Prof Prabhat Ranjan’s Blog

First, Why Can’t We Just ‘Burn’ Thorium?

This is the first question everyone asks, and it is a fair one. If we have so much thorium, why not simply put it in a reactor?

The answer is that thorium is ‘fertile’ but not ‘fissile’. Think of it this way: thorium is like a log of wood that will not light on its own. You need a spark. In nuclear terms, that spark is a neutron. When a neutron strikes a thorium-232 nucleus, it transforms through a two-step radioactive decay into uranium-233 — a remarkable fissile material that can sustain a chain reaction and produce energy.

The chain looks like this: thorium-232 absorbs a neutron, becomes thorium-233, which decays in 22 minutes into protactinium-233, which then decays over 27 days into uranium-233. That uranium-233 is your fuel.

This is what nuclear scientists call ‘breeding’ — using one type of nuclear reaction to produce the fuel for another. Dr. Homi Bhabha, the father of India’s atomic programme, designed our entire nuclear strategy around this concept sixty years ago. He called it the Three-Stage Programme. Stage One: use natural uranium in heavy water reactors to produce plutonium. Stage Two: use that plutonium in fast breeder reactors to produce more plutonium and begin converting thorium into uranium-233. Stage Three: run thorium-uranium-233 fuel cycles that are essentially self-sustaining. We are currently somewhere between Stage One and Stage Two. Stage Three — the thorium stage — is still decades away under conventional approaches.

The challenge is not the physics. The physics is settled and beautiful. The challenge is producing enough neutrons, of the right energy, efficiently enough to make thorium breeding economically viable at scale.

Five Ways to Breed Uranium-233 from Thorium

Over the past year, my team at ASPL Fusion has conducted a detailed technical assessment of every known method for converting thorium to uranium-233. Let me summarise what we found in plain terms.

1. Heavy Water Reactors — and the ANEEL Breakthrough

India’s 22 pressurised heavy water reactors — the workhorses of our nuclear fleet — can be loaded with thorium fuel alongside uranium. The thermal neutrons breed some uranium-233, but not enough to be self-sustaining. The Advanced Heavy Water Reactor (AHWR) being developed by BARC targets near-unity breeding. This is our most mature near-term option.

But something important happened recently that changes this picture substantially. A Chicago-based company, founded by Shri Mehul Shah, called Clean Core Thorium Energy (CCTE) has developed a fuel called ANEEL — Advanced Nuclear Energy for Enriched Life — that fits India’s existing PHWR fuel bundle dimensions without any reactor modification. ANEEL blends thorium with a small amount of High-Assay Low-Enriched Uranium (HALEU). The results from testing at Idaho National Laboratory are remarkable: burnup of 45–60 GWd per tonne, compared to just 7 GWd/tonne for natural uranium — six to eight times more energy from the same fuel load. Waste drops by over 85 per cent. And as the thorium in the pellet absorbs neutrons, it steadily converts to uranium-233, beginning to build India’s strategic fissile inventory right now, in reactors that are already running.

Dr. Anil Kakodkar, former chairman of the Atomic Energy Commission and one of India’s most respected nuclear scientists, has called this a transformational opportunity. NTPC, India’s largest power company, has approved a minority equity investment in CCTE. Larsen & Toubro signed a manufacturing partnership in 2024. The US Department of Energy granted CCTE an export licence for India in August 2025 — only the second such authorisation in twenty years.

There is one important caveat: ANEEL needs HALEU, which India does not currently produce. That enriched uranium would need to come from the United States, introducing a supply chain dependency that must be managed carefully. India should develop ANEEL fuel while simultaneously building the domestic enrichment capability to eventually produce HALEU independently. The two goals are not contradictory — they are sequential.

2. Fast Breeder Reactors (Our Stage Two)

Fast breeder reactors use higher-energy neutrons — the ones that are not slowed down by a moderator. These faster neutrons are better at producing more neutrons per fission, which means you can breed more thorium than you consume. The Prototype Fast Breeder Reactor at Kalpakkam, developed by IGCAR, is India’s most important near-term step. It will breed uranium-233 in thorium blankets surrounding the core while generating 500 megawatts of electricity.

3. Molten Salt Reactors (The Elegant Solution)

In a molten salt reactor, both fuel and coolant are dissolved in a fluoride salt mixture. The genius of this design is that you can continuously extract the protactinium-233 intermediate before it can accidentally absorb another neutron and waste the breeding reaction. China’s TMSR-LF1 at Wuwei, Gansu became the world’s first MSR to reach criticality since Oak Ridge’s MSRE shut down in 1969, achieving first criticality in October 2023 on uranium fuel. In October 2024 it went further — completing the world’s first thorium-to-uranium conversion inside an operating molten salt reactor, with Pa-233 detection confirming successful U-233 breeding. India has done significant molten salt research at BARC. This technology could ultimately offer the most efficient thorium breeding of any approach, but it has not yet been demonstrated at commercial scale anywhere in the world.

4. Accelerator-Driven Systems(ADS)

In an accelerator-driven system, a powerful particle accelerator fires protons at a heavy metal target, producing a shower of neutrons. These neutrons then drive a subcritical thorium assembly that cannot sustain a chain reaction on its own and is therefore inherently safe. The limitation is economic: the accelerator consumes significant electricity and produces no power of its own.

4b. Electron Accelerators and Dense Plasma Focus Devices

Two additional niche methods deserve brief mention. Electron accelerator-driven systems use bremsstrahlung photons from a high-power electron beam to drive photofission in thorium — a lower-cost accelerator alternative for small-scale breeding, though even more energy-negative than proton ADS. Dense Plasma Focus (DPF) devices, already operated at BARC and several Indian universities, produce short pulsed neutron bursts at very low cost (₹1–5 Crore for a university-scale device). These are not suitable for commercial breeding — their neutron yield is three orders of magnitude too low — but they are ideal for validating thorium irradiation geometry, neutron activation measurements, and uranium-233 separation chemistry before a full fusion device is commissioned. At ASPL Fusion, it may be worth exploring a DPF-based Phase 1.5 programme as a low-cost proof-of-concept for AERB pre-licensing engagement.

5. Fusion Neutrons (The Path We Are Building)

And now we come to fusion — the approach I find most compelling, and the one ASPL Fusion is actively developing.

When deuterium and tritium fuse — the same reaction that powers the sun — they release a neutron with 14.1 million electron volts of energy. This is far higher than any neutron produced by fission. And those high-energy neutrons do something remarkable when they strike thorium-232: they can split the thorium nucleus itself (fast fission), and they can cause a single neutron to produce two neutrons via a reaction physicists call (n,2n). The net result is that each fusion neutron is worth significantly more for thorium breeding than any fission neutron.

In a fusion-driven system, you place the thorium blanket surrounding the fusion plasma. The fusion neutrons breed uranium-233, and simultaneously breed the tritium fuel needed to sustain the fusion reaction. The blanket also absorbs the heat of the breeding reactions to generate electricity. In principle, you produce fissile fuel, generate power, and fuel your own fusion device — simultaneously.

There is a subtlety worth understanding. The blanket must do two things with the same neutrons: breed uranium-233 from thorium, and breed tritium from lithium to refuel the plasma. These goals compete directly. The solution our PRABHA-P3 blanket uses is a beryllium neutron multiplier zone between the lithium and thorium layers. Beryllium converts one high-energy neutron into two lower-energy ones via the Be(n,2n) reaction, providing the neutron budget to achieve both goals simultaneously. Getting this balance right is one of the central design tasks in Phase 3.

Why Fusion Neutrons Are Uniquely Powerful

Let me be more specific, because the physics here is worth understanding.

The reaction that matters most is what happens when a 14.1 MeV fusion neutron strikes a thorium-232 nucleus. There are three things it can do, and all three are useful:

• Breed uranium-233: The neutron is captured, and after two beta decays, you get U-233. This is the primary goal.
• Cause (n,2n) multiplication: The neutron has enough energy to knock two neutrons out of the thorium nucleus. So one neutron in, two neutrons out. This effectively doubles the neutron population available for breeding.
• Cause fast fission: Thorium-232 has a fission threshold of about 1.4 MeV. A 14.1 MeV neutron comfortably exceeds this, causing the thorium to fission and release more neutrons still.

No fission reactor produces neutrons energetic enough to trigger (n,2n) reactions in thorium at scale. Even fast breeder reactors, with neutrons at roughly 200,000 electron volts, fall short of the 14.1 million electron volt fusion threshold. This is not an engineering limitation. It is a fundamental difference in the physics of the two processes.

What ASPL Fusion Is Building

At ASPL Fusion, being incubated at the Institute for Plasma Research in Gandhinagar, we are developing a magnetic mirror fusion device called PRABHA (Plasma Reactor for Advanced Breeding and High-energy Applications). Our approach uses a Gas Dynamic Trap (GDT) magnetic mirror geometry, which is inherently simpler and more compact than a tokamak.

Our programme has four phases. Phase 1: a proton accelerator for Boron Neutron Capture Therapy cancer treatment — generating early revenue while we establish our neutron source expertise. Phase 2: a full deuterium-deuterium GDT fusion device for medical isotope manufacturing and thorium irradiation experiments. Phase 3: deuterium-tritium operation with a subcritical thorium blanket — the fusion-fission hybrid — breeding uranium-233 at meaningful scale while generating net electricity. Phase 4: our long-term vision — a deuterium-helium-3 tandem mirror, tritium-free fusion power in its cleanest form.

The Regulatory Reality and Why We Must Move Now

I want to be honest about the challenges, because intellectual honesty is what builds credibility.

There is a materials challenge worth naming directly. The 14.1 MeV neutrons that make D-T fusion so powerful for thorium breeding are also destructive to the steel wall facing the plasma. At these energies, neutrons cause helium bubbles to form inside metal grains — helium embrittlement — and displace atoms at rates roughly ten times higher than in a fission reactor. The PRABHA-P3 first wall will need replacement every three to five full-power years. We design for this from the start: modular wall cassettes, remote handling, clear waste management. India’s IGCAR has developed a reduced-activation steel called INRAFM specifically for fusion applications, and we will work closely with them.

The regulatory pathway for a fusion-fission hybrid in India is long. The Atomic Energy Regulatory Board’s multi-stage consent process, from site evaluation to full power operation, takes eight to twelve years for a first-of-a-kind device. The SHANTI Act of 2025 opened private sector participation in nuclear activities for the first time — a landmark change. But the regulations for a privately-operated fusion-fission hybrid producing bred uranium-233 are genuinely novel territory that AERB has not charted before.

This means we need to begin the regulatory conversation now, during Phase 2 operation, not when Phase 3 construction is ready to start. Early pre-licensing engagement with AERB, formal collaboration with DAE on uranium-233 material accountancy protocols, and participation in the IAEA’s Advanced Reactor Information System — these are not bureaucratic formalities. They are the critical path items that will determine whether India’s first fusion-fission hybrid is operational in the 2030s or the 2040s.

Under the current legal framework, bred uranium-233 cannot be owned by a private company. It must be transferred to DAE. Our business model, therefore, is not to sell uranium-233 as a commodity but to operate the breeding facility as a service to the nation — analogous to how private companies operate dedicated satellites or communication infrastructure for government use.

The International Race India Cannot Afford to Lose

Let me situate this in global context, because the competitive landscape is moving faster than most people realise.

China achieved the world’s first molten salt reactor criticality in 2023, at a facility in the Gobi Desert. They have committed to a 100-megawatt demonstration MSR by 2030 and a commercial gigawatt-scale plant by 2040. China’s thorium reserves in Inner Mongolia are smaller than India’s, but their programme is more advanced.

Belgium is building MYRRHA — a 100-megawatt accelerator-driven system that will be the world’s most advanced ADS demonstrator when it begins operations in 2036. A billion and a half euros of European investment.

Canada is testing thorium-uranium mixed oxide fuel in CANDU reactors, the same reactor technology that India’s PHWRs are derived from.

And the United States? They have 1,750 kilograms of uranium-233 sitting at Oak Ridge National Laboratory, a legacy of their 1960s thorium programme, which they are now systematically destroying because they have no plan to use it — except, quietly, one company does have a plan. Clean Core Thorium Energy’s ANEEL fuel, tested at Idaho National Laboratory, is the most advanced commercial thorium fuel programme in the world. And its primary target market is India.

Here is what I find striking about this landscape: every country is pursuing thorium breeding through fission-based methods — MSRs, ADS, fast reactors. Nobody is pursuing fusion-driven thorium breeding at demonstration scale. The field is completely open. India, through our PRABHA programme, has the opportunity to be the first country in the world to demonstrate commercially-viable fusion-fission thorium breeding. That is not an incremental advance. That is a genuine world-first.

I want to be clear about how ANEEL and the PRABHA programme relate to each other, because I am sometimes asked whether they compete. They do not. They operate at entirely different layers of India’s thorium strategy. ANEEL breeds uranium-233 in Stage I reactors that exist today — it is the near-term solution. The fusion approach breeds uranium-233 at much higher flux and without any dependence on imported enriched uranium — it is the long-term, strategically independent solution. India needs both. A country with our thorium reserves and our energy ambitions cannot afford to be dogmatic about which technology to back. We should back all of them that are credible, and both ANEEL and fusion-driven breeding are credible.

The Economics: Is This Actually Viable?

Science without economic viability is interesting but not transformative. So let me address the business case directly.

Our Phase 3 PRABHA-P3 fusion-fission hybrid is designed to generate multiple revenue streams simultaneously. Net electricity output of 50 to 150 megawatts at a wholesale tariff of four rupees per kilowatt-hour, with an 80% capacity factor, produces power revenue of roughly ₹250–500 Crore per year. Medical isotope production adds another ₹150 Crore per year. And the uranium-233 breeding, valued at approximately ₹8,000 per gram and transferred to DAE under a breeding service agreement, could contribute up to ₹400 Crore per year.

Total annual revenue: approximately ₹800–1,000 Crore per year from a single Phase 3 unit, against a capital cost of ₹8,000–12,000 Crore.

Two costs deserve explicit acknowledgement: decommissioning and waste disposal. The activated structural materials — first-wall modules, thorium blanket pebbles bearing trace U-232, tritium-contaminated components — are intermediate-level radioactive waste. Our 30-year decommissioning estimate is ₹700–1,400 Crore, or roughly ₹25–50 Crore per year annualised. It is real, it is budgeted, and it does not change the commercial conclusion. But it should be stated plainly.

There is an honest caveat: the first-of-a-kind premium. The first fusion-fission hybrid built anywhere in the world will cost two to three times what subsequent units cost. This is why our Phase 1 and Phase 2 revenues — from BNCT cancer treatment and medical isotope sales — are not merely nice to have. They are how we fund the technology development that makes Phase 3 possible without requiring a single additional equity raise after our Series A.

A Personal Note: Why This Matters

I have been asked many times why I left the comfortable world of technology policy to start a nuclear fusion company in my late sixties. The honest answer is that I looked at what we were building at TIFAC, at what India’s Technology Vision 2035 laid out as possible, and I realised that the most important gap was not in solar panels or electric vehicles or artificial intelligence — important as all of those are. The most important gap was in base-load energy that India can produce from its own resources, indefinitely, without importing fuel or emitting carbon.

Thorium is that resource. Fusion is the tool to unlock it. India is the country with both the thorium reserves and, at IPR and BARC and IGCAR, the scientific talent to make it happen. What has been missing is the private sector urgency — the willingness to take the risk, move faster than government timelines, and build the demonstration that converts potential into reality.

That is what we are attempting to do at PRABHA. We are not naive about the difficulty. Building a fusion device is genuinely hard. Building a fusion-fission hybrid with regulatory approvals in India is harder still. But the physics is real, the reserves are there, the talent exists, and the strategic imperative is clear.

India’s thorium future is not a distant dream. It is an engineering problem with a solution. We are working on it.