NDA Physics · Modern Physics

Nuclear Physics: Fission, Fusion, and Reactors

The nucleus stores enormous energy released by fission (splitting heavy nuclei) or fusion (joining light nuclei); reactors use controlled fission, and Einstein's E = mc² ties the released energy to lost mass.

All Modern Physics notes NDA Physics strategy

Why this matters

Five PYQs here, mostly EASY recall plus one MODERATE. The NDA tests the source of nuclear energy (fission, with E = mc² from Einstein), the principle of a reactor (CONTROLLED fission), the fuel (uranium minerals like pitchblende), and how radioactivity is measured (GM counter). Know fission vs fusion cold — they are the most-swapped distractors in the subtopic.

Concept 1 of 3

Nuclear energy — fission, fusion, and E = mc²

Intuition

A heavy nucleus like uranium can split into lighter pieces (fission), or very light nuclei like hydrogen can join into a heavier one (fusion). Either way, the products weigh slightly LESS than the starting material, and that lost mass becomes energy through Einstein's E = mc². Because c² is huge, even a speck of lost mass releases enormous energy.

Definition

Nuclear energy comes from a tiny loss of mass converted to energy:

Fission — a heavy nucleus (e.g. uranium-235) SPLITS into lighter nuclei, releasing energy. This is what power-reactor energy comes from.
Fusion — light nuclei (e.g. hydrogen isotopes) JOIN into a heavier nucleus, releasing even more energy. This powers the Sun and the hydrogen bomb.
Einstein's mass-energy equivalence $E = mc^2$ gives the energy released for the mass lost.

Mass-energy equivalence

E = mc^2

Eenergy released (J)
mmass lost / converted (kg)
cspeed of light, 3 x 10⁸ m/s

Worked example

Nuclear energy in a power reactor is generated by which process, and whose equation expresses the energy released?

Reactor energy comes from splitting heavy nuclei — nuclear fission.
The energy released equals the lost mass times $c^2$ , expressed by Einstein's $E = mc^2$ .

Answer:Nuclear fission; the energy is expressed by Einstein's E = mc².

Practice this conceptself-check · 4 quick reps

Try it yourself

Which process powers the Sun — fission or fusion — and which joins nuclei together?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
Reactor energy is generated by fission or fusion?
2.
Whose equation gives the energy released in nuclear reactions?
3.
Splitting a heavy nucleus is called?
4.
Joining light nuclei is called?

From the bank · past-year question

Example 1Modern PhysicsMODERATE

Nuclear energy is generated by

[Q137 · Sep · 2021]

Fission splits, fusion joins — and reactors use fission

Power reactors run on controlled FISSION (splitting uranium). FUSION (joining light nuclei) powers stars and is not yet a practical power source. Distractors swap these and also attach the wrong scientist (E = mc² is Einstein, not Rutherford/Bohr).

Concept 2 of 3

The nuclear reactor — controlled fission and its parts

Intuition

A reactor releases nuclear energy at a steady, safe rate by keeping the fission chain reaction CONTROLLED, not explosive. It does this with a few key parts: fuel (uranium), a moderator to slow neutrons, control rods to absorb excess neutrons, and a coolant to carry heat away. The exam asks for the basic principle (controlled fission) and which item does NOT belong.

Definition

A nuclear reactor generates power by controlled nuclear fission — a self-sustaining chain reaction held at a steady rate. Core parts:

Fuel — fissile material (uranium-235, plutonium-239).
Moderator — slows neutrons (graphite, heavy water).
Control rods — absorb neutrons to control the rate (cadmium, boron).
Coolant — carries heat away to make steam.

Reactor component	Role
Fuel (uranium-235)	Undergoes fission to release energy
Moderator (graphite, heavy water)	Slows down fast neutrons
Control rods (cadmium, boron)	Absorb neutrons to control the reaction rate
Coolant	Removes heat from the core
A mechanism to reduce CO₂ emission	Does NOT belong to a reactor NDA 2024 — the item that does NOT belong to a nuclear reactor is "a mechanism to reduce CO₂ emission" (reactors emit no CO₂ in the first place).

Principle = controlled fission. The odd-one-out trap is the CO₂-reduction mechanism — nuclear power produces no CO₂, so no such part exists.

Practice this conceptself-check · 4 quick reps

Try it yourself

What is the basic scientific principle behind a nuclear reactor?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
Basic principle behind a nuclear reactor?
2.
What do control rods (cadmium, boron) do in a reactor?
3.
What does a moderator do?
4.
Which item does NOT belong to a nuclear reactor: fuel, moderator, control rods, or a CO₂-reduction mechanism?

From the bank · past-year question

Example 2Modern PhysicsEASY

Basic scientific principle behind a nuclear reactor is

[Q81 · Apr · 2019]

Reactor = CONTROLLED fission (a bomb is uncontrolled)

The reactor's whole point is keeping fission controlled and steady. An uncontrolled fission chain reaction is an atomic bomb. The exam answer for a reactor is always "controlled fission."

Drill 1 more on the nuclear reactor — controlled fission and its parts

Concept 3 of 3

Nuclear fuel, measuring radioactivity, and radiation types

Intuition

Reactors burn uranium, which is mined from minerals such as pitchblende. Radioactivity — the spontaneous emission of radiation from unstable nuclei — is measured with a Geiger-Müller (GM) counter. The radiation comes in three flavours: alpha, beta, and gamma, ranked by penetrating power gamma > beta > alpha.

Definition

Recall facts for fuel, detection, and radiation:

Nuclear fuel mineral — uranium is obtained from pitchblende (a uranium ore).
Radioactivity is measured by a GM (Geiger-Müller) counter.
Radiation types: alpha (helium nucleus, +2), beta (electron, -1), gamma (high-energy EM, no charge). Penetrating power: gamma > beta > alpha; ionising power is the reverse: alpha > beta > gamma.

Alpha is stopped by a sheet of paper, beta by a few millimetres of aluminium, gamma needs thick lead or concrete. Penetration ranks gamma > beta > alpha; ionising power ranks the opposite way.

Radiation	What it is	Stopped by
Alpha (α)	Helium nucleus (2 protons + 2 neutrons), charge +2	A sheet of paper Most ionising, least penetrating.
Beta (β)	Fast electron, charge -1	A few mm of aluminium
Gamma (γ)	High-energy electromagnetic wave, no charge	Thick lead or concrete Least ionising, most penetrating.

Penetration: γ > β > α. Ionising power: α > β > γ. Alpha is stopped by paper; gamma needs thick lead.

Practice this conceptself-check · 4 quick reps

Try it yourself

Radioactivity is measured by which instrument, and which mineral supplies nuclear fuel?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
Radioactivity is measured by which instrument?
2.
Which mineral is used as nuclear fuel?
3.
Which radiation has the greatest penetrating power: alpha, beta, or gamma?
4.
Which radiation is stopped by a sheet of paper?

From the bank · past-year question

Example 3Modern PhysicsEASY

Radioactivity is measured by

[Q75 · Sep · 2017]

Penetration and ionising power run OPPOSITE ways

Gamma penetrates most but ionises least; alpha penetrates least (stopped by paper) but ionises most. Mixing up the two rankings is the classic radiation trap.

Pitchblende, not coal or limestone

Nuclear fuel comes from uranium ores such as pitchblende. Coal/petroleum are chemical (fossil) fuels, not nuclear. The exam answer for nuclear fuel mineral is pitchblende.

Drill 1 more on nuclear fuel, measuring radioactivity, and radiation types

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Nuclear energy — fission, fusion, and E = mc²
Mass-energy equivalence
$E = mc^2$

Reference tables (2)

The nuclear reactor — controlled fission and its parts5 rows

Reactor component	Role
Fuel (uranium-235)	Undergoes fission to release energy
Moderator (graphite, heavy water)	Slows down fast neutrons
Control rods (cadmium, boron)	Absorb neutrons to control the reaction rate
Coolant	Removes heat from the core
A mechanism to reduce CO₂ emission	Does NOT belong to a reactor NDA 2024 — the item that does NOT belong to a nuclear reactor is "a mechanism to reduce CO₂ emission" (reactors emit no CO₂ in the first place).

Principle = controlled fission. The odd-one-out trap is the CO₂-reduction mechanism — nuclear power produces no CO₂, so no such part exists.

Nuclear fuel, measuring radioactivity, and radiation types3 rows

Radiation	What it is	Stopped by
Alpha (α)	Helium nucleus (2 protons + 2 neutrons), charge +2	A sheet of paper Most ionising, least penetrating.
Beta (β)	Fast electron, charge -1	A few mm of aluminium
Gamma (γ)	High-energy electromagnetic wave, no charge	Thick lead or concrete Least ionising, most penetrating.

Penetration: γ > β > α. Ionising power: α > β > γ. Alpha is stopped by paper; gamma needs thick lead.

Watch out for (4)

Fission splits, fusion joins — and reactors use fission
→ Nuclear energy — fission, fusion, and E = mc²
Reactor = CONTROLLED fission (a bomb is uncontrolled)
→ The nuclear reactor — controlled fission and its parts
Penetration and ionising power run OPPOSITE ways
→ Nuclear fuel, measuring radioactivity, and radiation types
Pitchblende, not coal or limestone
→ Nuclear fuel, measuring radioactivity, and radiation types

Mastery check — 2 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Modern PhysicsEASY

Of the following, which does

\textbf{\text{not}}

belong to a nuclear reactor ?

[Q125 · Apr · 2024]

Example 2Modern PhysicsEASY

Which one of the following minerals is used as a fuel in nuclear power stations?

[Q60 · Apr · 2019]

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