Nuclear fission and fusion

Nuclear fission is the splitting of a large, unstable nucleus, such as uranium-235 or plutonium-239.

1. The nucleus absorbs a neutron. (Fission without this is rare.)
2. It splits into two smaller nuclei, of roughly equal size.
3. It gives out two or three neutrons and some gamma rays.
4. Energy is released: the new nuclei and neutrons move very fast.

The released neutrons can be absorbed by more uranium nuclei, making them split too. This is a chain reaction.

In a nuclear power station, the chain reaction is controlled so energy is released steadily. In a nuclear weapon, it is uncontrolled and causes an explosion.

Inside a reactor:

• Control rods absorb neutrons. Lowering them further into the reactor slows the chain reaction down; raising them speeds it up.
• A moderator, such as graphite or water, slows the neutrons down, so they are more likely to be absorbed by uranium-235 nuclei and cause fission.

The energy heats water to make steam, which turns turbines to generate electricity.

Nuclear fusion is the joining of two light nuclei, such as hydrogen, to form a heavier nucleus. Some of the mass is converted into energy, which is released as radiation.

Fusion powers the Sun and other stars. It is hard to do on Earth, because it needs extremely high temperatures and pressures to push the positive nuclei close enough to join.