Nuclear chain reaction

[xelasnave]

How do they start a nuclear chain reaction in a fission reactor?
I know we need a critical mass but what I dont know is where that first little particle comes from that splits the next and the next... do they have a gun to fire particles or is it a case that if you have a critical mass the reaction stars by itself.

AND... no I am not building one to replace the solar panels because it is wet...because it is wet I ask but it is along the line of idle curiosity.. I can not find how it starts.
alex:):):)

[leon]

Can't really help out here Alex, but it seems reasonable that it has to be started somehow, may it be a gun or other device, or even an extreme heat source . :shrug:

Leon :thumbsup:

[leon]

However Alex i did find this bit of info, it may be helpful.

Leon.

Fission chain reactions occur because of interactions between neutrons (http://en.wikipedia.org/wiki/Neutrons) and fissile (http://en.wikipedia.org/wiki/Fissile) isotopes (such as 235U). The chain reaction requires both the release of neutrons from fissile isotopes undergoing nuclear fission (http://en.wikipedia.org/wiki/Nuclear_fission) and the subsequent absorption of some of these neutrons in fissile isotopes. When an atom undergoes nuclear fission, a few neutrons (the exact number depends on several factors) are ejected from the reaction. These free neutrons will then interact with the surrounding medium, and if more fissile fuel is present, some may be absorbed and cause more fissions. Thus, the cycle repeats to give a reaction that is self-sustaining.
Nuclear power plants (http://en.wikipedia.org/wiki/Nuclear_power_plants) operate by precisely controlling the rate at which nuclear reactions occur, and that control is maintained through the use of several redundant layers of safety measures. Moreover, the materials in a nuclear reactor core and the uranium enrichment level make a nuclear explosion impossible, even if all safety measures failed. On the other hand, nuclear weapons (http://en.wikipedia.org/wiki/Nuclear_weapons) are specifically engineered to produce a reaction that is so fast and intense it cannot be controlled after it has started. When properly designed, this uncontrolled reaction can lead to an explosive energy release.

[DJDD]

perhaps teh reaction si initiated by the proximity of a radioactive isotope of uranium ? :shrug:

[DJDD]

actually, what i said was silly.
nuclear chain reactions are initiated by themselves.
remember the uranium is radioactive.

[xelasnave]

Thank you Leon for your help and the info.
alex

[xelasnave]

So you think it is critical mass... with no "gun" or whatever.
I have been reading and although I have covered a lot of ground I cant see it specifically stated but that is the feeling I have... but they have these diagrams with a little particle and I wondered where it came from originally.
Thanks for your thoughts.

alex:):):)

[DJDD]

exactly.
:)

[xelasnave]

Entia non sunt multiplicanda praeter necessitatem
alex

[DJDD]

are you sayign my post was unnecessary (sorry if I offended) or that my explanation was the simplest of those on offer?
:shrug:

or am I missing a joke? :doh:

[xelasnave]

I am so sorry... I was cutting and pasting and I pasted the wrong thing..that was something from an article on the man who offered tha razor to the throath of a complex idea..or theory.

The joke is on me and I cant find what I wanted to post..or remember it actually...but it was the simple answer is often the best.

alex

[xelasnave]

AND I can not be offended I never take things personal... comes from chosing to live alone I guess.

alex

[xelasnave]

AND I cant translate it by the way ...
but back to critical mass...
Can a block of lead have a critical mass I wonder...I have a reason for wondering.
In other words if ou had a block of lead big enough will we get a chain reaction... and lets place the lead in a high gravity environment.. building a new universe here and need that bit.

alex

[KenGee]

Alex a reactor doesn't use a critical mass, that in fact is called a bomb. A reactor uses control rods to keep the "pile" below this value, to have nice a "controlled" reaction. The reaction happens spontaneously, because it is radioactive, that's what radioactive things do decay. The nuclear decay gives off heat this heat is what is used to generate power.

[xelasnave]

Thank you Kenny:thumbsup:...
I am just fitting nuclear power into the push universe:lol::lol::lol:
alex:):):)

[g__day]

You take a bowling ball size lump of highly enriched Uranium and Plutonium. This has to be machined to be almost perfectly spherical and of constant density - you'll see why later. It has a hollow clyinder that is used to hold tritium rods that are fired into the ball just before just before implosion and neutron bombardment.

You send the tritium rods into the ball then implode this combined entity with a shaped charge (say surround the ball with an inch and a half of a two layer (slow than fast burn) C4 derivatives (miltary grade explosive with an extremely high burn rate - say 7,000 metres per second on the outer and 10,000 metres per second on the inner - with tungsten-rhenium drivers and on the otherside of this charge facing the ball is a 1cm thickenss of Berllyium - touch a 1mm thickness of U-235 surround a 10 kg mass og plutonium 239) that is ignited by 240 special denotators linked to krypton switches on precisely measured lengths (typically 1 metre) of cables to your denotator box containing high voltage capacitors linked to a timer. Your charges have to go off within a nanosecond (called a shake) of each others to crush the ball into an almost perfectly round much smaller ball. You want the bowling ball crushed to the size of a tennis ball. If the shaped charges aren't perfect your bang bang is goofed.

So the key challenge is mechanical - you need to refine P-239 (and avoid unstable P-240 which is likely to go off prematurely) crush it with a million atomspheres of pressure with a precisely shaped spherical wave that must be re-shaped to a planar wave implosion

About 5 shakes after your implosion you shoot high energy neutrons (from a zipper - a small cyclotron through a lithium-deuteride disk the size of your palm about 8 mm thick into the crushed ball though the Beryllium. This means you slow neutron source (travelling about 10% light speed hit the compressing P-239 core (already 10 times denser than lead and imploding) in trillions of places at once and fall under the Strong Nuclear force of the P-239 and get captured - turning it into unstable P-240) - bingo Nuclear reaction. The P-240 decays into two simpler atoms - but annihilating either a proton of a neutron and releasing 3 neutrons in the process - that will further incite P-239 or P-240 - runaway chain reaction follows at a geometric rate.

Crushing the ball into a smaller size raises its density above critical mass (which really should be critical density). The tritium explosion is racing in and out at 7,000 feet a second - but reaching chaing reaction takes less than 200 shakes - so the pressure wave of the shaped charge travels less than 2 feet before fission (run away chain reaction) has intiiated.

About 10% of your mass will be annihilated so a 10 kg ball releases about 1 * (3 * 10 ^ 8) ^ 2 newtons of energy.

Now if that ball if surrounded by a compressed source of hydrogen the fission reaction releases enough energy as heat (bewteen 10 - 100 million degrees celcius) to trigger a fusion reaction so you've gone from an atomic weapon (fission bomb) to a hydrogen bomb (fusion weapon) that releases between 10 - 1000 times more energy.

Lead doesn't fuse - it takes more energy than it releases - you need a really massive, and atomically unstable atom what wants to radioactive decay - to trigger fission. All you are doing is suping up the decay rate a trillion times or so!

(Source: Surprisingly accurate - Tom Clancy - The sum of all fears - pg 793 - 798) :)

[AGarvin]

In a nuclear reactor, the control rods are of a highly neutron absorbing element. When the reactor is shutdown, the rods are fully down in the reactor core essentially capturing the free neutrons. As they're raised, neutrons from the radioactive fuel begin to react with the fuel itself. By react I mean a fuel atom captures a free neutron and undergoes fission, releasing energy. Each atom that undergoes fission produces more free neutrons, so the reaction increases.

Reactors also use what are called moderators. A moderator is an element that slows down the neutron (a slow neutron is easier captured than a fast one). The neutron "bounces" off a moderator atom and imparts (loses) energy.

The thing to remember is that elements below iron on the periodic table release energy during fusion, but need energy to split (ie fission). Hydrogen releases the most energy, helium the next ..... upto iron. This is why main sequence stars sit fusing hydrogen into helium in their core, but once the hydrogen begins to run out, the core begins to collapse until it reaches a hot enough temp to fuse the helium ..... massive stars follow this down to iron cores ... no more fusion ... supernova. Smaller stars only fuse down to helium and carbon and leave a white dwarf behind, which is the "dead" stellar core.

Once you get past iron, it's fission that produces energy, with the far end of the periodic table near uranium producing the most energy.

Andrew.

[xelasnave]

Thanks for all that Matt and Andrew sure is interesting stuff.

alex

[maksutover]

The OPAL reactor in Lucas Heights initiates its reaction via a neutron source or gun. The fuel elements themselves which I think are 20% enriched dont spontaneously produce a chain reaction, as mentioned before this is a fission bomb.
In OPAL, the reactor is shut down every month or so for maintenance and fuel movement but the neutron gun will not be needed to restart the reaction due to secondary neutrons being present from the previous run which will suffice to initiate the reaction again.

[GeoffW1]

ASIO is watching you all :camera:

[Miaplacidus]

That first atom you ask about, Alex, is actually an interesting conundrum. The common explanation is really no explanation, logically speaking, namely that all radioactive material has a half-life, and that it is simply a question of probability: i.e. that within a given time frame a particular fraction of atoms will spontaneously split/disintegrate/decay. Logically, what we are describing here is an apparently causeless event (that is, if logic admits of the possibility of causeless events).
Of course, people unhappy with that description will perhaps argue that the first atomic splitting is triggered from some exogenous particle colliding with your radioactive material, but of course that remains pretty unprovable. Personally, I am happy to accept that there are causeless events in this universe, but some individuals find the proposition difficult to accept philosophically.

[GrahamL]

Anyone read the book on chenobal.. when all the alarms have been going for a while a tech guy slides down to to the reactor roof to find all the plugs ( hugely large blocks of concrete) all going up /down/down/up

I think he went with run.. :)

[AlexN]

Entia non sunt multiplicanda praeter necessitatem - entities must not be multiplied beyond necessity (roughly translated)

You were pointing out Ockhams Razor, This is otherwise paraphrased as "all other things being equal, the simplest answer is the best."

Nuclear reactions, as has already been mentioned, do not require a catalyst, as the very nature of any radioactive atom is to decay, and this decay leads to reactions in other atoms etc. etc. etc. Nuclear power plants control power and heat levels by raising and lowering the uranium rods into liquid vats... when raised, the uranium decays, and radiates heat in the process, generating power... when the rods get to a specific temperature, they are slowly lowered back into the liquid vats for cooling, and the process repeats....

Its pretty amazing stuff.. I was watching a doco on Discovery the other day, on the USS Ronald Regan (largest aircraft carrier in the US navy) and with its nuclear reactor, its able to run for 29 years straight without ever coming to port... 29 years... Brilliant technology in some hands, Terrifying in others...

[xelasnave]

Thanks for your input:thumbsup:.
I cannot accept a causeless event is causeless.

... but I can accept that there can be a cause which has not been observed and leaving an incorrect (presumably) impression of reality.

From the little reading I have done on the subject I did form an impression that at critical mass and with the material jacketed in graphite the reaction would start without any "starter"... Of course my morosophic view upon other matters would support that it may work "all by itself"...but this approach would suggest one should be able to have a large body of iron "heat up" if in a graphite jacket..I dont know how big each need be or the change in temp..if indeed there was a cahnge in temp...

I said elsewhere in general chat I suspect the heating up is as a result of the jackected material interupting the universal flow I speculate upon///anyways just thinking about unsupportable ideas more appropriate in general chat.

alex:):):)

[xelasnave]

That is interesting..you would think you either need it or not:shrug:..well I would think that:D:whistle:...thanks for the input on that:thumbsup:.

alex:):):)

[Miaplacidus]

The fundamental causelessness of radioactive decay is usually disguised (amongst much else) under seemingly scientific rubrics such as "quantum indeterminancy". Apparently it is not merely technically impossible to determine precisely the position and velocity of a subatomic particle, but rather something intrinsic to the fabric of our universe which makes it undoable. I.e. it isn't that these two bits of information aren't simultaneously measurable, but that in a very real sense both bits of information cannot exist in the one universe. Similarly, it is only our lamentably rational supra-atomic brains that prevent us from discerning the bleedingly obvious truth that Schrodinger's Cat is, as it were, simultaneously alive and dead up until that moment when somebody thoughtlessly opens the box. (I simplify somewhat.)
And no, I am not drunk. Good luck building the reactor, Alex. Invite me over when she goes critical and we'll have a barbie.
Cheers,
Brian.

[xelasnave]

I have seen piles of compost go critical so maybe there is hope;).
The cat that poor cat... I find it funny that the exception, as uncertainty seemed to start its life, has indeed become the rule... I read it comes from a proposition that at one point a particle can not be measured and at another that it can but such transforms math wise to it not being there and then being there... and that proposition was necessary to entertain the inflation theory... I think someplace else uncertainty refered to a probelm determining electron orbits...
So in princilple I am somewhat uncertain what the uncertainty principle says:whistle:.

IF iron has a certain radiactive decay then the sums must tell us if there is anything in my view... unfortunately I have not worked out the math to use yet....because I have only recently formed an interest in this area... but it will or it wont... I bet it will however...just a hunch as they say:D.

alex:):):)

[AlexN]

uncertainty is seemingly constant throughout anything...

Uncertainty would create problems determining electron orbits, due to not know the electrons position or velocity with any degree of accuracy at the direct moment of observation.. the closer you get to knowing either, the further you get from knowing the other.. to know an electrons orbit you would have to understand where the electron is and how fast it is going to calculate its orbit around the nucleus... Or at least, thats how I understand it..

You'll have to excuse my responses if they are a off, Im still learning about these things but trying hard to get my head around the idea of quantum mechanics.

[xelasnave]

Sorry Alex I missed this before.
I have been somewhat anti NP but I have developed an interest in it for interest sake so thanks for your input every bits helps.

alex

[xelasnave]

Hey I know zip about any of this I only repeat stuff I read or at least my impression of what I have read:)... for the most part it is so beyond me I dont know why I bother... but in time I learn something and move a little ways forwards.

alex:):)

[Galactic G]

All this talk about Nuclear chain reaction reminds me of the true story about David Hahn, the Radioactive Boy Scout.

If you've never read the story it's definitely worth the short read:- http://www.abc.net.au/science/articles/2004/07/15/1153968.htm?site=science/greatmomentsinscience&topic=latest

To start with David built himself a neutron gun from Americium-241 he found in Smoke Detectors. But alas, it didn't produce enough neutrons to start the reaction....so he went for plan B.

I won't spoil the rest of the story for you, but every time I read it it brings a smile to my face. (But not in an evil Dr No type way, more like that makes my carrot top experiment look lame.)

"Where's my shed gone, son?" I can only imagine the look on his parents face.

Now...bringing it back to Astronomy Science, I'm struggling to work out what is happening on Jupiter. It's a gaseous planet, approximately 90% hydrogen, but produces more radioactivity than any other planet in the solar system. How can that be?

As I understand it the Pioneer Probes discovered the amazing amount of radioactivity, and the Voyager Probes had to be reconfigured/redesigned to take it into account.

[xelasnave]

mmmm I did have a chemistry set ,,,

As to Jupiter what explainations are offerred ..is there nothing there that is satisfactory>

alex

[renormalised]

It's not so much radioactivity as it's trapped radiation in the planet's magnetic field, plus radiation being generated by the interaction between Jupiter and Io. In the same way that the Earth's field traps high energy particles emitted from the Sun, so Jupiter's much larger field does the same. Plus, sodium and sulphur particles erupted by Io's volcanoes gets trapped in the giant planet's magnetic field and they generate vast electrical currents which accelerate the particles in the field to high speed. This, then generates extra radiation (radio waves, x-rays etc) via particle collisions and such.

However, the core of the planet contains around 10 earth masses of rock and metal, so there will be an amount of radioactive minerals present in the core. That will contribute to the heating of the interior of the planet, though most of Jupiter's internal heat is likely derived from its formation, in the form of heat generated by the gravitational contraction of the planet.

[Wavytone]

To go critical the earliest reactors were based on a design in which enough fuel was packed in a given volume to exceed the threshold at which it will go critical, ie if unchecked a chain reaction would start spontaneously the moment the fuel exceed the threshold. The fuel was a static "pile" of material. To stop it literally melting while being assembled, a moderator medium was added which absorbs or slows neutrons to the point they cannot cause a reaction - depending on the type of reactor the moderating material included heavy water and/or rods of boron

There are two ways to increase the reaction rate - either:

(a) push more fuel into the heart of the reactor
(b) remove some of the moderating material.

This causes the reaction rate to increase and if unchecked it will melt the fuel, possibly igniting it, and possibly parts of the reactor vessel holding it. To stop that, either

(a) the fuel rods are withdrawn, or
(b) moderating rods are pushed in.

For these reasons fuel in modern reactors is packed as rods than can be pushed in or out, and the moderator includes a series of rods that can also be moved in or out.

For safety the reactor designs are supposed to include included a fail-safe arrangement such that if it overheated, moderator rods would fall into the reactor (using gravity) and some of the fuel drops out the bottom. If the fuel rods ignite the result is an assortment of highly radioactive powdery oxides (from the various isotopes present) - hence the need to encapsulate the Chernobyl reactor in a concrete sarcophagus to stop the stuff escaping into the environment. Evidently the russian design wasn't fail-safe.

[xelasnave]

I have asked and wondered if one could assemble a "critical mass" of say iron... any views why this would or would not work.

alex

[renormalised]

Wouldn't work....iron is stable (at least the most common isotopes are). Even if you had enough radioactive iron (Fe60) to do anything with, you'd have to pump that much energy into getting the iron to fuse or fission that it would be an exercise in futility trying to get it to go critical. You'd need the equivalent of the conditions in the core of a supergiant to do it. We don't have that kind of technology, yet. Fe60 normally decays via beta particle (an electron) emission into Co60 (cobalt) after about a half life of 1.5 million years.

The two elements that lie at the bottom of the nuclear binding energy curve, Fe58 and Ni62 (respectively) are the most stable of all elements. The most common isotope of iron, Fe56 (the iron you or I see everyday), forms from the decay of Ni56 formed in the cores of supergiant stars. Ni56 decays in about 6 days or so, into Co56 (cobalt), then into iron.

[xelasnave]

Thanks for that
alex