The upper part of the RPV is filled with the mixture of nitrogen gas and steam providing a surface in the primary circuit where liquid and vapor are maintained in equilibrium under saturated condition. The pressure of the primary system is equal to the nitrogen partial pressure plus the saturated steam pressure corresponding to the core outlet temperature. Thus the reactor operates at its own operating pressure matched with the system status.

The nitrogen gas partial pressure of 2 Mpa is chosen to maintain subcooling at the core exit in order to avoid boiling in the hot channel during transients.

The volume of gas space is large enough to prevent safety valves from opening during most severe design basis transient.

Reactor Coolant Pumps

The pumps are sealed type (i. e. glandless) canned motor pumps with added inertia to increase pump rundown time. With no shaft seals the small LOCA associated with seal failure in standard commercial designs is eliminated. With the primary water level lowered, they can be removed radially for servicing or replacement without having to remove the vessel closure head.