Please define "Critical Mass?

Answer 1

The minimum mass sphere of fissile material in which, once assembled, the neutron flux increases at an exponential rate due to neutron induced fission reactions. If the mass is less than this, free neutrons diffuse (leak) out of the mass too quickly for this to occur

Answer 2

In uranium fission reactions,the three neutrons produced in the fission may or may not be captured by the other uranium nuclei and the reaction may stop or slow down.

The escape of neutron out of the material, without getting captured, depends on surface area of material.

Greater the surface area of material, greater is the chance of escape.

But greater the volume of material, smaller is the chance of escape of neutron.

Thus surface area and volume affect the escape in opposite ways.

For a particular volume, the chances of escape are minimum.

The uranium fission reaction will not be sustained if volume is less than some certain volume. The volume of uranium below which the reaction will not sustain and ultimately stop, is called "CRITICAL VOLUME".

The MASS of critical volume is called "CRITICAL MASS".

If mass of uranium is less than critical mass, the fission process will slow down and stop

For sustained chain reaction,the mass of uranium should be greater than "critical mass"

Answer 3

Critical mass to what ? For nuclear fission, fusion, for stars... etc ? Generally when something succeeds a certain mass, as criteria for a new process, has achieved "critical mass".

Answer 4

It is the mass of u235 that once it is brought together will sustain fusion.