Ease of chain reaction for enriched uranium

[decafdave]

Critical mass is over 50kg so let's say I have 2 halves of a sphere of the isotope U-235, each weighing 30 kg. I drop one onto the other so that they form a supercritical sphere. No doubt a chain reaction would begin, but I assume it would produce energy on the level of a nuclear reactor rather than a fission bomb? Is it a matter of force when they collide? Just how soft is uranium?

My question stems from learning about the "gun type" fission bombs developed during WWII.
My guess is that uranium is a soft metal, and not much force is required to meld the two pieces together although I may be wrong.

 

[mathman]

The experiment you are describing is that of a bomb. A reactor is much more complicated. In a bomb the fission rate grows exponentially. In a reactor, it is necessary to keep it a constant level, using moderators, etc.

 

[decafdave]

So it's likely that is enough force to initiate the chain reaction? Any guess to the explosion's efficiency since there is no tamper? I know Little Boy didn't even achieve 2% but still yielded 14KT.

It's a good thing it's so difficult to isolate U-235!

 

[mathman]

It's not force that initiated the chain reaction, only having enough U235 together.

 

[Astronuc]

Critical mass is over 50kg so let's say I have 2 halves of a sphere of the isotope U-235, each weighing 30 kg. I drop one onto the other so that they form a supercritical sphere. No doubt a chain reaction would begin, but I assume it would produce energy on the level of a nuclear reactor rather than a fission bomb? Is it a matter of force when they collide? Just how soft is uranium?

My question stems from learning about the "gun type" fission bombs developed during WWII.
My guess is that uranium is a soft metal, and not much force is required to meld the two pieces together although I may be wrong.

Uranium is a hard metal and it undergoes dynamic strain hardening, which is one reason that it is used for armour piercing shells.

A critical mass of U-235 would get hot rather quickly, to the melting point. Gun type systems have a much larger mass than needed for supercriticality in order to increase yield before dispersion. Implosion devices use the compression of an explosion to increase the density of fissile material rapidly with an consequent increase in yield before dispersion.