The barrier system

The barrier system is based on the principle that there should be several barriers that complement and enclose each other. If one barrier fails, the next one takes over. If the next barrier fails, the next one takes over, and so on. The risk of a failure passing through all barrier systems is extremely small.

There are five barriers that protect the environment and surround the radioactive fission products.

The first barrier is the fuel itself, uranium oxide, a ceramic, non-water-soluble substance with a high melting point of about 2 800 degrees. The crystals in the fuel have a good ability to bind the fission products.

The second barrier is the zircaloy (zirconium alloy) tubes that encapsulate the fuel.

The third barrier is the reactor vessel and its associated piping system, the so-called primary system. The reactor vessel is made of thick steel.

The fourth barrier is the reactor containment, which consists of thick concrete with a gas-tight steel plate embedded in the concrete. The reactor containment must remain intact even if the fuel starts to melt. It also houses a water pool and sprinklers for cooling. There are also pipes to an emergency filter (known as a RAMA filter) in a separate building that can handle 99.9% of the radioactivity in the event of an accident. Radioactive material should therefore not be able to spread to the surroundings even if the internal barriers are breached in an accident.

The fifth barrier is the reactor building, which can withstand major stresses such as airplane crashes, earthquakes and floods.

In addition to this, the reactor has also been equipped with extra cooling systems. This is because it is important to be able to cool the core with water at all times to keep the two inner barriers intact. To keep the reactor vessel and containment intact, they must also be protected against excessive pressure. This is done by pressure reduction systems and pressure relief systems.