Three barriers

The fission of atomic nuclei, which takes place in a nuclear reactor to generate heat, also produces radioactive substances from which people and the environment must be protected. Nuclear safety is the set of technical and organisational provisions that are applied in the design, construction and operation of a nuclear plant to reduce the likelihood of an accident and to limit its consequences should it nevertheless occur.

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

  • control of the nuclear chain reaction, and therefore of the power generated,
  • cooling of the fuel, including removal of residual heat after the chain reaction has stopped,
  • containment of radioactive products.

Nuclear safety relies upon two main principles:

  • the availability of three protective barriers,
  • application of defense in depth

Three protective barriers

The concept of the “protective barriers” involves placing a series of strong, leak-tight physical barriers between the radioactive materials and the environment to contain radioactivity in all circumstances:

  • first barrier: the fuel, inside which most of the radioactive products are already trapped, is enclosed within a metal cladding;
  • second barrier: the reactor coolant system is enclosed within a pressurized metal envelope that includes the reactor vessel which houses the core containing the fuel rods;
  • third barrier: the reactor coolant system is itself enclosed in a thickwalled concrete containment building (for the EPR™ reactor, the containment is a double shell resting on a thick basemat, the inner wall being covered with a leak-tight metallic liner).

Maintaining the integrity and leaktightness of just one of these barriers is sufficient to contain radioactive fission products.

1. Fuel cladding

2. Reactor coolant boundary

3. Reactor containment