Nuclear power plants safety systems construction

Nuclear power plants are basically designed to prevent radioactive substances from being disseminated to the environment, either during normal operation or in case of disturbances. The safety systems must, however, be continually reviewed and improved, and lessons must be drawn from occurring events.

”Continuous improvements based on experience feedback is a very important part of the modernisation of our plants,” says Ann Berg, Head of Safety & Performance Business Unit Nuclear Power.

Safety work also includes telling people what happens and what is being done in nuclear power. The Swedish nuclear power plants have a number of safety systems to prevent major disruptions, such as:
· Instrumentation that monitors the reactor process
· Scram system
· Emergency cooling
· Residual effect cooling system

Several independent systems

The various safety systems shall firstly prevent errors and secondly counter disturbances. If disturbances do occur, the systems shall prevent them from leading to breakdowns, and if a breakdown still occurs, they shall reduce the consequences.

In order to alleviate the consequences in case of a breakdown, there are a number of barriers to prevent radioactive emissions. These barriers range from the fuel, via the reactor vessel and concrete containment to a special filter facility. The systems are built into several independent units, each of which alone must meet 100 percent of the security function. That is called redundancy. The parallel security systems are based on different techniques and different physical modes of action. That is called diversification.

Furthermore, the parallel safety systems are also separately placed. That is called physical separation. Of particular importance is that there is separate electricity supply to the various systems, so that a cable fire in one place for instance does not knock out all systems. Each parallel safety system is therefore fed with completely separated cables from their own separate reserve power unit – diesel generators or gas turbines.

The control rooms are also designed to minimise the impacts of possible human errors. For instance, important safety functions are automatic. The Swedish reactors have 30-minute rule: All actions that must be taken within 30 minutes after a serious disturbance should happen automatically. The operator thus gets time to analyze the incident before deciding on further action.

Barriers and filters

Surrounding the radioactive reactor fuel substances are several barriers to prevent emissions. The main ones are the ceramic structure of the fuel material, the fuel rod cladding, the reactor vessel, the dense shell of the containment, and the reactor building. In Sweden and other countries special filters have been installed to ventilate the enclosure. They shall be able to catch the absolutely greatest amount of the radioactive substances that would be emitted in case of a meltdown. If one has to reduce the pressure it is thus possible to release the excess pressure while the radioactivity stays in the filter.

At the Swedish nuclear power plants regular drills are being carried out according to a crisis management plan. They deal with different scenarios, including threats and field exercises involving several external parties, such as authorities, local government, police, and emergency services. Alarm systems and communications are regularly being tested.

More information about multi-barrier safety systems