Abstract: The primary coolant fluid is circulated within at least one primary loop between a steam generator and a pressure vessel. Within the steam generator, the primary fluid flows through a tube bundle and exchanges heat with a secondary fluid which is admitted in the form of water and discharged in the form of steam. After expansion within turbines and recovery in a condenser, the steam is then returned into the generator. The primary fluid circulating pumps are each driven by a turbine which is fed with steam taken directly from the steam generator or from the main outlet duct of the generator. The essential objective of the invention is to dispense with the use of an electric motor for driving each primary fluid circulating pump.