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.