At the upper part of RPV there is a space filled with the mixture of N2 and steam providing a surface in the primary circuit where liquid and vapor can be maintained in equilibrium under saturated condition for pressure control purpose during steady state operations and during transients The pressure of the pnrnarv svstem is equal to the N2 partial pressure plus the saturated steam pressure corresponding to the core outlet temperature A hydraulic instability way take place in some conditions m steam-water two phase flow under lower pressure and in natural circulating loop A N7 partial pressure of 0 6Mpa is chosen to maintain sub­cooling in the core outlet with large margin in order to avoid two-phase flow even m the hot channel dunng normal operation and transient The volume of upper space is large enough to prevent the safety valve from opening during most of the transients, such as loss of off-site power In the present design the safety valve is only opened in case of ATWS mduced by loss of mam heat sink The operational experiences gamed from NHR-5 shows that the N2 as a blanket gas does not cause a problem with water chemistry The concentrate of nitrate and nitnte m the coolant is around 5 ppb