Talk:Prompt critical

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Previously a disputed article -- comments read as follows -- This page currently needs a complete rewrite. For example, it confuses the terms prompt critical and supercritical, and talks of a delayed neutron flow control system which sounds impressive but is I suspect meaningless.

See critical mass.

However there's lots of good stuff here as well. Andrewa 02:26, 29 Nov 2004 (UTC)

See this external link for a good definition of supercritical (and I think they mention prompt-critical too). Andrewa 02:38, 29 Nov 2004 (UTC)

Listing on Wikipedia:Accuracy dispute is optional AFAIK, and I don't think necessary in this case. I'll attend to the problems in a few days' time hopefully, including renaming to prompt criticality and deleting the resulting redirect. Andrewa 02:45, 29 Nov 2004 (UTC)

OK, I've attempted a rewrite. Our coverage of the whole topic is still not terribly good. Critical mass is still a worry. But I've removed most of the overgeneralisations and fundamental misunderstandings I think. Andrewa 18:13, 29 Nov 2004 (UTC)

Thanks for the rewrite. I'm by no means an expert in nuclear engineering, but when I saw the words "went prompt critical" in the PL-1 article, I thought it would be nice to explain the term and did so to the best of my ability. Unfortunately in the process I confused myself thinking supercriticality was more powerful than prompt criticality. As for delayed neutron flow control system, that's what a graphite rod assembly is, though admittedly the name is too pompous and inappropriate for it. --Azazello 14:03, 1 Dec 2004 (UTC)

Sorry if my words above were a bit harsh, you seem to have the spirit of the Wiki well in hand! I think the results in this case are getting there.

I'm not a nuclear engineer either, but I have one in the family, worked in the industry for some years myself (in the computer department) and have a bee in my bonnet about some of the issues.

Not convinced about the graphite rod assemblies. Control rods are generally cadmium or boron neutron absorbers. In theory you could have a graphite rod assembly that made the pile more reactive when inserted, and use it to shut the pile down when it was removed, but I don't think that's ever been done for a power reactor, at least not deliberately. Something like it was done unintentionally with the RBMK, that was one of the design errors and was what actually triggered the first explosion, which was possibly inevitable by that time anyway. Andrewa 18:48, 1 Dec 2004 (UTC)

Cleanup and linguistic edits, plus incorporation of the concepts of prompt/delayed neutrons to distinguish supercriticality and prompt criticality (based on other versions floating around the web), otherwise without (I hope!) substantial content change. I have not removed the cleanup tag, since I notice that the article is scheduled for renaming to "... criticality", not "... critical" (and I don't know how to do that), and I have left the stub tag in place. -- Rudolf Cardinal, 12 June 2005.

Further additions, defining prompt criticality formally. Cleanup/stub tags tentatively removed. -- Rudolf Cardinal, 13 June 2005.

Reversion of part of opening paragraph - a previous edit made it sound as if a prompt critical assembly could be critical, or supercritical, or go bang. A prompt critical assembly, as I understand it, is a supercritical assembly that is supercritical needing only the contribution of prompt neutrons, and goes bang. You can't have a "merely critical prompt critical assembly", as I understand it - so a prompt critical assembly can't cause either a self-sustaining fission reaction or an exponential increase and an explosion. I've added a further explanation of prompt criticality as a subset of supercriticality, before the main explanation of subcritical/critical/supercritical/prompt critical. -- RudolfCardinal 17:26, 1 February 2006 (UTC)

[edit] Some rewriting

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)

I will review over the weekend. Thanks for the cleanup efforts! Georgewilliamherbert 00:27, 9 September 2006 (UTC)

Significant errors

This paragraph is abiguous/wrong: "In a prompt-critical (k > 1) assembly, the neutron activity increases exponentially by the factor k, and will cause an explosion if kept prompt-critical for long enough . . . "

This describes a supercritical condition, not a prompt-critical one (k(prompt) > 1 for prompt criticality). Supercriticality is not very relevent to prompt-criticality. Casual readers will be confused in the difference between prompt and delayed neutrons and their impact on criticality. It would be better to describe prompt-criticality here in terms of fast neutrons. (The section above is anonymous)

Errors in the reaction rate

There are some errors in the description of the reaction rate.

In a prompt-critical (k > 1) assembly, the neutron activity increases exponentially by the factor k, and will cause an explosion if kept prompt-critical for long enough. In contrast, in a subcritical assembly, each fission event triggers, on average, less than one new fission event (k < 1) and the activity decreases exponentially by the factor k. For example, if 2.4 neutrons are released per fission event, then if the probability of a neutron causing a further event is less than 1/2.4 = 0.42, the assembly is sub-critical and the neutron activity decreases exponentially with time.

It is not true that for a sub-critical assembly that the activity decreases exponentially with time; that would imply that the activity will drop asymptotically to zero. There is a steady-state activity whose level will increase with increasing k. This is due to the fact that spontaneous fission neutrons will trigger other neutrons; the total number of induced fission neutrons induced by a single spontaneous event is a function of k. When k>1, then this number is infinite and that results in an uncontrolled chain reaction.

If you inject a burst of neutrons into a sub-critical assembly, you will get a burst of activity that drops exponentially with time back to that steady-state level of activity.

I would therefore suggest rewriting this paragraph thus:

In a supercritical (k > 1) assembly, the neutron activity increases exponentially with time. If the assembly is supercritical but not prompt-critical, the increase will be fairly slow (e.g. double every few minutes). This was the case for the first controlled nuclear reaction at Chicago_Pile-1 built by Enrico Fermi. If the assembly is prompt-critical, the increase will extremely rapid and will cause an explosion if kept prompt-critical for long enough (meaning a few millionths of a second or less).

In contrast, in a subcritical assembly, each fission event triggers, on average, less than one new fission event (k < 1). While the level of neutron activity increases with increasing values of k, it will approach some steady state value and stay there, rather than increasing without limit like a supercritical assembly.

DavidGauntt 22:39, 14 June 2007 (UTC)DavidGauntt

I would agree with most of your recommended changes but disagree with citing the Chicago-Pile-1 as the only example of a supercritical reactor. ALL reactors must go supercritical to raise power. It is generally done in a controlled manner with k only slightly greater than 1.

AdamGott 14:45, 24 October 2007 (UTC)AdamGott

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