India’s Thorium Push Explained: Why Early Use in PHWRs Could Change Nuclear Strategy

Nuclear expansion: India has announced an ambitious Nuclear Energy Mission targeting 100 GW of installed nuclear capacity by 2047. Government planning envisages large indigenous PHWRs, light-water reactors, SMRs and additional fast breeder capacity as part of this expansion.

Fuel-security question: Rapid expansion means much greater requirements for nuclear fuel. India possesses domestic uranium resources, but its planned fleet is expected to require significant imports. A Parliamentary Committee projected roughly 5,400 tonnes per year of uranium oxide for a future PHWR fleet of around 27 GW under the assumptions examined in its report.

Domestic resource advantage: India simultaneously possesses large thorium resources, especially in monazite-bearing beach and inland deposits. Thorium therefore represents a potential long-term indigenous energy resource.

Recent technological milestone: The PFBR achieving first criticality gives the debate additional importance because Fast Breeder Reactors constitute the bridge between India's uranium-based first stage and its long-term thorium strategy.

Nuclear scientist: Anil Kakodkar is a former Chairman of India's Atomic Energy Commission and is presently Chancellor of the Homi Bhabha National Institute.

Core proposal: At the 6th International Climate Summit, he argued that thorium should begin to be introduced into India's expanding PHWR programme instead of waiting exclusively for large stocks of U-233 to emerge through the traditional FBR-to-third-stage sequence.

Key condition: He stressed that thorium should be irradiated in PHWRs in a manner that does not significantly raise electricity costs or increase uranium requirements.

Longer-term technologies: Kakodkar also emphasised development of high-burn-up fuel, access to suitably enriched uranium, thorium recycling and molten-salt reactor technologies over the coming years.

Element: Thorium is a naturally occurring radioactive metallic element. The overwhelmingly dominant natural isotope is Thorium-232.

Nuclear classification: Thorium-232 is fertile, not fissile. This distinction is fundamental.

Fissile material: A fissile isotope such as Uranium-235, Plutonium-239 or Uranium-233 can undergo fission with suitable neutrons and sustain a nuclear chain reaction.

Fertile material: A fertile isotope cannot normally sustain the required chain reaction directly but can absorb neutrons and be transformed into a fissile isotope.

Thus, having large thorium deposits does not mean India can simply load pure thorium into existing reactors and produce electricity.

The conversion occurs approximately as follows: Thorium-232 + neutron → Thorium-233

Thorium-233 undergoes beta decay → Protactinium-233

Protactinium-233 undergoes beta decay → Uranium-233

End product: Uranium-233 is fissile and can sustain a nuclear fission chain reaction.

Meaning of breeding: The process of converting fertile material into fissile material is called breeding.

Why an external fissile source is initially necessary: Before enough U-233 exists, neutrons must come from fission of another fissile isotope such as U-235 or Pu-239.

This is the basic reason India's thorium strategy requires interaction with the uranium-plutonium fuel cycle rather than a direct jump from thorium ore to a thorium-only reactor.

Monazite: India's principal thorium resource is associated with monazite, a phosphate mineral containing thorium and rare-earth elements.

Geographical occurrence: AMD has identified monazite in coastal beach sands, teri/red sands and inland alluvium in parts of Andhra Pradesh, Odisha, Tamil Nadu, Kerala, West Bengal, Jharkhand, Gujarat and Maharashtra.

Resource estimate: As of early 2026, AMD reported 13.15 million tonnes of monazite containing around 1.18 million tonnes of in-situ thorium oxide.

Exam caution: Claims about whether India ranks first, second or third globally in thorium “reserves” vary according to whether a source measures identified resources, reserves or different geological categories. For UPSC, the safer and more defensible fact is that India possesses very large thorium resources concentrated largely in monazite.

Resource mismatch: India historically possessed relatively limited readily usable uranium resources but much larger thorium resources.

Homi Bhabha's strategy: India's nuclear programme was therefore designed around a closed fuel cycle that progressively converts fertile material into fissile fuel and multiplies the energy obtainable from indigenous resources.

Long-term objective: The final purpose is not simply to operate three different reactor types. It is to create a self-sustaining domestic fissile-material inventory capable of supporting large-scale nuclear generation with thorium.

DAE continues to describe the three-stage programme as central to long-term nuclear energy security.

Reactor: Pressurised Heavy Water Reactors.

Fuel: Primarily natural uranium.

Moderator and coolant: Heavy water—D₂O—is used because deuterium absorbs relatively few neutrons, giving PHWRs excellent neutron economy.

Fission: The small U-235 fraction in natural uranium undergoes fission and releases energy.

Plutonium production: Some abundant U-238 absorbs neutrons and ultimately becomes fissile Pu-239.

Closed-cycle connection: After irradiation, spent PHWR fuel is reprocessed so that plutonium can be recovered for Stage 2.

The government says India has developed expertise across design, construction, operation and front- and back-end fuel-cycle activities for indigenous PHWRs.

Natural uranium composition: Natural uranium consists overwhelmingly of U-238 with only about 0.7% fissile U-235.

Heavy-water advantage: Ordinary light water absorbs more neutrons than heavy water. Heavy water's exceptionally low neutron absorption preserves enough neutrons to sustain the reaction even with the small U-235 fraction present in natural uranium.

Fuel flexibility: Good neutron economy and pressure-tube architecture also give PHWRs considerable flexibility to accommodate alternative fuel cycles.

This flexibility is one reason PHWRs are relevant to Kakodkar's proposal for earlier thorium irradiation.

Fast Breeder Reactors: Stage 2 uses fast-neutron reactors primarily fuelled by plutonium recovered from Stage 1.

Breeding principle: A breeder reactor is designed to produce fissile material from fertile material at a rate that can exceed its consumption of fissile fuel.

Uranium breeding: U-238 in the blanket can absorb fast neutrons and ultimately become Pu-239.

Thorium bridge: Thorium-232 can eventually be placed in breeding configurations to generate U-233 required for the third stage.

Thus, the second stage is not simply another electricity-producing stage—it is intended to multiply India's fissile fuel inventory.

Location: Kalpakkam, Tamil Nadu.

Capacity: 500 MWe.

Developer: Its technology was developed indigenously by the Indira Gandhi Centre for Atomic Research, while BHAVINI is responsible for the project.

Fuel: The PFBR uses uranium-plutonium mixed oxide fuel.

Coolant: It is a sodium-cooled fast reactor.

Milestone: The PFBR attained first criticality on 6 April 2026, which DAE regards as India's formal entry into the second stage.

Self-sustaining reaction: A reactor reaches criticality when, on average, one neutron from each fission causes another fission, allowing the chain reaction to sustain itself.

Subcritical: The neutron population progressively falls.

Critical: The neutron population remains steady.

Supercritical: The neutron population increases.

Important distinction: First criticality does not mean the reactor instantly begins full commercial electricity production. It is an important commissioning milestone followed by tests, progressive power increases and regulatory clearances.

Fast neutrons: Conventional thermal reactors slow neutrons down because slow or “thermal” neutrons are very effective at causing fission in isotopes such as U-235.

Fast reactor: An FBR largely avoids moderating neutrons and operates with a high-energy neutron spectrum.

Breeding benefit: Fast neutrons can provide favourable neutron economy for converting fertile U-238 into Pu-239 and thereby expanding fissile-material inventories.

Coolant choice: Sodium is useful because it transfers heat efficiently while slowing neutrons much less than water.

Thorium utilisation: Stage 3 envisages large-scale use of Thorium-232 with Uranium-233 bred during earlier stages.

Self-sustaining cycle: Once adequate U-233 inventory has been accumulated, thorium-based systems can recycle U-233 and breed additional fissile fuel from fresh Th-232.

End objective: The third stage is intended to make India's large thorium resource the foundation of long-term nuclear-energy self-reliance.

Thorium cannot start itself: Thorium-232 is fertile rather than fissile.

Fissile inventory constraint: Large-scale thorium reactors require significant quantities of fissile driver material—U-233, Pu-239 or enriched uranium—to initiate and sustain the breeding cycle.

Growth problem: Early planners therefore prioritised producing enough plutonium through PHWRs and multiplying fissile material through breeder reactors before deploying thorium at large scale.

Scale matters: A thorium system may sustain an existing energy-production level once an adequate fuel inventory exists, but rapidly creating the fissile stock required to expand a huge fleet is more difficult.

This is the fundamental reason the original programme was structured sequentially.

Much larger PHWR fleet: India's nuclear ambitions are now far larger than in the early decades of the programme. Kakodkar argues that tens of gigawatts of PHWR capacity provide a large platform in which thorium could be irradiated.

Parallel rather than sequential approach: Instead of treating Stage 1 → Stage 2 → Stage 3 as completely isolated blocks, thorium-bearing fuel could potentially begin accumulating practical experience and bred U-233 within PHWRs while FBR capacity continues to grow.

Time advantage: Such an approach could reduce the long delay before India develops mature commercial capability across fabrication, irradiation, reprocessing and recycling of thorium fuels.

Core idea: The proposal is therefore not “skip Stage 2”; it is “do not unnecessarily postpone all thorium-related deployment until Stage 2 becomes very large”.

Yes, experimentally: DAE states that thoria pellets have been used in the initial cores of operating PHWRs. Thorium-based fuel has also been irradiated in BARC research reactors.

Purpose: Such use has generated operational and material-science experience and enabled production of U-233.

Difference from commercial adoption: Experimental irradiation of thorium is very different from replacing a substantial fraction of commercial PHWR fuel with a thorium-bearing fuel cycle.

Therefore, Kakodkar's proposal involves scaling an area in which India already has research experience rather than beginning from zero.

Full identity: KAMINI is the Kalpakkam Mini Reactor located at IGCAR.

Power: It is a 30-kWt research reactor.

Fuel: It uses Uranium-233 fuel obtained through India's thorium-related fuel-cycle work.

Current status: IGCAR reports that KAMINI continues to operate for applications including neutron radiography, activation analysis, detector testing and irradiation experiments.

Strategic significance: KAMINI demonstrates that India has already completed the scientific chain from thorium irradiation to U-233 recovery, fuel fabrication and reactor utilisation at research scale.

It does not, however, constitute commercial-scale thorium electricity generation.

Technology demonstrator: BARC's Advanced Heavy Water Reactor (AHWR) is designed specifically to demonstrate large-scale thorium-fuel-cycle technologies.

Capacity: 300 MWe.

Design: It is a vertical pressure-tube reactor, boiling-light-water cooled and heavy-water moderated.

Fuel concept: The design uses thorium together with plutonium and U-233 in mixed oxide fuels.

Safety architecture: It incorporates passive and inherent safety features, including natural-circulation heat removal.

Role: AHWR is better understood as a gateway for validating thorium-cycle technologies than as proof that a large commercial Indian thorium fleet already exists.

Once-through cycle: In a once-through fuel cycle, spent fuel is removed from the reactor and ultimately managed as waste without routinely recovering usable fissile materials.

Closed cycle: India follows a strategy in which spent fuel is chemically reprocessed and useful fissile material is recovered and recycled.

For example: PHWR spent fuel → plutonium recovery → FBR fuel

Irradiated thorium → U-233 recovery → future thorium fuel

Strategic benefit: A closed cycle extracts much more energy from the original nuclear material and is essential to India's three-stage strategy.

Complexity: Reprocessing requires sophisticated chemistry, remote handling, safeguards, waste management and high safety standards.

Resource multiplication: Only a small fraction of the potential energy in natural uranium is extracted through ordinary once-through thermal-reactor operation.

Breeding: Reprocessing and breeder reactors allow U-238 and Th-232—fertile materials that constitute enormous energy resources—to be converted into fissile Pu-239 and U-233.

Kakodkar's argument: He has argued that recycling can increase utilisation of uranium resources many tens of times compared with a simple once-through approach.

UPSC perspective: India's nuclear strategy is therefore better understood as a fuel-cycle strategy, not merely a list of reactor projects.

Meaning: Burn-up measures how much energy has been extracted from a given quantity of nuclear fuel.

It is commonly expressed in units such as megawatt-days per tonne of heavy metal.

High burn-up: Higher burn-up means more energy is obtained before the fuel must be removed from the reactor.

Potential advantages: It can reduce the volume of spent fuel generated per unit of electricity and reduce refuelling requirements.

Challenge: Higher burn-up places greater demands on fuel cladding, materials, thermal performance and safety qualification.

Kakodkar's call for developing high-burn-up fuels is therefore a materials and reactor-engineering challenge, not merely a change in fuel chemistry.

Natural uranium: Contains only about 0.7% U-235.

Enrichment: Increases the fraction of fissile U-235.

Reason for combination: Because thorium itself is fertile, a thorium-bearing PHWR fuel may require fissile “driver” material such as enriched uranium or plutonium to provide neutrons.

Trade-off: Using higher-enrichment uranium could facilitate greater thorium utilisation but would introduce enrichment-supply, cost, licensing and safeguards considerations.

This is why “use thorium in PHWRs” does not mean loading reactors with thorium alone.

Separate development: ANEEL—Advanced Nuclear Energy for Enriched Life—is a proprietary thorium-uranium fuel being developed by Clean Core Thorium Energy for heavy-water reactors.

Composition: It combines thorium with enriched uranium rather than relying on pure thorium.

Indian exploration: NTPC and CCTE have been exploring its possible development and deployment in India, subject to government and regulatory approval.

Important distinction: Kakodkar's broader strategic argument for irradiating thorium in the large PHWR fleet should not automatically be treated as identical to adoption of ANEEL.

Government position: DAE stated in February 2026 that natural-uranium oxide would continue to be used in India's current PHWR fleet while the ANEEL proposal remained under exploration.

Concept: In a Molten Salt Reactor, nuclear fuel may be dissolved in a high-temperature molten salt or the salt may function as the coolant depending on the design.

Potential thorium role: A thorium-U-233 fuel cycle can be implemented in certain molten-salt breeder-reactor concepts.

Pressure advantage: DAE notes that thorium molten-salt breeder concepts can operate at near-atmospheric pressure, potentially providing some safety advantages compared with high-pressure water-cooled systems.

Maturity problem: DAE also clearly states that the technology is not yet mature. India is working on materials, molten-fluoride chemistry and component technologies.

Therefore, molten-salt reactors should be understood as a future R&D pathway, not an immediately deployable commercial substitute for PHWRs.

Energy security: Large domestic thorium resources could reduce long-run dependence on imported fissile resources.

Resource utilisation: A successfully closed Th-U-233 cycle could provide very large amounts of energy from a domestically available fertile material.

Waste profile: Thorium cycles can, depending on reactor and fuel-cycle design, generate lower quantities of plutonium and some long-lived transuranic elements than conventional uranium-plutonium cycles. BARC and DAE identify reduced long-lived waste as one potential advantage.

Climate relevance: Nuclear power provides low-carbon electricity that can complement variable solar and wind generation while supporting India's growing electricity requirements.

Strategic autonomy: A mature indigenous thorium cycle could reduce exposure to international fuel-market disruption.

Technology leadership: Few countries have pursued all stages of the thorium cycle—from mineral processing and irradiation to U-233 recovery and reactor research—as systematically as India.

Fuel versus reactor distinction: Thorium is a fuel-cycle material, not a reactor design. Safety depends on the whole system—coolant, moderator, pressure, containment, shutdown systems, fuel geometry and operating procedures.

Material advantages: Thoria has useful physical and thermal characteristics and certain thorium systems may permit favourable safety coefficients.

Passive safety: AHWR, for example, includes several passive safety features, but those features arise primarily from the reactor design rather than merely from the presence of thorium.

Correct conclusion: “Thorium reactor = automatically safe” is too simplistic for UPSC.

No.

Potential reduction: Some thorium fuel cycles can produce lower amounts of long-lived transuranic waste.

Fission products remain: Fission still produces highly radioactive fission products requiring safe management.

U-232 issue: U-233 produced from thorium is normally accompanied by some Uranium-232. Its decay chain includes intense gamma emitters such as Bi-212 and Tl-208.

Handling challenge: BARC notes that this high gamma radiation requires remote and shielded fuel fabrication and recycling.

Thus, thorium changes the waste profile; it does not make radioactive-waste management unnecessary.

Stable thoria: Thorium dioxide is chemically very stable, which is useful inside a reactor but makes dissolution during reprocessing more difficult.

Corrosion issue: BARC notes that fluoride additions can help dissolve thoria but can also increase corrosion of processing equipment.

Three-stream separation: Some thorium fuel cycles require sophisticated separation and management of uranium, thorium and plutonium streams.

Radiation: U-232 contamination causes high gamma fields, requiring remote operations.

These back-end challenges are among the main reasons commercial thorium deployment has progressed much more slowly than laboratory demonstrations.

No absolute immunity: U-233 is a fissile isotope and therefore requires strict safeguards and nuclear-material accounting.

Complicating factor: U-232 contamination and its intense gamma-emitting decay products make handling separated U-233 technically difficult and hazardous.

Balanced interpretation: Certain thorium fuel-cycle configurations may offer proliferation-resistance advantages, but it would be inaccurate to describe thorium as completely proliferation-proof.

Kakodkar's concern: Kakodkar argues that rapidly expanding global nuclear programmes could create a structural uranium-supply squeeze and that India should prepare early.

Global resource assessment: The OECD Nuclear Energy Agency and IAEA reach a more nuanced conclusion. Their latest Red Book says identified uranium resources are sufficient to support substantial nuclear growth through 2050 and beyond.

But availability is different from geology: New mines, mills and processing facilities require large investments and long lead times. Geopolitical problems and regulatory delays can cause market shortages even when uranium exists underground.

Indian vulnerability: India additionally faces its own imbalance between domestic production and a rapidly expanding nuclear fleet.

Hence the issue is better described as fuel-supply security, not simply physical exhaustion of global uranium.

Domestic production: Uranium Corporation of India Limited develops domestic uranium resources, while imported uranium is used in eligible safeguarded reactors.

Import sources: Parliamentary evidence cited Kazakhstan, Russia, Uzbekistan and Canada among India's uranium suppliers.

Future requirement: The Committee on Public Undertakings warned that PHWR expansion could sharply increase natural-uranium requirements and called for domestic mining, overseas resource acquisition and diversified supply.

Strategic implication: Thorium does not remove the need to secure uranium in the near term because India's current PHWR fleet and the fissile drivers required for thorium conversion still depend upon uranium-derived materials.

New legal framework: Parliament enacted the Sustainable Harnessing and Advancement of Nuclear Energy for Transforming India (SHANTI) Act, 2025.

Private participation: It allows wider licensed participation of non-government entities in parts of the nuclear-energy sector while retaining government licensing, regulatory oversight and controls over sensitive activities.

Regulation: The legislation gives statutory recognition to the Atomic Energy Regulatory Board.

Current implementation: As of July 2026, the government stated that rules under the Act were still being drafted.

Thorium connection: Expanding industry participation could potentially accelerate manufacturing, fuel-development and advanced-reactor R&D, but safety, safeguards and fuel-cycle controls remain essential.

Current contribution: Nuclear power supplied about 3.1% of India's electricity generation in 2024-25.

Expansion: Installed capacity of 8.78 GW is planned to rise to about 22 GW by 2031-32 and ultimately 100 GW by 2047.

Low-carbon firm power: Nuclear plants can generate electricity independent of sunshine and wind conditions and can therefore complement large renewable capacity.

Net-zero connection: The Nuclear Energy Mission is explicitly connected with India's long-term energy-security strategy and the goal of net-zero emissions by 2070.

Broader portfolio: Thorium should therefore be seen as one possible long-term component of an energy system that also includes renewables, storage, grids, energy efficiency and other low-carbon technologies.

Fuel qualification: New thorium-containing fuel must undergo extensive irradiation, materials testing and regulatory qualification before commercial use.

Fissile driver requirement: Thorium still needs U-235, Pu-239 or U-233 to sustain the reaction.

Reprocessing complexity: Recovering U-233 safely and economically from irradiated thorium is technically demanding.

U-232 contamination: Hard gamma radiation complicates fabrication, transport and recycling.

Economics: Any new fuel strategy must compete with established natural-uranium PHWR fuel while avoiding increases in electricity tariffs.

Supply contradiction: Some faster thorium routes may initially require enriched uranium, which itself creates a dependence on enrichment capability or external supply.

Industrial scale: Laboratory success and research-reactor operation do not automatically translate into thousands of commercial fuel bundles.

Regulation: New fuel behaviour must be assessed for reactivity coefficients, shutdown margins, thermal performance, accident behaviour and waste management.

Time: Nuclear fuel development and licensing generally require long experimental and regulatory cycles.

No: The breeder programme remains essential because India needs to multiply fissile material, especially if it ultimately wants a self-sustaining U-233/Th-232 cycle.

Complementary pathway: PHWR irradiation could supplement breeding and accelerate experience with thorium fuel.

FBR advantage: Fast breeders remain especially powerful for building fissile inventories from fertile materials.

Third-stage objective unchanged: Large-scale sustainable utilisation of thorium still depends on solving the entire closed fuel cycle—breeding, fabrication, reprocessing, recycling and waste management.

Therefore, the proposed change is best understood as making India's three stages more overlapping and iterative rather than strictly sequential.

Thorium is not a ready-made fuel: It is a fertile resource from which U-233 must first be created.

Three-stage programme is about fuel multiplication: PHWR → plutonium → FBR → U-233/thorium cycle.

PFBR is the bridge: Its 2026 criticality is important because breeder technology connects the uranium era to eventual large-scale thorium use.

Kakodkar's proposal is an acceleration strategy: It seeks to exploit India's future large PHWR fleet to begin thorium irradiation earlier.

Energy security is broader than resource abundance: A successful thorium programme requires reactors, enrichment or fissile drivers, reprocessing, fuel fabrication, waste management, regulation, economics and skilled manpower.

India should preserve the logic of its closed three-stage fuel cycle while testing whether selected PHWRs can safely accelerate thorium irradiation without reducing reactor economics or increasing uranium consumption disproportionately.

Thorium-bearing PHWR fuel should be introduced only after systematic irradiation testing, post-irradiation examination, reactor-physics validation and AERB safety approval.

The PFBR should be stabilised through commissioning and commercial operation, followed by carefully sequenced expansion of fast breeder capacity so that India's fissile-material inventory can grow reliably.

India should accelerate U-233 separation, remote fuel fabrication and thorium reprocessing technologies because the back end of the cycle is as important as reactor design.

The AHWR programme should be used to validate integrated thorium-cycle technologies, especially passive safety, fuel management, recycling and waste-management systems.

R&D on molten-salt reactors should continue, but deployment decisions should follow technological demonstration rather than assuming that an immature design will automatically solve current fuel-security problems.

India should simultaneously strengthen domestic uranium exploration and mining, diversify foreign uranium supplies and examine strategic stockpiles; thorium is principally a medium- to long-term solution rather than an immediate substitute for uranium.

Monazite extraction should be accompanied by strong coastal-environment safeguards, radiation protection and responsible rare-earth/thorium separation.

Universities, BARC, IGCAR, NPCIL, BHAVINI and the domestic nuclear industry should expand human-resource development in actinide chemistry, reactor materials, remote robotics, radiochemistry and fuel-cycle engineering.

The SHANTI Act's implementation rules should ensure that wider industry participation is accompanied by strong, independent and transparent nuclear-safety regulation.

India should evaluate thorium on full-system economics—capital costs, enrichment requirements, fuel fabrication, reprocessing, waste management and decommissioning—not merely on the low cost or abundance of the raw mineral.

The strategic goal should remain technology-neutral: thorium deserves accelerated development because of India's resource geography, but deployment should ultimately depend on safety, economics, scalability and energy-security gains.