IB Physics Nuclear Fission & Fusion FAQ

Fission and fusion sit in Theme E: Nuclear and quantum physics on the current Physics guide (first exams 2025). Fission is HL-only content (sub-topic E4); fusion is core to both SL and HL in E5, Fusion and stars. Both are tested through mass-energy calculations in Papers 1 and 2, and occasionally in Paper 3 data-based questions.

Fission (sub-topic E4) is HL-only in the current Physics guide — SL students aren't assessed on chain reactions, critical mass or reactor moderators. Fusion, by contrast, is core content for both levels under E5, Fusion and stars, though HL goes further into stellar nucleosynthesis and the proton-proton chain.

Fission splits a heavy, unstable nucleus (like uranium-235) into two lighter fragments plus neutrons, releasing energy from the mass lost. Fusion joins two light nuclei (like hydrogen isotopes) into a heavier one, also releasing energy — because both processes move nuclei closer to iron-56, the most tightly bound nucleus there is.

You need mass defect (Δm = sum of separate masses − mass of product), the mass-energy equation E = Δmc², and binding energy per nucleon to compare stability. The IB data booklet gives you c and atomic mass unit conversions (1 u = 931.5 MeV/c²), so you rarely calculate these from scratch.

Find the total mass before the reaction, subtract the total mass after, and convert that mass defect into energy using E = Δmc² (or Δm in u × 931.5 MeV). Always check your masses are in the same unit before subtracting — that's the single most common arithmetic slip I see in mock papers.

Students confuse total binding energy with binding energy per nucleon — the curve on the data booklet plots the latter, and that's what determines stability. I've marked scripts where a student correctly reads the graph but then argues fission of iron would release energy, which is backwards; iron sits at the peak.

Top scripts don't just plug numbers into E = mc² — they explain why the binding energy curve predicts which reactions release energy, and they connect fusion to stellar life-cycles (a favourite HL Paper 3 link). Practise past-paper data-based questions where you read values off a binding energy graph rather than being given them.

Both appear in Paper 1 (multiple choice, mostly conceptual) and Paper 2 (short and extended response, including mass-energy calculations). Paper 3 can include a data-based question on this theme, often pairing fusion with the Astrophysics option content at HL, since stellar fusion links directly to E5, Fusion and stars.

Not directly — you can't build a reactor or fusion device for coursework, so most students investigate related, measurable phenomena instead, like radioactive decay rates, half-life of a simulated isotope, or gamma absorption. A genuinely original angle here scores better on the Exploration criterion than a generic 'research report' on fission.

It's real physics with real engineering hurdles — projects like ITER in France aim to demonstrate net-positive fusion energy, but commercial fusion power stations are still decades away by most estimates. For the IB exam, your child needs the physics (mass defect, temperature requirements) rather than engineering feasibility debates, though HL Paper 3 sometimes touches on both.

Fusion is the engine behind stellar energy production, so E5, Fusion and stars, directly feeds into the HL Astrophysics option — questions on the proton-proton chain, stellar equilibrium and a star's life-cycle assume you already understand basic fusion energetics from the core theme. Revise them together rather than as separate topics.

Look for resources that pair concise notes with plenty of past-paper style calculation practice — this topic rewards repetition on mass-defect maths more than memorising definitions. On RevisionPrep, the Physics Revision Notes cover Theme E concisely, and the Topical Worksheets give graded fission and fusion calculations your child can self-mark before a mock.