Method for filling a main primary coolant system of a nuclear power plant with water and for draining it of air after said plant has been shut down, said main primary coolant system comprising a reactor vessel located in a pool of a reactor building, primary coolant pumps, at least one steam generator comprising a plurality of steam generator tubes that are located above the level of the reactor vessel, and a pressurizer, the method comprising the following steps: - a first putting into position of a head closing the reactor vessel, the head being of a false head type, and a sealing of said head where it joins the reactor vessel using sealing means; - an adjustment of the water level of the main primary coolant system to a low working range in order to bring the volumes of air contained in the tubes, in the pressurizer and in the reactor vessel into communication with one another; - a vacuum setting of the main primary coolant system by the suction of vacuum pumps connected to vents on the reactor vessel and on the pressurizer; - a filing up of the main primary coolant system to a vacuum level to a vacuum level and then brought to the atmospheric pressure of the main primary coolant system by opening vents (12, 13), by virtue of which the water of the main primary coolant system is drained into the tubes and then brought to the atmospheric pressure of the main primary coolant system by opening the vents, by virtue of which the water of the main primary coolant system is drained into the tubes; - a putting out of position of the head and the sealing means; - a filling of the reactor vessel and the pool with water; - a load of the reactor vessel with nuclear fuel; - a draining of the pool and second putting of the head into position for closing off the reactor vessel is put into position; then - a filling up of the main primary coolant system to the top of the pressurizer so as to allow the nuclear power plant startup operations to continue, - a first putting into position of a head closing the reactor vessel, the head being of a false head type, and a sealing of said head where it joins the reactor vessel using sealing means; - an adjustment of the water level of the main primary coolant system to a low working range in order to bring the volumes of air contained in the tubes, in the pressurizer and in the reactor vessel into communication with one another; - a vacuum setting of the main primary coolant system by the suction of vacuum pumps connected to vents on the reactor vessel and on the pressurizer; - a filing up of the main primary coolant system to a vacuum level to a vacuum level and then brought to the atmospheric pressure of the main primary coolant system by opening vents (12, 13), by virtue of which the water of the main primary coolant system is drained into the tubes and then brought to the atmospheric pressure of the main primary coolant system by opening the vents, by virtue of which the water of the main primary coolant system is drained into the tubes; - a putting out of position of the head and the sealing means; - a filling of the reactor vessel and the pool with water; - a load of the reactor vessel with nuclear fuel; - a draining of the pool and second putting of the head into position for closing off the reactor vessel is put into position; then - a filling up of the main primary coolant system to the top of the pressurizer so as to allow the nuclear power plant startup operations to continue,