Atomic Structure, Radioactivity & Nuclear Energy

Radioactivity is the spontaneous breaking up of an unstable nucleus, giving out radiation. It is a nuclear property, not a chemical one — heating, cooling, pressure or chemical combination cannot switch it on or off. Nuclei with Z greater than 82 (beyond lead) are all radioactive.

An unstable heavy nucleus rarely reaches stability in one step. It goes through a radioactive decay series — a chain of alpha and beta decays ending at a stable nucleus. The uranium-238 series ends at lead-206; the uranium-235 series ends at lead-207 and the thorium-232 series at lead-208. In every series, radon, a radioactive gas, appears as an intermediate — which is why radon accumulating in poorly ventilated basements is a real health concern.

Half-life (T) is the time in which half of a radioactive sample decays. It is a fixed fingerprint of each isotope — you cannot speed it up or slow it down. Crucially, half-life does not depend on how much sample you started with: 1 gram and 1 tonne of the same isotope have the same half-life.

Because half-lives are fixed, decay works as a clock. Carbon-14 dating (developed by Willard Libby) uses the ≈ 5,730-year half-life of ¹⁴C. A living plant or animal keeps taking in carbon, so its ¹⁴C level stays topped up; the moment it dies, intake stops and the ¹⁴C ticks down. Measure how much is left and you get the age. It works on once-living material — wood, bone, cloth, charcoal — up to roughly 50,000 years. For rocks and the age of the Earth, geologists use uranium dating: ²³⁸U decays to ²⁰⁶Pb with a half-life of about 4.5 billion years, long enough to date the planet itself.

Nuclear fission is the splitting of a heavy nucleus into two lighter ones, releasing energy and neutrons. It was discovered by Otto Hahn and Fritz Strassmann (1938) and explained by Lise Meitner and Otto Frisch. In a reactor, a slow (thermal) neutron is absorbed by uranium-235, which becomes unstable and breaks up, typically as: ²³⁵U + n → ¹⁴¹Ba + ⁹²Kr + 3 neutrons + energy (≈ 200 MeV).

Each fission gives out more neutrons than it consumed, and those neutrons split further nuclei — a chain reaction. For the chain to sustain itself the sample must be big enough that neutrons do not simply escape from the surface; that minimum amount of fissile material is the critical mass. Below it the reaction dies out; well above it, the reaction runs away.

• Fuel — natural or enriched uranium, or plutonium; it supplies the fissile nuclei.
• Moderator — heavy water (D₂O) or graphite (also light water, beryllium). It slows down fast neutrons so U-235 can capture them.
• Control rods — cadmium or boron rods that absorb neutrons. Push them in to slow the reactor, pull them out to speed it up.
• Coolant — heavy or light water, carbon dioxide, or liquid sodium in fast breeders. It carries heat away to make steam.
• Shielding — thick concrete and lead to stop radiation escaping.

Nuclear fusion is the opposite: light nuclei join to form a heavier one. This is what powers the Sun. In the proton–proton cycle, four hydrogen nuclei effectively combine, through intermediate steps, into one helium-4 nucleus, releasing positrons, neutrinos and a huge amount of energy — because the helium nucleus is lighter than the four protons that formed it, and the missing mass appears as energy. On Earth, fusion is desperately hard: positively charged nuclei repel each other, so you need temperatures of millions of degrees and a way to confine a plasma that hot without any container touching it. Experimental reactors use magnetic confinement (the tokamak; India's SST-1 and the international ITER project). Fusion produces far less long-lived radioactive waste than fission and its fuel (deuterium from sea water) is nearly unlimited — which is why the effort continues.

Dr. Homi Jehangir Bhabha is the architect (father) of India's nuclear programme. He founded the Tata Institute of Fundamental Research (1945), chaired the Atomic Energy Commission (AEC), set up in 1948, and guided the creation of the Department of Atomic Energy (DAE) in 1954, which is placed directly under the Prime Minister. The atomic establishment at Trombay (Mumbai) was renamed the Bhabha Atomic Research Centre (BARC) in his memory after his death in an air crash in 1966.

Bhabha's deepest insight was about resources. India has only modest uranium but among the world's largest thorium reserves, locked in the monazite sands of Kerala's beaches (also Tamil Nadu and Odisha). But thorium-232 is not fissile — it cannot burn directly in a reactor; it must first be converted into fissile uranium-233. So Bhabha designed a three-stage programme that begins with uranium and ends with thorium.

• Apsara (1956) — first research reactor in India and in Asia. Dhruva (1985) — India's largest research reactor. Both are at BARC, Trombay.
• Prototype Fast Breeder Reactor (PFBR) at Kalpakkam, Tamil Nadu is India's Stage 2 flagship, a 500 MWe sodium-cooled breeder. Kalpakkam also has the Fast Breeder Test Reactor (FBTR) and the Indira Gandhi Centre for Atomic Research (IGCAR).
• AERB (Atomic Energy Regulatory Board, 1983) is the safety regulator; NPCIL (1987) builds and operates power reactors; BHAVINI was created for fast breeder reactors.

On doctrine and diplomacy: after 1998 India adopted a policy of credible minimum deterrence with a declared no-first-use (NFU) commitment — India will not be the first to use nuclear weapons, but retaliation to a nuclear attack will be massive. The IAEA (International Atomic Energy Agency, 1957, headquartered in Vienna) promotes peaceful use and applies safeguards to civilian facilities. India has not signed the NPT (Nuclear Non-Proliferation Treaty, 1968), calling it discriminatory because it recognises as nuclear weapon states only those who tested before 1 January 1967, and asks everyone else to disarm without a time-bound commitment from the five. India has likewise not signed the CTBT (Comprehensive Test Ban Treaty, 1996), though it maintains a voluntary moratorium on further testing. The Indo-US civil nuclear deal broke this isolation: India separated its civil and military facilities, placed the civilian ones under IAEA safeguards, and in return received an NSG waiver in 2008 allowing it to import nuclear fuel and technology despite not being an NPT signatory.

Ionising radiation damages living cells and can break DNA. High doses cause radiation sickness — nausea, hair loss, damage to bone marrow; lower long-term exposure raises the risk of cancer and of genetic (heritable) damage. Rapidly dividing cells — bone marrow, the gut lining, a growing foetus — are the most vulnerable. This is also why radiation can be turned into a weapon against cancer: fast-dividing tumour cells are hit hardest.

• The three shields of protection: Time (stay near a source as briefly as possible), Distance (intensity falls sharply with distance), and Shielding (paper for α, aluminium for β, lead or concrete for γ).
• Workers wear lead aprons and carry film badges or dosimeters to record the dose they have received.
• After a reactor accident, potassium iodide tablets are distributed: ordinary iodine saturates the thyroid so it cannot absorb radioactive iodine-131.
• Spent fuel and reactor waste are stored under water, then vitrified (fused into glass) and buried deep — the hardest unsolved problem of fission power.