Nuclear Fission and Fusion: Radioactivity, Half-Life and Nuclear Reactors

The Atomic Nucleus: Protons, Neutrons and Binding Energy

Protons, Neutrons, Isotopes and the Nuclear Force

Every nucleus is made of protons and neutrons, together called nucleons. The number of protons is the atomic number Z, and the total number of nucleons is the mass number A. James Chadwick discovered the neutron in 1932, a neutral particle with almost the mass of a proton, and won the 1935 Nobel Prize for it. A free neutron is unstable, but inside a nucleus it is stable.

Atoms of one element that differ in mass are isotopes: their nuclei have the same number of protons but different numbers of neutrons, so they behave alike chemically. Hydrogen has three: ordinary hydrogen with a single proton, deuterium with one proton and one neutron, and tritium with one proton and two neutrons. Isotopes and the structure of the atom are covered in atomic structure and isotopes.

• Isobars: Nuclides with the same mass number, such as hydrogen-3 and helium-3.
• Isotones: Nuclides with the same number of neutrons but different atomic numbers.
• Size: Nuclear density is about 2.3 × 10 to the power 17 kilograms per cubic metre, the same for all nuclei and enormous compared with water’s 10 to the power 3.

A strong attractive force, the nuclear force, holds the nucleus together against the repulsion between protons. It is much stronger than the Coulomb force, acts only over a few femtometres, and is about the same between a neutron and a neutron, a proton and a neutron, or two protons, since it does not depend on electric charge.

Mass Defect and Binding Energy: E = mc Squared

A nucleus always weighs less than the protons and neutrons that make it up. For oxygen-16 the difference, the mass defect, is 0.13691 atomic mass units. Einstein's relation E = mc squared explains it: the missing mass is the energy that binds the nucleus, and the same energy must be supplied to break it into separate nucleons.

This energy is the binding energy, and dividing it by the number of nucleons gives the binding energy per nucleon, a measure of how tightly a nucleus holds together. It is nearly constant, about 8 MeV, for mass numbers between 30 and 170, peaks at about 8.75 MeV near A = 56, and is lower for very light and very heavy nuclei, 7.6 MeV for A = 238.

Radioactivity and Half-Life

Radioactive Elements and Becquerel's Discovery

A. H. Becquerel discovered radioactivity by accident in 1896. Pieces of uranium-potassium sulphate, wrapped in black paper, blackened a photographic plate through the paper and a sheet of silver, so something was being emitted that could pass through both. Marie and Pierre Curie followed, isolating radium and polonium, and Marie Curie became the first person to win two Nobel Prizes, for Physics in 1903 and Chemistry in 1911.

The activity of a sample, its number of decays per second, is measured in becquerels, after Becquerel; the older unit, the curie, equals 3.7 × 10 to the power 10 becquerels. Radioactive elements whose half-lives are short compared with the age of the universe, such as tritium and plutonium, are not found naturally in any quantity and are made in laboratories and reactors.

Alpha, Beta and Gamma Radiation

Unstable nuclei decay in three ways, and each changes the nucleus differently. The alpha particle is a helium nucleus, two protons and two neutrons bound together, so it carries two positive charges and has very nearly the mass of a helium atom.

• Alpha decay: A helium nucleus is emitted, so the mass number falls by 4 and the atomic number by 2; uranium-238 becomes thorium-234.
• Beta decay: An electron or a positron is emitted as a neutron turns into a proton or a proton into a neutron, so the mass number is unchanged.
• Gamma decay: An excited nucleus drops to a lower energy level and emits a high-energy photon, with no change in mass number or atomic number.

Beta decay also releases a neutrino or antineutrino, a neutral particle with very little or no mass that interacts so weakly with matter that it can pass through the Earth. Gamma rays usually follow an alpha or beta decay: cobalt-60, for example, beta-decays into an excited nickel-60 nucleus that then emits gamma rays of 1.17 and 1.33 MeV.

Half-Life, Radioactive Decay and Carbon Dating

The number of nuclei decaying each second is proportional to the number present, so radioactive decay follows an exponential law, unlike a batch of light bulbs that all burn out at about the same time. The half-life is fixed for each radionuclide: 12.5 years for tritium, 14.3 days for phosphorus-32 and 4.5 × 10 to the power 9 years for uranium-238.

Radiocarbon dating uses this regularity. A living plant or animal takes in carbon-14; once it dies, the carbon-14 decays with a half-life of about 5,730 years, so the carbon-14 left in wood or bone tells when it died. The oldest dates that can be measured reliably this way are about 50,000 years, the working limit of carbon dating.

Nuclear Energy: Nuclear Fission and Nuclear Fusion

Nuclear Fission and the Chain Reaction

Enrico Fermi found that neutrons striking uranium produce new radioactive elements, and when a neutron strikes a uranium-235 nucleus it splits into two nearly equal fragments, for example barium-144 and krypton-89, with three neutrons. Each such nuclear fission releases about 200 MeV, first as the kinetic energy of the fragments and then as heat.

Because each fission releases on average 2.5 neutrons, more than it uses, one fission can trigger others: a chain reaction, first suggested by Fermi in 1939. Uncontrolled and rapid, it is an atomic bomb; controlled and steady, it is a nuclear reactor. Nuclear energy is about a million times larger than chemical energy for the same mass: a kilogram of coal gives 10 to the power 7 joules, a kilogram of uranium in fission 10 to the power 14.

Nuclear Fusion in the Sun and in ITER

In nuclear fusion, light nuclei join into a heavier one. Two positive nuclei repel each other, so fusion needs extreme temperatures and pressures, found only in the cores of stars. The Sun burns hydrogen into helium through the proton-proton cycle: four hydrogen atoms end up as one helium atom and release 26.7 MeV. It has been doing so for about 5 billion years and has fuel for about as long again.

Controlled fusion on Earth is far harder, and many countries, India among them, pursue it because a fusion reactor is regarded as a future source of power. The largest such project is ITER, the International Thermonuclear Experimental Reactor, a tokamak under construction near Cadarache in southern France, meant to produce 500 MW of fusion power from about 50 MW of heating, a tenfold gain.

• Members: China, the European Union, India, Japan, Russia, South Korea and the United States; India formally joined in 2005.
• India’s share: 9.1 per cent of construction, delivered in kind through ITER-India at the Institute for Plasma Research, Ahmedabad.
• India’s components: The cryostat, cryolines, in-vessel shielding, and the cooling and cooling water systems.
• People and industry: About 200 Indian scientists and associates, with firms such as L&T, Inox India, TCS, TCE and HCL Technologies.

India's own fusion research began at the Institute for Plasma Research: its ADITYA tokamak was commissioned in 1989, and a 1995 decision led to SST-1, a superconducting steady-state tokamak built for 1,000-second operation. ITER itself is expected to reach first plasma in 2033 to 2034; one gram of deuterium-tritium fuel in fusion yields 90,000 kilowatt hours, the energy of about 11 tonnes of coal.

Difference Between Nuclear Fission and Fusion

A hydrogen bomb uses a fission bomb to create the temperature needed to fuse deuterium and tritium; the first thermonuclear device was exploded at Eniwetok Atoll on 1 November 1952. Fusion's fuel is light hydrogen isotopes and the Sun's fusion ends in helium, while fission leaves behind highly radioactive waste in its spent fuel.

Nuclear Reactors and India's Three-Stage Programme

Parts of a Nuclear Reactor: Fuel, Moderator, Heavy Water and Control Rods

Neutrons from fission are fast, about 2 MeV on average, and fast neutrons tend to escape without splitting another uranium-235 nucleus, while slow neutrons are far more likely to cause fission. A nuclear reactor therefore slows its neutrons with a moderator made of light nuclei, just as a marble hitting an identical marble head-on stops and hands over its energy.

• Fuel: Uranium-235 or plutonium; natural uranium in India’s first-stage reactors.
• Moderator: Water, heavy water (D2O) or graphite; India’s power reactors use heavy water, while the Apsara research reactor at BARC uses water.
• Control rods: Cadmium and other neutron absorbers that set the reaction rate; safety rods can be dropped in to shut it down quickly.
• Coolant: Carries the heat away; in a pressurised water reactor, water at about 600 K and 150 atmospheres feeds a steam generator that drives the turbine.

The ratio of fissions in one generation of neutrons to those in the generation before is the multiplication factor K. At K = 1 the reactor is critical, steady and self-sustaining; above 1 the power rises exponentially and the reactor can become supercritical, as at Chernobyl in 1986; below 1 the reaction dies away.

Thermal and Fast Breeder Reactors: Prototype Fast Breeder Reactor at Kalpakkam

A thermal reactor, like India's pressurised heavy water reactors, slows its neutrons with a moderator. A fast breeder reactor uses fast neutrons to sustain the chain reaction, so it needs no moderator, and besides generating power it breeds more fissile material than it consumes, turning uranium-238, which does not fission, into plutonium-239.

The 500 MWe Prototype Fast Breeder Reactor at Kalpakkam, designed by the Indira Gandhi Centre for Atomic Research and built by BHAVINI, attained first criticality, the start of a controlled fission chain reaction, on 6 April 2026. Its core of mixed oxide fuel is surrounded by a blanket of uranium-238 that fast neutrons turn into plutonium-239, and the blanket is designed to take thorium-232 later, to breed uranium-233.

Thorium and the Three Stages of India's Nuclear Programme

India has limited uranium but among the largest thorium reserves in the world, so the programme planned by Homi Bhabha works in three stages, each producing the fuel for the next. Thorium itself does not fission; it is fertile, and must first be turned into uranium-233 in a reactor before it can be used as fuel. The beach sands of Kerala and Odisha are rich in monazite, which contains about 8 to 10 per cent thorium.

1. Stage 1: Pressurised heavy water reactors burn natural uranium with heavy water as moderator, and their spent fuel yields plutonium-239.
2. Stage 2: Fast breeder reactors burn that plutonium and breed more fuel, including uranium-233 from thorium-232.
3. Stage 3: Reactors fuelled by uranium-233 draw on India’s thorium reserves at scale.

Thorium is about three times as abundant as uranium in the Earth's crust, and a thorium fuel cycle is expected to produce less nuclear waste; the uranium-233 it breeds is also hard to turn into a weapon. India's KAMINI is its first large research reactor to use uranium-233 as fuel.

Nuclear Power in India: Capacity, SHANTI Act and Small Modular Reactors

India's programme began around independence under Homi J. Bhabha. Its first reactor, Apsara, went critical on 4 August 1956, and CIRUS followed in 1960; a plutonium plant at Trombay brought the technology of reprocessing spent fuel, and research reactors such as DHRUVA and KAMINI followed. Today India is more than self-sufficient in heavy water production. The wider story of Indian science is told in science, technology and space.

• Capacity: 8.78 GW installed; 56,681 million units generated in 2024-25, 3.1 per cent of India’s electricity.
• Target: 100 GW by 2047 under the Nuclear Energy Mission of the Union Budget 2025-26, towards net zero by 2070; 22.38 GW expected by 2031-32.
• Small modular reactors: ₹20,000 crore for their development, with at least five indigenous units to run by 2033; BARC is designing the 200 MWe Bharat Small Modular Reactor and the 55 MWe SMR-55.
• SHANTI Act, 2025: The Sustainable Harnessing and Advancement of Nuclear Energy for Transforming India Act consolidates nuclear law, allows limited private participation under regulation, and gives the Atomic Energy Regulatory Board statutory recognition.

Imported uranium has shaped the programme. India agreed in 2005 to separate its civil and military nuclear facilities and to place its civil facilities under International Atomic Energy Agency safeguards; the Nuclear Suppliers Group's waiver of 6 September 2008 then opened access to fuel from other countries. Reactors that burn imported enriched uranium are under IAEA safeguards. Small reactors are followed up in small modular reactors and India's Bharat Small Reactor.

Uses and Hazards of Radiation

Radioisotopes in Medicine, Agriculture and Industry; RTGs

The main purpose of a research reactor is research rather than power generation, and research reactors are also an excellent source of radioisotopes, which are used in industry, medicine and agriculture. Carbon dating of old wood and bone, described above, is one everyday use of a naturally occurring radioisotope.

A radioisotope thermoelectric generator, or RTG, is a nuclear battery with no moving parts. It uses thermocouples to turn the heat of radioactive decay into electricity, so it is not a fission reactor at all, and with no moving parts to fail or wear out it is highly reliable. That makes it ideal for remote and harsh places, above all spacecraft travelling far from the Sun, where solar panels are impractical.

• Fuel: Plutonium-238, with a half-life of 87.7 years and low gamma and neutron emission.
• Origin of the fuel: Most plutonium-238 was made by irradiating neptunium-237, a by-product of producing weapons-grade plutonium-239.
• Missions: Pioneer 10 and 11, Voyager 1 and 2, Galileo, Cassini and New Horizons.

Radiation Safety, Nuclear Waste and Accidents

Nuclear power has real advantages: it needs a million times less fuel by mass than chemical fuels, and it does not produce the greenhouse gases that come from burning coal, wood, gas and oil. Its central problem is that spent fuel is highly radioactive and extremely hazardous to life, so reactors accumulate nuclear waste, fission products and heavy elements such as plutonium and americium, that needs elaborate handling and reprocessing.

• Chernobyl, 1986: Reactor 4 near Pripyat in Ukraine exploded on 26 April 1986, a reminder that reactor accidents can be catastrophic.
• Fukushima, 2011: On 11 March 2011 the Tohoku earthquake and tsunami cut the Fukushima Daiichi plant’s power and backup, the reactors could not be cooled, and radioactive material escaped.
• Liability: India’s Civil Liability for Nuclear Damage Act, 2010 set up no-fault liability to ensure compensation after a nuclear incident.
• Local opposition: The Kudankulam plant in Tamil Nadu faced delays from the opposition of local fishermen.

Sources

• NCERT: Physics Part II (Class XII), Nuclei
• PIB: Prototype Fast Breeder Reactor at Kalpakkam attains first criticality (7 April 2026)
• PIB: A New Chapter in India's Nuclear Journey (7 April 2026)
• PIB: Prime Minister and President of France visit ITER (12 February 2025)
• PIB: The SHANTI Bill, 2025 (19 December 2025)
• PIB: Parliament question, enhancing nuclear power capacity
• NASA Science: Radioisotope Thermoelectric Generators
• Bhabha Atomic Research Centre: Thorium Fuel Cycle
• Wikipedia: ITER
• Wikipedia: Institute for Plasma Research
• Wikipedia: Radioisotope thermoelectric generator
• Wikipedia: Plutonium-238
• Wikipedia: Thorium-based nuclear power
• Wikipedia: Nuclear power in India
• Wikipedia: India–United States Civil Nuclear Agreement
• Wikipedia: Radiocarbon dating
• Wikipedia: Alpha particle
• Wikipedia: Chernobyl disaster
• Wikipedia: Fukushima nuclear accident
• Wikipedia: Kudankulam Nuclear Power Plant
• UPSC: Civil Services Examination question papers