Nuclear reactors

18 March 2011

The RSC Radiochemistry Group has a wealth of accessible information on all aspects of nuclear power. A link to those resources can be found at the end of this document, but a short briefing on the more chemical aspects of nuclear power can be found below, adapted from the Radiochemistry Group's material.

How does a nuclear power station work?

Nuclear power stations are large steam engines. A heat source boils water, and the steam drives a turbine to produce energy. In a nuclear power station the heat source is a controlled nuclear chain reaction.

The reactors at Fukushima are Boiling Water Reactor (BWRs), one of several kinds of Light Water Reactors (LWRs).

In these reactors normal water is used as the coolant in the core, and the resulting steam drives the turbine.

What is a chain reaction?

A chain reaction is where the products of one reaction start another. In nuclear power reactors, this involves large atoms such as 235U that are known as 'fissile'. This means when they fission, or break apart, they emit neutrons which can make another atom fission, sustaining the chain reaction.

When the reaction is self-sustaining it is said to be critical. There is a certain amount of fuel needed for a reaction to become self-sustaining, and this is known as the critical mass.

What is nuclear fuel?

The composition of fuel rods in a nuclear power plant varies; at Fukushima, five of the six reactors use uranium oxide or UOX. The oxide is used because it has a much higher melting point than the metal itself.

The major sources of uranium are found in Africa, Australia, North America and
Russia. Only a few of the uranium ores known contain sufficient uranium (greater than 0.1%) to extract commercially. The most important are Pitchblende and Coffinite (silicate).

After arrival at the fuel manufacturing plant, the concentrated uranium ore needs further purification to reach nuclear grade. Purification removes neutron poisons, such as boron and cadmium, which would reduce the performance of the fuel in a nuclear reactor, and other impurities which impair the chemical and physical processes needed to produce finished fuel.

Naturally-occurring uranium ore contains mostly 238U. This is not fissile, so for nuclear fuel the ore must be enriched with the fissile 235U. The uranium fuel used in light water reactors is 'low-enriched uranium'.

What do the control rods do?

The control rods absorb neutrons. By raising or lowering the control rods, the speed of reaction can be altered, so a critical reaction can be maintained. To stop a reaction, the control rods are extended fully between the fuel rods and make it difficult for the chain reaction to continue. Control rods are made of boron, cadmium or hafnium, because these metals can absorb many neutrons.

What is a 'meltdown'?

A meltdown happens when the nuclear reaction goes supercritical, and gets so hot that the fuel melts. This presents a risk that the hot fuel will melt through the containment chamber, exposing the core to the outside world.

One concern with a meltdown is 'China Syndrome', where the fuel burns melts and continues a chain reaction once outside the reactor. It could become hotter and hotter and burn through the floor and into the Earth - but the Fukushima reactors are built to resist this, and it should not happen even with a serious meltdown.

A meltdown is not an explosion. The explosions seen so far at Fukushima have been hydrogen explosions. The hydrogen is made when steam reacts with the metal fuel rod casing - this is not a nuclear explosion.

Related Links

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Produced by the RSC's Radiochemistry Group

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