Nuclear Fission and Fusion
Fission splits a heavy nucleus and can release neutrons that sustain a chain reaction. Control rods absorb neutrons; moderators slow them in thermal reactors. Fusion joins light nuclei. Useful fusion requires high temperature, suitable fuel density and enough confinement time.
What a learner can do afterwards
- Describes how neutrons sustain a fission chain and distinguishes control rods from the moderator in a thermal reactor
- Explains how high temperature, fuel density and confinement time help make fusion reactions useful
- Compares fission and fusion inputs and products, and explains why either can leave radioactive material
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Fission splits a heavy nucleus into lighter nuclei. Uranium-235 can absorb a neutron and split, releasing energy and usually two or three neutrons. Some released neutrons cause further splits, forming a chain reaction. Others escape or are absorbed without fission. The chain stays steady when each split causes one further split on average. It grows when that average exceeds one. Some nuclei also split spontaneously.
Control rods absorb neutrons. Inserting them farther leaves fewer neutrons for fission and lowers fission power, with other conditions fixed. Withdrawing them can raise power. In a thermal reactor, a moderator slows neutrons, making further uranium-235 fissions more likely. Fast-neutron reactors use a different design without a moderator. Coolant carries heat away; that is a different job from regulating the chain.
Fusion joins light nuclei and can release energy. Hydrogen nuclei are positive and repel electrically. Higher temperature makes close encounters more likely. Useful fusion also needs suitable fuel density and enough confinement time. Gravity keeps dense fuel together in stars. On Earth, experiments use magnetic confinement or briefly compress fuel with lasers. They do not need a star's exact conditions.
Higher temperature raises nuclei's speeds, making close encounters more likely. Greater fuel density means more nuclei in a given volume, so encounters happen more often. Longer confinement keeps fuel and energy from escaping too quickly, allowing more fusion reactions before conditions are lost.
Spent fission fuel contains radioactive fragments, unused fuel and other heavy nuclei. Their lifetimes vary; some need isolation for thousands of years or longer. One studied fusion reaction joins deuterium and tritium, hydrogen isotopes, producing stable helium and a fast neutron. Tritium is radioactive, and neutrons can make reactor materials radioactive. Fusion waste depends on the fuel and materials; it is not always absent or short-lived.
Use the previous stop's distinction: shielding reduces irradiation, while containment limits radioactive material escaping and spreading contamination. Stopping a fission chain does not stop radioactive decay in the fuel. Decay still produces radiation and heat, so cooling and radiation protection remain necessary after shutdown.
Fission power stations use controlled chains to provide heat for electricity generation. Fusion experiments also release energy. Energy released in fuel is different from net electricity supplied by a whole plant. A fusion plant must sustain suitable fuel conditions, collect heat, protect materials and run equipment. Its electricity output must exceed the electricity used by the whole plant. One fusion event or hot fuel does not demonstrate this.
Fission splits heavy nuclei; fusion joins light ones. Control, cooling, radiation protection and whole-plant energy costs matter.
This lesson, question bank and flashcards were written for this dataset. OpenStax fission, fusion and nuclear-energy material informed the factual study.
The readings and flashcards were written for this dataset. OpenStax informed the factual study. Nuclear fission and Nuclear fusion are Khan Academy videos.
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Keeping a fission chain steady
When uranium-235 absorbs a neutron, it can split into lighter nuclei and release energy and more neutrons. A released neutron may cause another split, escape the fuel or be absorbed without causing fission.
A chain grows if each split causes more than one further split on average. It stays steady when the average is one. Counting the neutrons emitted in one event does not give a fixed doubling rule for the chain.
Control rods absorb neutrons to regulate the fission rate. A thermal reactor uses a moderator to slow neutrons. Fast-neutron reactors use a different design. Coolant removes heat.
Stopping the chain does not stop radioactive nuclei in the fuel from decaying. Their radiation and heat continue, so cooling, shielding and containment are still needed after shutdown.
Bringing light nuclei together
Light nuclei can join in fusion and release energy. Their positive charges repel, making close encounters difficult. Heating the fuel raises nuclei's speeds and makes close encounters more likely.
Greater fuel density means more nuclei occupy a given volume, so encounters happen more often at the same temperature. Confinement must keep fuel and energy from escaping too quickly. Maintaining suitable conditions for longer allows more fusion reactions before those conditions are lost. Temperature, density and confinement time therefore work together.
Gravity confines dense fuel in stars. Laboratory approaches use other methods. Magnetic fields can confine hot charged particles. Lasers can briefly compress a small fuel target. Neither method requires matching every condition inside a star.
The reacting nuclei determine the products. One studied reaction uses deuterium and tritium, two hydrogen isotopes, and produces helium and a neutron. Other fusion reactions have different products.
Releasing energy in fuel is only one part of generating electricity. A plant must operate steadily, collect heat, generate electricity and cover the electricity used by all its equipment.
Products, waste and protection
Spent fission fuel contains split fragments, unused fuel and other heavy nuclei. Its radioactive components have different half-lives. Some require isolation for thousands of years or longer. There is no single lifetime for all fission waste.
Deuterium-tritium fusion makes stable helium, but also uses radioactive tritium and releases fast neutrons. Neutron interactions can make reactor materials radioactive. The choice of materials affects the waste and how long it remains radioactive. Fusion therefore does not automatically mean no radioactive waste or waste that is always short-lived.
The previous topic's distinction applies to both processes. Shielding reduces exposure to radiation. Containment limits radioactive material escaping and spreading contamination. A useful comparison names the fuel, products and reactor materials rather than assigning the same waste properties to every design.
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