of neutrons are delayed by times on the order of seconds.  Bombs rely on prompt fast neutrons.  Reactors (whether fast or thermal) rely on delayed neutrons for their stability/controllability. NPguy (talk) 15:46, 28 September 2019 (UTC)
thread_title:  all in moderation 

message: A recent edit exchanged k-effective and reactivity, which I agree with.  There are many other places where k-effective is used that, for the same reason, could be changed to reactivity.  I believe that they should be changed. Gah4 (talk) 18:11, 24 August 2018 (UTC)
thread_title:  reactivity 

message: Reading Chicago_Pile-1 again, it isn't obvious that it ever went (delayed or not) critical.  Consider that they had a neutron source, maybe a pretty strong one.  They could then measure the overall k for the whole assembly, even when it was much less than one. Consider the case where k=0.9.  One neutron from the source will, on average produce 0.9 more. Those, then, will then produce 0.9^2, and from those produce 0.9^3. Summing the geometric series then shows that for each source neutron, you get 1/(1-k) neutrons.  With a good source, they might get the 1/2 watt with k of 0.9999 or so.  It seems that Fermi did understand delayed neutrons enough to know that they could build it where they did, but still had various manual and automatic safety systems. Gah4 (talk) 03:47, 24 September 2019 (UTC)
thread_title:  CP-1 

message: Should the criticality section mention neutron poison fission fragments?  As written, it seems to be true at the instant a reactor starts up, but after it is running, there are fission fragments that can also absorb neutrons, or decay products of those. Most famous is xenon poisoning, that makes restarting reactors soon after shutdown difficult or impossible. At delayed critical, enough neutrons have to be produced to balance along with those absorbed by such neutron absorbing reaction products, in addition to the non-fission absorption by uranium. Gah4 (talk) 01:32, 10 February 2020 (UTC)
thread_title:  poison 

message: I think I agree with the recent edit, removing unsafe describing the Chernobyl test. It wasn't the test that was unsafe, but powering up a reactor with xenon poisoning. The reactor had to be running at an appropriate energy to do the test. Getting to that energy caused the accident, not the test itself. As well as I know it, reactor operators are supposed to understand xenon poisoning, and its effect on starting up reactors. Gah4 (talk) 09:01, 9 October 2020 (UTC)
thread_title:  unsafe test 

message: The deadly Slotin accident is missing, 21 May 1946, wheras the comparable Daghlian accident is the first in this list.
https://en.wikipedia.org/wiki/Louis_Slotin#Criticality_accident  — Preceding unsigned comment added by 2003:C3:2F03:B800:F425:CE2E:274C:44CC (talk) 20:21, 22 November 2020 (UTC)
thread_title:  Slotin Accident 

message: The article notes that it is, on the average that each fission causes another one, presumably meaning time average. But there is also a spatial distribution. Reactors are designed, and refueled, to keep the power distribution somewhat equal throughout. An assembly might be way above critical on one side, and way below on the other side, so critical on average, but that isn't usually desired. The Oak Ridge drum described is likely such a case, where some fairly small part went prompt critical. Since the mean-free-path is fairly large, the size of the critical region is important. Gah4 (talk) 23:11, 22 November 2020 (UTC)
thread_title:  average 

message: There is a recent edit with the edit summary saying: made it sound like nuclear reactors could be controlled with just prompt neutrons, that is untrue.  I believe that isn't true.  That is, it is possible to keep the reaction stable without delayed neutrons, though it is harder. Control rods are a big part of stability, but they move at mechanical speeds, too slow to keep things stable. The main effect is thermal, and based on slow neutrons. As the fuel heats up, there atoms move faster, and the neutrons, too. The cross section is very sensitive to neutron energy (speed), and so decreases with temperature. Different reactor designs are more, or less, sensitive to the thermal affect. It seems that it is possible to build one where you can pull the control rods all the way out, and it won't melt down, just warm up a bit. On the other hand, the RBMK is not so stable, and gets worse at low enrichment. (Including after years of operation.) One of the fixes after Chernobyl is to increase the enrichment level for more stability. Well, also, thermal neutrons are slow, so even worst case power can't grow as fast as bombs. Gah4 (talk)
But another thing. A reactor is not homogeneous. Specifically, they are much larger than the mean free path. One part can be supercritical, and another part subcritical, for exact critical on the average. One side will be hotter, and the other cooler. So they move fuel rods around to equalize the reactivity. Gah4 (talk) 20:38, 17 June 2022 (UTC)
thread_title:  stability