How does a nuclear reactor work?

Fission

When a slow neutron collides with an atomic nucleus of uranium the neutron is absorbed and the nucleus disintegrates mostly into two parts. These fission products fly apart at great speed. When they are braked heat is generated. This heat is utilised in nuclear power stations for generating electricity.

Fission of uranium 235


Moderator

In addition every nuclear fission gives rise to two or three neutrons that travel at high speed. If these are to fission other atomic nuclei of uranium they must be braked. The function of this “brake” is undertaken in nuclear reactors by a so called moderator. This can be for example normal water or even carbon. When fast neutrons collide with the moderator atoms the neutrons slow down. If a chain reaction is to be maintained every fission must give rise to a neutron that in turn fissions an atomic nucleus. However fission gives rise to two or three neutrons. In other words the superfluous neutrons must be captured. In nuclear reactors this is the job of neutron absorbers, the so called control rods.

Controlling the chain reaction

The control rods are used to regulate the chain reaction. Depending on how far they are withdrawn from the reactor core where the chain reaction is taking place nuclear fissions are limited to a particular rate. When this rate is constant the reactor is “critical”: the consumption and generation of neutrons are in balance. The control rods can also be used to stop the chain reaction completely, i.e. shut down the reactor. The control rods need be advanced only a little way into the reactor core, and the chain reaction is suspended. Owing to the fundamental physical conditions there can be no explosive chain reaction in a reactor like the one at the HZB.

A research reactor is designed so that there are as many neutrons as possible available for physical and chemical analyses in addition to those needed to maintain the chain reaction. In this case part of the neutrons pass through beam holes to the various experimental setups. The heat generated by the chain reaction is a desired product in a nuclear power station. This is different for a research reactor: here only the neutrons are interesting. The heat is a waste product. The research reactor at the HZB is cooled with water.
In a nutshell we can say that every nuclear reactor has five essential components:

  • the fuel, uranium, that is sealed in rod- or plate-type fuel elements,
  • the moderator that envelops the fuel elements,
  • the control rods that depending on their positions capture a certain number of free neutrons and so regulate the intensity of the chain reaction,
  • the cooling system that draws away the heat generated by the nuclear fission,
  • the outer radiation shielding that absorbs radiation leaks.

How is the HZB research reactor structured?