Activation

A nuclear facility is a source of radiation, usually intense, from which the public and operating personnel must be protected. We must also limit the effect of this radiation on the facility's structures.

To achieve this, the source is surrounded by massive shields (so-called thermal and/or biological shields) which may include penetrations to ensure the necessary connections with the rest of the installation (circulation of fluids for heat extraction, purification, measurements, installation control, ventilation, sampling, etc.). As these penetrations can act as radiation leakage paths, they must not be allowed to predominate.

The protection systems are designed for the plant operating at full power; however, some parts are also protected for the shutdown plant, to enable inspection and maintenance. This is because remanent radioactivity can remain high, even after a long shutdown period (activity of fission products, activation of structural materials and fluids).

The slowing and absorption of radiation results in energy transfer to the materials involved in protection, which must be taken into account in the design to avoid undesirable temperature levels. Energy transfer can also cause damage to structural materials. In this respect, the pressurized water reactor (PWR) vessel is one of the reactor components whose mechanical characteristics and metallurgical condition are most closely monitored with regard to the risk of damage.

Protection studies include, on the one hand, the identification of radiation sources and, on the other, the calculation of radiation transfer to sensitive points. Primary radiation sources (those resulting directly from plant operation) create secondary sources after transport and attenuation in the surrounding materials, and the radiation thus created is in turn transported and attenuated. A complete protection calculation therefore consists of a sequence of source calculations and radiation transfer calculations. These studies are carried out on computer, using a succession of calculation software applications. In modern calculation schemes, this sequence is achieved by coupling software with different functionalities, whose sequence is managed by a control language. There are, however, approximate methods for manually calculating orders of magnitude.