[voerioc]

Why aren't atomic bombs burning hot? - A problem with nuclear fission theory

About nuclear plant, scientists tell us that the nuclear material absolutely needs to be refreshed by water. Otherwise it would be too hot and would melt everything surrounding it.

So, there is a little problem with atomic bombs then. When you see all the old footages and photos, you don't see any cooling system. Thus, the metal surrounding the nuclear material should melt. Especially since the nuclear material is more concentrated than the nuclear material used for nuclear plants. But no, it doesn't melt anything or seems to heat anything.

Another contradiction.

[FirstClassSkeptic]

If it gets so hot, why don't they have a super heater in the nuclear plant boiler?

There was a place on the net where they showed fuel rod manufacture. People were handling these rods by hand. There was also a picture of a hand holding a fuel pellet. I think it was the government, but I can't find that web page again.

[FirstClassSkeptic]

This looks like one, but it's not working now.

https://lh4.googleusercontent.com/-M-Bk ... l_rods.jpg

[It's OK now - Note added by rerevisionist]

[Ranb]

Nuclear material is a broad term indeed. None of the isotopes of uranium or plotonium have a short enough half life to generate the heat required to melt a portion of the pure material. Stack enough U-235 or Pu-239 together and you will get a chain reaction, but this is not melting; it is fission.

The reason a nuclear power plant needs constant cooling is that after operation there is what is called decay heat. As the fission products decay, heat is generated. as the time after shutdown increase, the amount of decay heat also goes down. When heat losses from ambient losses exceed the heat from decay, the cooling systems can be shutdown. This is when the reactor has achieved infinite grace period. Grace period is the time it takes from securing cooling flow to when a temperature limt is reached.

A nuclear power plant that has never been critical (operated with a self sustaining nuclear reaction) does not need cooling. A bomb does not need cooling either, unless the building it is in is on fire.

Pressurized water reactors normally use ordinary water as a coolant and moderator. The ziconium cladding protects the uranium oxide in the fuel plates or rods. Boilling water reactors directly boil the water in the core and pipe it to the steam turbines.

Ranb

[rerevisionist]

You're missing the whole point here. A critical mass is upposed to go more or less instantly into an explosion. However with say half the critical mass, U235 or whatever is fissioning and the process will continue will huge numbers of atoms, even if there aren't enough for an explosion. Therefore there will be energy given off and the thing should get 'burning hot'.

[Ranb]

You're missing the whole point here. A critical mass is upposed to go more or less instantly into an explosion. However with say half the critical mass, U235 or whatever is fissioning and the process will continue will huge numbers of atoms, even if there aren't enough for an explosion. Therefore there will be energy given off and the thing should get 'burning hot'.

No, completely wrong. The spontaneous fission rate of U-235 at subcritical mass is very low. Low enough that a gun barrel type bomb can be used. It is not until critical mass is achieved that the fission rate increases significantly. If critical mass of uranium is 52 kilograms, this does not mean that 26 kilograms of U-235 fissions at half the rate of a nuclear explosion at critical mass.

Uranium is either at critical mass or not, it either explodes or it does not. Uranium lacks a sufficient spontaneous fission rate at less than critical mass and the half life is far too long to generate enough heat to melt it at less than critical mass.

I would like to see what evidence you have to indicate otherwise.

Ranb

[rerevisionist]

OK. Try this thought experiment. Imagine a mass only just below 'critical mass'. From the point of view of virtually all the atoms the situation is almost indistinguishable from 'critical mass'. Any atom that undergoes fission will cause fission locally at the same rate as if the mass were critical. There must be fissions taking place almost as in a supposed bomb. Moreover this situation is supposed to remain for years, or at least between intervals where the stuff is replaced. With say half the 'critical mass' the local situations will still be the same.

What's needed really is some discussion of such things as the number of atoms, mean path of neutrons between fissions and the probability of a nucleus being struck actually fissioning. There's also a statistical issue - even if a critcal mass existed, and was stable enough not to melt or evaporate, and exact figure isn't realistic. But however you cut the cake here, the things will get hot.

[Ranb]

Your answer indicates you lack the most fundamental understanding about how nuclear fission works. There are different kinds of fission. Spontaneous fission takes place only in the largest nuclei such as those with an atomic number of 232 or more. Then there is fission from fast, intermediate, slow and delayed neutrons. Fast fission is also called prompt fission.

Prompt criticality is very undesirable in a nuclear power plant. When it happens there is a steam explosion if it is a water moderated reactor or a fire and explosion if it is sodium cooled. But in neither case is the explosion exactly like that of a nuclear bomb. In a nuclear power plant, fast intermediate, slow and delayed neutrons are all taking part in supplying the number of neutrons required to stay critical or slightly super-critical. Read abut the US Army’s SL-1 disaster.

In a bomb, the most desirable condition after assembly of the critical mass is prompt criticality before the slow neutrons can cause more fission that change the shape of the physics package and make the bomb fizzle. In this case assembly of the critical mass means to bring the pieces of the U-235 or Pu-239 together by high explosives quickly and compress them.

If there is no critical mass, there is no prompt criticality. Prompt criticality relies on enough fissionable material in close proximity so that the release of the 2-3 neutrons per fission can each result in more than one fission. This rapidly escalating effect is what causes the nuclear explosion. With more material in excess of critical mass, you can get a bigger blast, but if you are under critical mass, then you get no nuclear yield at all, just a small amount of radioactivity from the original uranium or plutonium scattered about by the high explosives in the bomb.

This is why it is called critical mass and not okie-odie mass. It is very critical to the operation, without that amount available, no atomic boom boom.

Ranb

[Heiwa]

... there is fission from fast, intermediate, slow and delayed neutrons. ...
In a nuclear power plant, fast intermediate, slow and delayed neutrons are all taking part in supplying the number of neutrons required to stay critical or slightly super-critical. Read abut the US Army’s SL-1 disaster.

In a bomb, the most desirable condition after assembly of the critical mass is prompt criticality before the slow neutrons can cause more fission that change the shape of the physics package and make the bomb fizzle. Ranb

Can you specify the velocities of fast, intermediate, slow and delayed neutrons and explain why slow neutrons pop up in an atomic bomb and manage to collide with atom cores far away producing fissions? How do you start the process? Using mechanical means? A hammer? Dynamite? To let a neutron out of the box?

[Ranb]

Based on what I read and was taught, fast neutrons from fission are traveling very fast, about a billion meters per second while a thermal neutron is going less than 3000 meters per second. Intermediate neutrons are between these two figures.

Some isotopes have a large microscopic cross section for absorption of intermediate neutrons. Intermediate neurons that undergo absorption/decay can make the isotope unavailable for fission.

If fast neutrons are moderated in a bomb, it can be a fizzle. I am not sure why slow neutrons will show up in a bomb unless a substance is used to moderate them. The usual method of initiating a chain reaction is by assembling the pieces at high speed with explosives.

Question for you. Where are you reading claims by responsible scientists that all of the fissionable material in a bomb fissions?

Ranb

[Heiwa]

Based on what I read and was taught, fast neutrons from fission are traveling very fast, about a billion meters per second while a thermal neutron is going less than 3000 meters per second. Intermediate neutrons are between these two figures.

Ranb

So a fast neutron is faster than light? Is it possible? And what is a thermal neutron? I thought there was only one type of neutron.

[Ranb]

I made a mistake on the calculator and did not check my work. Thanks for pointing that out. A fast neutron from fission moves at about 7% of the speed of light, this is about 21 million meters per second. Light moves early 300 million meters per second.

The characteristics of neutrons in a reactor are based on their energy and their source. Slow neutrons are in thermal equilibrium with their environment.

Ranb