Reactor, Accelerator, and Neutron-Generator Pathways: A Complementary View

Executive summary

Medical radioisotopes can be produced along at least three distinct physical pathways: research reactors, particle accelerators, and purpose-built neutron generators. Public conversation tends to collapse these into a competition. The production reality is that each pathway has a natural role, a set of isotopes it is good at, and a set it is poor at — and a resilient national system needs all three.

This note lays out the three pathways, what each does well, and why framing them as substitutes misreads the physics.

1. Pathway A — Research reactors

A research reactor produces neutrons as a by-product of a sustained fission chain reaction. These neutrons are thermalised — slowed to near thermal-equilibrium energies — and used to irradiate target materials loaded into the reactor core or surrounding irradiation facilities.

Strengths:

• Very high thermal-neutron flux, continuously, for long durations
• Excellent for neutron-capture production (n,γ) of isotopes such as Mo-99, Lu-177, I-131
• Decades of global operational experience; well-characterised radiochemistry

Trade-offs:

• Capital and regulatory weight is high — reactors are nation-scale assets
• Fleet ageing is a live concern; the publicly documented global Mo-99 supply relies on a small number of facilities, most commissioned decades ago (OECD-NEA, The Supply of Medical Radioisotopes, 2024–25)
• Thermal neutrons are not the right tool for every isotope — in particular, for isotopes produced via fast-neutron reactions

2. Pathway B — Particle accelerators (cyclotrons and linear accelerators)

Particle accelerators produce isotopes by bombarding target materials with charged particles — typically protons, deuterons, or alpha particles. They do not use a chain reaction; no fissile fuel is involved.

Strengths:

• Well-suited to positron-emitting diagnostic isotopes (F-18, C-11, N-13, O-15, Ga-68, Cu-64) and several therapeutic isotopes
• Can be sited close to hospitals, shortening the transport chain for short-lived nuclides
• Smaller footprint and lighter regulatory envelope than a reactor
• On-off operation: no fuel cycle, no spent fuel

Trade-offs:

• Typical yields per machine are modest compared to a high-flux reactor
• Beam time is a binding constraint; prioritisation between diagnostic and therapeutic campaigns is real
• Not every isotope has an economically viable accelerator route

In India, the national roadmap reflects this role. The Government of India’s Mega Science Vision 2035 — Nuclear Physics explicitly names accelerator-based production of medical radionuclides as a priority (§4.2.1), and allocates attention to accelerator-driven BNCT (§3.3) as a specific clinical application.

3. Pathway C — Neutron generators and accelerator-driven neutron sources

A neutron generator produces neutrons from a compact, dedicated reaction — most commonly deuterium–tritium (D-T), deuterium–deuterium (D-D), or proton-lithium (p-Li). Unlike a reactor, there is no chain reaction. Unlike a standard medical cyclotron, the primary product is a neutron beam, not a charged-particle beam.

Strengths:

• Produce neutrons at energy spectra unavailable from a thermal reactor — opening access to certain isotopes that require fast-neutron pathways
• Compact, quasi-distributable; regulatory classification is typically lighter than for a reactor
• Dual-use: the same neutron beam can serve medical applications, industrial neutron imaging, and materials testing
• Line up naturally with accelerator-driven sub-critical systems research (MSV2035 §4.1.3)

Trade-offs:

• Yield per unit is below large reactor facilities
• D-T operation requires tritium handling, which carries its own regulatory and supply-chain constraints
• Commercial operational experience is narrower than either of the first two pathways

One specific architecture within this category is the Gas Dynamic Trap (GDT) — an axisymmetric linear-mirror neutron source. It has roughly four decades of physics heritage at the Budker Institute of Nuclear Physics (BINP) in Novosibirsk, and was demonstrated at high field strengths in 2024 by the WHAM (Wisconsin HTS Axisymmetric Mirror) experiment at UW–Madison. The architecture is not exotic; it is well-characterised, and the regulatory envelope is lighter than that of a reactor.

4. What this means in practice

The three pathways are best understood as a portfolio, not a tournament. A partial map:

A country with only reactors is strong on some isotopes and blind on others. A country with only accelerators has the opposite gap. A country with only neutron generators is the narrowest of the three. Resilience follows from deploying all three, appropriately.

5. Why this is the right frame for India

India has decades of reactor-based radiochemistry expertise, anchored by DAE-linked institutions. It has a growing cyclotron base, particularly in urban medical centres. What it has historically lacked is a domestic neutron-generator layer at clinical and industrial scale.

The gap is not a failure of any one pathway. It is the absence of a third leg. Adding that leg does not replace the existing system — it extends it into isotopes and applications the first two cannot easily reach.

This is the work ASPL does: the neutron-generator layer, engineered within established regulatory and safety frameworks, designed to complement rather than compete with the reactor and cyclotron infrastructure that already exists.

6. What this piece is not

This piece is not a performance comparison, a benchmarking exercise, or a forecast. It does not claim one pathway is “better” than another — that question is always isotope-specific and application-specific. It does not claim timelines for any particular deployment; those are properly set by the regulators, not by commentary.

What we claim is narrow: the three pathways are complementary, India needs all three, and collapsing them into a binary debate does not help either the physics or the policy.

Sources & further reading

• Mega Science Vision 2035 — Nuclear Physics (DST, Government of India, commissioned by PSA)
• OECD-NEA: The Supply of Medical Radioisotopes
• Atomic Energy Regulatory Board (AERB)