India's Milestone in Nuclear Reactor Technology

What is a Nuclear Reactor?

A nuclear reactor generates electricity using the heat energy released from a nuclear fission reaction.

Can You Use Any Radioactive Material as Fuel?

No. This is a common misconception worth clearing up.

Radioactive and fissile are two very different things:

• Radioactive → The nucleus is unstable and decays, emitting radiation (alpha or beta particles)
• Fissile → The nucleus can be split by a neutron AND releases more neutrons to sustain a chain reaction

A reactor needs fissile material specifically — not just any radioactive material.

For example:

• Radium is highly radioactive but useless as reactor fuel
• Cobalt-60 is radioactive (used in cancer treatment) but cannot sustain a chain reaction

The Only 3 Practical Fissile Materials in the World

There are essentially only 3 practical fissile materials — so only these three can act as fuel for a nuclear reactor:

• U-235 — Natural (makes up only 0.7% of mined uranium)
• Pu-239 — Artificial (bred from U-238)
• U-233 — Artificial (bred from Th-232)

That’s it. The entire global nuclear industry runs on these three.

India’s Problem: Almost No Uranium

India has only about 1% of the world’s uranium reserves, so relying on U-235 alone isn’t a sustainable path for India’s long-term energy needs.

“So let’s get Plutonium from other countries instead?”

That’s a problem too. Plutonium is a nuclear bomb-grade material — the most tightly controlled substance on earth. No country will export it, even to close allies. The Nuclear Non-Proliferation Treaty (NPT) effectively prohibits transfer of separated plutonium between nations. You have to generate your own.

India’s Ace Card: Abundant Thorium

India holds approximately 25% of the world’s thorium reserves, found in the coastal sands of Kerala and Tamil Nadu. Thorium itself is not fissile, but it can be converted into U-233 (which is fissile) inside a reactor.

This is the entire foundation of Homi Bhabha’s brilliant 3-Stage plan — a ladder that takes India from its limited uranium all the way to a self-sustaining thorium-based fuel cycle.

Bhabha’s 3-Stage Nuclear Plan

Stage 1: Pressurised Heavy Water Reactor (PHWR)

• Uses natural uranium (with only 0.7% fissile U-235) as fuel
• Uses heavy water as both moderator and coolant
• The spent fuel contains Pu-239, bred from the abundant U-238 in natural uranium

Why heavy water and not regular water?

Regular (light) water contains hydrogen atoms, which are almost the same mass as a neutron. When a neutron collides with a hydrogen atom, it slows down drastically — like a billiard ball hitting another of equal size. This is useful for slowing neutrons, but light water also absorbs neutrons, wasting them.

Heavy water replaces hydrogen with deuterium (twice as heavy), making it a weaker moderator. More importantly, heavy water absorbs far fewer neutrons — so those saved neutrons go on to trigger more reactions and breed more Pu-239, making the reactor far more efficient.

How much plutonium does Stage 1 produce?

Not much — roughly 0.3 to 0.5 kg of plutonium per tonne of uranium fed in. This is why Stage 2 scaling takes decades. The slow accumulation of plutonium is the biggest bottleneck in the entire plan.

Stage 2: Fast Breeder Reactor (FBR)

• Uses Pu-239 from Stage 1 as fuel
• The core is surrounded by a blanket of U-238 and Th-232
• U-238 in the blanket breeds more Pu-239, sustaining the fuel cycle
• Th-232 in the blanket breeds U-233 (fissile) — the fuel for Stage 3
• Uses liquid sodium as coolant (no moderator)

Wait — if Pu-239 is consumed as fuel, why use it at all?

This is the key insight. You get out more plutonium than you put in. When Pu-239 fissions, it releases multiple neutrons. These neutrons bombard the U-238 blanket and convert it into fresh Pu-239. The breeding ratio is around 1.1 to 1.4 — meaning 1 kg of plutonium input eventually yields ~1.1 to 1.4 kg of plutonium output, while also generating electricity throughout. It’s like an investment that returns more than you put in, with electricity as a bonus.

Why liquid sodium as coolant — and why no moderator?

The word “Fast” in Fast Breeder Reactor refers to neutron speed. Fast neutrons are far better at converting U-238 into Pu-239 (breeding). To keep neutrons fast, you need a coolant that does not slow them down.

• Regular water slows neutrons dramatically — it would kill the breeding entirely
• Sodium nuclei are much heavier than neutrons, so neutrons bounce off sodium without losing much speed, like a tennis ball bouncing off a bowling ball

Sodium also has excellent heat conductivity and a boiling point of 883°C, so the reactor operates at atmospheric pressure — no pressurisation needed.

However, sodium comes with serious engineering challenges:

• It explodes on contact with water — requiring a complex three-loop design to keep sodium and steam completely separate
• It burns in air, requiring an inert argon atmosphere throughout
• It solidifies at 98°C, so the entire circuit must be kept heated even during shutdowns
• It is completely opaque, making inspection and fuel handling extremely difficult

These challenges are why building an FBR is one of the hardest engineering feats in nuclear technology.

Stage 3: Advanced Heavy Water Reactor (AHWR)

• Uses U-233 (bred in Stage 2) + Th-232 as fuel
• Uses heavy water as moderator
• Breeds its own U-233 from thorium inside the reactor — nearly a self-sustaining closed cycle

U-233 has a unique property: it is the only fissile material that can sustain breeding even in a thermal (slow neutron) reactor. This is why Stage 3 can use heavy water moderation instead of needing a fast reactor like Stage 2.

In theory, you keep feeding thorium in and the reactor keeps regenerating its own fissile fuel. In practice, it needs periodic reprocessing to remove accumulated fission byproducts (“nuclear ash”) that absorb neutrons and degrade the fuel rods over time. But the key point is that thorium is the only real consumable — and India has enormous amounts of it.

Why Stage 3 is the Real Endgame

Stages 1 and 2 still depend on uranium (even if indirectly). Stage 3 directly burns India’s most abundant resource — thorium — closing the loop on complete energy independence.

India’s known thorium reserves (~300,000 tonnes) could power the country at current consumption levels for centuries, with dramatically less long-lived radioactive waste compared to conventional reactors.

India’s Achievement

India has achieved criticality in Stage 2 — meaning the Fast Breeder Reactor (PFBR) at Kalpakkam reached a self-sustaining chain reaction in April 2026. We are among a very small group of nations to successfully build and operate an FBR, and arguably the first developing nation to do so. Several other nations that attempted FBRs abandoned them due to the enormous engineering complexity of sodium cooling.

Given India’s abundance of thorium, when Stage 3 comes online — decades from now — India will be self-sustainable in nuclear energy for centuries. This is the strategic vision Bhabha laid out in the 1950s, and India is steadily climbing the ladder he designed.