Containment System of Generation III+ Nuclear Power Plants

The general safety approach aims to a significant improvement of the safety of the next generation of nuclear power plants by means of:
- reduction of individual and collective dose to workers in all operational and accidental conditions;
- reduction of the number of significant incidents and potential radioactive releases through improvements of the equipment and systems used;
- significant reduction core melt frequency (less than 10‐5 per reactor per year).
Of the three basic safety functions that have to be ensured ‐ reactivity control, cooling the fuel and confining radioactive substances – containment system plays an active role in the last one and protects all the systems that perform the other two functions.
The "defence‐in‐depth" principle remains the fundamental principle of safety for the nuclear power plants of the next generation, with the implementation of several levels of protection, including successive barriers against the release of radioactive substances to the environment.
The aim is to ensure protection of the public and of the workers. This includes accident prevention as well as accident mitigation. New plants design shall aim at the practical elimination of accident situations which would lead to large early releases, in particular:
- accident sequences involving containment bypassing (via the steam generators or via circuits connected to the primary system which exit the containment), special attention shall be given to shutdown states and open containment building;
- global hydrogen detonations and in‐vessel and ex‐vessel steam explosions threatening the containment integrity; anyway the containment building must be designed to withstand the global deflagration of the maximum amount of hydrogen which could be contained inside in the case of core melt accidents and also to withstand a representative fast local deflagration;
- direct leakage from the containment building to the outside. Pipes liable to carry radioactive substances outside the containment building shall lead to peripheral buildings providing adequate confining capabilities.
Provisions shall be taken for permanent surveillance of the containment building leaktightness.
Due consideration must be paid to the different aspects of a spray system inside the containment building for severe accident situations. This system allows lowering both the pressure and the radioactive aerosols concentrations in the containment building; however a spray system reduces the inerting influence of steam increasing flame velocity of hydrogen combustion.
The basemat of the containment building shall be designed in such a way to prevent leakage of contaminated water and gases to the sub‐soil via cracks and provisions must be take to avoid penetration of "corium" inside it.