India's three-stage nuclear programme, conceived by Homi Bhabha, aims to exploit vast thorium reserves for long-term energy security. Stage 1 uses pressurised heavy water reactors (PHWRs) burning natural uranium; Stage 2 uses fast breeder reactors (FBRs) to convert thorium to U-233; Stage 3 employs advanced heavy water reactors running on U-233 and thorium. The Department of Atomic Energy promotes indigenous thorium-based technology for self-reliance. India holds approximately 25% of global thorium reserves.

India's nuclear fuel strategy is architecturally unique because it was reverse-engineered from resource reality, not geopolitics. India has very limited uranium deposits but roughly 10.98 million tonnes of thorium — the second-largest reserves on earth. Homi Bhabha designed a three-stage cascade in the 1950s to eventually make thorium, not uranium, the dominant fuel.

Stage 1 is already operational: 22+ PHWRs running on natural uranium produce plutonium-239 as spent fuel. Stage 2, now materialising with the Prototype Fast Breeder Reactor (PFBR) at Kalpakkam, uses plutonium-239 to breed more plutonium and simultaneously convert thorium-232 into fissile U-233. Stage 3 will then burn U-233 in Advanced Heavy Water Reactors (AHWRs), closing the fuel cycle.

The strategic importance is immense: once Stage 3 is mature, India becomes virtually fuel-independent. The PFBR at Kalpakkam, managed by BHAVINI (Bharatiya Nabhikiya Vidyut Nigam Limited), reached criticality in 2024, marking a milestone for Stage 2. AERB (Atomic Energy Regulatory Board) oversees safety. The Atomic Energy Act 1962 and Atomic Energy Commission (AEC) govern the entire programme. For UPSC, the examiner angle is almost always count-format statements testing which stage uses which fuel, which reactor type belongs where, and which organisation manages which component.