[voerioc]

Nuclear fission problem: Why isn't nuclear material (uranium 235) slowly consumed?

Another inconsistency we can notice is why the nuclear material inside the bomb isn't slowly (but quite fast in fact) consumed ? In a nuclear plant, they say the nuclear material is consumed in less than three years and then needs to be replaced by a new one. But, for atomic bombs, it seems that the same laws don't apply. You can keep it during thirty year without any problem of material depletion.

In fact, the nuclear material inside atomic bombs should be depleted way faster than the one from nuclear plants, since it is supposed to be more concentrated (I speak of the state of the bomb before the explosion of course). It should be consumed in less than two or one year.

It should be even worse than that, because after let's say a depletion of 30 %, the atomic material would be unusable as a bomb. It would be under the critical mass level. So, maybe that after only one year the bomb would be unusable.

[mooninquirer]

I think you are confusing a lot of things here. I'll look up the exact figure, but the half-life of uranium is in the hundreds of thousands of years, if not millions, and is very weakly radioactive. It is not going to fission on its own accord.
You are comparing that with the fissioning of the fuel in a power plant, and even THIS process will be halted with the withdrawal of the fuel rods, or with a meltdown in which the fuel no longer is assembled together with a moderator.

[voerioc]

I am not confusing anything. The nuclear material in the bombs is much more concentrated than the one of nuclear plants. This is why a nuclear plant can't explode like a nuclear bomb. Therefore, as there is a nuclear reaction in the plant, there should be a nuclear reaction in the bomb. And as the nuclear material in the plant is consumed in three years, the nuclear material in the bomb, which is more concentrated should be consumed even faster.

The reaction is dependent of the concentration of the nuclear material. If it isn't consumed fast in the ground, this is because it isn't concentrated enough. But when it is as much concentrated as it is in the plants, it is consumed quite quickly (three years). And as it is even more concentrated in atomic bombs, it should be consumed faster.

[rerevisionist]

The half-life of uranium is estimated at many million years (by calculating the amount of lead mixed with it) That's U238. They quote U235 as having a half-life 700M years - though I confess to being sceptical as to how that could be measured, because, if you have a chunk of it, there must be chain reactions, even if they peter out. (Probably U235 is considered to have a different decay path from U238). But the more U235, surely the more chain reactions there must be, even if they're insufficient to cause the whole thing to go up. That's the official view. So I think voerioc must be right.

[mooninquirer]

Are you talking about URANIUM ? You mentioned "in the ground." There actually IS a case of a naturally occurring nuclear reactor, were the natural uranium was depleted from its usual concentration of 0.7 % U 235 to a percentage less. This is because of the presence of a layering of ANOTHER material that acted as a MODERATOR, so that the fast neutrons released as a result of the fissioning of the U 235 would be slowed down, so they will be absorbed by other U 235 nuclei, and cause more fissioning. A moderator in a requirement in a nuclear reactor.

This naturally occurring nuclear reactor was in a uranium deposit in Gabon, Africa.

[FirstClassSkeptic]

The half-life of uranium is estimated at many million years (by calculating the amount of lead mixed with it) That's U238. They quote U235 as having a half-life 700M years

It seems to me to be a fundamental error to assume that the half life for geological deposits of uranium would be the same for concentrated uranium. Because with the uranium concentrated, there would be more chances for one decay to release neutrons and start another decay. Thus half life would decrease as the uranium was more concentrated.

[rerevisionist]

First class, yes that was what I was trying to say --
But the more U235, surely the more chain reactions there must be, even if they're insufficient to cause the whole thing to go up

[voerioc]

First class, yes that was what I was trying to say --
But the more U235, surely the more chain reactions there must be, even if they're insufficient to cause the whole thing to go up

Yes. It's the point.

There is only 0,7 % of uranium 235 in the rocks of uranium mines. Then, it is consumed after many millions years.

There is 3 % of uranium 235 in the material of nuclear plants. Then, it is consumed after 3 years.

And there is 80 % of uranium 235 in an atomic bomb. Then, it should be consumed after... what ? One month ? One week ? One day ? (in fact it should be much quicker than my previous supposition of one year).

But no, the atomic bomb can wait quietly during twenty years before being used without any problem of nuclear material depletion. I like the science behind nuclear bombs. It's like magic.

[mooninquirer]

Now, now, remember that U 235 can ONLY fission with SLOW neutrons, and yet the fissioning itself always yields fast neutrons. The fast neutrons have to pass through a moderator first, to slow them down, before they can be absorbed by other U 235 nuclei.

Read this article about the naturally occurring nuclear chain reaction, which happened to use water as a moderator :

http://www.en.wikipedia.org/wiki/Natura ... on_reactor


Also, there are DIFFERENT kinds of decay involved here. Aside from the the very special case described above which permitted a relatively quick self sustaining chain reaction, involving slow neutron absorption resulting in fissioning, the decay referred to when they speak of the half life of uranium in the millions of years is alpha particle decay, and very rare occasions of spontaneous fission, without the absorption of any neutron. The alpha decay is spontaneous, but it does NOT result in a chain reaction, because that alpha particle will have NO impact upon whether any other uranium nucleus is going to fission or undergo any decay. In fact, the alpha particle will NOT come into contact with any uranium nucleus, because like charges repel. An alpha particle is basically a helium nucleus --- two protons and two neutrons, so it has a net positive charge, and it will therefore be repelled by uranium nuclei.

In the very rare cases of spontaneous fission of U 235, there will be ONLY fast neutrons released, and this will NOT result in a chain reaction consuming the U 235 nuclei, because those fast neutrons would have to pass through a moderator to slow them down. So, without the moderator, the decay is many, many orders slower than in a reactor.

Assembling a mass of pure U 235 together will NOT result in its decaying any faster than naturally occurring uranium of
99.3 % U 238 and 0.7 % U 235, because it will not have a moderator to slow down the fast neutrons.

Here is the full, official citation of why Enrico Fermi won the Nobel Prize in Physics in 1938 : " For his demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons."

[voerioc]

Now, now, remember that U 235 can ONLY fission with SLOW neutrons, and yet the fissioning itself always yields fast neutrons. The fast neutrons have to pass through a moderator first, to slow them down, before they can be absorbed by other U 235 nuclei.

So, you are telling me that according to the own principles of orthodox nuclear science, those bombs wouldn't explode. Are you kidding me ?

[mooninquirer]

Yes, of course I am saying that nuke bombs would not explode. That is what this forum is about. And I say this based on an understanding of all of physics ---- classical, modern, kinetic theory, you name it. Nuclear bombs exploding are WW II psy-op and government propaganda. Science is something whose experiments can be reproduced. But independent laboratories are prevented from exploding nuclear bombs to test this out. So, we are left with the theory. And nuke bombs exploding does not fit in with the theory of nuclear fission, as it is applied to nuclear power plants, and experiments done on nuclear fission.

There are a few other reasons besides the slow neutron vs fast neutron problem, which might apply to only uranium. They cannot deny it, because of Enrico Fermi's Nobel citation, as well as his Nobel lecture, in which he discussed the need to slow down fast neutrons by passing them through a moderator. The cat was out of the bag for uranium. It is possible that the slow neutron problem applies to plutonium as well, but plutonium was developed AFTER the quest for a possible nuclear bomb was undertaken, so it is possible that with plutonium, the data has been altered. I have to think about this. It depends upon how much university laboratories have access to plutonium, because experiments can be done to verify whether fissioning will occur with fast neutrons.

But even if plutonium ( usually the isotope mentioned for power plants or nuke bombs is Pu 239 ), fissions with fast neutrons, there are still problems as to why it will not explode, or why it will not explode with a force anywhere near what is claimed. This problem is that people are applying chemical thinking to nuclear phenomena. In chemical explosions, the exothermic reaction releases heat, which further stimulates more reactions to occur. But in the case of nuclear fissions which release heat, this heat is totally irrelevant to other nuclear fissions. The only thing that is relevant is whether another nucleus absorbs a neutron, and this is made much more difficult by too much heat, which will melt the metal, and cause it to lose its essential spherical shape for a critical mass.

Now, as for the claim that each fission results in on average 2.5 neutrons being released, this is true, but those diagrams illustrating a nuclear chain reaction are VERY misleading. They have the nuclei the size of marbles, and lined up like bowling pins, so that you see the two neutrons from each fission hitting the NEXT nuclei ! But the fact is, that if the nuclei of U 235 were the size of a marble ( about a half inch ), then the whole U 235 atom, with its outer electron clouds, would be the size of Yankee stadium ! And the neutrons would be the size of mustard seeds. One of these things emitted is just NOT going to hit the "next nucleus" ----- it will have to travel very far, passing by millions of other nuclei before hitting another nucleus.
So, a chain reaction is a very SPARSE process.

There is another reason why the general public might accept that nuclear bombs explode ---- they see those diagrams of nuclei being split, and they see it flying apart. This is true, IN A VACUUM. In a mass of metal, the fission fragments will be trapped in the lattice of the metal. The fission fragments will acquire their own electron clouds from the electron cloud of the former fissile atom. Because of this, they will NOT go very far, and the atoms of the fission fragments will collide with the atoms of surrounding atoms. The electron clouds will collide. This will create a lot of vibration of the atoms surrounding the fission event. Now, HEAT, on the atomic scale, is exactly the kinetic energy of atoms. This can be the kinetic energy of linear motion, or of atoms vibrating back and forth. However, none of these other vibrating atoms is going to fission as a result of the neutrons released by that fissioning. All that we will have is heat. So, it is a very misleading diagram of a nuclear chain reaction. The WHOLE lattice of atoms should be drawn, as well as the scale. But, the problem is that if the nuclei were drawn to scale on a piece of paper, then they could only be seen with a magnifying glass, if not a microscope.

Watch this lecture. Although the professor does not conclude that nuke bombs are a hoax, of course, he does give a lot of the background in nuclear fission, including the enormous spacing, and the essential shape of a sphere for a critical mass. Notice that he assures the audience that nuke power plants will not explode like nuke bombs. So, rhetorically, I wish to use this to our advantage, and say, " if nuke power plants cannot possibly explode, then neither can "nuke bombs." "

Note how the professor is very flustered in answering a student's question as to why a nuke bomb will explode and not a nuke plant. And note that in the end, he actually ADMITS the same thing that I would tend to think ---- that initially a nuke explosion is no greater than an equivalent amount of chemical explosive, but that its greater blast effect is the result of a SECONDARY fissioning in mid air. But I say this is a contradiction, because in order for a a chain reaction to take place in the first place, the subcritical pieces had to be brought together in a compact, critical mass ---- so there will NOT be this secondary fissioning. The nuclei are very sparse to begin with, and if it is hard to shoot and hit ducks assembled on a pond, it is going to be harder to hit ducks after they fly away in all directions.

Type into the youtube space : UC BERKELEY PHYSICS NUKES

[voerioc]

Yes, of course I am saying that nuke bombs would not explode. That is what this forum is about. And I say this based on an understanding of all of physics ---- classical, modern, kinetic theory, you name it.

Of course this is what this forum is about. But that's not the point here. The point is : when you have criticized my idea, you never said that it was only your point of view. Therefore, you have made people believe my idea is false according to the official nuclear science. Which is not the case at all.

According to official science, the bomb can explode. Therefore, my idea that the nuclear material should be slowly consumed is perfectly valid (same thing about the idea that it should be burning hot).

Off course it isn't true according to your point of view. But the forum and this present topic is not about discussing your views about nuclear science, but the views from official nuclear science.

Off course, your theory is interesting. It adds another discrepancy about atomic bombs. But you should develop this idea in a topic of your own.