Maarg Manthan · Topic 13.1

Nuclear Fission and Nuclear Fusion

Science & Technology › Nuclear Technology and Energy · Topic 13.1

Nuclear fission is the splitting of a heavy nucleus, such as uranium-235 or plutonium-239, into two lighter nuclei when it absorbs a neutron, releasing about 200 million electron volts of energy and two or three more neutrons, which can split other nuclei and sustain a chain reaction. Nuclear fusion is the joining of two light nuclei, such as the hydrogen isotopes deuterium and tritium, to form a heavier nucleus, helium, and a neutron, releasing about 17.6 million electron volts. The energy comes from a small loss of mass, according to Einstein's equation E equals mc squared. All present nuclear power stations use fission, while fusion powers the Sun and the hydrogen bomb and is the target of experimental reactors such as ITER, in which India is a partner.

5 min read · UPSC Prelims · Science & Technology · Nuclear Technology and Energy

At a Glance

Fission Splitting heavy nuclei
Fusion Joining light nuclei
Fission fuel U-235, Pu-239
Fusion fuel Deuterium and tritium
Fusion project ITER, India a partner
Exam link UPSC Prelims, Science & Technology: Nuclear Technology and Energy

Where Does It Fit?

This is the first topic of the macro on nuclear technology and energy. It sets out the two nuclear reactions on which power, weapons and stars depend, before the macro moves to the Indian programme. Previous topic: GSAT-17 as ISRO’s Heaviest Communication Satellite. Next topic: Stages of India’s Nuclear Fuel Cycle.

How Do Fission and Fusion Differ?

Feature	Fission and fusion
Process	Fission splits a heavy nucleus. Fusion joins light nuclei
Fuel	Uranium-235, plutonium-239, uranium-233. Deuterium and tritium
Energy per reaction	About 200 MeV for fission, 17.6 MeV for deuterium-tritium fusion, but fusion gives more energy per kilogram of fuel
Conditions	Fission occurs at normal temperature with a neutron. Fusion needs temperatures of over 100 million degrees Celsius to overcome repulsion between nuclei
Chain reaction	Fission can be self-sustaining. Fusion must be continuously heated and confined
Waste	Fission produces long-lived radioactive waste. Fusion produces little long-lived waste
Use	Fission: power reactors and atom bombs. Fusion: Sun, hydrogen bomb, research reactors

How Are Fission Reactors Controlled and What Is Fusion Research?

Chain reaction: each fission releases neutrons that can cause further fissions; if on average one neutron per fission causes another, the reactor is critical
Critical mass: the minimum amount of fissile material to sustain a chain reaction
Moderator: a material such as heavy water, graphite or ordinary water that slows neutrons to the speed at which they cause fission in U-235
Control rods: rods of boron or cadmium absorb neutrons to control or stop the reaction
Coolant: a fluid that removes heat from the core
Fusion confinement: magnetic confinement in a tokamak and inertial confinement by lasers
ITER: International Thermonuclear Experimental Reactor being built at Cadarache in France by 35 nations including India, China, the EU, Japan, Korea, Russia and the United States
NIF ignition: in December 2022 the National Ignition Facility in the United States achieved fusion ignition with laser fusion, producing more energy than the laser energy delivered to the target

What Else Should You Remember?

Discovery of fission: Otto Hahn and Fritz Strassmann found it in 1938, and Lise Meitner and Otto Frisch explained it
First reactor: Enrico Fermi’s Chicago Pile-1 achieved a chain reaction on 2 December 1942
Mass defect: the mass of the products is slightly less than the mass of the reactants, and the difference is released as energy
Enrichment: natural uranium has only 0.7 per cent U-235, and it is raised to 3 to 5 per cent for most power reactors
Fertile materials: U-238 and thorium-232 can be converted to plutonium-239 and uranium-233, as in India’s Three-Stage Civil Nuclear Programme
Tokamak: a doughnut shaped magnetic chamber that confines hot plasma. India’s SST-1 at the Institute for Plasma Research, Gandhinagar, is a tokamak
Plasma: the fourth state of matter, a hot ionised gas
ITER timeline: first plasma has been delayed to the 2030s, so check the current schedule
Indian role in ITER: India joined in 2005 and supplies parts such as cryostat and cooling systems
Hydrogen bomb: uses fission to trigger fusion, giving much greater yield than an atomic bomb
Energy in stars: the Sun fuses hydrogen to helium in its core
Safety: fusion cannot run away like a fission chain reaction, which makes it inherently safer, but has not yet given net electrical power
Delayed neutrons: a small share of neutrons are released a little after fission, which gives operators time to control the reaction
Meltdown risk: if cooling fails, decay heat can damage the fuel, so reactors have backup cooling and safety systems

Why Does It Matter for the Exam?

Differences: fuel, conditions, waste
Components: moderator and control rod functions
Projects: ITER and NIF
Current affairs link: fusion milestones and ITER schedule
Common errors: saying that fusion is used in present power plants or that control rods slow neutrons

As an analytical point, fission delivers carbon-free power today with waste and safety issues, while fusion offers a clean source that is still an experiment. India supports both through its fission programme and its role in ITER.

Why Is This Topic Important?

Basics Fission and fusion
Parts Moderator, rods
Future ITER

What Came Next?

The next topic covers the stages of India’s nuclear fuel cycle. Next topic: Stages of India’s Nuclear Fuel Cycle.

Key People and Terms

Fissile Able to sustain fission
Moderator Neutron slowing material
Control rod Neutron absorbing rod
Plasma Hot ionised gas
Tokamak Magnetic fusion device
Mass defect Missing mass turned to energy

Exam Corner

Points to Remember

Natural uranium has 0.7% U-235.
Fusion needs over 100 million degrees.
First chain reaction: 1942, Fermi.
India joined ITER in 2005.
Moderator slows neutrons; control rods absorb them.

Do Not Confuse With

Moderator and control rod Slows neutrons versus absorbs neutrons
Fission and fusion Splitting versus joining
Fissile and fertile Can fission versus can be converted

Memory Hook

Split the heavy, join the light; moderator slows, rods absorb, ITER dreams of the Sun.

Mains Angle

Questions ask for differences and components. These points help.

Today Fission power
Tomorrow Fusion
India ITER partner

A Question You May Face

An original practice question, not a past paper question.

“Distinguish between nuclear fission and fusion and discuss the prospects of fusion energy and India’s role in it.”

How to Answer

Differences Fuel, conditions.
Reactors Moderator, rods.
Fusion ITER, NIF.
India Partner and SST-1.

GS Relevance

Prelims: fission versus fusion, chain reaction, moderators, ITER, SST-1. GS Paper 3: science and technology, nuclear energy.

Frequently Asked Questions

What is nuclear fission?

Nuclear fission is the splitting of a heavy atomic nucleus, such as uranium-235, into two smaller nuclei after it absorbs a neutron. The process releases a large amount of energy and extra neutrons, which can split further nuclei and set up a chain reaction. It is the process used in all present nuclear power plants.

What is nuclear fusion?

Nuclear fusion is the joining of two light nuclei, such as deuterium and tritium, to form a heavier nucleus with release of energy. It powers the Sun and the hydrogen bomb. It needs temperatures above 100 million degrees Celsius, so a controlled fusion reactor for power has not yet been achieved.

What is the role of a moderator in a reactor?

A moderator is a material that slows down fast neutrons released by fission to speeds at which they are more likely to cause fission in uranium-235. Heavy water, ordinary water and graphite are used. India's Pressurised Heavy Water Reactors use heavy water as moderator, which allows them to use natural uranium.

What is ITER?

ITER, the International Thermonuclear Experimental Reactor, is a tokamak being built at Cadarache in southern France by 35 countries, including India, to prove that fusion can produce more energy than it uses. It is the largest fusion experiment in the world, and India supplies components such as the cryostat.

What did the National Ignition Facility achieve in 2022?

In December 2022, the National Ignition Facility in the United States used 192 lasers on a tiny fuel capsule and for the first time produced more fusion energy than the laser energy delivered to the target, called ignition. This was an important scientific milestone, though still far from a practical power plant.

PYQ Practice — Statement Analysis

1 Fusion needs very high temperatures to overcome the repulsion between nuclei.

True

Over 100 million degrees.

2 Control rods slow down neutrons in a reactor.

False

They absorb neutrons.

3 ITER is being built in France.