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the navy taught me, in 1979, that SCRAM meant Super-Critical Reactor Axe Man. of course, you can't always believe the navy. — Preceding unsigned comment added by (talk) 21:17, 26 October 2013 (UTC)[reply]

Old comments


Searching on google show's the term to be Safety Control Rod Axe Man. Also referenced in Control Rod. Amazon also has quite a large number of books on it. Linky. Makes more sense too imo.. (Feel free to change it otherwise if any other sources contradict that..) Nikwong 14:02, 28 October 2007 (UTC)[reply]

I've heard the term used as Supercritical Reactor Axe Man.

I've heard US Navy nukes use the above term. Oversoul 01:35, 2 December 2005 (UTC)[reply]

After reading the jargon file, I tend to agree with the nukes more than the jargon file. "Safety cut rope axe man" doesn't even make sense. "Supercritical reactor axe man" does. I'll look into this more. Oversoul 01:40, 2 December 2005 (UTC)[reply]

I've read that at the Chicago-reactor (1st reactor) there were also a "suicide team" of people standing on a balcony with buckets of "neutron-killer" to poure over the reactor if something happened. Can someone confirm/refute this?

In the Wikipedia entry on Norman Hilberry I notice the following:

On December 2, 1942, he was present for the start up of Chicago Pile-1, the world's
first nuclear reactor to achieve criticality. Hilberry stood ready with an axe to cut
the scram line, a manila rope connected to control rods that could quickly shut the
reactor down.[1][2]

Dick Kimball (talk) 13:30, 12 January 2016 (UTC)[reply]

  1. ^ "CP-1 Goes Critical". Department of Energy. Archived from the original on November 22, 2010.
  2. ^ Wellock, Tom. "Putting the Axe to the 'Scram' Myth". United States Nuclear Regulatory Commission. Retrieved December 25, 2015.



I have never in many years of nuclear power operations, both navy and civilian, heard a scram referred to as a "trip." Citation, please? ➥the Epopt 16:49, 25 February 2007 (UTC)[reply]

[1] should give you enough citations to satisfy your disbelief; I'll add a cite to Nuclear Tourist... -nbach 22:30, 25 February 2007 (UTC)[reply]

Text issue


"On a SCRAM, 99.3% of the reaction is almost immediately stopped. The remaining 0.7% continues to generate "decay heat" for some time"

I'm not exactly where this number was pulled from but it is most assuredly wrong and misleading. A scram adds a massive amount of negative reactivity into the core and this obviously makes it highly subcritical. In a couple of minutes the neutron induced fission reactions will be many orders of magnitude below any threshold to measure their heat generation. Saying that 99.3% of the reaction is stopped is misleading at best. This number is probably referencing the heat generation produced by the reactor, but it is wrong as well. If you separate the prompt and delayed parts of a fission reaction apart you will find that 6.5% of the energy is delayed (excluding neutrinos which don't add heat). Thus if you scram the reactor, you will initially have at least 6.5% of the initial power output. I say "at least" because you have to sum up the decay heat from all of the different nuclides in the core at the time of shutdown. This means you have to know the power history. If your core was operating at 100% power for a long time and then shut down, you will have to add in the decay heat from the long-lived radioactive nuclides. And if your core was just started up, pushed to 100% power and then scrammed, you will not have had time to build up a large concentration of long-lived radioactive nuclides. Thus in the first case you will start out above 6.5% and this value will decrease as short-lived nuclides decay and only long-lived nuclides remain. From there it will slowly decrease based on the half-lives and concentrations of all of the different radioactive nuclides in the core. In the second case, you will start out at 6.5% and this value will rapidly drop to almost zero as very little long-lived nuclides exist.--Burzum 22:06, 19 May 2007 (UTC)[reply]

I think the point here was that the delayed neutron fraction (i.e. beta) is about .7%, so after after a scram there is still an elevated flux in the reactor from delayed neutrons causing fissions. Most of the precursors have short half lives, so after a few minutes there is essentially no flux in the reactor, as you imply. However, the original statement was correct that while 99.3% of the reactions will be stopped (essentially) immediately, about .7% of the steady-state neutron population is not prompt, so neutrons will continue to be produced and fissions will continue to happen until all of the precursors have decayed. I will clarify this point in the article. --nbach (talk) 09:38, 18 November 2007 (UTC)[reply]

Dubious--Super-Critical Reactor Axe Manila


This claim definitely needs a citation. I have heard many different expansions of the SCRAM acronym, but this is a new one to me. It doubly needs the citation because it is in an etymology section. Cheers.--Burzum 08:11, 14 October 2007 (UTC)[reply]

I've heard two though the years. "Safety, Cut the Rope, Axe Man" and "Safety Control Rod, Axe Man". I've never heard of the others. —Preceding unsigned comment added by (talk) 03:36, 28 August 2008 (UTC)[reply]

I attended Navy Nuclear Power School in NTC Orlando and was told the Super-Critical Reactor Ax Man version. Not saying it's true, just that NPS is likely the source of this version. — Preceding unsigned comment added by 2601:602:9201:2C00:5529:ADF8:7EC4:B03C (talk) 02:11, 15 June 2016 (UTC)[reply]

Reactor Response


This section of the article is wrong (there really isn't a better word to describe its condition right now). I'll try writing an article on reactivity that will more clearly describe how prompt neutrons and delayed neutrons work in a nuclear reactor and how they affect the effective neutron lifetime which will be important to understanding how a massive insertion of negative reactivity will affect the core of a nuclear reactor. The fact that we don't have a decent article covering reactor kinetics is probably causing too much confusion among people who have little experience in nuclear engineering. Until then this section will need to be toned down. For anybody that is confused about what I'm talking about Volume 2 of the DOE Fundamentals Handbooks on Nuclear Physics and Reactor Theory describes how a scram works in more detail. If you understand how reactor kinetics, the failures of this section will be obvious but difficult to explain without another article on reactor kinetics. Cheers.--Burzum (talk) 14:51, 31 January 2008 (UTC)[reply]

What exactly is wrong with it? I agree that an article about kinetics would be good, but I believe the information under reactor response is correct. --nbach (talk) 07:23, 3 February 2008 (UTC)[reply]
It is flat out wrong. A scram means that a large negative insertion of reactivity (a fractional change in reactor power per effective neutron generation) has occurred. The delayed neutrons will tend towards the decay constant of the longer lived nuclides because the massive change in fission rate will mean that delayed neutron precursors (fission products that go on to produce delayed neutrons) produced prior to the scram will be more strongly represented than those after the scram since the reaction rate after the scram is too low to produce enough delayed neutron precursors to produce enough short lived delayed neutrons (which will rapidly decay away leaving a larger fraction of longer lived delayed neutron precursors over time). These longer lived delayed neutron precursors will dominate until the reactor falls into the region where subcritical multiplication occurs. Scramming a reactor is more complicated than shutting of a coal plant. It is really difficult to give comparable examples and only a discussion of reactor kinetics can give it even a half-hearted explanation. The best that I can say is that the neutron response after scramming a reactor has more in common with discharging a capacitor than with flipping a switch, though even this example makes me cringe by its crudity. In any case, like the description of when you say that you have discharged a capacitor, these delayed neutron precursors do not have any set cutoff date where once they were significant and now they aren't. Typically a reactor is said to be shutdown when subcritical multiplication is significant. Cheers.--Burzum (talk) 20:02, 11 February 2008 (UTC)[reply]
That is essentially what the article says. Immediately after scram, the neutron population significantly decreases (and so does the power) because of the short life of fast neutrons, but "we cannot shut the reactor down any faster than on a period of , determined by the longest-lived delayed neutron precursor. In U-235 fueled thermal reactors, sec." (Duderstadt & Hamilton p. 245). I don't believe the article says anything inconsistent with this. If you don't have Duderstadt, you'll find nearly the same analysis (without PKA equations) in the DOE handbook, p. 31-2. Also, I'm confused as to what you mean by "subcritical multiplication is significant;" the reactor has only one state; it can only be critical, subcritical, or supercritial. Do you just mean that is very subcricial? --nbach (talk) 06:59, 19 February 2008 (UTC)[reply]
Subcritical multiplication means that the reactor power will stabilize due to the effects of source neutrons even though is less than 1. And one period does not mean that your reactor is shut down. What D&H is saying is that you can't shut down the reactor "faster than" how the decay constant of your long lived delayed neutrons affects the reactor period. It isn't saying that the reactor will shut down in 80 seconds. What it is saying is that reactor period will approach -80 s since after the initial large amount of negative reactivity inserted will go to zero, the reactivities will cancel out (beta bar will be ignored since it is small) and only the long term delayed neutrons will be around leaving . This doesn't mean the reactor is shut down. The startup rate can be calculated as , thus your long lived delayed neutron precursors are limiting the rate at which the reactor shuts down. What you would see in a graph is a large negative spike in the SUR, it would then come up to about -1/3 DPM, and once subcritical multiplication took over it would slowly level off at zero. I hope this clears this up. Cheers.--Burzum (talk) 17:37, 19 February 2008 (UTC)[reply]
Oh, and in case you are curious why the SUR will come up to zero due to subcritical multiplication, the full reactor period equation is (if I recall correctly):. Obviously as the fission rate () drops the effective source neutron rate will become more important (which occurs at very low powers).--Burzum (talk) 18:01, 19 February 2008 (UTC)[reply]

Safety systems

There were multiple safety systems in place at the Chicago pile, with some electrically-controlled control rods as well as vessels containing a cadmium solution available to stop any reactions if necessary. Therefore, the job of the “SCRAM” to drop another control rod by the force of gravity was most likely superfluous.

This ORNL page describes an account by Wally Koehler, who was actually standing on top of the pile when it went critical. He describes the "SCRAM" as the FIRST line of defense, with him and two others standing by with "vessels" (buckets) of cadmium sulfate in case that proved insufficient. So, not superfluous. Since that's a first-hand account, and it's on the ORNL site, I think maybe we should consider it authoritative? Graft | talk 03:39, 19 December 2008 (UTC)[reply]

A few years ago, while working at Hanford, I had the pleasure of talking with a retiree. He was one of the other two men, standing on top of the pile, along with Wally Koehler. His story of SCRAM was essentially the same as Wally's. Now we've got two of the three men with buckets to dump saying the same thing. They were backups (Defense in Depth) in case the control rod stuck, after the axe man cut the rope. Of course, if they had to have dumped their buckets, the only next step would have been to tear the pile apart and clean up everything, then rebuild it. —Preceding unsigned comment added by (talk) 15:47, 25 October 2009 (UTC)[reply]

Oldest Reference


The oldest reference that I have found citing the origin of the word SCRAM is: Raymond L. Murray, "The Etymology of 'Scram,'" Nuclear News, 31, August 1988, pp 105-107; "Letters," Nuclear News, 31, December 1988, pp 17-18. Now this is referenced in a document titled "Glossary of Nuclear Criticality Terms LA-1 1627-MS UC-714 Issued: October 1989". This 2nd document may be found here. However I can find no online copies of the original document. SunSw0rd (talk) 23:46, 13 October 2010 (UTC)[reply]



Move etymology to top to be congruent with other articles? (talk) 03:00, 5 April 2011 (UTC)[reply]

Change name of article?


Right now, the article name is 'Scram', but the term it is about is either 'scram' or 'SCRAM'. It seems to me that the title should be changed to 'SCRAM'. —Preceding unsigned comment added by (talk) 02:08, 13 May 2011 (UTC)[reply]



I have made a minor edit to the etymology section referencing a recent NRC article that disputes the tradition history of the term. Hopefully others will be able to rework the section in the light of this. Sidefall (talk) 12:35, 24 May 2011 (UTC)[reply]

Iodine pit


I've added Iodine pit to "see also", but it'd be better to include a line or two about it in the "reactor response" section. The basic issue is that, for some reactor designs, the reactor cannot be restarted for tens of hours after a rapid shutdown from full power. Perhaps someone who is better versed on this topic could add a brief explanation.   Will Beback  talk  22:48, 28 May 2011 (UTC)[reply]

Under the bleachers


I have changed this to read "under the spectator seating" as the term "bleachers" is not generally understood outside North America. (talk) 21:03, 10 May 2014 (UTC)[reply]

"scram" or "SCRAM"


I believe the title, as it is an acronym, should be changed to SCRAM. Also, both "SCRAM" and "Scram" are used several times throughout the article. I think these should be changed for consistency to SCRAM, but they should also match the title of the page.Fhutch21 (talk) 22:12, 5 May 2016 (UTC)[reply]



Means what and this is on someones ankle for a home monitor. ???? Explained definition! 2601:845:8100:350:4114:82B:A2EF:A3F2 (talk) 23:42, 9 April 2024 (UTC)[reply]