Safety
Boiling water reactors

The REB has three containment barriers:

• fuel cladding ;

• the primary circuit envelope, comprising the vessel, its extensions (rod control mechanism envelopes, etc.) and the water and steam pipes connecting to the vessel as far as the feedwater isolation valves and non-return valves and the double steam isolation valves located at the containment penetrations;

• the containment system delimiting a sealed volume completely surrounding the vessel and the second barrier piping up to the isolation valves.

In PWRs, the steam generator tubes form part of the primary circuit envelope (static barrier). On the BWR, the...