Topic 15: Nuclear Technology — Fission, Fusion, Reactors & Non-Proliferation

This file is a complete GS-III account of nuclear science & policy. It explains nuclear fission & fusion, radioactivity & isotopes, nuclear reactors & their types (PHWR, FBR, PWR, SMR), India's three-stage nuclear power programme & the thorium plan, nuclear power in India (NPCIL, Kudankulam), nuclear safety & waste (Chernobyl, Fukushima), the non-proliferation regime (NPT, IAEA, NSG & the 2008 nuclear deal), and non-power applications in medicine, agriculture & industry — with labelled diagrams, tables and real dated PYQs with full model answers. (Basic atomic physics is in Topic 7.)

UPSC Prelims · Mains GS-III Fission · Fusion ~35 min read 3-Stage & Thorium NPT / IAEA / NSG

1.Fission & Fusion
2.Radioactivity & Isotopes
3.Nuclear Reactors & Types
4.Three-Stage Programme
5.Thorium & Fuel Cycle
6.Nuclear Power in India
7.Nuclear Safety & Waste
8.Non-Proliferation Regime
9.Non-Power Applications
10.Current Affairs Link
11.Prelims PYQs
12.Mains PYQs + Model Answers
★15-Minute Revision Box

Conceptual Clarity — How UPSC Tests Nuclear Technology

This is a high-frequency area bridging pure science & strategic policy. Sort your prep into three question-types:

Definitional / static — "what is fission?", "what is a fast breeder reactor?", "which isotope is fissile?". Recall of concepts & terms.
Statement-elimination — reactor types, three-stage-programme sequence, NPT/NSG facts where one detail decides the answer.
Applied / current — a reactor, mission or treaty in the news (Kudankulam, thorium, NSG, fusion/ITER) linked to a fundamental — the strong GS-III energy & strategic-autonomy bridge.

Highest-frequency themes: fission vs fusion · U-235/U-238/Pu-239/U-233 & fissile vs fertile · reactor types (PHWR/FBR/SMR) · three-stage programme & thorium · Kudankulam & NPCIL · NPT/IAEA/NSG & the 2008 deal · radioisotopes in medicine/agriculture.

1. Nuclear Fission & Fusion

Fig 15.1 — In fission, a slow neutron splits a U-235 nucleus into lighter fragments, releasing heat and 2–3 fresh neutrons; these split further nuclei, sustaining a controlled chain reaction in a reactor (uncontrolled in a bomb).

Nuclear fission = a heavy nucleus (e.g. U-235, Pu-239) splits into lighter nuclei when struck by a neutron, releasing large energy (from mass–energy, E=mc²) plus 2–3 neutrons that sustain a chain reaction. This powers reactors & atom bombs.
Nuclear fusion = light nuclei (e.g. hydrogen isotopes deuterium & tritium) join to form a heavier nucleus (helium), releasing even more energy — it powers the Sun & the hydrogen bomb.
Fusion vs fission: fusion needs extreme temperature/pressure, produces little long-lived waste & abundant fuel, but is not yet commercially viable (research reactors like ITER, tokamaks). Fission is proven but leaves radioactive waste.
Critical mass = minimum fissile mass needed to sustain a chain reaction; a moderator slows neutrons to make fission more likely.

Prelims facts: fission = splitting (reactors & atom bomb); fusion = joining (Sun & hydrogen bomb); fusion releases more energy per unit mass & less waste but is not yet commercial; ITER is the international fusion project. Both convert a tiny mass into huge energy (E=mc²).

2. Radioactivity & Isotopes

Radioactivity = spontaneous emission of radiation from unstable nuclei as alpha (α, helium nuclei), beta (β, electrons) & gamma (γ, high-energy waves) rays. Penetration: gamma > beta > alpha; ionising ability the reverse.
Isotopes = atoms of the same element with the same protons but different neutrons (e.g. U-235 & U-238; C-12 & C-14). Some isotopes are radioactive (radioisotopes).
Half-life = time for half of a radioactive sample to decay — used in carbon dating (C-14) & medical dosing.
Fissile vs fertile: fissile nuclei readily undergo fission with slow neutrons (U-235, Pu-239, U-233); fertile nuclei (U-238, Th-232) themselves don't fission but can be converted into fissile material in a reactor.

Isotope	Nature	Role
U-235	Fissile (0.7% of natural U)	Reactor & bomb fuel
U-238	Fertile	→ Pu-239 (Stage II)
Pu-239	Fissile (man-made)	FBR fuel, weapons
Th-232	Fertile	→ U-233 (Stage III)
U-233	Fissile (man-made)	Thorium-cycle fuel

Prelims traps: only U-235 is naturally fissile (and only 0.7% of natural uranium); U-238 & Th-232 are fertile — converted to Pu-239 & U-233 respectively. Half-life underlies carbon dating; gamma rays are the most penetrating.

3. Nuclear Reactors & Their Types

A nuclear reactor controls a fission chain reaction to release heat, which boils water to spin a steam turbine & generate electricity. Key parts: fuel (U/Pu), moderator (slows neutrons — heavy water/graphite/light water), control rods (absorb neutrons — boron/cadmium, to control/stop the reaction), coolant & shielding.
PHWR (Pressurised Heavy Water Reactor) — India's workhorse; uses natural uranium fuel with heavy water (D₂O) as moderator & coolant. Indigenous, no enrichment needed.
PWR (Pressurised Water Reactor) — uses enriched uranium & light water (e.g. Kudankulam, Russian VVER).
FBR (Fast Breeder Reactor) — uses fast (unmoderated) neutrons & breeds more fissile fuel (Pu-239 from U-238) than it consumes; core of India's Stage II (PFBR at Kalpakkam).
SMR (Small Modular Reactor) — compact, factory-built reactors (up to ~300 MW) for flexible, safer deployment; a global & Indian focus area.

Reactor	Fuel	Moderator
PHWR	Natural uranium	Heavy water (D₂O)
PWR / VVER	Enriched uranium	Light water
FBR	Pu-239 / U-238 blanket	None (fast neutrons)
SMR	Enriched (varies)	Varies (compact)

Prelims traps: India's mainstay = PHWR (natural uranium + heavy-water moderator); a breeder makes more fuel than it burns using fast neutrons (no moderator); control rods (boron/cadmium) absorb neutrons to control the reaction; heavy water is a moderator, not a fuel.

4. India's Three-Stage Nuclear Power Programme

Conceived by Dr Homi Bhabha, the programme aims to use India's limited uranium but vast thorium reserves through three linked stages, each feeding the next.
Stage I — PHWRs: burn natural uranium (U-235) in heavy-water reactors; the spent fuel yields plutonium-239.
Stage II — Fast Breeder Reactors: use that Pu-239 to breed more Pu-239 (from U-238) and to convert Th-232 → U-233 in a thorium blanket, building up a U-233 stockpile.
Stage III — Thorium reactors: use U-233 with thorium for a nearly self-sustaining thorium cycle — tapping India's huge thorium reserves for long-term energy security.

Prelims facts: the three-stage programme (Homi Bhabha) sequence = PHWR (U-235 → Pu-239) → FBR (Pu-239 breeds Pu & makes U-233 from Th-232) → Thorium reactors (U-233 + Th). It exists because India has little uranium but abundant thorium (monazite sands of Kerala/Odisha).

5. Thorium & the Fuel Cycle

India holds among the world's largest thorium reserves (in monazite beach sands), but only modest uranium — the entire three-stage logic flows from this.
Thorium (Th-232) is fertile, not fissile — it must first be converted to fissile U-233 inside a reactor before it can generate power. Hence thorium cannot be used directly in Stage I.
Advantages of thorium: abundant, produces less long-lived waste & less weapons-usable plutonium, better proliferation resistance.
AHWR (Advanced Heavy Water Reactor) is designed to demonstrate large-scale thorium use. Reprocessing & the closed fuel cycle (recycling spent fuel) are central to India's approach.

GS-III hook: thorium is India's strategic energy trump card — but because it is fertile, unlocking it needs the breeder (Stage II) to create U-233 first. Delays in Stage II (FBR commissioning) are the bottleneck for the thorium age; hence interest in imported uranium (post-2008 deal) to expand Stage I.

6. Nuclear Power in India

NPCIL (Nuclear Power Corporation of India Ltd) operates India's power reactors; the DAE (Department of Atomic Energy) oversees the programme; BARC is the premier R&D body.
Nuclear power is a small but firm, low-carbon share of India's electricity mix, mostly from indigenous PHWRs plus imported LWRs.
Kudankulam (Tamil Nadu) = India's largest nuclear plant, Russian VVER (PWR) units built with Russian cooperation.
Other stations: Tarapur (India's oldest), Rawatbhata, Kaiga, Kakrapar, Kalpakkam (with the PFBR), Narora. Government has approved a fleet-mode expansion of indigenous 700 MWe PHWRs.

Prelims facts: reactors are run by NPCIL under the DAE; Kudankulam (Tamil Nadu) uses Russian VVER (PWR) reactors; Tarapur is the oldest; the PFBR (prototype fast breeder) is at Kalpakkam; indigenous PHWRs are being built in "fleet mode" (700 MWe).

7. Nuclear Safety & Waste Management

Major accidents: Chernobyl (1986, USSR — reactor explosion, worst civil nuclear disaster) & Fukushima (2011, Japan — earthquake+tsunami caused meltdowns; reshaped global safety & siting norms). India's own Three Mile Island reference point is 1979 (USA).
Radioactive waste is classified low/intermediate/high-level; high-level waste stays hazardous for millennia — managed by vitrification (locking in glass) & deep-geological storage.
Regulation: the AERB (Atomic Energy Regulatory Board) is India's nuclear safety regulator. Debate exists over making it fully independent.
Civil Liability for Nuclear Damage Act, 2010 channels accident liability to the operator (with supplier recourse) — a sticking point for foreign suppliers.

Prelims traps: Chernobyl (1986, USSR) & Fukushima (2011, Japan, tsunami-triggered) are the landmark disasters; India's regulator = AERB; high-level waste is managed by vitrification + deep geological disposal; the 2010 Civil Liability Act provides supplier recourse, which deterred some foreign vendors.

8. The Non-Proliferation Regime

NPT (Non-Proliferation Treaty, 1968) recognises only 5 nuclear-weapon states (US, Russia, UK, France, China) & bars others from weapons. India has NOT signed the NPT, calling it discriminatory; India is a responsible nuclear power with a "No First Use" doctrine & a voluntary test moratorium.
IAEA (International Atomic Energy Agency) promotes peaceful nuclear use & runs safeguards/inspections to prevent diversion to weapons.
NSG (Nuclear Suppliers Group) controls exports of nuclear material & technology. India seeks membership (backed by most, blocked mainly by China over the NPT-signature issue).
The 2008 India–US Civil Nuclear Deal (123 Agreement) + the NSG waiver ended India's nuclear isolation, letting India trade in civil nuclear fuel/tech while keeping military facilities outside safeguards (separation plan). Other regimes: CTBT (test ban — India hasn't signed), MTCR/Wassenaar/Australia Group (India is a member of these three).

Prelims facts: India is not a signatory to the NPT or CTBT but follows No First Use & a test moratorium; the 2008 NSG waiver + 123 Agreement opened civil nuclear commerce; India is a member of the MTCR, Wassenaar Arrangement & Australia Group but not the NSG (China blocks). IAEA safeguards apply to India's civil (not military) reactors.

9. Non-Power Applications of Nuclear Technology

Medicine: radioisotopes for diagnosis (technetium-99m, iodine-131 scans, PET using fluorine-18) & therapy (cobalt-60 & radiotherapy for cancer); sterilisation of medical equipment.
Agriculture: mutation breeding for improved crop varieties, radiation-based food preservation/irradiation (onions, spices), the Sterile Insect Technique for pest control, and tracers for fertiliser/soil studies.
Industry: gamma radiography to detect flaws in metals/welds, thickness & level gauges, and nucleonic gauges.
Other: carbon dating (archaeology), hydrology tracers, and space (radioisotope thermoelectric generators). India's BARC supplies radioisotopes; Bhabhatron is an indigenous cancer-therapy machine.

GS-III hook: nuclear technology is not just power — radioisotopes (Co-60, I-131, Tc-99m) drive cancer care, food irradiation extends shelf life & cuts waste, and industrial gauging aids quality control. This "atoms for peace" dimension often anchors Mains value-addition & Prelims one-liners.

10. Current Affairs Link (2024–2026)

Nuclear energy & strategy are recurring tech-policy stories — verify the latest before the exam. check for latest update or data

Nuclear expansion: fleet-mode 700 MWe PHWRs, SMR plans & moves to open the sector to private/foreign participation apply Sections 3 & 6. check for latest update or data

Fast breeder & thorium: progress on the PFBR at Kalpakkam (Stage II) & thorium-cycle milestones apply Sections 4–5. check for latest update or data

Fusion & diplomacy: ITER progress and NSG-membership/liability-law debates apply Sections 1 & 8. check for latest update or data

Recent theme	Fundamental it tests	Why it matters for UPSC
Fleet-mode PHWRs / SMRs	Reactors	GS-III clean-energy transition.
PFBR at Kalpakkam	Three-stage / breeder	GS-III energy security.
NSG membership	Non-proliferation	GS-II/III strategic autonomy.
ITER / fusion	Fusion	GS-III frontier energy.

Recurring exam hooks: fission vs fusion · fissile vs fertile · PHWR/FBR/SMR · three-stage & thorium · Kudankulam/NPCIL · Chernobyl/Fukushima & AERB · NPT/IAEA/NSG & the 2008 deal · radioisotopes.

11. Prelims PYQs

Objective questions anchored to genuinely tested UPSC themes on nuclear technology. Each carries a worked rationale.

UPSC Prelims — Fission vs fusion

Q: The energy of the Sun is produced by —

(a) nuclear fission of uranium
(b) nuclear fusion of hydrogen
(c) chemical burning
(d) radioactive decay only

Answer: (b) The Sun fuses hydrogen nuclei into helium (fusion), releasing enormous energy; fission (splitting) powers reactors and atom bombs.

UPSC Prelims — Fissile material

Q: Which of the following is a naturally occurring fissile material?

(a) Uranium-238
(b) Thorium-232
(c) Uranium-235
(d) Plutonium-239

Answer: (c) U-235 (0.7% of natural uranium) is the only naturally fissile isotope; U-238 & Th-232 are fertile; Pu-239 is man-made in reactors.

UPSC Prelims — Indian reactor

Q: India's indigenous PHWRs primarily use —

(a) natural uranium fuel with heavy water as moderator
(b) enriched uranium with graphite moderator
(c) plutonium with light water
(d) thorium directly

Answer: (a) The Pressurised Heavy Water Reactor uses natural uranium fuel with heavy water (D₂O) as both moderator and coolant — no enrichment needed.

UPSC Prelims — Three-stage

Q: The third stage of India's nuclear power programme is based on —

(a) natural uranium
(b) plutonium-239
(c) thorium and uranium-233
(d) enriched uranium

Answer: (c) Stage III uses thorium (Th-232) with fissile U-233 bred earlier — tapping India's vast thorium reserves. Stage I = U-235 (PHWR), Stage II = Pu-239 (FBR).

UPSC Prelims — Breeder reactor

Q: A Fast Breeder Reactor is one that —

(a) uses only thorium fuel
(b) produces more fissile material than it consumes
(c) requires no fuel
(d) uses light water as moderator

Answer: (b) A breeder uses fast (unmoderated) neutrons to convert fertile U-238 into fissile Pu-239, breeding more fuel than it burns; India's PFBR is at Kalpakkam.

UPSC Prelims — Control rods

Q: In a nuclear reactor, control rods (of boron/cadmium) are used to —

(a) speed up neutrons
(b) absorb neutrons and control the reaction
(c) act as fuel
(d) cool the core

Answer: (b) Control rods absorb excess neutrons to control or stop the chain reaction; the moderator (e.g. heavy water) slows neutrons, and the coolant removes heat.

UPSC Prelims — Non-proliferation

Q: With reference to India and the nuclear regime, which is correct?

(a) India has signed the NPT
(b) India is a member of the MTCR, Wassenaar Arrangement and Australia Group but not the NSG
(c) India has signed the CTBT
(d) India is a founding NSG member

Answer: (b) India has not signed the NPT or CTBT; it joined the MTCR, Wassenaar Arrangement and Australia Group, but NSG membership is blocked (mainly by China).

UPSC Prelims — Radioisotope

Q: Cobalt-60 is widely used in —

(a) cancer radiotherapy and sterilisation
(b) making nuclear fuel
(c) carbon dating
(d) generating electricity

Answer: (a) Cobalt-60 emits gamma rays used in cancer radiotherapy and to sterilise equipment/food; C-14 (not Co-60) is used in carbon dating.

Prelims — anticipated themes

Likely: fission vs fusion & ITER · fissile vs fertile (U-235/U-238/Th-232/Pu-239/U-233) · PHWR/FBR/SMR · three-stage sequence · Kudankulam/PFBR · AERB & liability law · NPT/NSG & 2008 deal · radioisotopes. check for latest update or data

12. Mains PYQs + Model Answers

Nuclear technology anchors GS-III questions on energy security, self-reliance & strategy. The frameworks below show how to deploy it analytically.

Mains GS-III 15 marks · 250 words

Q: Explain India's three-stage nuclear power programme and its significance for energy security.

Model Answer

Introduction: Conceived by Homi Bhabha, the three-stage programme (Section 4) leverages India's limited uranium but vast thorium to build energy self-reliance.
The three stages:
- Stage I — PHWRs burn natural uranium, yielding Pu-239; Stage II — FBRs breed Pu-239 and convert Th-232 to U-233; Stage III — thorium reactors run on U-233.
Significance: converts abundant thorium into usable fuel → long-term energy security, low-carbon power, strategic autonomy & reduced import dependence.
Challenges: slow FBR commissioning, technical complexity of the thorium cycle, uranium shortage in Stage I, safety & cost.
Way forward: accelerate the PFBR & AHWR, use imported uranium (post-2008 deal) to expand Stage I, and add SMRs.
Conclusion: the programme is India's roadmap to a self-reliant, thorium-powered low-carbon future.

Mains GS-III 15 marks · 250 words

Q: Nuclear energy is a double-edged sword. Discuss its role in India's clean-energy transition and the safety concerns involved.

Model Answer

Introduction: Nuclear power (Sections 6–7) offers firm low-carbon electricity but carries safety & waste risks — a double-edged sword.
Role in clean-energy transition:
- Reliable base-load, low lifecycle emissions, complements intermittent solar/wind, supports net-zero & energy security.
Safety & other concerns: accident risk (Chernobyl, Fukushima), long-lived radioactive waste, cost & time overruns, public acceptance & the liability law deterring suppliers.
Mitigation: strong independent regulation (AERB), robust siting & passive-safety designs, SMRs, vitrification & deep disposal, transparency.
Conclusion: with rigorous safety, nuclear energy is a valuable pillar of India's low-carbon mix.

Mains GS-III / GS-II 15 marks · 250 words

Q: Examine India's stand on the global non-proliferation regime and the significance of the 2008 civil nuclear deal.

Model Answer

Introduction: India (Section 8) is outside the NPT, viewing it as discriminatory, yet upholds a responsible record (No First Use, test moratorium).
India's stand:
- Rejects discriminatory NPT/CTBT; seeks recognition as a responsible nuclear power; member of MTCR/Wassenaar/Australia Group; seeks NSG entry.
Significance of the 2008 deal: the 123 Agreement + NSG waiver ended nuclear isolation, enabling civil fuel/technology imports while separating military facilities — boosting energy & diplomatic standing.
Continuing hurdles: NSG membership blocked by China, liability-law concerns, uranium supply.
Conclusion: the deal was a diplomatic breakthrough; NSG entry remains the unfinished agenda of India's strategic autonomy.

Mains GS-III 10 marks · 150 words

Q: Beyond electricity, how does nuclear technology serve society? Illustrate.

Model Answer

Introduction: Nuclear technology (Section 9) has wide "atoms for peace" applications beyond power.
Applications:
- Medicine — cancer radiotherapy (Co-60), diagnostic scans (Tc-99m, I-131), equipment sterilisation.
- Agriculture — mutation breeding, food irradiation, sterile insect technique.
- Industry & science — flaw detection, gauging, carbon dating, hydrology tracers.
India's role: BARC supplies radioisotopes; Bhabhatron aids affordable cancer care.
Conclusion: nuclear science advances health, food security & industry — not just energy.

Mains GS-III — anticipated themes

Likely: three-stage & thorium for energy security · nuclear in the clean-energy transition & safety · India & non-proliferation / NSG / 2008 deal · SMRs & private participation · non-power applications. check for latest update or data

15-Minute Revision Box

Must-Remember Facts — Nuclear Technology

Science:

Fission = splitting (reactors/atom bomb); Fusion = joining (Sun/H-bomb, ITER)
Fissile: U-235 (natural), Pu-239, U-233 (man-made)
Fertile: U-238 → Pu-239; Th-232 → U-233
Half-life → carbon dating; γ most penetrating

Reactors:

PHWR = natural U + heavy water (India's mainstay)
FBR = fast neutrons, breeds fuel (PFBR, Kalpakkam)
Control rods (boron/cadmium) absorb neutrons
SMR = small modular, up to ~300 MW

India's programme:

3-stage (Homi Bhabha): PHWR → FBR → Thorium
Thorium is fertile; needs U-233 first (via breeder)
NPCIL/DAE/BARC; AERB = regulator
Kudankulam = Russian VVER (PWR); Tarapur oldest

Policy & uses:

India NOT in NPT/CTBT; No First Use; in MTCR/Wassenaar/Australia Group; not NSG
2008 deal = 123 Agreement + NSG waiver
Chernobyl (1986), Fukushima (2011); vitrification for waste
Radioisotopes: Co-60, I-131, Tc-99m (medicine/agri/industry)

Highest-frequency themes: fission vs fusion · fissile vs fertile · PHWR/FBR/SMR · three-stage & thorium · Kudankulam/NPCIL · AERB & safety · NPT/IAEA/NSG & 2008 deal · radioisotopes.

Item	Value
U-235 in natural U	~0.7%
Chernobyl	1986, USSR
Fukushima	2011, Japan
Nuclear deal	2008 (123 + NSG)

Term	Key idea
Fission	Splitting
Fusion	Joining
Fissile	U-235/Pu-239/U-233
Fertile	U-238/Th-232

Item	Note
PHWR	Natural U + D₂O
PFBR	Kalpakkam (Stage II)
Kudankulam	Russian VVER
AERB	Regulator

Topic 15: Nuclear Technology — Fission, Fusion, Reactors & Non-Proliferation

On this page

Conceptual Clarity — How UPSC Tests Nuclear Technology

1. Nuclear Fission & Fusion

2. Radioactivity & Isotopes

3. Nuclear Reactors & Their Types

4. India's Three-Stage Nuclear Power Programme

5. Thorium & the Fuel Cycle

6. Nuclear Power in India

7. Nuclear Safety & Waste Management

8. The Non-Proliferation Regime

9. Non-Power Applications of Nuclear Technology

10. Current Affairs Link (2024–2026)

11. Prelims PYQs

12. Mains PYQs + Model Answers

Model Answer

Model Answer

Model Answer

Model Answer

15-Minute Revision Box

Must-Remember Facts — Nuclear Technology