The objective of these safety barriers is to prevent any release of radioactivity into the environment and protection against the ionising radiation in all operating modes, so during failures too.
A ceramic form of nuclear fuel assures the first barrier - pellets, which are capable to bind the irradiation products. The second barrier is represented by a sealed metal tube creating an envelope for the nuclear fuel. The next third barrier is created by a closed primary cooling system including the reactor pressure vessel, preventing to the release of primary coolant containing radioactive products into the environment at all design temperatures and pressures. And the last, fourth barrier is created by a hermetic box - containment, which collects the radioactivity leaked out through eventual breaks in the first or second barrier
Hence, there is an additional steel-concrete envelope surrounding the reactor vessel to protect the personnel against direct irradiation.
The individual safety barriers at nuclear power plant are completed by an extensive safety system. The function of safety systems is to shut down the reactor. Next, to stop the chain reaction, assure the residual heat removal from the rector core, keep the integrity of primary circuit and reduce the pressure in the containment to assure the function of the individual safety barriers in all operating conditions, even at maximum designed accident, i.e. primary loop break. Finally, reactor power is controlled by reducing, respectively increasing number of thermal neutrons in the reactor core. In practice it is done by inserting, respectively withdrawing control rods into and from reactor core. Reactor trip means rapid interruption of fission reaction, what is the basic requirement of nuclear safety. Reactor is for this reason equipped with safety system, which comprises control rods, arranged into a few groups. Actuation of this system is realised by interrupting power supply for electric drives, which otherwise maintain control rod groups in their working position, and all control rods fall into reactor core, interrupting fission process within period of 10 to 12 seconds and thus shutting reactor down. That brings me to the end of my presentation. Thank you for your attention and if you have any questions, I`ll be glad to answer them. .