A nuclear power plant contains various systems aimed not at producing electricity but focused solely on the safety of the facility in case of transitory incidents or accidents foreseen in the design.
Reactor control and protection systems
A nuclear power plant has suitable instrumentation for monitoring the behaviour of the reactor and measuring its most important parameters (neutron flux, temperatures, pressure, coolant level in the vessel, etc.). This information is processed by reactor control systems that keep its functioning stable by controlling, among other parameters, the position of the control rods, that is, their level of insertion into the core.
The relevant parameters that affect the reactor’s functioning also have established limit values so that if any of them is exceeded a signal is immediately generated to automatically shut down the reactor. This causes the rapid insertion of the control rods (an action called reactor SCRAM).
The system that produces this action is the reactor protection system. The safety systems (cooling, ventilation, power supply, isolation of the containment building, etc.) can also be automatically started by the scram signal if the conditions set for their start-up are reached.
The controlled nuclear reaction in the reactor core generates a large amount of heat. During normal operation, this heat is used to generate electricity.
After a reactor is shut down, it continues to produce heat even if there are no fission reactions, due to the residual heat from the fission products. All power plants therefore have residual heat removal systems with their own circuits, pumps and heat exchangers that allow the transport of this energy to the plant’s heat sink.
Technological safeguards systems
The application of the principle of defence in depth in the design of nuclear power plants includes a series of safety systems called technological safeguards designed to prevent or mitigate the consequences of “postulated” accidents (those considered credible and for which these systems are designed).
One of the main objectives of the nuclear safety assessments undertaken by the CSN is to supervise the proper design, maintenance, testing and operation of this type of systems.