Fusion-Fission Hybrid Archives - Prof Prabhat Ranjan's Blog

India is writing the rulebook for fusion

Two draft instruments will decide, far more concretely than any policy statement, whether fusion energy has a path in India. Having read them line by line, I want to explain what I found — because the most important issue turns on one word.

On 14 August the Department of Atomic Energy and the Atomic Energy Regulatory Board placed two draft instruments in the public domain: the SHANTI Rules, 2026 and the SHANTI Regulations, 2026, made under the Act passed last year. Comments close on 4 September. Anyone may file them.

These are not glamorous documents. They are long, technical, and written in the flat register that subordinate legislation demands. But they will determine, far more concretely than any policy statement, whether fusion energy has a path in India. I have spent the past fortnight reading them line by line, and I want to explain what I found — because the single most important issue turns on one word.

A note on where I stand, before you read any further. I am Co-founder and Chief Executive Officer of ASPL Fusion Pvt. Ltd., a private company that would be a potential applicant under the rules I am about to discuss. I also chair the committee constituted by the Central Electricity Authority to prepare a national roadmap for fusion-based power generation. What follows is written in my personal and professional capacity and not on behalf of that committee. Some of what I argue for would benefit companies like mine. I have tried throughout to argue on grounds that are neutral as between technologies and as between companies, and you should judge whether I have succeeded.

India’s Thorium Question Has More Than One Answer

The Prototype Fast Breeder Reactor went critical at Kalpakkam in April. It is a good moment to ask what we actually mean when we say “thorium” — because it is not one technology but at least six distinct engineering routes, and all of them are answers to the same question: how does India get more U-233, sooner?

On the many roads from monazite sand to megawatt-hours — and where a fusion-driven neutron source fits in.

On 6 April 2026, the Prototype Fast Breeder Reactor at Kalpakkam attained first criticality. For those of us who have spent our working lives around India’s nuclear and plasma programmes, it was a moment to sit with quietly. Construction began in 2004. First operation was once planned for 2010. The reactor is now running, and India has formally entered the second stage of the programme Homi Bhabha sketched in 1954.

I want to use that milestone to ask a question we do not ask often enough: when we say “thorium,” what exactly do we mean?

Building on What We Have: How India’s Atomic Energy Establishment Can Launch a Private Fusion Industry

In his previous blog, Prof. Ranjan argued that America’s new fusion roadmap marks a change of posture — the state stepping back from building the first power plant and instead building the shared infrastructure that lets private companies build it. Here he asks the natural follow-up for India: not what our public institutions should build, but what they should open. A surprising share of what a private fusion industry needs already sits inside the Department of Atomic Energy.

Building on What We Have — Prof. Prabhat Ranjan

A companion essay. This piece continues an argument begun in “Build, Innovate, Grow: What America’s New Fusion Roadmap Means for India.”

In my last essay I argued that the deepest lesson of America’s new fusion roadmap is a change of posture — the state stepping back from building the first power plant, and instead building the shared infrastructure that lets private companies build it. That argument invites an obvious question for India: if our public institutions are to enable a private fusion industry, what exactly should they build?

After some months of looking closely, I think the more urgent question is what they should open. A great deal of what India needs already exists, scattered across the units of the Department of Atomic Energy. The fastest and cheapest way to accelerate fusion here is not a greenfield campaign of new institutes; it is to upgrade, repurpose, and — above all — open a defined slice of the capability the DAE has spent six decades building.

I do not write this from the outside. I carried out my own doctoral fusion research at Berkeley; spent nine years as a scientist at the Saha Institute of Nuclear Physics, where I worked on India’s first tokamak, commissioned in 1987; and later led the ADITYA tokamak at the Institute for Plasma Research, along with the operation and control group of its SST-1 superconducting tokamak. Several of the units I describe below I have worked inside, and I have watched this establishment’s plasma capability grow from a single small tokamak into a national programme. What follows is, in part, an argument that India underestimates what it already owns.

A platform, not a building

The American roadmap calls its shared infrastructure the Tritium-Blanket Development Platform — a distributed network of test stands and loops, public and private, that any developer can draw on because no single company can justify building them alone. India can assemble an equivalent almost entirely from assets it already holds. The right unit of thinking is not a new national laboratory but a platform — a coordinated set of shared user facilities, each anchored in an existing DAE unit and opened, under clear rules, to vetted private developers. Let me walk through fusion’s hardest gaps and where, in the DAE, each could be addressed.

The Neutron Bridge

At INEF 2026 I realised how little the fission and fusion communities still talk to each other. Yet a fusion device is, before it is ever a power plant, an intense source of fast neutrons — and that surplus can breed U-233 from India’s thorium, burn long-lived waste, and drive a sub-critical hybrid that never goes critical. Fission problems with a fusion answer.

The Neutron Bridge — Part 1 | Prof. Prabhat Ranjan

I spent two days at the India Nuclear Energy Forum at IIT Bombay this May, and I came away with one impression more strongly than any other. The people who build and run our fission plants — some of the most capable nuclear engineers anywhere — have, for entirely understandable reasons, not been watching what has happened in fusion over the last five or six years. And the fusion community, equally guilty, has rarely bothered to explain itself in terms a reactor physicist would find useful.

That is a shame, because the two fields are closer to needing each other than either side seems to realise. I want to use this note to close a little of that gap, written deliberately for colleagues who think in cross-sections and neutron budgets rather than press releases.

What changed in fusion while you were busy keeping the grid running

For most of our careers, “fusion” meant one thing: a very large tokamak, decades away, funded by governments. ITER is real and important, but it shaped a perception that fusion is monolithic and remote. That perception is now out of date in two specific ways.

First, the magnet problem has largely been solved by materials, not by scale. High-temperature superconductors — REBCO tapes in particular — let us reach the field strengths that used to require enormous low-temperature magnets, in machines a fraction of the size. A high field at a small radius changes the entire economic argument, because confinement and reaction rate scale steeply with field. This is why the credible new private programmes are an order of magnitude smaller than ITER and still expect meaningful performance.

Second, and more relevant to this audience, the field has rediscovered configurations the fission community would find refreshingly simple. I work on the axisymmetric magnetic mirror — the Gas Dynamic Trap lineage from Budker Institute in Novosibirsk, recently validated by the WHAM experiment at Wisconsin and its commercial successor. A linear mirror is not a closed toroidal device. It is, conceptually, a long straight magnetic bottle. It is easier to build, easier to maintain, and — crucially — it is an excellent neutron source even when its energy gain is modest. Hold that last point; it is the whole argument.

The deeper shift is one of intent. A generation of fusion programmes asked only one question: when do we put electricity on the grid? The newer programmes ask a different one first: what is fusion good for before it is a power plant? And the honest answer is that a fusion device is, first and foremost, a controllable, intense source of fast neutrons. That reframing is what should interest you.

Why a fusion neutron is different

A deuterium–tritium reaction releases 17.6 MeV, of which 14.1 MeV leaves as a neutron. Compare that with the roughly 2 MeV average of a fission neutron. The factor of seven in energy is not a curiosity — it opens reaction channels that are simply closed to a thermal or even a fast fission spectrum.

Energy comparison of a fission neutron at about 2 MeV and a D–T fusion neutron at 14.1 MeV, showing the high-energy reaction channels — (n,2n) multiplication and fast fission of minor actinides — that only the fusion neutron reaches. — The fusion neutron is born about seven times more energetic than a fission neutron, clearing the thresholds for neutron multiplication and hard-spectrum fission that a reactor spectrum cannot.

At 14 MeV you get neutron multiplication almost for free. Inelastic and (n,2n) channels in beryllium and lead mean a blanket can return more neutrons than it receives. That surplus is the resource. In a critical reactor every neutron is spoken for — you live or die by the six-factor formula and you have almost no margin to spend neutrons on anything that does not sustain the chain. A fusion source hands you a neutron budget you do not have to balance against criticality.

What you choose to do with that surplus — breed fuel that has eluded our thorium programme for decades, burn the waste that fills our repositories, and generate power without the assembly ever going critical — is exactly where our two fields meet.

Unlocking India’s Thorium Age : How ASPL Fusion Can Accelerate India’s Three-Stage Nuclear Programme

On 6 April 2026, India’s PFBR at Kalpakkam achieved first criticality — entering Stage 2 of the three-stage nuclear programme. But Stage 3, the thorium endgame, remains 45–65 years away. The bottleneck is ²³³U, which must be bred from thorium and currently depends entirely on the slow FBR build-out. Fusion-fission hybrid technology offers a parallel route: fusion neutrons driving a subcritical thorium blanket, independently of the FBR fleet, potentially compressing the wait by 20–30 years.

📣 Breaking — 6 April 2026 India’s Prototype Fast Breeder Reactor (PFBR) at Kalpakkam, Tamil Nadu achieved first criticality at 20:25 IST — marking India’s formal entry into Stage 2 of the three-stage nuclear programme. This historic milestone, coming just as this article goes to press, makes the question of accelerating Stage 3 more urgent and more actionable than ever. ASPL Fusion’s Project PRABHA is designed to be exactly that accelerant.

India sits atop the world’s largest thorium reserves — enough to power the country for centuries. Yet the gateway to this abundance, Stage 3 of the three-stage nuclear programme, remains decades away under the current trajectory. ASPL Fusion’s fusion-fission hybrid technology offers a credible, privately-funded shortcut: producing ²³³U directly from thorium using fusion neutrons, bypassing the central bottleneck of the Fast Breeder Reactor fleet, and potentially compressing a 40-year wait into 20.

1. India’s Three-Stage Vision: Brilliant Design, Slow Execution

When Dr. Homi Bhabha conceived India’s three-stage nuclear programme in the 1950s, it was a work of strategic genius. India had almost no uranium but enormous thorium deposits — estimated at ~25% of the world’s total reserves. The plan was to harness fission step by step, each stage feeding the next, culminating in a self-sustaining thorium-uranium fuel cycle that could power India essentially forever.

Stage	Reactor Type	Fuel In / Out	Strategic Purpose
Stage 1	PHWRs (Pressurised Heavy Water)	Natural U-238 → Pu-239	Exploit domestic uranium; breed plutonium
Stage 2	Fast Breeder Reactors (FBRs)	Pu-239 + Th-232 → ²³³U	Multiply fissile inventory; introduce thorium
Stage 3	Advanced Heavy Water Reactors (AHWRs)	²³³U + Th-232 → Power	Exploit vast thorium reserves at scale

The logic is elegant. The execution has been painfully slow. Stage 1 is mature — 22 PHWRs now operate across India. Stage 2 is inching forward: the Prototype Fast Breeder Reactor (PFBR) at Kalpakkam achieved first criticality on 6 April 2026 after decades of delays. Stage 3, the thorium endgame, remains a distant prospect. At current rates, meaningful Stage 3 deployment may not arrive until 2070–2090 — over half a century away.

The central bottleneck is fissile material inventory. Stage 3 reactors require an initial loading of ²³³U — a material that does not occur in nature. It must be bred from thorium by bombarding it with neutrons. Under the classical pathway, the only source of those neutrons at scale is the FBR fleet. But FBRs take decades to build, and their early fuel is precious Pu-239 from Stage 1 — itself in limited supply. It is a slow compound-interest problem, and India is impatient.

2. The Missing Accelerant: An External Neutron Source

To understand the bottleneck, it helps to understand the breeding reaction itself. When a thorium-232 nucleus absorbs a neutron, it does not immediately fission. Instead, it undergoes a two-step transmutation: first to protactinium-233, then to uranium-233 — a fissile material that can sustain a chain reaction just as uranium-235 or plutonium-239 can. This is the nuclear alchemy at the heart of Stage 3. The challenge is neutrons: you need a sustained, high-intensity neutron flux to irradiate thorium at scale, in sufficient quantity to produce meaningful amounts of ²³³U. Classically, only a working fission reactor can supply that flux — which is why Stage 3 has always depended on Stage 2 FBRs. But what if India could breed ²³³U without waiting for the FBR fleet? What if a neutron source powerful enough to drive a thorium blanket could be built faster, cheaper, and with private capital?

This is precisely the proposition at the heart of fusion-fission hybrid technology — and it is the core strategic value of ASPL Fusion’s Project PRABHA.

What is a fusion-fission hybrid?

A fusion-fission hybrid marries two nuclear processes. Fusion — the joining of light nuclei (deuterium and tritium, or deuterium alone) — releases vast energy and, crucially, a flood of energetic neutrons. Fission — the splitting of heavy nuclei like thorium or uranium when struck by neutrons — releases further energy and breeds new fissile material. In a conventional fission reactor, a precise critical mass of fissile fuel must be maintained to keep the chain reaction going, demanding expensive enrichment and creating inherent safety challenges.

In a fusion-fission hybrid, the fusion core acts as a powerful, controllable neutron gun, firing neutrons into a surrounding “blanket” of thorium. The blanket undergoes breeding reactions, but it never reaches criticality on its own: its neutron multiplication factor k_eff is kept below 1.0 by design. This means the reaction halts the instant the fusion source is switched off — an intrinsic safety property no conventional reactor can match, since no self-sustained runaway chain reaction is physically possible. However, subcritical systems still accumulate decay heat from fission products after shutdown, requiring continued cooling; they are not free from thermal-hydraulic safety requirements. Yet the neutron economy within the blanket can be made substantial. The neutron multiplication factor M = 1/(1−k_eff) = 33 means that one source neutron ultimately produces approximately 33 neutrons across all cascade generations — not 33 breeding reactions. Of these, a fraction (typically 10–30%, depending on blanket design and spectrum) are usefully absorbed in Th-232 to initiate the transmutation chain to ²³³U. The remainder sustain the multiplication, compensate for parasitic absorption in structural materials, and manage leakage. This neutron amplification, combined with a carefully optimised multi-zone blanket, is what enables meaningful ²³³U production from a modest fusion source.

ASPL Fusion’s PRABHA-Hybrid is a subcritical fusion-fission hybrid with the following design parameters:

k_eff = 0.97 — deeply subcritical, inherently safe
Neutron multiplication M = 33× — one source neutron produces ~33 neutrons in cascade across all generations (M = 1/(1−k_eff)); a fraction of these are usefully absorbed in Th-232 for ²³³U breeding
Thorium breeding zone: TBR ≥ 1.05 — tritium self-sufficient
~80 kg of ²³³U per unit per year from the thorium zone — a design-target based on internal neutronics calculations at ~53 MWth blanket power; subject to detailed validation accounting for Pa-233 capture losses, spectrum effects, and geometric efficiency
33 MWe net electrical output at 87.5% capacity factor (253 GWh/yr)

The neutron source is a Gas Dynamic Trap (GDT) driven system, with possible collaboration with BINP Novosibirsk — the world leader in tandem mirror devices. This is not speculative physics; GDT devices have operated for decades. ASPL is the first Indian private company to access this technology for a commercial fusion application.

Viksit Bharat Needs Firm Power: Why Fusion Must Be in India’s Energy Plan Now

India’s electricity demand will nearly quadruple by 2047. Solar and wind cannot carry that load alone. The time to build the next generation of baseload is not 2040 — it is today and why fusion must be a strong part of it.

India’s electricity demand will nearly quadruple by 2047. Solar and wind cannot carry that load alone. The time to build the next generation of baseload is not 2040 — it is today.

Disclosure: The author is co-founder of ASPL Fusion, a private Indian fusion company. This article presents an industry perspective on national energy policy. The policy arguments are grounded in publicly available data; the ASPL programme description in Section 5 is provided as a concrete illustration of what a domestic fusion development path can look like in practice.

At a Glance — For Decision-Makers

708 GW peak demand and 2,100 GW total capacity needed by 2047 — the equivalent of building a new US power grid from scratch.

Intermittent renewables alone cannot provide firm power — grid stability requires large-scale dispatchable baseload even with abundant storage.

Fusion now has serious private capital behind it: Microsoft, Google, and OpenAI are structuring power purchase agreements with fusion companies. India risks being a technology importer.

Three budget-cycle asks for 2026: fusion in National Energy Plan · dedicated component development fund · AERB regulatory engagement mandate.

Key Number

India’s Central Electricity Authority projects peak power demand reaching 708 GW by 2047 — four times today’s installed capacity. Meeting it requires 2,100 GW of generation: close to the entire current installed capacity of the US and EU combined. That is the scale of infrastructure build India must manage across the next two decades.