Method and system for maintaining pressure in a nuclear power plant primary loop during startup or shutdown

An automated method for maintaining pressure within a nuclear power plant primary loop, the method comprising the steps of:

• partially filling a portion of a pressurizer vessel, in fluid communication with the primary loop, with a liquid for maintaining pressure in the primary loop;
• circulating a primary coolant through the primary loop with a reactor coolant pump while controlling the pressure in the pressurizer vessel bye:
• (i) automatically injecting an inert gas by a first automated device, operatively connected to the pressurizer vessel, into the pressurizer vessel when the pressure in the pressurizer vessel is less than a first predetermined pressure;
• and
• (ii) automatically venting the gas by a second automated device, operatively connected to the pressurizer vessel, from the pressurizer vessel when the pressure in the pressurizer vessel is greater than a second predetermined pressure;
• and
• circulating the primary coolant through the primary loop with a reactor coolant pump after:
• (i) sensing the pressure and the temperature in the pressurizer vessel;
• and
• (ii) isolating the first automated device and the second automated device from the pressurizer vessel when the sensed temperature and pressure in the pressurizer vessel is suitable for operating the primary loop with the automated devices offline.