In the
nuclear field, a chain reaction is what occurs when you get a
critical mass of some
fissionable material, like
uranium or
plutonium.
If you are making a
nuclear reactor, you get lots of heat, which makes lots of
steam, which drives a
turbine, which makes lots of
megawatts of lovely
electricity for the
utility companies to charge an
arm and a leg for.
If you are making a
nuclear bomb, you get
LOTS of heat and radiation, which makes lots of
fire and
burns and
smoke and a really big
mushroom cloud and a nasty
pressure wave, which make lots and lots and lots of
death and
destruction and
radiation sickness, and lots of
cancer and
mutations too, which means that in the end you lose an
arm and a leg there as well.
The moral of the story? Stay away from chain reactions - they are bad for your
limbs.