Nuclear Physics and Radiation Safety

8.1 Nuclear Reactions

Binding energy: Energy needed to completely separate a nucleus into its protons and neutrons. The more tightly bound a nucleus, the more stable.

Iron-56 (Fe-56) has the highest binding energy per nucleon — the most stable nucleus.

E = mc² (Einstein, 1905 Special Relativity): Mass-energy equivalence. Small mass defect in nuclear reactions converts to enormous energy. 1 a.m.u. ≈ 931 MeV of energy.

Nuclear Fission

• Heavy nucleus + neutron → two medium nuclei + 2–3 neutrons + ~200 MeV
• Example: U-235 + n → Ba-141 + Kr-92 + 3n + 200 MeV
• Chain reaction: Released neutrons cause further fissions. Controlled in nuclear reactors; uncontrolled in nuclear weapons.
• Critical mass: Minimum amount of fissile material to sustain chain reaction.

Nuclear Reactor Components:

• Fuel: Enriched U-235 (3–4% enrichment for power reactors)
• Moderator: Slows neutrons to thermal (slow) speeds — heavy water (D₂O), graphite, ordinary water
• Control rods: Absorb excess neutrons — cadmium or boron; inserted/withdrawn to regulate reaction rate
• Coolant: Removes heat — water, heavy water, CO₂, sodium; heat drives steam turbines
• Containment: Reinforced concrete/steel dome preventing radiation leakage

India's Nuclear Programme

• Three-stage programme (Homi Bhabha): Stage 1 (natural uranium → Pu-239), Stage 2 (Pu-239 breeder reactors), Stage 3 (thorium → U-233 fuel cycle).
• India has among the world's largest thorium reserves — 25% of global reserves in Kerala, Tamil Nadu, AP.
• 22 operational reactors, ~6,780 MWe capacity (2025); target: 100 GWe nuclear capacity by 2047.
• Rajasthan Atomic Power Station (RAPS), Rawatbhata: 6 units, ~1,180 MWe — India's second-largest nuclear station.

8.2 Nuclear Fusion

Fusion reaction (Deuterium-Tritium): D + T → He-4 + n + 17.6 MeV

• Requires 100 million °C — hotter than Sun's core — to overcome Coulomb repulsion between nuclei.
• Fusion produces He-4 as primary waste (inert gas); only short-lived activated components.

ITER (International Thermonuclear Experimental Reactor):

• Under construction in Cadarache, France; 35 countries participating including India.
• Target: Q = 10 (output 10× input).
• First plasma: expected 2025; full D-T operation: 2035.

NIF (National Ignition Facility), USA:

• Achieved ignition (Q > 1) in December 2022 — first time fusion produced more energy than delivered by lasers.
• Historical milestone in fusion research.

8.3 Radiation Safety

Units of Radiation Measurement:

• Becquerel (Bq): Activity — 1 disintegration per second.
• Gray (Gy): Absorbed dose — 1 joule energy absorbed per kg tissue.
• Sievert (Sv): Effective dose — accounts for biological effectiveness of different radiation types. 1 Sv = 1 J/kg (for equal-harm comparison).

ALARA Principle (As Low As Reasonably Achievable): Guiding principle of radiation protection.

Dose limits (ICRP — International Commission on Radiological Protection):

• Radiation workers: 20 mSv/year averaged over 5 years (100 mSv max in 5 years)
• General public: 1 mSv/year
• Natural background radiation (India average): ~2.4 mSv/year (higher in Kerala's monazite belt — 20–30 mSv/year in some areas, yet no increase in cancer rates — used to argue for radiation hormesis)

Three fundamentals of radiation protection:

1. Time: Minimise time spent near radioactive source. Dose ∝ time.
2. Distance: Maximise distance from source. Dose ∝ 1/distance².
3. Shielding: Appropriate shielding material: alpha — paper/skin; beta — thin aluminium; gamma/X-ray — lead, concrete.

Radiation accidents and lessons:

• Chernobyl (1986): Graphite-moderated RBMK reactor design flaw + operator error → steam explosion → radioactive fire. ~30 direct deaths; long-term thyroid cancer increase in Ukraine, Belarus, Russia.
• Fukushima (2011): Tsunami disabled cooling → hydrogen explosions → radioactive release. No direct radiation deaths, but mass evacuation.
• Lessons: Passive safety systems (cooling without external power), strengthened containment, transparent emergency protocols.

---