📌 Topic 01 of 6 · Chapter 03 · Nuclear Technology
Nuclear Fission, Fusion & Reactors
Fission vs fusion, PHWR BWR PWR FBR, moderators, coolants — complete notes for UPSC & PSC exams.
⚛️ Nuclear Fission vs Fusion
| Feature | Nuclear Fission | Nuclear Fusion |
|---|---|---|
| Process | Heavy nucleus (U-235, Pu-239) splits into smaller nuclei | Light nuclei (H-2, H-3) combine to form heavier nucleus |
| Energy released | Large amount | Much larger amount (3-4× more than fission) |
| Temperature needed | Moderate (chain reaction) | Extremely high (~100 million°C) |
| Radioactive waste | Significant; long-lived | Minimal; short-lived |
| Current use | Nuclear power plants; atomic bombs | Hydrogen bombs; power plants under research (ITER) |
| Fuel | Uranium-235, Plutonium-239 | Deuterium (H-2), Tritium (H-3) |
⚛️ Types of Nuclear Reactors
| Reactor | Moderator | Coolant | Fuel | Used in India |
|---|---|---|---|---|
| PHWR (Pressurised Heavy Water Reactor) | Heavy water (D₂O) | Heavy water | Natural uranium | Yes — most common; Rawatbhata, Kaiga, Kalpakkam, Narora, Kakrapar |
| BWR (Boiling Water Reactor) | Light water (H₂O) | Light water (boils) | Enriched uranium | Yes — Tarapur (US-supplied) |
| PWR (Pressurised Water Reactor) | Light water | Light water (pressurised) | Enriched uranium | Yes — Kudankulam (Russian VVER) |
| FBR (Fast Breeder Reactor) | None (fast neutrons) | Liquid sodium | Plutonium-239 | Under construction — Kalpakkam (PFBR) |
| AHWR (Advanced Heavy Water Reactor) | Heavy water | Boiling light water | Thorium + U-233 | Under development — Stage 3 of India’s programme |
⚛️ Key Components of a Nuclear Reactor
- Fuel rods — contain fissile material (U-235 or Pu-239)
- Moderator — slows down fast neutrons to thermal speeds (heavy water, light water, graphite)
- Control rods — absorb neutrons to control reaction rate (boron or cadmium)
- Coolant — removes heat from reactor core (water, heavy water, liquid sodium)
- Reflector — reflects neutrons back into core (graphite, beryllium)
- Shielding — protects workers from radiation (concrete, lead)
- Steam generator — converts heat to steam to drive turbines
⭐ Why India chose PHWR?
India chose PHWR technology because it uses natural uranium (without enrichment). India has limited uranium reserves but can use natural uranium directly in PHWRs. Heavy water (D₂O) is a better moderator than light water — absorbs fewer neutrons, allowing natural uranium to sustain a chain reaction. India produces heavy water at plants in Baroda, Tuticorin, Talcher, and other locations.
India chose PHWR technology because it uses natural uranium (without enrichment). India has limited uranium reserves but can use natural uranium directly in PHWRs. Heavy water (D₂O) is a better moderator than light water — absorbs fewer neutrons, allowing natural uranium to sustain a chain reaction. India produces heavy water at plants in Baroda, Tuticorin, Talcher, and other locations.