Ch 6 · 5.2.4 Fission and Fusion · O Level Physics 5054
Topic 6 of 8 in Nuclear Physics — O Level Physics 5054 — 49 lessons.
Introduction
There are two ways to release energy from nuclei: split a big one, or join two small ones. Nuclear power stations split uranium nuclei. The Sun and every other star shine by joining hydrogen nuclei. (Syllabus section 5.2.4.)
Key ideas — Fusion
Nuclear fusion is the joining of two smaller (light) nuclei to form a larger nucleus, with the release of energy.
🧑🏫 Teacher's explanation
Imagine two powerful magnets with the same poles facing each other. They naturally want to push away from each other with great force. To get them to touch and stick, you have to push them together extremely hard and very fast. This is exactly what happens inside a star.
📖 Definitions
🔍 Worked example — Easy
A single fusion reaction involves two hydrogen nuclei combining to form one helium nucleus. If the mass of the two hydrogen nuclei is 3.016 u and the mass of the resulting helium nucleus is 3.014 u, calculate the mass defect Δ m in atomic mass units (u).
🔍 Worked example — Standard
In a fission reaction, a ^235_92U nucleus absorbs a neutron and splits into ^144_56Ba and ^91_36Kr. How many neutrons are released in this specific reaction?
🔍 Worked example — Exam-style
A nuclear reactor uses control rods to manage the chain reaction. If the rate of neutron absorption by control rods increases, what happens to the number of fission events per second, and why?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Fission
Nuclear fission is the splitting of a large nucleus into smaller ones.
🧑🏫 Teacher's explanation
Imagine a large, overstuffed water balloon. If you poke it with a tiny needle, the tension becomes too much, and the balloon suddenly bursts into several smaller pieces, splashing water everywhere. In the nucleus, the 'tension' is the electrical repulsion between protons. When a neutron hits a heavy nucleus like Uranium-235, it acts like that needle, making the nucleus so unstable that it snaps apart.
📖 Definitions
🔍 Worked example — Easy
A single ^235_92U nucleus absorbs one neutron and splits into ^141_56Ba and ^92_36Kr. Calculate the total number of neutrons released in this single fission event.
🔍 Worked example — Standard
In a nuclear reactor, a fission event produces 3 neutrons. If these 3 neutrons go on to cause 3 more fission events, and each of those produces 3 neutrons, calculate the total number of neutrons produced in the second generation of fission.
🔍 Worked example — Exam-style
A nuclear reactor uses control rods to manage the chain reaction. If the number of neutrons causing fission is N, and each fission produces k neutrons, the number of neutrons in the next generation is N × k. If k = 2.5 and we want to maintain a steady state (where N remains constant), what must the average number of neutrons from each fission that actually cause a new fission be?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Chain reaction
Each fission releases 2 or 3 neutrons. If these go on to be absorbed by other U-235 nuclei, each causes another fission, releasing more neutrons, and so on. This self-sustaining process is a chain reaction.
🧑🏫 Teacher's explanation
Imagine a room full of people standing around, each holding a single balloon. If one person pops their balloon, they accidentally throw two more balloons into the crowd. If those two people then pop their balloons, they throw four more. Very quickly, the room is filled with popping balloons and flying rubber. This is an uncontrolled chain reaction.
📖 Definitions
🔍 Worked example — Easy
A single U-235 nucleus undergoes fission and releases 3 neutrons. If each of these neutrons goes on to cause exactly one new fission event, how many total fission events will have occurred after the second generation of fissions?
🔍 Worked example — Standard
In a controlled nuclear reactor, the goal is to maintain a steady power output. If a fission event produces 2.5 neutrons on average (due to some being absorbed or escaping), but only 1 neutron is needed to sustain the reaction, what is the effect on the chain reaction if the number of neutrons released per fission increases to 3?
🔍 Worked example — Exam-style
A nuclear reactor uses control rods to absorb neutrons. If the number of neutrons produced per fission is n, and the probability of a neutron causing a new fission is p, the number of fissions in the next generation is n × p. If n = 3 and p = 0.4, calculate the number of fissions in the next generation from a single initial fission.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe the process of fission in a U-235 nucleus. [2] (b) Explain how the chain reaction is kept controlled in the reactor. [2] [4 marks]
Key ideas — Controlling the reaction in a nuclear reactor
The whole reactor is surrounded by thick concrete shielding to absorb radiation.
🧑🏫 Teacher's explanation
Imagine a room full of people throwing bouncy balls at each other. If everyone throws the balls at high speed, they might fly past each other without hitting anyone. This is like fast neutrons in a reactor; they are moving too quickly to be 'caught' by a U-235 nucleus. To make the reaction happen, we need to slow those balls down so they are more likely to hit a target. This is what a moderator does.
📖 Definitions
🔍 Worked example — Easy
A single U-235 nucleus absorbs a neutron and splits into two daughter nuclei and 3 neutrons. If this happens in a chain reaction, how many neutrons are released in total after 2 generations of fission (starting from the first event)?
🔍 Worked example — Standard
In a reactor, the control rods are lowered into the core. State the effect this has on the number of neutrons available to cause fission and explain why this is necessary.
🔍 Worked example — Exam-style
A nuclear reactor uses water as both a coolant and a moderator. Explain the dual role of water in the reactor core.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
The control rods absorb fewer neutrons, so more neutrons are available to cause fission. More than one neutron from each fission goes on to cause another, so the rate of fission (and the energy released per second) increases. They must be lowered again to return to a steady rate.
🔍 Worked example — Easy
A single U-235 nucleus absorbs a neutron and splits into two daughter nuclei. If this process releases 200 MeV of energy and produces 3 neutrons, calculate the total energy released if this happens in 10 separate fission events.
🔍 Worked example — Standard
In a nuclear reactor, a fission event produces 2 neutrons. If the reactor is in a steady state, how many neutrons must be absorbed by the control rods for every 100 fission events to ensure the reaction does not increase?
🔍 Worked example — Exam-style
A nuclear reactor uses control rods to manage the reaction. If the control rods are moved out of the core, describe the effect on the neutron population and the energy output rate.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) The joining of two smaller nuclei to form a larger nucleus, releasing energy; in the Sun and other stars. (2) It becomes unstable and splits into two smaller daughter nuclei, releasing two or three neutrons and a large amount of energy. (3) The neutrons released by each fission go on to cause further fissions, which release more neutrons, so the process keeps itself going. (4) (a) slows down neutrons so they are more likely to cause fission; (b) absorb neutrons to control the rate of fission; (c) carries thermal energy away from the reactor core.