Fission and Fusion » Nuclear Fission

Nuclear reactions as a source of energy

Burning coal is a chemical reaction. The energy comes from atoms rearranging themselves. Nuclear reactions are different. They happen inside the nucleus of an atom, and they can release far more energy from the same amount of material.

In Nuclear Decay you saw that an unstable nucleus can emit radiation. That is one kind of nuclear reaction. There are three you need to know about:

• Radioactive decay: an unstable nucleus emits radiation and releases energy.
• Fission: a large nucleus splits into smaller nuclei (this lesson).
• Fusion: small nuclei join together (see Nuclear Fusion).

All three can be a source of energy. This lesson is about fission, because it is the reaction used in nuclear power stations.

Key terms:

• Nuclear reaction: a change that happens in the nucleus of an atom and can release energy.
• Fission: the splitting of a large nucleus into two smaller nuclei.
• Fission products: the particles left after a nucleus has split.
• Daughter nuclei: the two smaller nuclei made when a nucleus splits.

How a uranium-235 nucleus is split

The fuel is uranium-235 (U-235). In Atomic Structure and Isotopes you met it as an isotope of uranium: 92 protons and 143 neutrons, so a mass number of 235. Its nucleus is large, and a neutron hitting it can split it in two.

Fission happens in these steps:

1. A neutron travels towards a U-235 nucleus.
2. The neutron collides with the nucleus and is absorbed by it.
3. The nucleus is now very unstable, so it splits into two smaller nuclei.
4. Two or three neutrons are released at the same time.
5. Energy is released.

Imagine a wobbly water drop. Add a little more water and it breaks into two smaller drops, flicking off some spray. The neutron is the extra water, the two drops are the daughter nuclei and the spray is the spare neutrons. It is only a picture, not what really happens, but it helps you remember the order.

Where does the energy go?

The two daughter nuclei fly apart at very high speed. So the energy released ends up as the kinetic energy of the fission products. This is what the spec asks you to know.

The fast-moving particles bump into the atoms around them and make those atoms vibrate more. In other words, the surroundings get hotter. This is why fission is useful as a source of energy for heating.

What fission produces

Fission of U-235 always produces:

• two daughter nuclei, which are smaller nuclei and are radioactive;
• a small number of neutrons, usually two or three.

The daughter nuclei are not a fixed pair. U-235 can split in several different ways. The daughter nuclei are radioactive, which means they are unstable and will go on to decay by emitting radiation, as you saw in Alpha, Beta and Gamma Radiation. That is one reason the waste from nuclear power is a problem (see Dangers of Ionising Radiation).

An example equation

One way that U-235 can split is shown in this equation. The small numbers are the mass number (top) and the atomic number (bottom), as in Balancing Nuclear Equations:

¹₀n + ²³⁵₉₂U → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n

Here the daughter nuclei are barium-141 and krypton-92, and three neutrons are released. You do not have to learn this pair of nuclei. Check that it balances:

	Left-hand side	Right-hand side
Mass numbers	1 + 235 = 236	141 + 92 + (3 × 1) = 236
Atomic numbers	0 + 92 = 92	56 + 36 + (3 × 0) = 92

Both totals match, so the equation is balanced.

Exam-style question

(a) State what must hit a nucleus of uranium-235 to start fission. [1 mark]

(b) Describe what happens to the nucleus during fission. [3 marks]

(c) State what form the energy released takes. [1 mark]

(d) A U-235 nucleus absorbs a neutron and splits into krypton-89 (atomic number 36), barium (atomic number 56) and three neutrons. Show that the barium nucleus has a mass number of 144. [2 marks]