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Physics C‑class Mid‑importance

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Replaced the introduction:

A criticality accident is when nuclear fission is unexpectedly and accidentally triggered. The term refers to the achievement of criticality in fissile material, such as uranium, plutonium, or thorium, which releases an often fatal amount of beta radiation (neutrons).

There are a number of things wrong here. For a start, it's not just any fission reaction. It must be a chain reaction, otherwise it's neither a criticality nor likely to be dangerous. Fission is happening all the time, spontaneous fission of Pu-241 for example is the main problem in trying to make weapons from low grade plutonium. For another, thorium is neither fissile nor capable of criticality. Thirdly, I see there's already been a dispute (but no discussion here) on whether beta radiation is a problem in this context. It's not, the problem is the neutrons. They are not the same thing.

I've flagged the article as disputed because there are a number of equally suspect claims throughout the article and they'll take a while to check. Rather than stubify the whole thing, I've left this material in for the moment, but it means the article now has a good introduction followed by some very shaky stuff! Andrewa 03:48, 25 Oct 2004 (UTC)

• Your point on the chain reaction is well-taken -- I misclarified. I had thought that thorium-232 is fissile but now that I look into a bit more it seems that it is just fertile. And I seem to have gotten mixed-up about beta radiation, for some reason I think was thinking of it being neutrons but again, I see that's not the case. Ugh, I occasionally get slightly turned around on such things, it is embarassing. Nevertheless, the historical aspects ought to be just fine, for whatever that's worth; that's more my area than the physics. --Fastfission 20:32, 25 Oct 2004 (UTC)

Fair enough, on that basis I've removed the dispute notice. Andrewa 00:33, 26 Oct 2004 (UTC)

Light current and removal of images and information

Hi, regarding your recent removal of informaion on the article I'd like if you could clarify your reasoning for doing so. Firstly, the statement which you removed regarding the Slotin incident:"Immediately realizing what had happened he quickly disassembled the device, likely saving the lives of seven fellow scientists nearby." is not some random speculation. The other scientists recieved very severe doses of radiation due to this criticality (see the slotin article) and if it had not been stopped they would have unquestionably received lethal doses within mere seconds.

Second, the removal of the image of an external beam of high energy charged particles from a cyclotron is a completely plausable model for what an air glow of a criticality accident looks like. It is not likely at all that an actual image of a criticality accident will ever be taken and this is the closest we will probably ever have to knowing what one looks like. The mechanism for the production of ionized air in a criticality accident (mostly via the photoelectric effect and compton scattering due to high energy x-rays and gamma rays is indescernable from the ionization of air in the image produced by irradiation of high energy charged particle radiation. There is no good reason to remove either of these things from the article. --Deglr6328 01:19, 12 September 2006 (UTC)

"Immediately realizing what had happened he quickly disassembled the device, likely saving the lives of seven fellow scientists nearby." is not some random speculation.

THis implies some knowledge by our editors. We do'nt have that knowledge. Im happy for the quote to stay, but not this speculation by some anon editor.

Also the cyclotron pic doesnt actually have anything to do with the page does it (apart from the color being similar to something else.) Again this is speculation about what a criticality might look like and is therefore not fact. It is misleading to include it.--Light current 01:21, 12 September 2006 (UTC)

What if we remove the word "seven"? I think we can say with a fully reasonable degree of certainty that if Slotin received 2100 REM in a criticality of duration of certainly less than ~2s and two other scientists in the room also suffered the effects of severe radiation exposure (Dr. Graves received something like 350-400 REM) then it is obvious and sane to conclude at least a few other men present at the experiment were mere seconds away from getting lethal doses and the quick action of Slotin probably saved them. I'm not trying to deify him or something, the way he did the experiment was reckless and stupid but his quick action did save lives. --Deglr6328 01:31, 12 September 2006 (UTC)

Surely this says it all and presumably is veriable:

Said Robin Connor, a professor of physics at the University of Manitoba, "Slotin's personal sacrifice undoubtedly saved those who were in the room with him. For himself, there was no hope of recovery, something he must have known at once." We need speculate no more 8-)--Light current 01:46, 12 September 2006 (UTC)

ok that's fine with me. now about the cyclotron image.

I am still unclear on what you mean when you say: "Also the cyclotron pic doesnt actually have anything to do with the page does it (apart from the color being similar to something else.)". The image has direct application to the article I would argue. Nothing is being presented by the inclusion of the image which is misleading (I have always taken extreme caution to ensure my edits do not mislead).

Let us consider the effect of ionization of the air. The consituents of air are more or less the same no matter where you are on the planet and were undoubtedly typical at the site of all criticality accidents that I can recall reading about. It is the particular elemental composition of the air of course, which gives rise to the specific and uniqe emission spectrum of an excited mixture of said gasses. In this case, of mostly nitrogen and oxygen, it is blue (sparks, corona discharge, external cyclotron beam). All criticality accident witnesses who see the airglow event report a blue haze or glow. This is air ionization produced by high energy radiation. Here, we have an image of air ionization produced by irradiation from high energy radiation. They are the same phenomenon. Save for the bragg peak effect exhibited with the cyclotron, the diffuse transparent blue glow is what those people who have seen an accident saw. I think it is a very interesting and valuable part of the aricle, it gives insight where no other substitute can. --Deglr6328 01:48, 12 September 2006 (UTC)

I think we need other comments about the applicability of this picture.--Light current 03:37, 12 September 2006 (UTC)

Ok. list at rfc maybe.--Deglr6328 11:38, 12 September 2006 (UTC)

Yes, if you would do the honours! 8-)--Light current 11:41, 12 September 2006 (UTC)

k. done.--Deglr6328 18:48, 13 September 2006 (UTC)

Thx--Light current 19:17, 13 September 2006 (UTC)

hmmm no one seems to care.--Deglr6328 06:17, 16 September 2006 (UTC)

Well I dont really want to get into an argument over it, so well just leave it for now pending any comments. OK? BTW, thanks for being so cooperative and pleasant to deal with. I wish all my edit conflicts were as easy to deal with!--Light current 16:37, 16 September 2006 (UTC)

Ok, I've just seen the RfC. In my view, the photo is fine provided that a source can be found for the statement that: "Due to the very similar mechanism of production, the blue glow is thought to resemble the "blue flash" seen by Harry Daghlian and other witnesses of criticality accidents." If a source can't be found, it's original research. Best wishes, Jakew 18:42, 1 October 2006 (UTC)

As it turns out, Slotin's quick action my have been for naught. I've read that the heating of the core probably dislodged the beryllium shell before Slotin could have reacted by simple expansion. However, it is also noted that within a few more seconds, after a bit of cooling, the halves would have come back together, causing another neutron drenching. The man did his level best, and there's no point in dissing him over his reactions. SkoreKeep (talk) 01:12, 13 January 2014 (UTC)

Explosion error – Fukushima and Chernobyl

From Chernobyl entry: A second, more powerful explosion occurred about two or three seconds after the first; evidence indicates that the second explosion resulted from a nuclear excursion.

From this entry: In the history of atomic power development, fewer than a dozen criticality accidents have occurred in collections of fissile materials outside nuclear reactors, but most of these have resulted in death, by radiation exposure, of the nearest person(s) to the event. However, none have resulted in explosions.

Somebody is wrong about criticality (nuclear excursions) causing explosions. —Preceding unsigned comment added by 71.184.150.132 (talk) 04:55, 16 March 2011 (UTC)

I invented a neologism "moderated nuclear explosion" some three years ago, describing these events and WP:SYNTHesized an article to include stuff like uranium hydride bomb. The article was promptly deleted as WP:OR. (The deleted article can be found here: User:Petri Krohn/moderated.)

Looking at the pictures of Fukushima Dai-ichi reactor number 4 today, it seems that what happened at Chernobyl has happened again – if in a smaller scale. There is no question, that an explosion occurred. The mechanism cannot be explained as a steam explosion, as it implies no criticality. A criticality accident – by definiton – implies no explosion. -- Petri Krohn (talk) 21:46, 16 March 2011 (UTC)

The most massive criticality excursion accident, Chernobyl, isn't present in this article!--62.202.134.232 (talk) 12:59, 8 November 2013 (UTC)

"Chernobyl liquidators" remark lacks context

This sentence--which concludes the penultimate paragraph of the "blue glow" subsection--cannot possibly be understood based only on the portion of the article preceding it:

It would be also interesting to remark that the ozone smell was said to be a sign of high radioactivity field through Chernobyl liquidators.

No previous mention of ozone nor Chernobyl. Is the phrase "Chernobyl liquidators" familiar to most readers? If not, then its first appearance ought to link to the relevant article.drone5 (talk) 07:22, 14 April 2011 (UTC)

See Liquidator (Chernobyl). --beefyt (talk) 04:35, 25 March 2013 (UTC)

Problem with Heat

This article questions the heat felt by people with a criticality accident occurs. If a nuclear reaction is taking place then of course heat will be produced. Heat is generated in a nuclear bomb and from the sun etc. I don't understand why the article suggestions that it is in the minds of the people witnessing the event. 124.254.78.121 (talk) 05:23, 30 May 2012 (UTC)

But this is no bomb. Heat from stopped fission fragments should have been mostly trapped in the core, which didn't even melt. A few cm of air around the core should have trapped fission fragments from the surface. Do you think people that far away would have felt heat from a ball of metal that size, even if it were glowing red-hot? Probably not-- there's just not enough radiating surface area. And a "wave" of such heat is doubly unlikely. Once red hot, that much metal would takes some time to cool to less than that. Normal thermal cooling events for masses like this (6.2 kg of metal, with a very small surface area) don't happen on that time scale, by natural convection cooling, or by radiative cooling. The physics is just wrong.

More likely that the people actually felt some nerve firing from the blast of neutrons hitting them and ionizing things in the skin, and reported it as "heat" (or "temperature increase"). SBHarris 21:58, 30 May 2012 (UTC)

nonsense cleanup needed

The article currently has this text:

Nuclear reactors operate at exact criticality. When at least one dollar of reactivity is added above the exact critical point (the point where neutrons produced is balanced by neutrons lost per generation) then the chain reaction does not rely on delayed neutrons, and the rate of change of neutron population increases exponentially as the time constant is the prompt neutron lifetime.

The "dollar" in the paragraph above shows there's some creeping nonsense in the article, can a subject expert please read it and eliminate this and less obvious cases? BACbKA (talk) 10:43, 17 June 2013 (UTC)

The term "dollar" as a fraction of reactivity scaled in terms of delayed neutron input, is due to Louis Slotin. . A cent is a 1/100 of a dollar. Delayed critical reactors operate in the "cent" region from 1 cent to 99 cents. When a nuclear reactor exceeds 100 cents = a dollar it is no longer dependent on delayed neutrons, and then its power doubling time is only controlled by the half life of neutrons in the reactor, which is milliseconds or less, so there is no longer any hope of controlling it in real time, and that is what makes for a prompt critical reactor vs. a delayed critical one. Controllable nuclear reactors operate at some fraction or percent of a reactivity dollar (or, if you will, in some price in cents). Probably this term needs its own wikipage, but you can see nuclear reactor control and control physics pages. SBHarris 03:20, 18 June 2013 (UTC)

Wow! thanks a lot for the clarification. And it's actually explained in another place --- I guess I shouldn't have read just the few paragraphs I was interested in, but the whole article in the first place. Sorry for the bother. BACbKA (talk) 06:02, 18 June 2013 (UTC)

Definition of the term Criticality

Presently there is a disambiguation page for Criticality but no dedicated page providing a succinct definition. I was looking for an explanation of the milestone in the construction and operation of a nuclear power plant, where they say "achieved criticality on day/month/year". Thoughts anyone?--Graham Proud (talk) 16:23, 23 January 2014 (UTC)

For example, the Reactor Details page from the International Atomic Energy Agency lists the First Criticality Date for Hanbit-1 in Korea as 31 January 1986.--Graham Proud (talk) 06:12, 24 January 2014 (UTC)

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