nuclear fission

[RossGr]

"Why does a neutron make a nucleus unstable?"

Each element has a optimal number of neutrons and protons any isotopes are inherently unstable, they want to lose the extra neutrons in order to gain the most stable configuration. This has to do with minimum energy levels.

"Why do we use neutrons?"

Really this is a pretty silly question (sorry) When an unstable isotope decays, it does so by ejecting neutrons, So we have NO CHOISE, We did not CHOOSE to use neutrons. They are the particles which are the products of neuclear decay. In the fission process an enerjectic neutron penetrates the nuclus of U 238 casusing it to become unstable, because of the instability it breaks apart forming 2 daughter atoms (I don't recall which elements) plus enerjectic neutrons and energy in the form of gamma rays and prehaps other photons. The enerjetic neutrons are now free to destabilize other atoms of U238, do the reaction runs. We did not choose to use neutrons, they drive the reaction, this is simply the way it works.

Why does it work this way? I have no clue, (nor does any Physicist) it simply does, Physics cannot answer why, only how.

 

[f95toli]

This is only partly true. I agree that one reason for using neutrons is that neutrons are that neutrons are emitted when U238 breaks.
However, this is only one of many possible reactions when a core breaks. In other elemens an alpha-particle (He-hucleaus), a beta (an electron), a positron, a neutrino or even some heavier particles (such as a Li-nucleus) might be emitted. U238 and plutonium are the only elements that are used for bombs and reactors simply because they emit neutrons whith an energy that can sustain a chain-reaction.
An element such as Americanum (you might have some at home in your smoke-detector) emitts alpha particles when it decays, hence it can not create a chain reaction.

So in a way we did choose to use neutrons, simply by choosing which elements to work with.

 

[silverpig]

Originally posted by: bwanaaa
AHA! thank you f59toli. An interesting problem that parallels the present difficulty in maintaining a sustainable fusion rxn. But that begs the question of why a neutron add to a nucleus makes it unstable? There are for example stable isotopes that result from neutron bombardment. Was it dum luck that neutrons thrown into U238 result in fission? Probably not, the observation of radioactive decay had been known for a while and certainly catalogued by the various elements. The use of neutrons was an educated guess? or was it because it was the only other ingredient in their kitchen? After all, why not try injecting neutrons into a plasma core? might that not facilitate (or 'catalyze' to use a chemical term) fusion. Would an intermediate inherently unstable compound be more likely to progress to fusion than trying to squeeze 2 hydrogen atoms together? Think of it as lowering the activation energy. That might make the present day "magnetic bottle" approach to fusion not only more easily attainable, buut also more controllable.

As for the2700 tests done so far, I wonder about the effect of of the long lived by products. Would it not be ironic that now, as we approach the dawn of the stem cell era (stem cell age?) where we can renew our body parts indefinitely, that the only impediment might be the toxic soup we live in?

Muon catalysed fusion is being explored. Basically the atom is given a muon. A muon is a negatively charged particle with a mass greater than that of an electron, but much less than that of a nucleon (proton or neutron). The muon orbits the nucleus inside the electron shells and thus "hides" the charge of the target nucleus from the approaching nucleus. It allows the approaching nucleus to get much closer to the target nucleus before electrostatic repulsion becomes a huge factor and makes it more likely that the nuclei will fuse.

Unfortuneatly, it is difficult to create enough muons and put them all in orbits around nuclei long enough to sustain a power producing fusion reaction.

 

[Wintermute76]

Originally posted by: bwanaaa
Would an intermediate inherently unstable compound be more likely to progress to fusion than trying to squeeze 2 hydrogen atoms together? Think of it as lowering the activation energy. That might make the present day "magnetic bottle" approach to fusion not only more easily attainable, buut also more controllable.

Hydrogen requires the least amount of energy to undergo fusion. As you progress up the periodic table, you need to input more energy to acheive fusion. Stars get up to iron before energy output falls off and they burn out.

Stars and fusion

 

[topjetboy]

I imagine that hydrogen is the easiest to fuse because you are dealing with only one proton in the nucleus. Try squeezing two helium nuclei together and your dealing with 2 protons and 2 neutrons.. Probably requires double the pressure. In terms of muon catalyzed fusion, I thought the toroidal magnetic bottle is used to contain a plasma. Wouldnt the free floating electrons serve the same purpose? However, they are probably too low energy. Goose their energy with a vandeGraf . Maybe a lightning bolt shot into the plasma would hide the proton charge long enough to permit a couple of nuclei to fuse?

However, I was thinking of an electrically neutral unstable intermediate. For example, neutron bombardment creates unstable (and often unnameable and very short lived) intermediates. Isnt that why deuterium and tritium are used anyway?

 

[silverpig]

Originally posted by: topjetboy
I imagine that hydrogen is the easiest to fuse because you are dealing with only one proton in the nucleus. Try squeezing two helium nuclei together and your dealing with 2 protons and 2 neutrons.. Probably requires double the pressure. In terms of muon catalyzed fusion, I thought the toroidal magnetic bottle is used to contain a plasma. Wouldnt the free floating electrons serve the same purpose? However, they are probably too low energy. Goose their energy with a vandeGraf . Maybe a lightning bolt shot into the plasma would hide the proton charge long enough to permit a couple of nuclei to fuse?

However, I was thinking of an electrically neutral unstable intermediate. For example, neutron bombardment creates unstable (and often unnameable and very short lived) intermediates. Isnt that why deuterium and tritium are used anyway?

Muon catalyzation is a neutral unstable intermediate. Replace an electron with a muon and you have some sort of odd atom...

 

[KenGr]

Originally posted by: SinfulWeeper

Originally posted by: KenGr

Originally posted by: SinfulWeeper
I dunno know who came up with the idea of how to build the bomb. What I do know is this, when all the atoms split. The entire chain reaction is done in less than 20 billionths (or was is trillionths?) of a second. Done... nada... no more...
The explosion you see is the 'waste' of that fraction of a second.

The person who figures out to keep that waste contained and stored will be shot and killed by the oil companies because of the threat it poses to them. Think of that one for a while.

Fission is fission, whether it's a reactor or a bomb. The difference in geometry just means that the reactor, in the worst case, can slightly supercritical (this is how power is increased - with a small supercritical transient) while the bomb can go massively supercritical. If the fissile material is the same, the reaction is the same.

Actually, the concept of using a "bomb" for power production was fairly well developed some years ago. Essentially you would develop a large underground compartment and set off, at regular intervals, small bomb blasts. This would create high pressure and temperature which could be tapped at a steady rate for power production. There's no reason it can't work but it holds no real economic advantage over power reactors. Also, it's completely unacceptable from a proliferation viewpoint, so don't expect to see this one proposed.

Yes, blasting a bomb has been in use for power production for a long time.

However you missed my point. The 'waste' or explosion is the byproduct of wasted energy. The most common type waste is in the form of your every day autombile engine. An explosion is controlled in the cylinder and contained in there.
What I was suggesting on why a person would be assainated by the oil companies is really simple. Either it be nuclear or conventional, would be to make a form of power production that exceeds 75 percent efficency and done cheaply. When the atoms split or join in nuclear weapons, and let off that energy. It is so overwhelming that current techology can not let it be contained and harnessed in its pure form. Sure there are nuclear power plants. But that is a prime example of inefficiency. It does not use the power from the 'core', instead it makes steam to turn a turbine.

Well, bombs for power is an old concept but it hasn't been in use.

I think the real point being missed is the concept of "wasted energy". Whether the energy (heat, pressure) results from a reactor or explosion or burning fuel, it must be converted to electricity. When a Uranium or Plutonium atom splits in a reactor it releases exactly the same energy it releases in a bomb. As you learn in Thermodynamics 101, processes based on existing cycles like Carnot, Rankine, etc. are limited in efficiency by the temperatures at which they operate. These same processes would be used in a bomb based system unless you could develop a solid state heat to electricity converter. Actually these exist (think thermocouple or radioactive decay satellite power generation) but the efficiency is so low that there is no practical large scale application. If and when these devices are developed, the big energy companies would be thrilled since this would encourage the use of fossil fuel generators as much as nuclear - maybe more so. Also, 75% efficiency is not that much of a breakthrough. Gas combined cycle plants exceed 60% and advanced high temperature nuclear plants approach 50%. Due to thermodynamic cycle limits, further increases will depend on material advances such as ceramic based heat transfer materials which rules out the "done cheaply" part of the equation.

 

[Sahakiel]

I also would agree that Einstein had little direct involvement in the Manhattan Project. His greatest contribution was signing his name to someone else's letter and lending his reputation to convince Roosevelt to back the scheme. Even he had no idea of how to use his famous equation to do anything.

As for who came up with the idea of nuclear fission, that seems to be under debate and will probably never be resolved. On the other hand, self-sustainable fission reactions is probably attributed to Fermi. The first sustainable reaction occured in the early 1940's in Chicago. Luckily, it only just barely contained enough neutrons to sustain itself by a rather slim margin, ran out of fuel pretty quickly and died down, otherwise Chicago would've been the site of the first nuclear bombardment. Who knows, maybe a few more cities as well before someone realized data and test procedure should be recorded offsite, preferrably a few states over, before testing bombs in the lab.

From what I remember, U-238 is too stable for bombs or reactors. It emits neutrons at a steady rate while decaying. If U-238, which pretty much accounts for the entire supply of Uranium in the world, were capable of "nuclear cascade", the entire Uranium supply in the crust would've wreaked havoc ages ago. U-235, which accounts for about 0.7% of Uranium deposits, is quite capable of sustaining a chain reaction by itself. Naturally occuring Uranium needs to be refined quite a bit to increase the concentration of U-235 for weapons grade, which is something like 80% or so. On the other hand, U-238 can also be refined to produce Pu-239. I think Pu-239 may occur naturally, but in such small quantities the amount is dwarfed by U-234.

Side note: I heard a rumor the shotgun technique was considered so reliable the design was never tested before field use.

 

[capybara]

yes, PL239 occurs naturrally mixed in with the u238, and can be centrifuged out.
this is why iraq's possession of a gas centrifuge caused so much alarm.
....ps: besides the shotgun, another approach that worrked was a lens
that focused energy from a conventional explosion onto the 239.

 

[silverpig]

Originally posted by: Sahakiel
I also would agree that Einstein had little direct involvement in the Manhattan Project. His greatest contribution was signing his name to someone else's letter and lending his reputation to convince Roosevelt to back the scheme. Even he had no idea of how to use his famous equation to do anything.

As for who came up with the idea of nuclear fission, that seems to be under debate and will probably never be resolved. On the other hand, self-sustainable fission reactions is probably attributed to Fermi. The first sustainable reaction occured in the early 1940's in Chicago. Luckily, it only just barely contained enough neutrons to sustain itself by a rather slim margin, ran out of fuel pretty quickly and died down, otherwise Chicago would've been the site of the first nuclear bombardment. Who knows, maybe a few more cities as well before someone realized data and test procedure should be recorded offsite, preferrably a few states over, before testing bombs in the lab.

From what I remember, U-238 is too stable for bombs or reactors. It emits neutrons at a steady rate while decaying. If U-238, which pretty much accounts for the entire supply of Uranium in the world, were capable of "nuclear cascade", the entire Uranium supply in the crust would've wreaked havoc ages ago. U-235, which accounts for about 0.7% of Uranium deposits, is quite capable of sustaining a chain reaction by itself. Naturally occuring Uranium needs to be refined quite a bit to increase the concentration of U-235 for weapons grade, which is something like 80% or so. On the other hand, U-238 can also be refined to produce Pu-239. I think Pu-239 may occur naturally, but in such small quantities the amount is dwarfed by U-234.

Side note: I heard a rumor the shotgun technique was considered so reliable the design was never tested before field use.

U-238 is used in reactors all over the place. They need heavy water, D2O, and are mainly in use here in Canada. We also sell the reactors all over the world.

 

[KenGr]

Originally posted by: capybara
yes, PL239 occurs naturrally mixed in with the u238, and can be centrifuged out.
this is why iraq's possession of a gas centrifuge caused so much alarm.
....ps: besides the shotgun, another approach that worrked was a lens
that focused energy from a conventional explosion onto the 239.

TIME OUT

We've gone seriously off track here in the last few posts. Uranium 238 is NOT fissionable. For very complex reasons, even numbered isotopes (U-236,U238, PL240, etc.) while certain odd numbered isotopes are fissionable (U-233, U-235, PL239 plus a thorium isotope which escapes me at this moment). When U238 absorbs a neutron, it becomes PL-239. This is the breeder reactor reaction to create more fuel. A previous post suggested that CANDU heavy water reactors could use U-238 as fuel because of the heavy water moderator. Not true. The heavy water is a more efficient moderator allowing use of fuel with very low enrichment of U-235. However, the fuel is still U-235.

PL239 does not occur naturally (except possibly in the past in some natural reactors in unique uranium deposits). Plutonium is created when Uranium-238 is placed in a high neutron flux (i.e. in a reactor). In weapons production reactors (like North Korea's) U-238 is put into the core for limited durations to produce PL-239 but limit the amount of PL-240 and other isotopes created. This allows the Plutonium to be separated from the Uranium by chemical means and avoids the use of centrifuges. Centrifuges are required for isotope separation to create highly enriched U-235 from natural Uranium or to separate P-239 from spent light water reactor fuel where it will be seriously contaminated with other Plutonium isotopes. When you hear of a country procuring equipment to build centrifuges, that generally means they are working on a Uranium bomb, not a Plutonium weapon.