Talk:Void coefficient

Magnox containment

Does Magnox have full containment or it is more like in RBMK type reactor?

Looks like it doesn't have a secondary containment building: [1] [2] [3] (but note that at least two are anti-nuclear activist sites). But comparing them to RBMK is kind of like comparing apples and oranges. This is not to say that they're safe, of course! But they have rather different failure modes. (Besides, the later RBMK, like the Chernobyl reactor actually did have partial secondary containment...) Anyway, I'm still looking for some authoritative technical descriptions of a Magnox reactor, to see if they actually do have some analogous backup containment system. But I'm not too hopeful.

This should maybe go on Talk:Magnox reactor and ultimately on Magnox reactor itself. --Andrew 05:00, Dec 23, 2004 (UTC)

Yes, there's a discussion at Talk:Magnox#Lack of secondary containment, and no, there's no secondary containment. Andrewa 18:24, 19 June 2007 (UTC)

BWR design

It is not at all clear what is meant by the discussion of BWRs under Reactor Design. The statement that BWRs "generally have a negative void coefficient" is true. However the remainder of the sentence ("but if these coefficients are too large, a stuck valve may lead to a rise in pressure, a liquefication of the coolant, and a drastic rise in temperature") seems confused or garbled. It's not clear what "stuck valve" means in this context -- stuck open? stuck closed? just does not move? -- I think the intended idea is something like: Sudden closure of a main steamline valve may ["will" during normal operation] lead to....; also "drastic rise in temperature" would more properly be "sudden rise in power level". My comment here is intended as a warning not accept this particular part of this entry as worded.--BoHawk 21:39, 23 Dec 2004 (UTC)

Under the Explanation the following sentence is also misleading: "In boiling-water reactors with large negative void coefficients, a stuck steam valve means the pressure rises, the water begins to condense, and the thermal output rises until a pipe bursts or something melts." A key element of BWRs is pressure control. In a BWR after reaching normal operating pressure, as power level is increased, the steam flow is increased to maintain essentially constant pressure. This is done by automatic (or sometimes manual) positioning of turbine control and/or bypass valves. The sentence that I am challenging seems to be getting at a thought that loss of ability to open a valve to control pressure could lead to loss of pressure control; loss of pressure control could lead to pressure increase and consequent reduction of the voids in the core; reduction of voids will lead to power increase and more pressure increase and this will continue until something breaks. In a generalized sense, this is true; however, by design BWRs include safety valves to limit pressure increases and include scrams (to shutdown the reactor) on both high pressure and high power conditions.--BoHawk 21:39, 23 Dec 2004 (UTC)

Ah. I put that sentence in, after reading an article by the CANDU people [4]. (It's the one referenced in the article). More should certainly be said about the safety systems, I agree, but the point is that a large negative void coefficient can lead to instablility, just as a large positive one can. (Reactors with positive void coefficients also have safety systems designed to deal with runaways, and they normally work fine. But having a large coefficient makes them more necessary and more difficult to build; and sometimes they fail.) If you'd care to elaborate, it sounds like you know more about the workings of BWRs than I do. --Andrew 04:43, Dec 23, 2004 (UTC)

I do have a lot of experience with design, operation and analysis of GE-designed BWRs. I may, if time allows, try to improve both this article and the one on Boiling Water Reactors (where I've already made a few changes). The following are the best web sites that I currently know where one can find fairly detailed, and reliable information on BWR design: http://book.nc.chalmers.se/KAPITEL/CH20NY3.PDF

http://www.engr.sjsu.edu/jrhee/me210/Table%20of%20Contents.pdf (on my browser this TOC links to the listed chapters -- to the entire document can be accessed from this page)

http://www.nuc.berkeley.edu/designs/abwr/

I am grappling, somewhat with a concern on whether the Wikipedia is a useful document that is worth spending some time on or whether it is more likely to become a source of plausible, but not necessarily correct, information that could be inappopriately used. So, I added the links above to allow someone to go closer to the real technical sources. (As an example of "plausible" but not really correct -- the current definition of "void coefficient" is plausible and sort-of OK, but not really right. In more precise, but possibly-too-technical terms, the void coefficient is more properly described as a measure of the "reactivity" change caused by a change is void content or moderator density -- but, then you get into the issue of "what is reactivity"?)--BoHawk 21:39, 23 Dec 2004 (UTC)

That's great, I hope you do contribute. I wrote most of the current article, and it sounds like what's there isn't quite right. I do hope I wasn't too misleading, but I hope you'll help improve the article - we could really use your help. I haven't time just now to read those references, but I am curious, so I'll try to get to them soon.

As for your concerns about the reliability of Wikipedia, I understand them and can sympathize. In my experience, Wikipedia is generally as reliable and compendious as I would expect from a general-purpose encyclopedia, that is, not perfect, but good enough for non-specialists. I think, from your description, that what's here now is not quite right but close enough to give a casual reader the correct idea; if you stick around, it can be correct. My experience with the rest of Wikipedia is that it was clear when not to trust articles too much (i.e., written by a non-expert) and when to trust articles more; and there are many areas where it's very solid.

As for what to do about this definition, I'd suggest giving the right definition; if it's necessary to explain reactivity, well, just link to a not-yet-existent article or wait for someone to fill it in here. I'd stick those links at the bottom of the article, too, so people can find the real data once they've read the article.

I'll try to get to the article when I can, but if you'd like to work on it, better someone who really knows what they're doing! --Andrew 08:37, Dec 24, 2004 (UTC)

Definition

I've started modifying this page some by expanding, and changing in one relatively important way, the basic definition. The "important" change is to get rid of the idea that void coefficient has to do with the rate of power change. It really has to do with the amount of power change. The rate of power change is related to the rate of change in void content, the magnitude of total reactivity change cause by the change in voids, and whether the total reactivity change is less than or greater than the reactivity tied up in the delayed neutron fraction. (I'm working from memory on this last statement, so my terminology may not be quite correct; but the issue is whether or not the amount of reactivity change is enough to cause the reactor to become prompt critical. I'll try to fix the terminology -- if incorrect -- later.) If you feel that I am getting too technical here, let me know and I will try to use simpler terminology. My goal is to try to get roughly the right idea communicated to an average high school science student.--BoHawk 17:17, 26 Dec 2004 (UTC)

Stability

The negative void coefficient of some of the reactors designed in the United States, such as the reactor in Three Mile Island and (if I'm not mistaken) SL-1, is taken by many people as something that makes both reactors inherently better, safer and "more stable" then Sovietic RBMK used in Chernobyl. But, obviously, this stability can't mean much, as the three reactors suffered serious incidents.

What good is this "stability" given by the negative void coefficient when the reactor can suffer meltdown anyway in the case of a LOCA? This is (if I'm not mistaken) what happened in TMI. What is the point of stressing this difference between the designs of these two USA reactors and the Sovietic one, when in the end they can all blow up anyway? This doesn't look like "stability" to me. -- NIC1138 (talk) 17:22, 22 April 2008 (UTC)

Notice how Chernobyl exploded and TMI just melted down? There is a huge difference there and that is risk to the public. Sure, the core melted at TMI, but almost everything that could have gone wrong did go wrong and nobody outside of containment received anything more than an x-ray's worth of dose. The negative void coefficient doesn't necessarily mean stability, it means passive safety. Basically if all controls to the reactor are lost it will shut it self down. It is impossible to compare a melt down to what happened at Chernobyl. —Preceding unsigned comment added by 74.192.22.246 (talk) 20:10, 2 September 2008 (UTC)

• Some of the credit here must go to Edward Teller, who long suspected that positive void coefficients in a nuclear reactor were a Bad Thing from an engineering perspective and spent many years working behind the scenes making sure no such design was licensed for construction within the US. As you note, Chernobyl tragically proved that he was correct. —Preceding unsigned comment added by 69.41.40.24 (talk) 12:53, 13 March 2011 (UTC)

Scope

This Page's introduction and explanation sections are grossly verbose and talk about topics such as reactivity in depth.

This page should stay in scope by only taking directly about the coefficient and it's effects.

The following excerpt, for example, can be found on the reactivity page and is absolutely not needed to describe void coefficient

"Reactivity, in the nuclear engineering sense (not to be confused with chemical reactivity), measures the degree of change in neutron multiplication in a reactor core. Reactivity is directly related to the tendency of the reactor core to change power level: if reactivity is positive, the core power tends to increase; if it is negative, the core power tends to decrease; if it is zero, the core power tends to remain stable. The reactivity of the core may be adjusted by the reactor control system in order to obtain a desired power level change (or to keep the same power level). It can be compared to the reaction of an automobile as conditions around it change (for instance, wind intensity and direction or road slope), and therefore the corresponding counter-measure that the driver applies to maintain road speed or execute a desired maneuver." Angelmarauder (talk) 06:49, 5 February 2020 (UTC)