Understanding Critical Mass and its Role in Nuclear Fission

[Mr_Bojingles]

I can't quite understand critical mass. They say in a fission bomb they have to separate two blocks of fissile Uranium of subcritical mass so they won't undergo spontaneous fission.

What exactly is critical mass? Is it just the amount of Uranium held in one place at one time and if so why does it have to be under the critical mass. Even if it is a small amount and it spontaneously underwent fission would it not cause a smaller chain reaction and blow up that smaller amount of Uranium??

Why is it that a large amount of Uranium in one place is in danger of undergoing spontaneous fission? What would cause one of the atoms to split and cause a chain reaction?



Another question I have is when they propel the two blocks of fissile Uranium at each other what is it that causes the chain reaction? I thought the only way to cause the fission of a Uranium was bombarding it with neutrons. Is propelling Uranium particles at each other the same thing as propelling neutrons at the atoms?

 

[Gruppenpest]

Gun assembly
This more like the method you have described. Actually not that many bomb designs used it. Once implosion-type A-bombs became more compact, it became obsolete as the method has inherent safety issues and is difficult to make efficient in terms of yield for the amount of fissile material used.

Implosion
This is basically the better way to go about it.

Critical mass depends on the material configuration (geometry)(a sphere is optimal), the presence of a surrrounding tamper that can reflect neutrons back, and density. The "beauty" of implosion is that it causes the density of the fissile core to shoot way up. It also uses the optimal geometry too.

 

[Gruppenpest]

Why is it that a large amount of Uranium in one place is in danger of undergoing spontaneous fission? What would cause one of the atoms to split and cause a chain reaction?

http://en.wikipedia.org/wiki/Spontaneous_fission

U-235 (and U-238 even) can simply fission at random even without absorbing a slow neutron or getting struck by a fast neutron. The rate isn't very fast, but it does happen.

http://en.wikipedia.org/wiki/Critical_mass