First Shutdown System (FSS)

The first shutdown system is designed to extinguish the nuclear reaction, when abnormal or deviated from normal situations occur, and to maintain the core sub-critical during all shut-down states.

This function is achieved by dropping a total of 25 neutron-absorbing elements into the core by the action of gravity. Each neutron absorbing element is a cluster composed of a maximum of 18 individual rods which are put together in a single unit. Each unit fits well into guide tubes of each fuel element assembly.

The System is composed of two different sets of absorbing elements. The "Fast Extinction System - FES" contains 6 assemblies capable of causing the immediate extinction of the core during any operational or accidental situation. The absorbing elements of FES are kept fully withdrawn during normal operation and they are completely introduced into the core few seconds after a reactor trip signal occurs.

The second set of absorbing elements (19) is part of the Adjust and Control System (ACS). This system compensates long term reactivity changes produced by fuel burn-up and allows short-term reactivity changes to regulate the reactor power. In case of reactor trip, the absorbing elements are dropped into the core in less than a minute and the negative reactivity is enough to keep the reactor in shut down mode with an appropriate safety margin during all core cooling system conditions.