Overview

Nuclear reactor safety requires that three functions should be fulfilled at all times:

• The chain reaction must be controlled and so must the power generated.

• The fuel must be cooled during and after operation, including after the chain reaction has stopped, as residual heat remains in the reactor core caused by continued radioactive disintegrations and fissions.

• Radioactive products must be retained and controlled. The safety philosophy relies upon two main principles: three protective barriers and so-called defence in depth.

The three protective barriers are intended to contain radioactivity in all circumstances. The first barrier is the fuel, inside which most of the radioactive products are already trapped. The fuel is contained within a metal cladding (magnox, steel or zircaloy) which presents a barrier to stop the products escaping.

The second barrier is the reactor coolant system, housed within a containment enclosure which includes the reactor vessel containing the core constituted by the fuel within its cladding. For the third barrier the reactor coolant system is also enclosed in a containment including a biological shield made of very thick concrete.

For the European PWR now under construction in Finland, for example, this construction is a double shell resting upon a thick basemat, whose inner wall is covered with a leak-tight metal liner.

3.1 Defence in depth

The concept of defence in depth is defined by the International Atomic Energy Agency as a hierarchical deployment of different levels of equipment and procedures in order to maintain the effectiveness of physical barriers...