Though this discussion has so far focused on the neutrons that are produced by nuclear fission, the fact that large quantities of energy are released must not be overlooked. This is reflected in the large kinetic energy that is imparted to the fast-moving neutrons and fission products as they dispersed from the fissioning atoms; energy is also released in the form of gamma radiation or gamma rays. As the fission products and fast neutrons travel through the neutron moderator and slow down, much of their kinetic energy is converted into thermal energy, or heat. Gamma rays are also attenuated by the moderator, resulting in the production of heat. In consequence, the core of a nuclear reactor needs to be cooled continuously in order to keep the structural elements of the core and the fuel assemblies from overheating or even melting.
Schematic representation of the primary (light blue) and secondary (green) cooling systems at MNR.
In many cases, the reactor’s neutron moderator also acts as a coolant. For example, at MNR, the reactor pool water (the moderator) absorbs the heat energy that is given off in the core. In order to prevent it from over-heating and boiling, the pool water is not stagnant: it circulates continuously downwards through the reactor core and out via a special aperture at the bottom of the pool, from where it is directed by the primary pump to a heat exchanger where its heat energy is transferred to the secondary cooling system before it is routed back into the reactor pool (see diagram above). The cooling towers (see images below) are designed to act as heat exchangers, removing heat (thermal energy) from the secondary cooling system (green) and transferring it to the atmosphere – the final destination for the energy created in the MNR core. Because the pool water (primary cooling system, light blue) does not come in direct contact with the cooling towers, any contaminants in the pool water cannot escape to the atmosphere. The cooling towers are located adjacent to the reactor containment building, wedged in between the Arthur Bournes Building and the Nuclear Research Building.
The south end of the Nuclear Research Building with the reactor containment building and the cooling towers visible.