DSE Physics: Nuclear Energy — Tutor Lesson Plan (fission, fusion, E = mc²)

2026-11-01

Tutor lesson-prep guides · Secondary physics

Students can recite E = mc² but answer where the energy comes from with it comes out of the atom. The real source is the small mass that disappears.

Objectives — The student can describe a fission chain reaction and the job of control rods, calculate energy released from a mass defect, and compare fusion with fission including the three rules of radiation protection.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Atomic structure	Protons, neutrons, electrons	Mass sits in the nucleus
Conservation of energy	Energy changes form	Kinetic and internal energy
Units	MeV and joules	1 MeV = 1.6 x 10^-13 J

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Prerequisite check	What is inside a nucleus	Names the nucleus
8-22 min	Fission	U-235 absorbs a slow neutron	Draws one generation
22-38 min	Mass defect	Delta m into energy	3.20e-11 J
38-50 min	Fusion	Light nuclei join, very hot	Two similarities and differences
50-56 min	Reactor and protection	Rods, shielding, distance, time	Matches three rules
56-60 min	Exit check	Three questions marked	Works alone

03 Teaching script (read it out)

Scene 1: Fission runs on neutrons, and it chains

A U-235 nucleus absorbing a slow neutron splits into two fragments and releases two or three neutrons.
Those neutrons trigger further splits: that is a chain reaction, and it sustains itself while more than one neutron per split goes on to cause another.
Control rods (boron, cadmium) absorb spare neutrons; the moderator (water, graphite) slows fast neutrons down. Keep the two jobs apart.

Student answers what happens to the rate when a control rod is withdrawn (it rises).

Scene 2: The energy comes from the missing mass

Masses before and after the reaction are not equal, and the difference is what becomes energy: E = mc squared.
One U-235 fission releases about 200 MeV, a mass defect of roughly 0.215 u (about 3.56e-28 kg), which is 3.20e-11 J.
Burning coal gives about 2.9 x 10^7 J per kilogram, so the same energy needs only about 0.0 kg of uranium fuel: several orders of magnitude apart.

Student computes the energy for a mass defect of 0.2 u (0.2 x 931.5 = about 186 MeV).

Scene 3: Fusion is harder but denser

Deuterium and tritium fuse into helium plus a neutron, and the nuclei need very high temperature to overcome electrostatic repulsion.
The Sun runs on fusion; fusion waste is shorter lived, but ignition is very hard to engineer.
Radiation protection: less time, more distance, thicker shielding such as concrete or lead.

Correct this: fusion is not waste free, its waste simply has a much shorter half-life.

04 Question types and worked steps

Type	Example	Steps	Answer
Fission equation	U-235 + n gives Ba-141 + Kr-92 + 3n	Conserve mass number and atomic number	Three neutrons
Energy	Delta m = 0.2 u	E = mc squared, or x 931.5 MeV per u	About 186 MeV
Half-life	Waste after four half-lives	Halve each time	6.2% left
Comparison	Fusion against fission	Fuel, conditions, waste	One point each

Sanity check (teach this): Mass numbers must balance on both sides of the equation, and so must atomic numbers. If energy is wanted in MeV while mass is in kilograms, convert first, otherwise the answer is out by 10^13.

05 Independent practice (3 questions)

Question	Answer	What to watch
Convert 200 MeV to joules	3.20e-11 J	200 x 10^6 x 1.6 x 10^-19
Percentage left after four half-lives	6.2%	Halve four times
Why reactors use slow neutrons	They are absorbed by U-235 far more easily	The moderator slows them

06 Common mistakes and what to say

Mistake	Student writes	Say this
Control rods called a moderator	says rods slow neutrons down	Rods absorb neutrons; the moderator slows them.
Missing c squared	uses E = delta m	Without c squared the answer is 10^17 times too small.
Fusion called waste free	claims zero radioactivity	Fusion waste is short lived, not absent.
Half-life treated as linear	zero left after four half-lives	Each half-life halves the amount; it never reaches zero.

07 Homework (paper, section and question numbers)

DSE Physics practice paper (nuclear energy) — Section A questions 7 to 9: fission equations, energy, half-life
Secondary physics chapter index — All chapters

08 Exit check (last 5 minutes)

Question	Answer	Decision
200 MeV in joules	3.20e-11 J	All right: move to radiation and society
Job of a control rod	Absorbs spare neutrons	One wrong: re-teach the reactor
Percentage after four half-lives	6.2%	Two wrong: repeat scene 2

09 Tutor FAQ

Memorise 931.5 MeV per u?

The paper usually gives the conversion; knowing the size and the method matters more.

Which releases more, fission or fusion?

Per kilogram fusion does, but every commercial reactor today is fission.

Is all radiation harmful?

Dose decides risk, and background radiation exists; the three rules cut the dose.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.