How a chain reaction builds

1. Each fission of a uranium-235 nucleus throws out two or more neutrons, and those neutrons are what makes a chain reaction possible.
2. If one of them is absorbed by another uranium-235 nucleus, that nucleus becomes unstable and splits in turn, releasing more neutrons and more energy.
3. The reaction is therefore self-sustaining: once it has started, it supplies its own trigger and needs nothing from outside.
4. Not every neutron goes on to cause a fission, because some escape from the edge of the fuel and some are absorbed by nuclei that do not split.
5. What matters is therefore the average number of neutrons from each fission that go on to cause another fission, and everything about running a reactor comes down to holding that number where it is wanted.

Keeping the reaction steady

1. If more than one neutron per fission causes a further fission, the number of fissions grows with every generation, so the rate of energy release rises faster and faster and the reaction runs away.
2. If fewer than one does so, each generation is smaller than the one before, so the reaction dies out and the reactor shuts itself down.
3. If exactly one does so, each generation is the same size as the last, so the number of fissions per second stays constant and energy is released at a steady rate.
4. That last case is a controlled chain reaction, and it is the condition a power station holds its reactor in for months at a time.
5. A steady rate of fission gives a steady rate of energy transfer, which is what allows a turbine and generator to run at a constant output.

The moderator

1. The neutrons that come out of a fission are moving extremely fast, and a fast neutron is very likely to shoot past a uranium-235 nucleus without being captured.
2. Left as they are, too few of them would be absorbed, so the chain reaction would not keep going.
3. The fuel rods are therefore surrounded by a moderator, usually blocks of graphite or a tank of water, filling the space between them.
4. A neutron leaving a fuel rod collides many times with the nuclei of the moderator, and it transfers some of its kinetic energy at each collision.
5. After enough collisions the neutron is travelling slowly, and a slow neutron spends far longer passing a nucleus, so it is much more likely to be absorbed.
6. Slowed neutrons that wander back into a fuel rod are therefore absorbed by uranium-235 nuclei, and the chain reaction is sustained.
7. The moderator slows neutrons down and does not absorb them, which is the single most important thing to keep straight in this topic.

The control rods

1. Control rods are made from a material that absorbs neutrons strongly without splitting, such as boron or cadmium.
2. They hang between the fuel rods and can be raised and lowered by motors while the reactor is running.
3. Every neutron a rod absorbs is one that can no longer reach a uranium-235 nucleus, so the rods set how many neutrons are left to keep the reaction going.
4. Lowering the rods further in absorbs more neutrons, so fewer fissions happen each second and the rate of energy release falls.
5. Raising the rods further out absorbs fewer neutrons, so more fissions happen each second and the rate of energy release rises.
6. Detectors in the core measure the neutron level continuously and the rod position is adjusted automatically, which holds the average at one neutron per fission causing the next.
7. In an emergency the rods are dropped fully into the core, absorbing so many neutrons that almost none reach a fuel nucleus and the chain reaction stops.

From fission to electricity

1. The energy released by the controlled chain reaction raises the thermal energy of the fuel rods, so the core becomes extremely hot.
2. A coolant, usually water under high pressure or carbon dioxide gas, is pumped through the core and heated as it passes the fuel rods.
3. The hot coolant flows to a heat exchanger, where it transfers thermal energy to water in a completely separate circuit before returning to the core.
4. Keeping the two circuits separate matters, because the coolant has been through the reactor and must not be allowed to carry radioactive material out to the rest of the station.
5. The water in the second circuit boils, producing high-pressure steam.
6. The steam is directed onto the blades of a turbine and makes it spin.
7. The turbine shaft turns a generator, which transfers energy electrically to the National Grid through transformers.
8. The used steam is condensed back to water in a condenser cooled by river or sea water, and pumped round to be boiled again.
9. The reactor is only the source of thermal energy, so from the heat exchanger onwards a nuclear station works in exactly the same way as a coal or gas station.

The products are radioactive

1. The daughter nuclei left after a fission have too many neutrons for nuclei of their size, because they inherited a neutron-rich mixture from the much larger uranium nucleus.
2. Those nuclei are unstable, so they undergo radioactive decay and emit ionising radiation, which is why the products of nuclear fission are radioactive.
3. Used fuel taken out of the core is therefore intensely radioactive, and some of the isotopes in it stay active for a very long time.
4. Parts of the reactor structure also become radioactive over the years, because their nuclei absorb neutrons and become unstable.
5. Used fuel is therefore moved by remote handling behind thick shielding, and it is this radioactivity that creates the waste and decommissioning problems a nuclear station has to plan for.