E.4
Fission
Energy release in spontaneous and neutron-induced fission, chain reactions, nuclear reactor components and the management of radioactive waste.
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Key Concepts, Fission
What is Nuclear Fission?
Nuclear fission is the splitting of a heavy nucleus into two smaller daughter nuclei, releasing energy. Neutron-induced fission (used in reactors) occurs when a slow-moving thermal neutron is absorbed by a fissile nucleus such as uranium-235. The compound nucleus becomes highly unstable and splits, typically producing two daughter nuclei of unequal mass, two or three fast neutrons, and gamma radiation. Energy is released because the binding energy per nucleon of the products is greater than that of the original nucleus, so mass is converted to energy via E = mc².
Calculating Energy Released in Fission
The energy released is calculated from the mass defect: the difference between the total mass of reactants (uranium nucleus plus neutron) and the total mass of the products. Convert the mass defect Δm in atomic mass units to energy using 1 u = 931.5 MeV, or convert to kg and use E = Δm × c². A typical uranium-235 fission event releases about 200 MeV. For comparison, burning one carbon atom releases only a few eV, so nuclear fission releases roughly 50 million times more energy per reaction.
Chain Reactions
Each fission event releases 2-3 neutrons. If these cause further fissions, a chain reaction develops. In an uncontrolled chain reaction, the number of fissions grows exponentially (as in a nuclear weapon). In a controlled chain reaction (in a reactor), exactly one neutron from each fission causes a further fission on average, sustaining a steady rate of energy release. A critical mass of fissile material is needed: too small and neutrons escape without causing further fissions. Fast fission neutrons are less likely to cause further fissions in U-235 and must be slowed first by a moderator.
Nuclear Reactor Components
A nuclear reactor has four key components. The moderator (water or graphite) slows fast fission neutrons to thermal speeds so they can be absorbed by U-235. Control rods (boron or cadmium) absorb neutrons and are raised or lowered to control the reaction rate. The coolant (water, CO₂ or liquid sodium) carries thermal energy away from the core to the heat exchanger. The heat exchanger transfers heat to a secondary water circuit to produce steam and drive turbines. Thick concrete and lead shielding surrounds the reactor to absorb radiation.
Radioactive Waste and Long-Term Storage
Fission products are highly radioactive, generating heat and emitting alpha, beta and gamma radiation. High-level waste (spent fuel rods) remains dangerously radioactive for thousands of years. Short-term storage involves cooling ponds at the reactor site, allowing the most active short-lived isotopes to decay. Long-term proposals include vitrification (sealing waste in glass) and deep geological disposal in stable rock formations. The challenge is that the timescales involved (tens of thousands of years) far exceed any human institution.
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Frequently Asked Questions, IB Physics Fission
What is Fission in IB Physics? ↓
Energy release in spontaneous and neutron-induced fission, chain reactions, nuclear reactor components and the management of radioactive waste. This topic is part of Theme E (Nuclear & Quantum Physics) in the current IB Physics syllabus.
Is Fission SL or HL in IB Physics? ↓
Fission is covered by both SL and HL students in the current IB Physics syllabus. HL students study additional depth and extension content beyond the SL core.
What equations do I need for IB Physics Fission? ↓
The key equations for Fission are covered in the concept tutorial above. For a structured set of notes with all equations, conditions and worked examples, the GradePod Exam Pack includes a revision note template for every topic.
What are common exam mistakes in IB Physics Fission? ↓
Common mistakes are covered in detail in the exam technique video above. The GradePod Exam Pack also includes exam-style questions with mark schemes so you can see exactly how marks are awarded and where students typically drop them.
How do I revise Fission for the IB Physics exam? ↓
Follow the GradePod three-step method. First, watch the concept tutorial and tick off each learning objective on the checklist above as you go. Second, watch the exam technique video to see how IB-style questions are answered under exam conditions. Third, use the Exam Pack to practise independently with knowledge questions, exam questions and mark schemes. That's it. It works. I promise.