Fission and Fusion » Chain Reactions and Nuclear Reactors

From one fission to a chain reaction

In Nuclear Fission you saw that a neutron hits a uranium-235 nucleus, which splits and releases two or three more neutrons. Those spare neutrons are the key to this lesson.

Each new neutron can strike another U-235 nucleus. That nucleus splits and releases more neutrons, which can strike more nuclei, and so on. This self-sustaining series of fissions is a chain reaction.

Think of a row of dominoes. One falls and knocks over the next, which knocks over the next. You only had to start it once. In the same way, one neutron starts the process and the neutrons from each fission keep it going.

Key terms:

• Chain reaction: a series of fissions in which the neutrons released by one fission strike other U-235 nuclei and cause them to split.
• Controlled chain reaction: a chain reaction kept at a steady rate.
• Uncontrolled chain reaction: a chain reaction in which the number of fissions grows very quickly.
• Control rods: rods that absorb neutrons to slow the reaction down.
• Moderator: a material that slows down fast neutrons so they are more likely to cause fission.
• Shielding: thick material around a reactor that absorbs neutrons and gamma radiation.

Controlled and uncontrolled

What matters is how many of the neutrons from each fission go on to cause another fission. Some neutrons escape from the fuel and some are absorbed by other materials, so not every neutron causes a new fission.

You can see why growth is so fast with a simple count. Suppose each fission causes two more. One fission becomes 2, then 4, then 8, then 16. After only ten steps there are over a thousand fissions, and the number keeps doubling. That is an uncontrolled chain reaction.

In a power station, we do not want a sudden burst of energy. We want a steady supply. So the reactor must keep the average at one. Two things in the reactor do this job: the control rods and the moderator.

Control rods

The control rods are made of a material that absorbs neutrons (boron is a common one). A neutron that is absorbed by a control rod cannot cause fission.

• Lower the rods further into the reactor: more neutrons are absorbed, fewer fissions happen, and the reaction slows down.
• Raise the rods out of the reactor: fewer neutrons are absorbed, more fissions happen, and the reaction speeds up.

The operators move the rods up and down to keep the chain reaction steady. If they push the rods all the way in, so many neutrons are absorbed that the reaction stops. This is how a reactor is shut down.

The moderator

The neutrons released by fission are travelling very fast. A fast neutron is less likely to be absorbed by a U-235 nucleus, so it is less likely to cause fission. A slower neutron is much more likely to cause fission.

The moderator solves this. It surrounds the fuel. The fast neutrons collide with the atoms of the moderator and lose speed. Then they are much more likely to cause fission when they meet another U-235 nucleus. The moderator can be graphite (a form of carbon) or water.

Shielding

A reactor core gives out neutrons and gamma radiation. Both are ionising radiation, and you saw in Dangers of Ionising Radiation that this can damage cells. So the reactor is surrounded by shielding: thick concrete and steel. The shielding absorbs the neutrons and gamma radiation, which protects the workers in the power station and the public outside.

A reactor, part by part

You may be asked to describe a reactor in words. Build the picture from the middle outwards:

Part	What it is	What it does
Fuel	Rods or pellets containing uranium-235 in the middle of the reactor	Its nuclei split, which releases energy
Moderator	Graphite or water surrounding the fuel	Slows fast neutrons so they are more likely to cause fission
Control rods	Neutron-absorbing rods that move in and out between the fuel	Absorb neutrons to speed up or slow down the reaction
Shielding	Thick concrete and steel around the whole reactor	Absorbs neutrons and gamma radiation

The energy released by the fission heats the reactor, and this heat is used to boil water to make steam in the power station.

Exam-style question

(a) Explain what is meant by a chain reaction in a nuclear reactor. [2 marks]

(b) State the difference between a controlled chain reaction and an uncontrolled chain reaction. [2 marks]

(c) Describe the job of the control rods and of the moderator. [4 marks]

(d) State what the reactor shielding is made of and why it is needed. [2 marks]