Nuclear science: the atom, radioactivity, fission and fusion

The first principle

The nucleus of an atom holds protons and neutrons together with the strong nuclear force, and a nucleus weighs a little less than the particles it is made of. That missing mass is the binding energy, and by Einstein's relation E = mc² a tiny loss of mass releases an enormous amount of energy. Middle sized nuclei such as iron are the most tightly bound, so energy is released either by splitting a heavy nucleus (fission) or by joining two light ones (fusion), and unstable nuclei release it on their own as radioactivity.

The core

The atom and its nucleus

• Atomic number and mass number. The number of protons, the atomic number, fixes the element, while protons plus neutrons give the mass number. Isotopes of an element have the same number of protons but different numbers of neutrons, so they behave alike in chemistry but differently in the nucleus: uranium-235 splits easily, while uranium-238, which makes up about 99.3 per cent of natural uranium, mostly does not.
• Binding energy per nucleon rises steeply from hydrogen to iron and then falls slowly towards uranium. Splitting a uranium nucleus into two middle sized ones therefore releases about 200 million electronvolts, tens of millions of times the energy released when one carbon atom burns.

Radioactivity

• An unstable nucleus decays to a more stable one and emits radiation as it does. In alpha decay it throws out a helium nucleus, in beta decay a neutron turns into a proton and emits an electron (with an antineutrino), and gamma rays are high energy photons released as the new nucleus settles.
• Penetrating power differs. Alpha particles are stopped by a sheet of paper or the outer layer of skin but are very harmful if swallowed or breathed in, beta particles are stopped by a few millimetres of aluminium, and gamma rays need thick lead or concrete.
• Half life is the time in which half the nuclei in a sample decay. It cannot be changed by heat, pressure or chemistry, and it ranges from fractions of a second to billions of years: iodine-131 has about 8 days, cobalt-60 about 5.3 years, caesium-137 about 30 years, carbon-14 about 5,730 years and uranium-238 about 4.5 billion years.
• Measuring radiation. The becquerel counts decays per second, the gray measures energy absorbed per kilogram of tissue, and the sievert weighs that dose by the type of radiation and the organ, so it is the unit used for health risk.

Fission and the chain reaction

• Fission happens when a uranium-235 nucleus absorbs a slow neutron, splits into two lighter nuclei and releases two or three fresh neutrons along with energy. If at least one of those neutrons causes another fission, a chain reaction sustains itself.
• Critical mass is the least amount of fissile material that can sustain a chain reaction, because in a smaller piece too many neutrons escape. A reactor keeps the reaction exactly balanced with control rods and usually a moderator, such as heavy water, that slows neutrons; a weapon instead assembles a supercritical mass very fast, which is why reactor fuel, with only a few per cent uranium-235 or natural uranium, cannot explode like a bomb.
• Fissile and fertile. Fissile nuclei (uranium-235, plutonium-239, uranium-233) split with slow neutrons, while fertile nuclei (uranium-238, thorium-232) can be turned into fissile ones by absorbing a neutron. This is the basis of India's three stage programme, which its own note explains.

Fusion

• Fusion joins light nuclei. The most practical reaction fuses deuterium and tritium, two heavy forms of hydrogen, into helium and a fast neutron, releasing about 17.6 million electronvolts. Nuclei repel each other electrically, so they fuse only at temperatures of about 100 million degrees Celsius, where matter is a plasma of bare nuclei and electrons.
• Confining a plasma that hot is the central problem, because no material wall can touch it. A tokamak holds the plasma in a ring shaped magnetic field, while inertial confinement crushes a tiny fuel pellet with lasers; the National Ignition Facility in the United States first got more fusion energy out of a pellet than the laser energy put into it in December 2022.
• The advantages are an almost limitless fuel (deuterium from sea water and tritium bred from lithium), no long lived high level waste of the kind fission leaves, and no possibility of a runaway chain reaction. The disadvantages are that no device has yet produced more power than the whole plant consumes, and that fast neutrons make the reactor walls radioactive.

Dimensions

• Energy and climate. Fission already gives steady low carbon electricity, and fusion, if it succeeds, would add an almost unlimited source; the power reactor note covers plant types, safety and law.
• Health. The same radiation that harms cells is used to find and kill cancers, so the issue is dose and control, not presence.
• Agriculture and food. Radiation induces useful mutations in seeds, sterilises insect pests and preserves food, reducing post harvest loss without leaving the food radioactive.
• Security. Fissile material can make weapons, so enrichment and reprocessing are tightly controlled; the nuclear triad sits in the missiles note and the non proliferation treaties in GS2 International Relations.
• Environmental. Radioactive waste, uranium mining and accidents such as Chernobyl (1986) and Fukushima (2011) show the costs, which disaster management covers.

In practice

Radiation in daily use

• Medicine. Iodine-131 treats thyroid disease, technetium-99m is the most widely used tracer for diagnostic scans, cobalt-60 machines and linear accelerators treat cancer, and lutetium-177 is used to target some tumours from inside the body. The Bhabha Atomic Research Centre (BARC) built the Bhabhatron, an Indian cobalt-60 teletherapy machine, and the Tata Memorial Centre under the Department of Atomic Energy (DAE) is one of India's main cancer hospitals.
• Agriculture. BARC had released 70 crop varieties developed by radiation induced mutation, the Trombay varieties, by December 2024, including groundnut, mustard, pulses and rice, and in 2025 India's first mutant banana variety, developed with BARC, was released.
• Food irradiation. Gamma rays or electron beams kill insects and microbes and stop sprouting in onions and potatoes, and Krushak, at Lasalgaon in Maharashtra, was set up for onions; irradiated food is marked with the Radura logo.
• Industry and research. Radiography checks welds and castings, radioactive tracers find leaks in pipelines, carbon-14 dating dates organic remains up to about 50,000 years, and smoke detectors use a tiny alpha source.

Fusion research and India

• ITER, the International Thermonuclear Experimental Reactor being built at Cadarache in France, is a joint project of China, the European Union, India, Japan, Korea, Russia and the United States. It aims to produce about 500 MW of fusion power from about 50 MW of heating, a gain of ten. India contributes about one eleventh of the cost, mostly in kind, including the cryostat, the giant steel vessel that encloses the machine, which Larsen and Toubro built. Under the baseline revised in 2024, research operation is to begin in 2034 and deuterium–tritium operation in 2039.
• At home, the Institute for Plasma Research in Gandhinagar runs the tokamaks ADITYA and the Steady State Superconducting Tokamak (SST-1), and serves as ITER-India, the domestic agency for ITER.

Rules for radiation safety

• The Atomic Energy Regulatory Board (AERB) licenses every radiation source, from power plants to hospital X-ray machines, under the Atomic Energy (Radiation Protection) Rules 2004, and follows the dose limits of the International Commission on Radiological Protection (ICRP): 20 millisieverts a year averaged over five years for workers and 1 millisievert a year for the public, counting neither natural background nor medical exposure. Every dose must also be kept as low as reasonably achievable (ALARA). Natural sources give the average person about 2.4 millisieverts a year worldwide, about half of it from radon gas, according to the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), and the monazite coast of Kerala has some of the highest natural background levels in the world.

Issues and way forward

• Radiophobia. Fear of anything nuclear blocks food irradiation, plants and even cancer facilities; teaching that dose decides harm, and labelling clearly, can build trust.
• Orphan sources. Lost or scrapped sources harm people who do not know the danger, as when cobalt-60 from a discarded Delhi University machine reached a scrap market at Mayapuri in Delhi in 2010 and killed one person; strict licensing, source tracking and checks at scrapyards prevent this.
• Isotope supply. Medical isotopes such as molybdenum-99, the parent of technetium-99m, come from a few ageing reactors worldwide, so research reactors and cyclotrons at home matter for health security.
• Fusion timelines. ITER's delays show that fusion power is decades away, so it should be pursued as long term research without diverting money from fission and renewables that can cut emissions now.
• Skills. Nuclear medicine, radiation oncology and radiation physics need many more trained people, especially outside the big cities.

Common confusions

• Radiation and radioactivity. Radioactivity is the property of unstable nuclei, while radiation is the energy they or other sources give off; food irradiated with gamma rays is exposed to radiation but does not become radioactive.
• Fissile and fertile. Fissile nuclei split with slow neutrons and sustain a chain reaction, while fertile nuclei must first absorb a neutron and be transformed.
• Half life and lifetime. After one half life half the sample remains and after two half lives a quarter, so radioactivity never drops to zero at a fixed date.
• Nuclear fission and nuclear fusion. Fission splits heavy nuclei and runs every power reactor today, while fusion joins light nuclei, powers the Sun and is still experimental on Earth.
• Gray and sievert. The gray is absorbed energy, while the sievert adjusts that energy for the kind of radiation and the organ, so only the sievert measures risk.

For the exam

• Keyword bank: isotope, binding energy, mass defect, E = mc², half life, alpha beta gamma, chain reaction, critical mass, moderator, fissile and fertile, plasma, tokamak, ITER, mutation breeding, food irradiation, sievert and ALARA.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction with data: The Bhabha Atomic Research Centre had released 70 crop varieties developed through radiation induced mutation by December 2024, a reminder that nuclear science feeds farms as well as power plants (DAE, 2024).
  • Introduction with a source: Under its revised baseline of 2024, ITER, in which India is one of seven members, plans to begin research operation in 2034 and deuterium–tritium fusion in 2039.
  • Conclusion: Nuclear science is neither a danger to be feared nor a cure for everything, because dose and control decide harm, and India's task is to use radiation widely in medicine, farming and power under a strong, independent regulator.
  • Conclusion: Fission can cut India's emissions now and fusion may transform energy later, so steady investment in both, with skilled people and public trust, is the sound course.