The active zone of the reactors is usually surrounded by a mantle consisting of the moderator (water, graphite, or another substance as previously mentioned). Thus, the number of the escaping neutrons can decrease because this mantle reflects a part of the escaping neutrons. The escaping neutrons influence the effective neutron multiplication factor. The application of the reflector can decrease the size of the reactor, and the fuel can be utilized more economically.

7.1.1.2 Coolants

The greatest part of the energy released in the fission is the kinetic energy of the fission products (Table 7.2). While the fission products are slowing down, the fuel

rods are heated. In operation, the temperature of the fuel rods may rise above 1000° C, and then they have to be cooled permanently. Therefore, the active zone always contains coolant.

Because of the strong neutron radiation in the active zone, the coolant becomes radioactive. For this reason, the coolant must be circulated in a closed system, which is called a “primary circuit.” The primary coolant is pumped into a heat exchanger full of tubes. Heat is transferred through the walls of these tubes to the lower-pressure secondary coolant located on the sheet side of the heat exchanger, where it evaporates to pressurized steam (a steam generator). In this way, the cool­ants in the primary and secondary circuits do not touch each other directly, so the secondary coolant remains inactive.

The pressurized steam formed in the steam generator is fed through a steam tur­bine, which drives the electric generator. In the meantime, the secondary coolant (a mixture of water and steam) is cooled down and condensed in a condenser. Then the condensed steam is pumped back into the steam generator (Figure 7.2).

In reactors moderated by light or heavy water, the moderator acts as a coolant too. In graphite-moderated reactors, the coolant is a gas (CO2 or He) or water. In some reactors, molten metals (e. g., sodium), salts, organic solvent, He, or steam is applied as the coolant.

In the water reactors, the coolant is continuously purified by ion exchangers to remove the dissolved radioactive ions (e. g., cesium and iodide ions). The colloidal or greater solid particles are filtered.