Guidelines for Reactor Safety
A nuclear power plant must be constructed so that radioactive substances cannot be discharged in an uncontrollable manner. The reactor containment should be intact even during a core accident and a stable final state should be able to be reached, so that the core can be kept cooled and under water.
Reactor safety is based on the following principles:
The barrier system
The barrier system is based on the principle that there should be several barriers that complement and surround each other, so that if one barrier does not work then the next one will take over. If the next barrier does not function either, then the one after it takes over. The risk that a problem will pass through all of the barrier systems is therefore very small.
Levels of defence in depth
These levels entail designing and operating nuclear power plants with the highest protection possible against technical errors, human mistakes and uncontrollable events outside the plants, such as fire, earthquakes, lighting, sabotage and etcetera. The principle includes five different safety levels. These work in approximately the same way as barriers – if one falters then safety is maintained through the others.