The flow rate in the reactor primary system is achieved by natural circulation. The driving forces obtained by the differences in the density along the circuit are balanced by friction and produce losses, resulting a flow rate in the core that allows for sufficient thermal margin to critical phenomena.

The driving force for the coolant natural convection is produced by the location of the steam generators above the core. The coolant acts also as moderator.

Self-pressurisation of the primary system in the steam dome is the result of the liquid-vapour equilibrium: the core outlet bulk temperature corresponds to saturation temperature at primary pressure. Heaters and sprinkles typical of conventional PWRs are thus eliminated.