Fission Reactors and Fusion in the Laboratory
A chain reaction is controlled by moderating neutrons and absorbing the surplus, while fusion needs temperatures high enough to beat the Coulomb barrier and confinement long enough to pay for itself. The physics of each explains the engineering.
What a learner can do afterwards
- Explains the role of moderator, control rods and coolant in a reactor
- States the conditions fusion needs and why confinement time enters
- Compares the energy released per kilogram and the waste produced by each route
1 · Read
Fire a neutron into uranium-235 and the nucleus wobbles and splits into two lighter fragments plus two or three neutrons. The menu varies: barium plus krypton one time, xenon plus strontium the next. Mass numbers balance, the 236 nucleons going in equal the fragments plus the neutrons, and the fragments weigh less than the starting nucleus, so the missing mass becomes energy.
Those fresh neutrons can split further nuclei, a chain reaction. Exactly one triggered fission per fission on average holds it steady; fewer dies out and more runs away. Water slows fast neutrons to keep splits likely, cadmium control rods swallow the surplus, and coolant ferries heat to power. Fermi's Chicago pile proved it in 1942.
Fusion builds up instead: light nuclei join into heavier ones, releasing energy. But nuclei repel electrically, so only brutal heat gives them the speed to touch. Heat alone is not enough: dense fuel must stay confined long enough to pay back, so temperature, density, and time enter together.
Per kilogram fusion releases far more and leaves no long-lived actinide waste, yet it is harder to sustain. The Sun shows the scale: its proton-proton chain turns four protons into helium plus about 26 MeV, and even so the Sun loses only a tiny fraction of its mass over billions of years.
Fission splits heavy nuclei in a controlled chain, while fusion joins light ones that must stay hot, dense, and confined.
More to read
10.5 Fission
A fission reactor is a controlled chain reaction. Each uranium split releases neutrons, and on average exactly one of them must trigger the next split to hold steady. The moderator slows neutrons so they split nuclei more readily, while control rods swallow extras to trim the rate. Coolant hauls the heat to turbines that make electricity.
10.6 Nuclear Fusion
Fusion forces light nuclei together until the strong force grabs them, paying out the binding difference. The catch is electric repulsion: only million degree speeds let nuclei touch. Even then they must stay packed long enough to collide, which is why confinement time joins temperature and density as a requirement. Magnetic bottles and laser crush both chase that triple goal.
32.6 Fission
Fission of one uranium nucleus frees roughly two hundred MeV, millions of times a chemical burn per atom. Reactor design is mostly neutron accounting: geometry, enrichment, and poisons decide where each neutron goes. Subcritical assemblies fade, supercritical ones surge, and operators live in the narrow middle. Safety systems are built to push the balance toward shutdown.
22.4 Nuclear Fission and Fusion
Per kilogram, fusion fuel outyields fission fuel several fold, and its ash is helium rather than long lived waste. Fission waste stays hazardous for millennia and needs guarded storage. Fusion dodges that burden but demands extreme machines that have not yet broken even at scale.
Explore more
2 · Watch
به انگلیسیTake it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.