In respect to nuclear fission and nuclear fusion what is meant by the term critical mass?

Answer 1

The term critical mass does not relate to nuclear fusion. Nuclear fusion is the fusing, the joining, of two or more nucleons or nuclei to create a heavier nucleus. It takes enormous energy to set up the conditions that will make this happen. Fusion occurs naturally in stars, and is the mechanism that powers them up. Stars operate in an equilibrium wherein nuclear fusion tries to force everything apart and gravity holds everything together.

Nuclear fission is the splitting of atoms, the splitting of atomic nuclei, and it can be looked at as the opposite of fusion. In fission, certain materials - and of them, only uranium-235 occurs naturally - will, when a certain minimum amount is brought together, begin to fission. They will spontaneously begin to fission because that certain minimum amount, the critical mass, has been brought together. The natural decay of the radionuclide releases neutrons, and when a critical mass is brought together, the naturally released neutrons now can build a chain reaction. The material goes critical because critical mass has been reached.

Note: We're giving thorium the day off here (which does not fission well itself but is usually converted in a reactor to uranium-233), and plutonium can be found with uranium only in the most minute amounts.

A link is provided to an article on critical mass posted by our friends at Wikipedia, where knowledge is free.

Basically, critical mass is the level of mass that something reaches to make something happen.

As a solid metal sphere inside a sphere of uranium, the critical mass of plutonium is 6.4 kg, the core of the MK-III atomic bomb (Gadget at Trinity & Fatman at Nagasaki) was 6.2 kg and became a supercritical mass when imploded using chemical explosive lenses. To ensure a good yield and not depend on natural spontaneous neutron production (which might cause a fizzle) at the optimal moment of supercriticality, a neutron source fired a pulse of neutrons to start the chain reaction.

There is a sense where stellar fusion has critical mass: a protostar whose mass is too low cannot ignite fusion in the first place and becomes a brown dwarf. However the term critical mass is not normally used to describe this stellar mass threshold. There is also a sense where neutron star and black hole formation processes have critical mass, but that is a topic for a different category on another day.

Answer 2

The minimum amount needed to achieve a desired effect whatever that is for your particular situation. Like, critical mass in diversity at your company may mean 25% non whites etc. When you've hit 25%, you've achieved critical mass in that situation.

Answer 3

The smallest amount of material that can sustain a chain reaction. :)

Answer 4

K(multiplication factor/reproduction factor) >1 (Mass of fissile material to be supercritical)