Nuclear power – How a nuclear reactor produces electricity

News reports last week suggested that members of the political party currently in government in Australia – the Australian Labor Party – are recommending a debate about the future of nuclear power generation and a rethink to their opposition to nuclear energy. While Australia does not have any nuclear power stations, there are around 450 operating stations worldwide. France, for example, generates around 75% of its power from nuclear stations. If Australia is going to start a debate about nuclear power, it makes it a fitting time to look at just how a nuclear power reactor works, what nuclear waste really is, and whether they as carbon friendly or unfriendly as people think.

 

How a nuclear power station works

The key factor for a nuclear reaction in a power station is Uranium, a naturally occurring chemical. Uranium is found in many forms, the most common of which is called Uranium-238, which makes up around 99% of total Uranium. The type used for generating nuclear power however, Uranium-235, only makes up around 0.7% of total Uranium.

 

In a nuclear power station, Uranium-235 is bombarded with extremely small particles called neutrons. When Uranium-235 is struck by a neutron, it absorbs it and becomes extremely unstable, causing it to split apart. This splitting of Uranium-235 releases a large amount of energy as, as well as three more neutrons and a large amount of radioactivity. The actual splitting is referred to as fission.

A neutron (blue particle at the top) strikes a Uranium-235 atom, causing instability and splitting into two smaller atoms. This splitting releases energy and three neutrons.

 

This reaction occurs underwater in a nuclear power station. This is for two reasons: firstly to control the speed of the neutrons, and secondly to keep the temperature of the Uranium under control. The energy released from the Uranium-235 splitting heats the water surrounding it – this heated water is then used as a heat source to boil another separate vessel of water and produce steam. This steam is then used to turn electricity generators, producing the electricity. In essence, a nuclear power station is just an extremely large super-powered kettle, boiling water to produce steam.

 

Schematic of a nuclear reactor. The uranium-235 fuel rods (6) heat water which is then piped (7) to a separate vessel (8). In this vessel, cold water (13) is heated and the resulting steam (12) is used to turn power turbines (not pictured). The reactor water is returned to the reaction vessel (10).

 

The neutrons released from the reaction continue to travel onwards until they strike more Uranium-235, causing the same reaction over and over – a chain reaction. In an ideal situation, one of those three neutrons will strike another Uranium-235 atom, and one of the three neutrons released from that fission reaction will strike another. This situation, where only one out of three neutrons causes another fission reaction is very stable, for every neutron causing a reaction, there is only one resulting neutron causing another reaction, meaning the number of reactions happening does not increase or decrease.

 

The major factor controlling how many resultant neutrons strike another Uranium-235 atom is the amount of Uranium-235 available to act as a target. If there is less Uranium-235 there are less targets for the neutrons to strike and it is called “subcritical”, the number of reactions will reduce and the energy production will diminish. If there is too much Uranium-235 there are many targets for the neutrons and it is called “supercritical”, more than one neutron may strike another Uranium-235, increasing the number of reactions occurring and increasing the amount of energy being released. If this is not controlled, the amount of energy can continue to increase until it causes an explosion. When only one neutron strikes another Uranium-235 it is called “critical mass”, with no increases or decreases of the energy produced.

 

In practice, a nuclear power station will actually be supercritical, it will have more Uranium-235 than required to keep the one-to-one ratio. The levels of the reactions are controlled using control rods made from a mixture of metals. These control rods are pushed into the water and literally soak up the neutrons being released by the splitting of Uranium. By adding more control rods the number of follow-up reactions is reduced by removing more neutrons. Removing control rods has the opposite effect, with more neutrons being allowed to create reactions. Using these control rods the energy production can be closely controlled, keeping the nuclear reactions at a stable level, and also controlling the amount of steam produced and electricity output from the station. This allows the station to increase or decrease electricity production in response to demand.

 

Control rods controlling the nuclear reaction. On the left, control rods are used to reduce the nuclear reaction by preventing neutrons from causing chain reactions. On the right, the control rods have been withdrawn, allowing more neutrons to cause fission and increased energy production. These control rods are capable of completely stopping the nuclear reaction, and can be used to shut-down the nuclear station almost instantly if the chain reactions increase out of control.

 

As mentioned earlier, Uranium-235 only makes up around 0.7% of total Uranium. However, for use in a power station the amount of Uranium-235 needs to be around 2-3%. To increase the levels of Uranium-235, before being sent to the power station Uranium is enriched to increase the proportion of Uranium-235. After processing the Uranium is formed into pellets approximately 2.5cm long, with the normal lifespan of these pellets in a nuclear power station around 3-5 years, after which the amount of Uranium-235 remaining in the pellet has reduced to a level which is no longer a critical mass. Following this time, the rods are removed from the power generator and become waste.

 

As a point of comparison – nuclear weapons require at least 90% Uranium-235 to be effective. This means that Uranium power station fuel is woefully insufficient to be used as a weapon without extensive processing.

 

Nuclear power stations are extremely safe when designed and operated correctly. However, they do produce toxic waste, and the next article will look at this waste and ask the question whether nuclear power is as carbon friendly as its supporters believe.

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