Reactor safety

A large amount of radioactive substances accumulate in the fuel of a nuclear reactor during operation. These substances must be kept apart efficiently from the environment. The first barrier is the ceramic fuel, which absorbs the fission products in the fuel efficiently. The second barrier is the fuel cladding. The next is the pressure vessel of the reactor, where the reactor core is housed. The fourth barrier is the gastight containment building that encircles the reactor system, and the fifth barrier is the reactor building, where the containment building is situated.

The radiation energy in the radioactive substances that accumulate in the fuel still generates heat even after shutting down the reactor. This decay heat power is initially a few per cent of the thermal output of the reactor when active, but it gradually falls.

The fundamental point for reactor safety is that the chain reaction of the uranium fission and the power it generates are always under control and that the dissipation of the decay heat caused by the fuel´s radioactivity can be handled in all situations.

A nuclear power plant is equipped with multiple safety systems that are used to detect disruptions and bring them under control quickly. An automatic scram stops the reactor operating, if necessary, by driving the control rods into the reactor core in a matter of seconds. The decay heat power is eliminated by means of a high-pressure emergency core-cooling system that feeds water into the reactor at high pressure. As the pressure in the reactor decreases, the emergency core cooling transfers to an injection system in the core. Both systems and their auxiliary systems are divided into sub-systems independent of each other, so that safety is assured even if one of them develops a fault.