Nuclear island is the heart of the plant. It houses the nuclear system that produces the steam (NSSS – Nuclear Steam Supply System).
Nuclear island in 1.3 m thick reinforced concrete is designed to withstand any (highly unlikely) internal explosions, earthquakes and floods and to contain leakage of radioactive material from the innermost barriers (fuel and primary circuits).
In addition to all this, the reactor building is protected by a second reinforced concrete structure, 1.3 m thick, designed to withstand the impact of large civil aircraft in the case of accident/terrorist attack.
Inside it, the primary circuit comprises the following elements:
- The reactor, containing the nuclear fuel and the control systems;
- The pumps, for the heat carrying fluid, which ensure circulation of the coolant liquid (water);
- The pressurizer, used to control the pressure inside the primary circuit;
- The steam generators, where heat is exchanged between the primary circuit and the secondary circuit that drives the turbo generator.
The primary circuit is divided into four independent loops, each with a steam generator and pump.
Reactor buildings housing EPRs are larger than those housing other kinds of PWR; this increases the time available for emergency in the event of water leaks caused by accidental rupture of the primary circuit, because the pressure inside the building increases more slowly. In any case, the system is designed to prevent any escape of contaminated water from the primary circuit towards the exterior.