Safety barriers in a nuclear power plant are supplemented by a safety system.Their main task is to shut down the reactor – to stop the chain fission reaction, to ensure residual heat removal from the reactor core, to ensure integrity of the primary circuit, and to reduce pressure in the containment to protect safety barriers - in all operating conditions, therefore, as well as in case of accidents and emergencies considered in power plant’s design and without negative environmental impact. We recognize passive and active safety systems.
These systems need an energy source for their operation, e.g. electricity, compressed air or fluid. An example of active systems are the high and low pressure emergency core cooling system and a spray system which reduce pressure in the containment.
The high-pressure and the low-pressure emergency systems serve for maintaining coolant pressure in the primary circuit, and thus ensuring heat removal from the reactor core under extraordinary operating conditions.They consist of boron acid tanks and pumps ensuring make-up of solution/coolant to the primary circuit in case of a loss of reactor coolant.Increase of boron concentration in the primary coolant terminates the fission reaction because boron absorbs free neutrons evoking the fission.If high-pressure system pumps are not sufficient for making up water into the primary circuit, low-pressure pumps, which have a higher flow rate, would also start to continuously supply coolant to the primary circuit continuously would be started. After emptying the tanks, they pump water from floors of containment, where the water leaked through a leaking pipe of the primary circuit.
If pressure in the containment increases, for example when the primary circuit is ruptured, the spray system is activated, which reduces the pressure in these compartments by spraying the areas – cooling them down to condensate the steam.
These systems do not need any power source for their operation.They consist of hydro-accumulators and a vacuum-barbotage system.
If coolant pressure in the reactor drops below the value at which boron acid solution in hydro-accumulators is maintained, solution from the hydro-accumulators is discharged to the reactor, it will flood the reactor core, and thus ensure heat removal from the core.
The bubbler condenser system is a set of twelve floors of covered trays filled with boron acid solution situated in a bubbler condenser tower.Boron acid solution forms a water closure with relatively high total flow rate section and small hydraulic resistance. Lower area of the water closure is connected with the steam generator compartment; the upper area of the water closure is connected with four retention air traps through check valves.In case of loss of coolant accident in the containment, steam-gas mixture flows through a water closure where it cools down and steam phase condensates.Non-condensing air and radioactive gases flow through the series of check valve to the air locks where they remain captured. Then they are treated in HVAC systems.
All safety important devices are backed up three times (3 x 100%) and are ready for immediate operation under normal operating conditions. They are independent from each other and spatially isolated. To fulfil the necessary safety function, it is sufficient for just one of the three redundant systems to work.