Patent Number: 056617690
Section: summary

The present invention relates to a system for depressurising plants operating with pressurised steam. The system is of the type in which cold water, contained in a reservoir at a higher level than the pressurised plant, is injected under gravity. It is known that this type of system is used to ensure the maximum safety in the case of an incident in a pressurised plant which requires the immediate depressurising of the plant itself. A typical application is in the field of water-cooled nuclear reactors where research into more advanced safety measures is a fundamental part of any development programmed. The depressurisation consists of the injection into the pressurised plant of a large quantity of cold water which condenses the steam present and lowers the temperature with a consequent drop in the internal pressure. After this depressurisation it is then possible to activate further systems for alleviating the incident. The depressurising system must, however, activate without any possibility of failure. Such failure could have devastating effects in that the further alleviating systems would be ineffective. Depressurising systems which operate by injection of a coolant liquid under gravity are known. Such injection is, however, initiated through the use of automatic logic enabling signals from a control systems or manually by operators. This then requires the intervention of suitable actuator members such as valves. The need for an enabling signal, itself, however, introduces a probability of failure. Moreover the activation of the injection requires the presence of external energy sources. This also introduces a failure probability. Finally the actuators also have a failure probability. For these reasons, recent design developments in the nuclear field have been directed to the devising of passive safety systems. Such systems are able to carry out the functions assigned to them only on the basis of physical laws by virtue of adequate mechanical-functional design. The technical problem at the basis of the present invention is that of providing a depressurising system for pressurised plants which overcomes the above problems explained in the prior art, while satisfying this requirement. This problem is solved according to the invention by a depressurising system for plants operating with pressurised steam and including a steam head, by injection of cold water under gravity from a reservoir located at a position higher than the pressurised plant and having a delivery duct for delivery to the pressurised plant, characterised in that it includes an ejector which has an inlet section, an outlet section and a narrow section, a condenser which has an inlet and an outlet, a first connector duct which puts the reservoir in communication with the steam head, a second connector duct which puts the inlet section of the ejector in communication with the steam head, a third connector duct which puts the outlet section in communication with the inlet of the condenser, a first injector duct which puts the outlet of the condenser in communication with the pressurised plant, and in that the delivery duct forms a syphon, the reservoir communicating with the pressurised plant through the delivery duct, and through the narrow section of the ejector. The main advantage of the present invention lies in the fact that it operates passively, relying solely on physical laws by virtue solely of its structural characteristics. A further advantage is due to the fact that the realisation of the invention requires the assembly of components which are simple to design. A further advantage of the present invention lies in the fact that it is adapted to a wide range of pressurised plant including any type of pressurised water or boiling water nuclear reactor. Further characteristics and advantages of the invention will become clearer from the description of one embodiment of the depressurising system for pressurised plants applied to a pressurised light-water nuclear reactor made below with reference to the appended drawings, given by way of non-limitative example.