Nuclear Energy: Reactors, Fuel Cycle and India's Three-Stage Programme
Fission and fusion, reactor parts and types, the nuclear fuel cycle, Bhabha's three-stage plan, PFBR, SMRs and the SHANTI Act 2025 for CGPSC Prelims and Mains.
Prerequisites
- Conventional Energy Technology: a nuclear plant is a thermal plant whose heat comes from fission instead of coal.
- Chemistry in Everyday Life: atoms, isotopes and radioactivity.
- Conventional Energy (Geography): locations of India’s nuclear power stations.
The Hook: Why Does India Want Thorium?
Chhattisgarh’s steel plants at Bhilai and its aluminium smelter at Korba run on coal-fired power. Under the Nuclear Energy Mission, the government wants small reactors that heavy industries like these could one day use as captive, low-carbon power plants.
Meanwhile, at Kalpakkam in Tamil Nadu, the Prototype Fast Breeder Reactor (PFBR) reached first criticality on 6 April 2026, taking India into the second stage of a plan drawn up by Homi J. Bhabha in the 1950s. The goal of that plan is to run India’s reactors on thorium, an element found in the beach sands of Kerala and Odisha. Why thorium, why three stages, and how does a reactor work at all?
Why it matters: Prelims asks reactor components (moderator, coolant, control rods), reactor types, fuel cycle facts, the three stages and recent milestones (PFBR, SHANTI Act). Mains GS-II asks about nuclear energy’s role, safety and India’s programme.
The Physics: Fission and Fusion
Isotopes are atoms of the same element with different numbers of neutrons. Natural uranium is about 99.3% U-238 and only about 0.7% U-235.
Energy from mass: in both fission and fusion, the products weigh slightly less than the starting nuclei. The missing mass becomes energy by Einstein’s relation \( E = mc^2 \). Because \(c^2\) is enormous, a tiny mass gives a huge amount of energy.
Nuclear fission (नाभिकीय विखंडन)
A slow (thermal) neutron strikes a U-235 nucleus, which splits into two lighter nuclei, releasing about 200 MeV of energy and 2–3 new neutrons.
$$^{235}_{92}\mathrm{U} + {}^{1}_{0}n \rightarrow {}^{141}_{56}\mathrm{Ba} + {}^{92}_{36}\mathrm{Kr} + 3\,{}^{1}_{0}n + \text{energy}$$- The new neutrons can split more nuclei: a chain reaction.
- Critical (k = 1): each fission causes, on average, exactly one more. This steady state is what a reactor wants; “first criticality” means a reactor has achieved a self-sustaining chain reaction for the first time.
- Supercritical (k > 1): the reaction grows (an uncontrolled version is a bomb). Subcritical (k < 1): it dies out.
- Fissile nuclei split with slow neutrons: U-235, Pu-239, U-233. Fertile nuclei do not, but can be converted into fissile ones by absorbing a neutron: U-238 → Pu-239, Th-232 → U-233.
Nuclear fusion (नाभिकीय संलयन)
Light nuclei join to form a heavier one. The easiest reaction fuses deuterium and tritium (two isotopes of hydrogen):
$$^{2}_{1}\mathrm{H} + {}^{3}_{1}\mathrm{H} \rightarrow {}^{4}_{2}\mathrm{He} + {}^{1}_{0}n + 17.6\ \text{MeV}$$- This powers the Sun and stars.
- On Earth it needs temperatures above 100 million °C, so the hot plasma is held in a magnetic cage called a tokamak.
- ITER (International Thermonuclear Experimental Reactor) is being built at Cadarache, France. Its seven members are the EU, China, India, Japan, South Korea, Russia and the USA; India joined in 2005.
- India’s own tokamaks are Aditya and SST-1 at the Institute for Plasma Research, Gandhinagar.
- Fusion produces no long-lived high-level waste and has no runaway chain reaction, but no fusion device has yet supplied commercial electricity.
| Point | Fission | Fusion |
|---|---|---|
| Process | Heavy nucleus splits | Light nuclei join |
| Fuel | U-235, Pu-239, U-233 | Deuterium, tritium |
| Conditions | Achieved in ordinary reactors | Extreme temperature and pressure |
| Waste | Long-lived radioactive waste | Much less; mainly activated structures |
| Status | Commercial since the 1950s | Experimental (ITER) |
Inside a Nuclear Reactor
A nuclear power plant (परमाणु बिजलीघर) is a thermal power plant: fission heat makes steam, steam spins a turbine, the turbine drives a generator. What differs is the “boiler”.
| Component | Job | Common materials |
|---|---|---|
| Fuel | Fissile material in sealed rods | Natural or enriched uranium oxide; MOX (uranium–plutonium oxide) |
| Moderator (मंदक) | Slows down fast neutrons so they can cause more fission in U-235 | Heavy water (D₂O), ordinary (light) water, graphite |
| Control rods (नियंत्रक छड़) | Absorb neutrons to control or stop the chain reaction | Boron, cadmium, hafnium |
| Coolant (शीतलक) | Carries heat away from the core to make steam | Light water, heavy water, liquid sodium, CO₂ or helium |
| Reflector | Bounces escaping neutrons back into the core | Beryllium, graphite, water |
| Shielding and containment | Stops radiation and holds any release inside a thick concrete and steel dome | Concrete, steel, lead |
graph LR
A["Fuel rods
(fission heat)"] --> B["Coolant carries heat"]
M["Moderator slows neutrons"] --> A
C["Control rods absorb neutrons"] --> A
B --> D["Steam generator"]
D --> E["Turbine"]
E --> F["Generator → grid"]
E --> G["Condenser / cooling water"]Reactor types
| Type | Fuel | Moderator / coolant | Indian example |
|---|---|---|---|
| PHWR (pressurised heavy water reactor) | Natural uranium | Heavy water for both | India’s workhorse: units of 220, 540 and 700 MW; Kakrapar has the first indigenous 700 MW PHWRs |
| BWR (boiling water reactor) | Enriched uranium | Light water; water boils in the core | Tarapur 1 and 2 (1969), India’s first nuclear power station |
| PWR / VVER (pressurised water reactor) | Enriched uranium | Light water under high pressure | Kudankulam (Russian VVER, 1,000 MW units), India’s largest station |
| FBR (fast breeder reactor) | Plutonium–uranium (MOX), surrounded by a U-238 (later thorium) blanket | No moderator; liquid sodium coolant | FBTR (Kalpakkam, 1985); PFBR (500 MWe, Kalpakkam) |
| AHWR (advanced heavy water reactor) | Thorium with U-233 / plutonium | Heavy water moderator, light water coolant | 300 MWe design by BARC for the third stage |
| RBMK | Low-enriched uranium | Graphite moderator, water coolant | Not used in India; the Chernobyl design |
The Nuclear Fuel Cycle
graph LR
A["Mining
Jaduguda, Tummalapalle"] --> B["Milling
yellowcake U₃O₈"]
B --> C["Conversion and
enrichment (for LWRs)"]
C --> D["Fuel fabrication
NFC Hyderabad"]
D --> E["Reactor"]
E --> F["Spent fuel
(cooling pools)"]
F --> G["Reprocessing
separate Pu and U"]
G --> D
G --> H["High-level waste
vitrification, storage"]| Stage | What happens | India |
|---|---|---|
| Mining | Uranium ore is mined | Uranium Corporation of India Ltd (UCIL): Jaduguda (Jharkhand), Tummalapalle (Andhra Pradesh). Indian ores are low-grade |
| Milling | Ore is crushed and chemically processed into yellowcake (U₃O₈) | UCIL mills |
| Enrichment | Raises the share of U-235. Light-water reactor fuel: about 3–5%; weapons-grade: about 90% | Not needed for PHWRs; India’s enrichment plant is at Rattehalli, near Mysuru |
| Fuel fabrication | Uranium oxide pellets in zircaloy tubes, bundled into fuel assemblies | Nuclear Fuel Complex (NFC), Hyderabad |
| Reactor | Fission; some U-238 turns into Pu-239 | 24 reactors (July 2026) |
| Reprocessing | Spent fuel is dissolved and plutonium and uranium are separated (PUREX process) for reuse | Trombay, Tarapur, Kalpakkam |
| Waste management | High-level liquid waste is fixed in glass (vitrification) and stored; long-term plan is a deep geological repository | BARC |
Open vs closed cycle: in an open (once-through) cycle, spent fuel is treated as waste. In a closed cycle, it is reprocessed and the plutonium and uranium are used again. India follows a closed fuel cycle, because its three-stage programme depends on recovering plutonium.
Radioactivity basics: alpha particles are stopped by paper, beta by a thin aluminium sheet, gamma rays need thick lead or concrete. Activity is measured in becquerel (Bq); the biological effect of a dose in sievert (Sv).
India’s Three-Stage Nuclear Power Programme
Why three stages? India has limited, low-grade uranium but some of the world’s largest thorium reserves, in monazite beach sands of Kerala, Odisha, Andhra Pradesh and Tamil Nadu. The Atomic Minerals Directorate has established about 11.93 million tonnes of monazite, containing roughly 1 million tonnes of thorium oxide; India’s share of world thorium is often cited as about a quarter. But thorium is fertile, not fissile: it cannot sustain a chain reaction by itself. It must first be converted into U-233, and that needs a lot of neutrons from another fissile fuel. The three stages build up that fissile stock step by step.
graph TD
A["Stage 1: PHWRs
Fuel: natural uranium
Output: power + plutonium-239"] --> B["Stage 2: Fast Breeder Reactors
Fuel: plutonium (MOX)
Blanket: U-238 → more Pu-239
then thorium → U-233"]
B --> C["Stage 3: Thorium reactors
Fuel: thorium + U-233
Self-sustaining thorium cycle"]| Stage | Reactor | Fuel → product | Status (2026) |
|---|---|---|---|
| 1 | PHWR | Natural uranium → power; U-238 in the fuel becomes Pu-239 | Mature; most of India’s 24 reactors |
| 2 | Fast breeder reactor (FBR) | Pu-239 (as MOX) → power; a U-238 blanket breeds more Pu-239 than is used. Later a thorium blanket breeds U-233 | FBTR (Kalpakkam, 1985) proved the technology; PFBR (500 MWe, built by BHAVINI) reached first criticality on 6 April 2026 |
| 3 | Thorium-based reactors (e.g. AHWR design) | Th-232 + U-233 → power; thorium keeps breeding U-233 | Research stage. KAMINI (30 kW, Kalpakkam, 1996) runs on U-233 fuel |
Key conversion reactions:
$$^{238}\mathrm{U} + n \rightarrow {}^{239}\mathrm{U} \xrightarrow{\beta^-} {}^{239}\mathrm{Np} \xrightarrow{\beta^-} {}^{239}\mathrm{Pu}$$$$^{232}\mathrm{Th} + n \rightarrow {}^{233}\mathrm{Th} \xrightarrow{\beta^-} {}^{233}\mathrm{Pa} \xrightarrow{\beta^-} {}^{233}\mathrm{U}$$India’s Nuclear Programme: Institutions and Milestones
| Year | Milestone |
|---|---|
| 1948 | Atomic Energy Commission set up (Homi J. Bhabha its first chairman) |
| 1954 | Department of Atomic Energy (DAE) created; Atomic Energy Establishment, Trombay (renamed BARC in 1967) |
| 1956 | Apsara at Trombay: first research reactor in Asia |
| 1969 | Tarapur: India’s first nuclear power station (BWRs) |
| 1974 / 1998 | Pokhran-I / Pokhran-II nuclear tests |
| 1983 | Atomic Energy Regulatory Board (AERB) set up |
| 1985 | FBTR at Kalpakkam reaches criticality |
| 1987 | NPCIL (Nuclear Power Corporation of India Ltd) formed to build and run power reactors |
| 2008 | India–US civil nuclear agreement; Nuclear Suppliers Group (NSG) waiver allows India to import fuel and reactors despite not signing the NPT |
| 2025 | Nuclear Energy Mission (Budget 2025-26); SHANTI Act passed in December |
| 2026 | PFBR first criticality, 6 April 2026 |
India’s fleet (as of July 2026): 24 reactors with 8,780 MW at seven sites (Tarapur, Rawatbhata, Kalpakkam, Narora, Kakrapar, Kaiga, Kudankulam), run by NPCIL. About ten more units are under construction. Nuclear is still under 2% of India’s installed capacity of about 552 GW.
- Nuclear Energy Mission (Union Budget 2025-26): target of 100 GW of nuclear capacity by 2047; ₹20,000 crore for research and development of Small Modular Reactors (SMRs), with at least five indigenous SMRs operational by 2033.
- Bharat Small Reactors (BSR): the proven 220 MW PHWR design adapted, with a smaller land footprint, as captive power plants for industries such as steel, aluminium and metals.
- BARC’s SMR designs: BSMR-200 (about 200 MWe, pressurised water reactor) and SMR-55 (55 MWe), plus a small high-temperature gas-cooled reactor intended for hydrogen production.
- SHANTI Act, 2025 (Sustainable Harnessing and Advancement of Nuclear Energy for Transforming India), assent 20 December 2025:
- Replaces the Atomic Energy Act, 1962 and the Civil Liability for Nuclear Damage Act, 2010.
- Opens building, owning and operating nuclear power plants to qualified private companies and joint ventures, ending a government monopoly of over six decades.
- Keeps strategic activities (enrichment beyond notified limits, reprocessing and spent-fuel management, heavy water, high-level waste) with the Central Government.
- Gives the AERB statutory status.
- Channels liability to the operator and removes the supplier “right of recourse” that had deterred foreign and private suppliers under the 2010 Act.
- Draft SHANTI Rules, 2026 were released in August 2026.
Chhattisgarh angle: Chhattisgarh has no nuclear power station (as of 2026); its grid is coal-dominated. The BSR and SMR plans are aimed at exactly the kind of industry the state hosts: steel (Bhilai, Raipur, Raigarh) and aluminium (BALCO, Korba). Captive small reactors could one day replace coal-fired captive plants, though siting, water and public acceptance would decide that.
Analysis for Mains: Should India Expand Nuclear Power?
| For | Against / concerns |
|---|---|
| Low-carbon base load: runs round the clock at high capacity factor, unlike solar and wind | High capital cost and long construction times; cost overruns (PFBR took about two decades) |
| Very small land and fuel footprint per unit of energy | Safety: Three Mile Island (1979), Chernobyl (1986), Fukushima (2011) shaped public fear |
| Energy security: thorium reserves can last centuries in a closed cycle | Radioactive waste needs safe storage for thousands of years |
| Strategic technology base; SMRs for industry decarbonisation | Local opposition (Kudankulam protests, Jaitapur), land acquisition, cooling water |
| SHANTI Act brings private capital and global suppliers | Liability and regulatory independence must be credible; proliferation concerns |
Way forward: complete stage 2 (PFBR, then more FBRs), fleet-mode 700 MW PHWRs, SMRs for industry, a credible independent AERB, transparent safety communication, and a long-term waste repository.
Memory Tricks
Don’t Confuse
| This | vs | That |
|---|---|---|
| Moderator: slows neutrons | vs | Control rods: absorb neutrons |
| Fissile: U-235, Pu-239, U-233 | vs | Fertile: U-238, Th-232 |
| PHWR: natural uranium, heavy water | vs | PWR/BWR: enriched uranium, light water |
| FBR: no moderator, liquid sodium coolant | vs | Thermal reactors: need a moderator |
| FBTR (1985): test reactor | vs | PFBR (500 MWe): prototype, criticality 6 April 2026 |
| Tarapur: first (1969), BWR | vs | Kudankulam: largest, Russian VVER |
| NPCIL: builds and runs most reactors | vs | BHAVINI: built the PFBR · AERB: regulator |
| Fission: splitting, commercial | vs | Fusion: joining, experimental (ITER, Cadarache) |
| SHANTI Act 2025: new nuclear law | vs | Atomic Energy Act 1962, CLNDA 2010: replaced laws |
Common Mistakes
- “Thorium is a fissile fuel that can be used directly in reactors.” False. Thorium is fertile; it must first be converted to U-233.
- “Control rods slow down neutrons.” False. That is the moderator. Control rods absorb neutrons.
- “Fast breeder reactors use heavy water as moderator.” False. They have no moderator and use liquid sodium as coolant.
- “PHWRs need enriched uranium.” False. Their heavy-water moderator lets them use natural uranium.
- “Tarapur is India’s largest nuclear plant.” False. It is the oldest; Kudankulam is the largest.
- “Fusion power is already supplying electricity.” False. ITER is experimental.
- “The SHANTI Act lets private companies do everything in the fuel cycle.” False. Strategic activities such as reprocessing, heavy water and high-level waste stay with the Central Government.
Practice
All questions below are original practice questions written in CGPSC style. They are not previous-year questions.
Level 1: Foundation
- In a nuclear reactor, the material that slows down neutrons is called the ________.
- Natural uranium contains only about ________ % of the fissile isotope U-235.
- The second stage of India’s nuclear programme is based on ________ reactors.
Answers
- Moderator (heavy water, light water or graphite).
- 0.7%.
- Fast breeder reactors (PFBR, Kalpakkam).
Level 2: Prelims MCQs
Q1. Consider the following statements regarding India’s three-stage nuclear power programme:
- The first stage uses pressurised heavy water reactors fuelled by natural uranium.
- The second stage uses fast breeder reactors in which plutonium is the main fuel.
- In the third stage, thorium is used directly as a fissile fuel without any conversion.
Which of the statements given above is/are correct?
- (a) 1 only
- (b) 1 and 2 only
- (c) 2 and 3 only
- (d) 1, 2 and 3
Answer and explanation
Q2. Match List I (Reactor component) with List II (Material commonly used):
| List I | List II |
|---|---|
| A. Moderator | 1. Liquid sodium |
| B. Control rod | 2. Heavy water |
| C. Coolant in a fast breeder | 3. Mixed uranium–plutonium oxide |
| D. Fuel in a fast breeder | 4. Boron or cadmium |
- (a) A-2, B-4, C-1, D-3
- (b) A-4, B-2, C-1, D-3
- (c) A-2, B-4, C-3, D-1
- (d) A-2, B-1, C-4, D-3
Answer and explanation
Q3. Which one of the following statements about the Prototype Fast Breeder Reactor (PFBR) is correct?
- (a) It is located at Tarapur and is fuelled by natural uranium
- (b) It uses heavy water as moderator and coolant
- (c) It is a 500 MWe reactor at Kalpakkam that reached first criticality in April 2026
- (d) It is a fusion reactor built with ITER partners
Answer and explanation
Q4. The SHANTI Act, 2025 replaced which of the following laws?
- Atomic Energy Act, 1962
- Civil Liability for Nuclear Damage Act, 2010
- Environment (Protection) Act, 1986
Select the correct answer using the code given below:
- (a) 1 only
- (b) 1 and 2 only
- (c) 2 and 3 only
- (d) 1, 2 and 3
Answer and explanation
Q5. Which of the following isotopes is/are fissile?
- Uranium-233
- Uranium-238
- Plutonium-239
- Thorium-232
- (a) 1 and 3 only
- (b) 2 and 4 only
- (c) 1, 2 and 3 only
- (d) 1, 3 and 4 only
Answer and explanation
Level 3: Mains Answer Writing
Q1. (60 words) Why did India adopt a three-stage nuclear power programme?
Model answer outline
- Resource logic: limited, low-grade uranium; large thorium in monazite sands (Kerala, Odisha, AP, TN).
- Problem: thorium is fertile, not fissile; needs U-233 made in reactors.
- Solution: Stage 1 PHWRs (natural U → Pu); Stage 2 fast breeders multiply Pu and breed U-233 from thorium (PFBR criticality April 2026); Stage 3 thorium–U-233 reactors.
- Goal: long-term energy independence.
Q2. (150–200 words) “Nuclear energy is essential for India’s net-zero goal, but its expansion faces significant hurdles.” Discuss in the light of recent policy changes.
Model answer outline
- Intro: India has 24 reactors with 8,780 MW (July 2026), under 2% of capacity, but targets 100 GW by 2047 and net zero by 2070.
- Why essential: low-carbon, round-the-clock base load to complement solar and wind; small land use; thorium for long-term security; SMRs for hard-to-abate industry.
- Recent policy: Nuclear Energy Mission (₹20,000 crore SMR R&D, five SMRs by 2033); BSRs for captive industrial use; SHANTI Act 2025 (private participation, statutory AERB, operator liability); PFBR criticality (April 2026).
- Hurdles: high cost and delays; safety perceptions after Fukushima; waste management; local protests and land; fuel supply; need for a credible independent regulator.
- Chhattisgarh angle: steel and aluminium clusters (Bhilai, Korba) are natural users of captive small reactors, but the state has no nuclear plant and would need water, siting and public consent.
- Conclusion: Nuclear expansion is feasible if cost, safety and trust are managed together; it complements, not replaces, renewables.
Quick Revision Table
| Fact | Detail |
|---|---|
| Natural uranium | ~99.3% U-238, ~0.7% U-235 |
| Fission energy | ~200 MeV per U-235 fission; 2–3 neutrons |
| D-T fusion | 17.6 MeV; tokamak; ITER at Cadarache, France; India joined 2005 |
| Fissile / fertile | U-235, Pu-239, U-233 / U-238, Th-232 |
| Moderator / control rods / FBR coolant | D₂O, H₂O, graphite / boron, cadmium / liquid sodium |
| PHWR | Natural U, heavy water; India’s workhorse; 700 MW units |
| First / largest station | Tarapur (1969, BWR) / Kudankulam (VVER) |
| Fleet (July 2026) | 24 reactors, 8,780 MW, 7 sites; NPCIL |
| Fuel cycle sites | Jaduguda, Tummalapalle (mining); NFC Hyderabad (fuel); Rattehalli (enrichment); Trombay, Tarapur, Kalpakkam (reprocessing) |
| Closed fuel cycle | Reprocess spent fuel to recover Pu and U |
| Three stages | PHWR (nat. U) → FBR (Pu, breeds) → thorium + U-233 |
| FBTR / PFBR / KAMINI | 1985 / 500 MWe, criticality 6 Apr 2026, BHAVINI / U-233 fuelled, 30 kW |
| Thorium | Monazite sands; ~11.93 MT monazite (AMD) |
| Nuclear Energy Mission | 100 GW by 2047; ₹20,000 crore SMR R&D; 5 SMRs by 2033 |
| SMRs | BSR (220 MW PHWR, captive); BSMR-200; SMR-55 |
| SHANTI Act 2025 | Assent 20 Dec 2025; replaces AEA 1962 + CLNDA 2010; private entry; statutory AERB |
Key Takeaways
- A nuclear plant is a thermal plant heated by fission; the moderator slows neutrons, control rods absorb them.
- India’s PHWRs run on natural uranium because heavy water is such a good moderator.
- The three-stage programme exists because India has little uranium but abundant thorium, which is fertile, not fissile.
- PFBR’s criticality (6 April 2026) moved India into stage 2.
- Nuclear Energy Mission (100 GW by 2047, SMRs) and the SHANTI Act 2025 (private entry, statutory AERB) are the new policy frame.
What’s Next
- Energy Resources — Quick Revision: the whole chapter on one page
- Space and Defence Technology: DRDO, nuclear doctrine and strategic technology
- Environmental Pollution and Waste Management: radioactive and hazardous waste
- Conventional Energy (Geography): where the reactors are