A large amount of radioactive substances accumulate in the fuel of
a nuclear reactor during its operation. These substances must be
efficiently kept isolated from the environment.
Various isolation barriers are used for this purpose. The first barrier is the ceramic fuel, which efficiently absorbs the fission products in the fuel. The second barrier is the fuel cladding. The next is the reactor pressure vessel in which the reactor core is housed. The fourth barrier is the gas-tight reactor containment that encircles the reactor system. The fifth barrier is the reactor building.
The radiation energy in the radioactive substances that accumulate in the fuel will still generate heat even after the reactor has been shut down. This “residual heat” is initially a few percent of the thermal output of an active reactor, but it will gradually decrease.
Starting point: management of a chain reaction and the residual heat
The fundamental issue in reactor safety is that the chain reaction of the uranium fission and the power it generates must always be kept under control and that the dissipation of the residual heat caused by the fuel’s radioactivity must be handled under all circumstances.
A nuclear power plant is equipped with multiple safety systems that are used to detect disruptions and bring them quickly under control. If necessary, an automatic reactor trip will stop the reactor by driving the control rods into the reactor core in a matter of seconds.
Removal of the residual heat begins in the emergency injection system that feeds water into the reactor at high pressure. As the pressure in the reactor decreases, the emergency core cooling will be transferred to the core spray system. 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 fails.