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.
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
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 gives the energy released for the mass lost.
Mass-energy equivalence
- Eenergy released (J)
- mmass lost / converted (kg)
- cspeed of light, 3 x 10⁸ m/s
Worked example
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
Example 1 · Modern Physics · Nuclear Physics
Fission splits, fusion joins — and reactors use fission
Concept 2 of 3: The nuclear reactor — controlled fission and its parts
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). |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
Example 2 · Modern Physics · Nuclear Physics
Reactor = CONTROLLED fission (a bomb is uncontrolled)
Concept 3 of 3: Nuclear fuel, measuring radioactivity, and radiation types
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. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
Example 3 · Modern Physics · Nuclear Physics
Penetration and ionising power run OPPOSITE ways
Pitchblende, not coal or limestone
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
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). |
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. |
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
Test yourself on Modern Physics
10 past NDA questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.