Fission and Fusion: Two Ways to Release Nuclear Energy

0:00How can the tiny nucleus at the centre of an atom release energy?

0:05A nucleus is made of protons and neutrons, packed tightly together.

0:10There are two ways: fission splits a large nucleus, and fusion joins small nuclei.

0:17Fission uses a large nucleus such as uranium-235, which has 235 protons and neutrons.

0:27A neutron hits the nucleus and is absorbed.

0:30The nucleus splits into two smaller nuclei, two or three neutrons, and energy.

0:36These neutrons can split more uranium nuclei. Here, one split causes three, then nine.

0:44This is called a chain reaction, and it can grow very fast.

0:49In a nuclear reactor, control rods absorb some of the neutrons.

0:54Then each split causes just one more, on average, so the reaction stays steady.

1:00In fusion, small nuclei join to make a larger one.

1:05Here, hydrogen-2 and hydrogen-3 join to make helium-4, giving out a neutron and energy.

1:13Fusion powers the sun, whose core is about 15 million degrees Celsius.

1:19Kilogram for kilogram, fusion releases even more energy than fission.

1:25But fusion is not yet a practical power source on Earth.

1:29In both, what comes out has slightly less mass than what went in.

1:35That missing mass has become a lot of energy.

1:38Fission splits a large nucleus, fusion joins small nuclei, and both turn a little mass into a lot of energy.