Defence in depth

Defence in depth involves three levels of action:

• First level: requires that the project should be conceived according to principles of prudence and high quality design and building, that operation should be diligent according to the protocols and experience to prevent failures and damage.
• Second level: involves use of means of surveillance to detect any anomaly that deviates from normal operation of the plant, as well as the corresponding shutdown systems. It includes all the instruments for measuring and controlling sensitive physical readings such as water pressure, temperature, etc. Reactor protection systems are also part of this level: for example, immediate shutdown of the reactor (SCRAM), which introduces neutron absorbers by inserting the control rods into the core and letting in borated water (boron is an excellent neutron absorber).
• Third level: provides for the adoption of action to mitigate the consequences and prevent core meltdown. It includes safety systems (redundant) to automatically restore the reactor to a safe and shutdown state and to guarantee containment of the radioactive material. It responds to incidents considered during the design phase.
• Beyond: defence in depth goes further, and supplies the complementary measures and practices required to minimise the consequences of a “serious accident” in the (unlikely) event that all three more internal barriers fail.

Three consecutive physical barriers contribute to prevent radioactive leaks towards the exterior.