# Talk:Neutron moderator

WikiProject Physics (Rated Start-class, Mid-importance)
This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
Start  This article has been rated as Start-Class on the project's quality scale.
Mid  This article has been rated as Mid-importance on the project's importance scale.

## Why are fast neutrons LESS likely to cause fission?

parrfin is waxx!!!!!! From the article:

"Faster neutrons are much less likely to cause further fission....The newly-released fast neutrons, moving at roughly 10% of the speed of light, must be slowed down...if they are to be likely to cause further fission in neighbouring uranium nuclei and hence continue the chain reaction."

That seems counterintuitive. A nonexpert like me would think that a faster-moving object (neutron, whatever) would be MORE likely to cause its target to break apart (i.e., fission) on impact. Can someone explain this? 24.6.66.193 18:08, 7 May 2007 (UTC)

It is counterintuitive, but only because we tend to assume that a faster-moving object has as much chance of hitting a given target as a slow-moving object, and therefore the only difference is the greater energy delivered by the faster object. This is certainly true if you're talking about, say, a gun pointing at a stationary target, and two types of projectiles (slow and fast).
Now, however, put yourself in the target's shoes and think about which is easier to catch - a baseball at 95 mph, or a baseball lobbed at slow speed. The slower ball is easier to catch basicallyl because it spends more time in the vicinity of your hand, giving your brain more time to effect the correct muscle movements to catch the ball. (And even if the fast ball hits your hand it might just sail on through since it would hurt too much to hang on to it.)
Similarly, a slow neutron that spends more time in the vicinity of a nucleus is more likely to be caught by that nucleus. If it can do this and still have enough energy to induce the nucleus to fission, that's a win-win situation. This is the case with U-235. Whitlock 23:01, 7 May 2007 (UTC)
For another nonexpert like me i would also think that fast neutrons are more likely to cause reactions but would i be correct in comparing it to reaching the escape velocity for a planet? The faster you are the easier you can break away from a gravitational body but in the case of a fission reaction you want to be able to get close? But another thing that I don't get is that under 'layer cake' configurations hydrogen bombs can even split uranium-235 with super fast neutrons? Isn't that the antithesis of it? 203.218.26.155 (talk) 14:49, 4 September 2008 (UTC)
Yes, if you were trying to get caught in the gravitational field of a planet you'd slow down in the planet's vicinity, just like a neutron is slowed down in the vicinity of a nucleus that we want to capture it (except that for the neutron-nucleus interaction the Strong Nuclear Force (SNF) is involved, instead of gravity). A fission chain reaction in U235 (or Pu239) can be sustained with fast neutrons as well, and this is the principal of a fast-neutron reactor as well as nuclear weapons. It's not as efficient because the probability of fission is much smaller, but other factors benefit: (1) for the fast-neutron reactor the higher neutron energy leads to more neutrons being released per fission event, which leads to higher neutron economy and the possibility of breeding or waste (actinide) burning (note: this is also possible, to a lesser degree, in heavy-water-moderated reactors like CANDU, also because of high neutron economy); (2) for a nuclear weapon the goal isn't efficient use of fissile material, but getting as many fission events to occur in as short a time as possible -- which means no time for neutron moderation, and making use of almost pure U-235 (HEU). Whitlock (talk) 19:48, 4 September 2008 (UTC)
To my understanding, Enrico Fermi found that slower neutrons were far more effective in causing fission, because what happens is that the neucleus "catches" a nuetron that gets added to its neucleus. This stretches out the neucleus which weakens the force that keeps the neucleus together. Remember that protons repel each other due to sharing positive charges, but neucleur forces are greater than the forces that repel them. Now given an already unstable neucleus, by adding the additional nuetron weakens the neuclear forces enough that the forces that repel protons becomes greater and causes the breakdown of the nucleus. This is called fission. —Preceding unsigned comment added by 76.89.132.250 (talk) 11:15, 27 February 2008 (UTC)

## Carbon compounds

I have just added the carbon dioxide [1] mention under the general wording "carbon compounds" because of the earlier mention of hydrocarbons and because it struck me that others, e.g. carbon tetrafluoride, might be realistic. That is, I can't rule them out, so I chose a wording that covers everything. However, this should really be firmed up, and more detail should be given about how carbon dioxide is used - I suspect in liquid or supercritical form for the density, in something like a CANDU arrangement or even an analogue of an Aqueous homogeneous reactor, but I don't know. I also suspect it would have to operate at lower, less efficient temperatures for pressure reasons. Even so, it might be enough cheaper than heavy water to be justified in research reactors or as a bridging technology to kickstart breeding a starter stock of fissile material for later thorium breeder reactors. All this is valuable information which must be out there somewhere, so I hope someone who knows can follow this up and flesh it out. PMLawrence (talk) 12:33, 17 October 2008 (UTC)

## Clarification of the lede

The lede paragraph currently states:

In nuclear engineering, a neutron moderator is a medium which reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235.

Is this correct? If the purpose of the moderator is to reduce the speed of fast neutrons, wouldn't the intent be to render the material incapable of sustaining a nuclear chain reaction? N2e (talk) 13:57, 23 October 2009 (UTC)

Why don't you look at the graph of fission cross section which clearly shows that fission probability is highest for slow neutrons. Fast neutrons have too much energy to be 'captured' by a nucleus to cause a reaction. Kbrose (talk) 16:20, 23 October 2009 (UTC)

More specifically, at low neutron energy the fission cross section for U-235 rises more than the capture cross section for U-238 does, allowing lower-enriched uranium to go critical. Without absorption by U-238 even a small mass of U-235 can go critical even with fast neutrons, as in a nuclear weapon. --JWB (talk) 17:56, 23 October 2009 (UTC)

## Erroneous/unclear graph of fission cross section in reactor moderators section

According to the graph a 1keV neutron has a Fission cross section of about 6 barns and a 1MeV neutron about 1 barn. But, on the Uranium-235 page the second paragraph contains the statement

'Its [uranium's] nuclear cross section for slow thermal neutrons is about 1000 barns. For fast neutrons it is on the order of 1 barn.'

So either the y scale on the graph is supposed to be logarithmic (which it isn't at the moment) or there is some other problem with either.Pondermotive (talk) 23:26, 30 April 2011 (UTC)

Actually I think I am wrong here, 1KeV is still much hotter than thermal temperatures, which are a about 1/10-1/100th of an eV for a neutron I think. The graph would be more useful if it extended to thermal temperatures as it would convey more information, perhaps marking on the range of common energies from neutrons emitted from a fission event and thermal energies for neutrons to give more intuitive reference points. —Preceding unsigned comment added by Pondermotive (talkcontribs) 11:21, 1 May 2011 (UTC)

## Incorrect Math

There's a discrepancy between the top table and the first paragraph which lists, "regular (light) water (roughly 75% of the world's reactors), solid graphite (20% of reactors) and heavy water (5% of reactors)". The table says there are 29 heavy water reactors (8%) and 29 graphite reactors (8%). KenJackson.US (talk) 21:52, 25 November 2011 (UTC)

## Common

I think moderator is common for all nuclear fuels even in breeder reactor (thorium). But in this article they mentioned only U 235 in first para. Moderator is also used in U-233 also. see this en:Molten salt reactor.

In my view take that U-235 in first para may correct. Is it right?--தென்காசி சுப்பிரமணியன் (talk) 06:48, 1 January 2012 (UTC)

## Boltzmann constant

The Boltzmann constant (kB) should be identified in the first equation given in the "Moderation" section:

$E=\frac{1}{2}mv^2=\frac{3}{2}k_B T$ ($k_B$ is the Boltzmann constant)

or something like that.

While we're at it, the rest of the parameters should be named as well. It's ordinary good encyclopedic communication to describe the parameters in equations. It's surprising how many people wouldn't know what m or v or T is without labeling or good context. It's our duty to communicate, not to presume our own level of knowledge (nearly) on lay readers.

108.7.160.131 (talk) 16:36, 21 May 2013 (UTC)

### Misplaced locking

It's a little over reaction to lock a page for sock puppetry, I think. The guilty party is one person (and his socks), not the article. Injunctive action should be toward that person, not the article. And also, the editing by socks must be disruptive, or warring, etc.. If the editing by socks is good, the fact that it was done via socks is irrelevant. I say this because the single reason given for the lock is "persistent sock puppetry". In and of itself that ain't necessarily such a bad thing, disruption would need to go along with it to compel action. Disruption may well be implied by "persistent sock puppetry" which would make it a reasonable reason for action, but still the action should be toward the disruptor, not the article. 108.7.160.131 (talk) 16:36, 21 May 2013 (UTC)

## Formula for ξ

$\xi= \ln\frac{E_0}{E}$ and $n=\frac{1}{\xi}(\ln E_0-\ln E)$ imply $n=1$. Something is wrong there. --Chricho ∀ (talk) 01:06, 28 June 2013 (UTC)