How does a thermal nuclear reactor work?

The thermal nuclear reactor is one of many systems used to produce energy (electricity). Like many systems (furnaces for example) it does this by heating water to produce steam. This steam drives turbines which in turn drive generators. These produce the electricity.

A nuclear reactor consists of three main parts: a reactor core, a pump and a heat exchanger.


1) The Reactor Core

The reactor core is the main unit in a nuclear reactor and it is where all the heat energy is produced. This is achieved by a process called nuclear fission. The fissile material (i.e the fuel for the process) is in this case Uranium-235.

The reactor core constitutes of 3 parts:

           Fuel rods – where the fissile material is held. As mentioned, the fuel in this case is Uranium-235, however, an isotope of Uranium (i.e the same number of protons but a different number of neutrons) is also present in the fuel rods, Uranium-238.

           Control rods – made of boron, these control the rate of reaction.

           Moderator – This slows down neutrons produced in the reaction. The importance of this is explained later on.

What happens in the reactor core?

         The Fission Reaction – Slow moving neutrons are collided with the U-235 contained in the fuel rods. The uranium absorbs the neutron but quickly becomes unstable, the nucleus is too large to hold itself together. The nucleus therefore immediately decays, producing on average 3 fast moving neutrons and two smaller, more stable nuclei, known as daughter nuclei. The two daughter nuclei have less binding energy per nucleon (look this up if you do not know what this means) and so energy is released. This energy is carried away by the neutrons as kinetic energy and so explains why they are fast moving.

          The Moderator – In order to produce a chain reaction, so that no more energy is needed to cause further fission, these neutrons must go on to produce further fission themselves. However, they are moving far too quickly to be accepted by a Uranium-235 nucleus. This is where the moderator comes in. The moderator is usually water or heavy water (water with the hydrogen isotope deuterium replacing the hydrogen molecules). Collisions of the fast moving neutrons with this moderator cause them to lose kinetic energy. Around 50 collisions are required to reduce the neutrons to thermal speeds at which they are able to produce further fission.

In the collisions, the kinetic energy of the neutrons does not simply disappear, as we know energy cannot be destroyed. Instead, the energy is transferred to the kinetic energy of the moderator particles, causing them to speed up. Increasing kinetic energy of molecules is another way of saying the temperature increases. The slowing down of the fission neutrons therefore causes the moderator to heat up, which is important in producing the electricity as we will see later.

          The Control Rods – the boron control rods are a VITAL part of the reactor and are what distinguish the process from that of a nuclear bomb. See, if all the neutrons were allowed to go on to produce further fission, the process would develop as follows:

- 1 neutron would cause 1 reaction, thus producing 3 more neutrons (on average)

- These 3 neutrons would each cause another reaction, producing 9 more neutrons in total.

- These 9 neutrons would each cause further fission, producing 27 more neutrons in total.

- By the 10th generation, the reactions will produce nearly 60,000 more neutrons.

All this occurs in a split second causing rapid expansion, indistinguishable from an explosion. As mentioned, this is what happens in a nuclear bomb.

The job of the control rods is simply to ensure that only one neutron on average from each reaction goes on to produce further fission, this is the critical condition. They do this by absorbing the extra neutrons. The depth to which the control rods are inserted is controlled automatically to absorb more or less neutrons if necessary.


2) The Pump

The role of the pump is very simple, it pumps the moderator from the reactor core to the heat exchanger and back again. 


3) The Heat Exchanger

At the heat exchanger, the moderator flows through a set of convoluted tubes, surrounded by water, turning the surrounding water to steam. The convolution maximises the time the moderator spends in contact with the water and ensures as much heat energy is absorbed from the moderator as possible, before it is pumped back to the reactor core to be heated up again.

The steam produced is used to drive the turbines, which turn the generators and thus electricity is produced.

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