Context:- India has reached the second stage of its three-stage nuclear programme.

Stage-1

Natural uranium is used as fuel in a heavy water reactor, producing plutonium and  depleted uranium.

Stage-2

It uses thorium as fuel along  with a mixture of plutonium and natural uranium in small quantities, which produces U-233.

Stage-3

Thorium and uranium fuel are used in this. Under this, thorium turns into U-233 which again acts as fuel in the reactor.