Differentiate between nuclear fission and nuclear fusion. Give one example for each with nuclear reaction.

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Q. Differentiate between nuclear fission and nuclear fusion. Give one example for each with nuclear reaction.

CBSECBSE Class XII Board 2026Subjective· 3mImportance★★★★★

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Concept understanding — Nuclear Fission

Nuclear fission is the process by which a heavy nucleus splits into two smaller nuclei of roughly comparable mass, releasing a very large amount of energy, discovered in 1938 by the German scientists Otto Hahn and Fritz Strassmann (published January 1939; Lise Meitner and Otto Frisch supplied the theoretical fission interpretation the following month), who found that bombarding uranium with a neutron caused it to break apart in this way. The fission process is always accompanied by the release of additional free neutrons, and the released energy is many orders of magnitude greater than any ordinary chemical reaction can produce.

A representative uranium-235 fission reaction is

^{235}_{92}U+{}^{1}_{0}n\ \to\ {}^{236}_{92}U^{*}\ \to\ {}^{141}_{56}Ba+{}^{92}_{36}Kr+3\,{}^{1}_{0}n+Q

Absorbing a slow (thermal) neutron leaves the uranium nucleus in a short-lived excited state, $^{236}_{92}U^{*}$ , which lasts no more than about $10^{-12}$ s before splitting into two "daughter" fragment nuclei plus, on average, about 2.5 free neutrons; working through the mass bookkeeping for a typical fission channel gives an energy release of roughly 200 MeV per event.

Chain reaction. Since each fission produces roughly three fresh neutrons, and each of those neutrons can go on to trigger fission in yet another uranium-235 nucleus, a self-multiplying chain reaction can develop, with the number of fission events growing rapidly (roughly geometrically) generation after generation. Left completely unchecked, this is an uncontrolled chain reaction, releasing its entire stored energy within a tiny fraction of a second - the principle behind the atomic bomb. If instead the average number of neutrons allowed to trigger further fission is deliberately damped down to exactly one per event, the result is a controlled chain reaction that proceeds steadily and can be harnessed for useful power - exactly what happens inside a nuclear reactor. …

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Nuclear fission splits a heavy nucleus into lighter ones (releasing energy), while nuclear fusion joins light nuclei into a heavier one (releasing even more energy). Fission example: $^{235}_{92}\mathrm{U} + n \rightarrow ^{141}_{56}\mathrm{Ba} + ^{92}_{36}\mathrm{Kr} + 3n + \text{energy}$ . Fusion example: $^{2}_{1}\mathrm{H} + ^{3}_{1}\mathrm{H} \rightarrow ^{4}_{2}\mathrm{He} + n + \text{energy}$ .

The core difference between fission and fusion lies in what happens to the nucleus — and why energy is released. Both processes convert a tiny amount of mass into energy, following Einstein’s $E = mc^2$ , but they work in opposite directions.

Nuclear fission is the splitting of a heavy, unstable nucleus (like uranium-235 or plutonium-239) into two or more lighter nuclei, along with neutrons and a large amount of energy. It is the principle behind nuclear power plants and atomic bombs. The key insight: heavy nuclei have a lower binding energy per nucleon than medium-mass nuclei, so when they split, the fragments are more tightly bound, and the mass defect releases energy.

Nuclear fusion is the joining of two light nuclei (like isotopes of hydrogen) to form a heavier nucleus. This releases even more energy per unit mass than fission. Fusion powers the Sun and stars. The reason: light nuclei have even lower binding energy per nucleon than medium ones, so fusing them into a more stable nucleus (like helium) releases a huge amount of energy. Fusion requires extremely high temperatures and pressures to overcome electrostatic repulsion.

Let’s break down the differences systematically.

Definition and process
- Fission: A heavy nucleus absorbs a neutron, becomes unstable, and splits into two medium-mass fragments, releasing 2–3 neutrons and energy.
- Fusion: Two light nuclei overcome Coulomb repulsion (at millions of degrees) and merge into a single heavier nucleus, releasing a neutron and energy.
Energy released
- Fission: About 200 MeV per event (for uranium-235). Energy per unit mass is roughly $8 \times 10^{13}$ J/kg.
- Fusion: About 17.6 MeV per deuterium-tritium reaction, but energy per unit mass is about $3.4 \times 10^{14}$ J/kg — roughly 4 times more than fission.
Fuel availability
- Fission: Uses rare heavy elements like uranium-235 (0.7% of natural uranium) or plutonium-239 (bred from uranium-238).
- Fusion: Uses abundant light elements — deuterium (extracted from seawater) and tritium (bred from lithium). Fuel is virtually limitless.
By-products and safety
- Fission: Produces long-lived radioactive waste (e.g., strontium-90, cesium-137) that requires careful storage for thousands of years. Risk of chain reaction runaway (meltdown).
- Fusion: Produces mostly helium (non-radioactive) and some neutron-activated materials. No chain reaction risk — if containment fails, the reaction simply stops.
Conditions required
- Fission: Can occur at room temperature with a critical mass of fissile material. Controlled in reactors using control rods.
- Fusion: Requires temperatures of ~100 million K and high pressure (as in stars or tokamaks). Currently not yet commercially viable for power generation.
Examples with nuclear reactions

Fission example (one common reaction of uranium-235): …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set ANNUAL1 mark
$Q. Give one point of difference between nuclear fission reaction and nuclear fusion reaction.$
›Reveal solutionSolution

Fission splits a heavy nucleus (easy to trigger); fusion joins light nuclei (needs extreme heat to overcome Coulomb repulsion).

Nuclear fission is the splitting of a single heavy nucleus (e.g. uranium-235) into two lighter fragments, which can be triggered at ordinary temperatures/conditions, for example by the absorption of a slow (thermal) neutron. Nuclear fusion, on the other hand, is the combining of two light nuclei (e.g. hydrogen isotopes) into a heavier nucleus; because both nuclei are positively charged and repel each other strongly, fusion requires extremely high temperatures (of the order of $10^7$ – $10^8$ K, as found in stellar cores) to give the nuclei enough kinetic energy to …

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CBSE 2023Set F1 markMCQ
$Q. The function of moderator in nuclear reactor is to (A) Slow the speed of neutrons (B) Fast the speed of neutrons (C) Slow the speed of electrons (D) Fast the speed of electrons$
›Reveal solutionSolution

A moderator slows fast neutrons to thermal speeds so they are more readily captured for fission.

Fission of $^{235}$ U releases fast neutrons (~2 MeV). These fast neutrons have a low probability of causing further fission. A moderator (heavy water, graphite, or ordinary water) contains light nuclei that slow the neutrons down by repeated elastic collisions, bringing them to thermal energies (~0.025 eV), where t …

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CBSE 2023Set B1 markMCQ
$Q. Bhabha Atomic Research Centre is situated in$
(i) New Delhi
(ii) Mumbai
(iii) Kolkata
(iv) Bangalore
›Reveal solutionSolution

BARC, India's premier nuclear research institute, is situated at Trombay in Mumbai.

The Bhabha Atomic Research Centre (BARC) was established in 1954 by Dr. Homi J. Bhabha (originally as the Atomic Energy Establishment, Trombay) and renamed BARC in 1967 in his honour. It is India's largest nuclear research facility, working on nuclear reactor technology, radioisotope production and …

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CBSE 2023Set ANNUAL1 mark
$Q. What is nuclear fission? Write a nuclear fission reaction.$
›Reveal solutionSolution

Nuclear fission is the splitting of a heavy nucleus into two medium-mass nuclei with release of neutrons and energy, illustrated by the classic $^{235}$ U fission reaction.

Solution:

Nuclear fission is the nuclear reaction in which a heavy nucleus (such as $^{235}_{92}\text{U}$ ), on absorbing a slow (thermal) neutron, splits into two nuclei of roughly comparable (medium) mass, along with the emission of two or three fast neutrons and a large amount of energy (since the binding energy per nucleon of the fragments is higher than that of the parent nucleus).

A typical fission reaction:

${}_{92}^{235}\text{U} + {}_{0}^{1}n \longrightarrow {}_{56}^{144}\text{Ba} + {}_{36}^{89}\text{Kr} + 3\,{}_{0}^{1}n + Q$

…

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CBSE 2023Set ANNUAL1 markMCQ
$Q. An atom bomb works on the principle of$
(a) nuclear fusion
(b) nuclear fission
(c) α-decay
(d) β-decay.
›Reveal solutionSolution

An atom bomb releases energy by splitting heavy nuclei (nuclear fission), not by fusing light nuclei.

An atom bomb (fission bomb) works by an uncontrolled chain reaction of nuclear fission: a heavy, fissile nucleus such as $^{235}_{92}\text{U}$ or $^{239}_{94}\text{Pu}$ absorbs a neutron, becomes unstable, and splits into two lighter nuclei plus 2–3 fast neutrons, releasing a large amount of energy (from the mass defect, via $E=\Delta m c^2$ ). Each released neutron can trigger further fissions in nearby nuclei, causing a rapidly multiplying, uncontrolled chain reaction tha …

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CBSE 2022Set ANNUAL1 markMCQ
$Q. The material which is used as a moderator in nuclear reactor is$
(a) uranium
(b) heavy water
(c) cadmium
(d) plutonium.
›Reveal solutionSolution

A moderator slows down fast neutrons released in fission to thermal energies (needed to sustain the chain reaction) via elastic collisions; heavy water (and graphite) do this efficiently while absorbing very few neutrons.

In a nuclear (fission) reactor, neutrons released by fission of $^{235}\text{U}$ are fast (high energy, ~MeV), but the fission cross-section is much larger for slow (thermal) neutrons. A moderator's job is to slow these fast neutrons down through repeated elastic collisions. Energy transfer per collision is maximum when the target nucleus has a mass close to that of the neutron, so light nuclei (hydrogen, deuterium, carbon) make good moderators — and, crucially, they must also have a low neutron-absorption cross-section so they don't remove neutrons from the chain reaction. …

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CBSE 2020Set ANNUAL1 mark
$Q. Write the function of coolant used in nuclear reactor.$
›Reveal solutionSolution
The coolant's job is to remove the enormous heat produced by fission in the reactor core and transport it out safely and usefully.

In a nuclear reactor, the controlled fission chain reaction in the fuel rods releases a large amount of thermal energy. A coolant (such as water, heavy water, liquid sodium, or a gas like CO $_2$ /helium, depending on reactor design) is circulated through the reactor core.

Functions of the coolant:

It absorbs the heat generated in the core by flowing past the fuel rods.

It transports this heat out of the reactor core to a heat exchanger/steam generator, where the heat is used to convert water into high-pressure steam.

This steam then drives a turbine coupled to a generator, producing electricity. …
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CBSE 2018Set ANNUAL1 markMCQ
$Q. The explosion of atom bomb is based on the principle of :$
(a) fusion reaction
(b) uncontrolled fission reaction
(c) thermonuclear reaction
(d) controlled fission reaction
›Reveal solutionSolution

An atom bomb releases its energy through an uncontrolled chain of nuclear fission reactions.

When a heavy nucleus such as $U^{235}$ or $Pu^{239}$ absorbs a neutron, it splits (fission) into two lighter nuclei, releasing energy and 2-3 more neutrons. If the multiplication factor $k>1$ and nothing limits the reaction, these neutrons trigger further fissions in an exponentially growing, uncontrolled chain reaction, releasing an enormous amount of energy in a fraction of a second — an explosion. This is exactly the mechanism of an atom bomb.

…

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Q.Differentiate between nuclear fission and nuclear fusion. Give one example for each with nuclear reaction.

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