The safety barriers in nuclear power plants are supported by safety systems. Their main function is to shut down the reactor – stop the fission chain reaction, ensure the removal of residual heat from the reactor, ensure the integrity of the primary circuit and reduce pressure in the containment to protect safety barriers – in every operational mode, even the most severe emergency envisaged in the power plant design, without any negative impact on the power plant’s surroundings. These systems can be divided into active and passive safety systems.
These do not require any external energy source to function. They comprise hydro-accumulators and a vacuum bubble condenser system.
If the pressure of the coolant in the reactor falls below the pressure that keeps the boric acid solution in the hydro-accumulators, the solution will flow into the reactor, flooding the reactor core and ensuring the removal of heat from the core.
The bubble condenser system consists of twelve levels of covered trays filled with boric acid solution installed in the bubble condenser tower. The boric acid solution forms a water seal with a relatively large overall flow cross-section and low hydraulic resistance. The lower space of the water seal is connected to the steam generator box, the upper space of the water seal is connected through check valves to four gas traps. In the event of an accident involving the leakage of coolant from the primary circuit into the containment, the steam-gas mixture passes through the water seal, where the steam phase cools and condenses. Non-condensing air and radioactive gases pass through the check valves into the gas traps, where they remain localised. They can then be cleaned through the ventilation system.
All safety-critical devices are backed up three times (3 x 100%) and are ready for immediate activation in normal operation. They are independent of each other and separated from each other in space. The required safety function can be performed by any one of the three systems.
