1. Types of Radiation
Substances, whether naturally occurring or not, may consist of atoms that are unstable – that is to say that they may go spontaneous transformation into more stable product atoms by the release of energy. Such substances are said to be radioactive, and the transformation is known as radioactive decay.
In addition to naturally occurring unstable substances, other substances with stable atoms can be made radioactive by bombarding them with nuclear particles. The activation products produced in a nuclear reactor are an example of these. The fission products during reactor operation are also mostly radioactive.
Radioactive decay is accompanied by the release of energy – this can occur in the emission of charged particles and gamma-rays (γ). The three main types of radiation emitted are alpha-radiation (α), beta-radiation (β) and gamma-radiation (γ). Neutrons (n) are a forth type of particulate radiation. Each type of radiation has different distances it can travel in air, and different materials it can penetrate as illustrated in Error! Reference source not found..
Alpha-radiation consists of two neutrons and two protons i.e. helium nuclei. Beta-radiation comprises of high speed electrons that originate from the nucleus of an atom, and gamma-radiation is a form of electro-magnetic radiation: this form of radiation consists of packets of energy (photons) transmitted as waves.
2. Nuclear Fission
Nuclear fission is the process by which heat is produced in a nuclear reactor. In nuclear fission the nucleus of an atom is split into two parts (fission products) whereby energy in the form of heat is released. This can occur naturally where inherently unstable heavy nuclei undergo spontaneous fission, or fission can be generated by bombarding nuclei with neutrons. Whilst the former is possible it is relatively rare, and the latter is the method in which energy is obtained from the fission of Uranium in a nuclear reactor.
Naturally occurring Uranium consists of mainly two isotopes: U-235 (less than 1%) and U-238 (remaining 99%). It is the only the U-235 atoms that are capable of undergoing fission to any great extent. Enriched uranium is where the percentage of the U-235 atoms in the metal has been increased: for some nuclear power applications this percentage increase or enrichment is generally between 3 and 5 percent (Low Enriched Uranium or LEU).
When a U-235 atom captures a slow neutron it splits into two parts and two or more fast neutrons are released. These fast neutrons, as the name suggests, are travelling so fast that the probability of hitting another U-235 atom is very low. To increase the probability of a continued reaction they must be slowed down (i.e. slow neutron) to a speed at which the fission reaction can be maintained (i.e. a chain reaction). This is achieved by means of a moderator.
By this definition a moderator is a material capable of slowing down fast neutrons. Materials that are employed as moderators are light water (PWR reactors), graphite (AGR and Magnox reactors) and heavy water (CANDU reactors). The moderator, together with other components, forms the reactor core which is responsible for maintaining a controlled nuclear fission chain reaction to produce heat to generate the steam required to drive turbine generators.
