New Delhi, April 8 -- India's prototype fast breeder reactor at Kalpakkam, Tamil Nadu, has gone 'critical,' heralding the second stage of a three-stage nuclear power generation programme envisaged by Homi J. Bhabha.

When a reactor goes critical, it means that a single atomic fission event within it releases enough neutrons to produce exactly one other fission, on average. This is enough to sustain the process of breaking uranium into lighter elements, destroying some mass and creating energy at a rate defined by Einstein's famous equation, E=mc².

If the fission is sub-critical, every fission of a uranium atom would not lead to another fission and the chain reaction would fizzle out. If the reactor goes super-critical, each fission ...