Question

Define/explain the terms critical mass/chain reaction as they apply to both a nuclear bomb and a nuclear power plant. Briefly describe how both an atomic bomb and a nuclear power plant work.

Answer 1

A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, the nuclear fission cross-section), its density, its shape, its enrichment, its purity, its temperature, and its surroundings.

If a chain reaction is started, then if  the reaction will sustain itself, it is known as "critical", and the mass of radioactive element required to produced the critical condition is said to be a "critical mass".

now, the principle of working of nuclear power plant:

A really heavy atom—a stable kind of uranium called uranium-235 has a nucleus with 92 protons and 143 neutrons.If a neutron is fired at uranium-235, it is turned into uranium-236: an unstable version of the same atom (a radioactive isotope of uranium) with 92 protons and 144 neutrons . Uranium-236 is too unstable to live for long , so it splits apart into two much smaller atoms, barium and krypton, releasing quite a lot of energy and releasing three spare neutrons at the same time.

Now the spare neutrons can crash into other uranium-235 atoms, making them split apart too. And when each of those atoms splits, it too will produce spare neutrons. So a single fission of a single uranium-235 atom rapidly becomes a chain reaction—a huge amount of energy is released.

The only difference that atomic bombs hasve is that the reaction is uncontrolled in their case.