Patent Number: 053496175
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 diagrammatically shows in section the vessel of a pressurized water nuclear reactor. In this reactor, which is provided with the residual power removal apparatus according to the invention, there are obviously the basic components for such installations, namely several inlets 1 supplying cold water at the top of the vessel 2, said water flowing in accordance with the path indicated by the arrows F in the drawing, i.e. firstly downwards into an external annular compartment 3 and then in accordance with a hairpin path rising through the core 4, where heating takes place and the heat will be used in the not shown steam generator. For this purpose the hot water leaves the vessel through one or more pipes 5, after traversing the external annular compartment 3 with the aid of per se known devices. The circulation system described hereinbefore relates to the primary water circuit, which then supplies the not shown steam generator. The steam which is produced is used for the operation of the turbines and then one or more alternators. In order to remove the residual power or after-power produced by the core 4 after a chain reaction shutdown, the apparatus according to the invention essentially comprises a supplementary or third ferrule 7 defining with the internal ferrule 6 of the external compartment 3 a complementary annular space 8. According to the invention, said annular space 8 has one or more heat exchangers 9 autonomously supplied by a second heat transfer fluid entering at 13 and leaving at 14. This secondary heat transfer fluid makes it possible to remove from the vessel the desired power and in turn exchanges its heat with a random known cold source, e.g. an air cooler, condenser, etc. This secondary heat transfer fluid can be of a random nature and can be in single or two-phase form. In the case of a water reactor, in advantageous manner said secondary fluid is constituted by ordinary water. In order that such an apparatus is effective, it is necessary for the water to continuously circulate in the reactor vessel between the core and the exchanger and for said circulation to disturb the normal reactor operation to the minimum extent. The present invention describes the internal arrangement of the vessel in order to bring about this circulation. It is necessary for the hot water of the core to be able to flow along the exchanger 9 and for this purpose two means are provided. The first means is a first orifice 12 located in the lower part of the complementary annular space 8 linking the latter with the downward part of the cooling cold water prior to its entry into the core. The second means is a second orifice 11 located in the third ferrule 7 above the exchangers 9 and below the intake tube 1 and outlet tube 5 for the primary water. Usually there are several orifices 11 and there are a certain number of holes distributed over the periphery of the third ferrule 7. Thus, the complementary annular space 8 communicates with the space located above the core 4 and known as the upper plenum. The flow of hot water to be cooled in the complementary annular space 8 along the exchangers 9 takes place under the effect of two different processes which, according to the particular case, may act simultaneously or separately depending on whether the primary water does or does not flow in the reactor. Thus, if said primary water does flow, its rate is sufficient to create at the first orifice 12 a vacuum in the complementary space 8, which sucks the hot water into said space from the upper plenum 10 through orifices 11. However, if the flow of primary water is stopped, the system is dimensioned and designed in such a way that the residual heat released by the core is adequate for producing a thermosiphon effect, which leads to a circulation of the hot water in accordance with a downward path in the complementary annular space 8. As has been already stated, the cold heat transfer fluid enters by the pipe 1, descends through the annular space 3 and rises into the core 4, where it is heated and loses pressure before passing out of the vessel at 5. In the lower part of the compartment 3, a vacuum is created in the annular space 8. As this vacuum exceeds the pressure drop in the core 4, the fluid in 8 is then sucked by the fluid from 3. Therefore there is a downward heat transfer fluid path in the annular space 8. When the fluid from the loop 1 is no longer available (e.g. during a break or stoppage of the not shown pumps), a natural flow is established between the core and the exchangers 9 via the orifices 11 and 12. The interest of this arrangement of the components in the vessel is that the fluid passing through the space 8 is moved by complementary forces, one being the forced convection due to the main circulation from the loop 1, whilst the other is natural convection. One or other of the two forces is necessary. There is no flow direction reversal during the passage from one force to the other or during the disappearance of one of the forces. Therefore this apparatus makes it possible to extract power from the reactor, no matter what the pressure and temperature levels of the primary heat transfer fluid and no matter what the state of the loops. It is pointed out that if the pressure drops of the core 4 become excessive, a complementary device for improving the vacuum effect should be installed at the link 12. For example, such devices are shown in FIGS. 2 and 3. FIG. 2 shows the first embodiment of the means for increasing the vacuum, the opening at 12 having been partially closed by means 30 and replaced by a series of small openings 15 positioned radially on the ferrule 6. By means of a series of cylindrical or almost cylindrical radial pipes 16, which radially traverse the annular space 3, these openings 15 communicate with the external compartment 3. On each of the pipes there are two longitudinal slots 17 for communication with the annular space 3. The azimuth position of these slots forms an angle .phi. close to 80.degree. with the main downward flow direction in the annular space 3, so as in this way to create a greater vacuum effect. As the suction effect is also linked with the speed of the downward fluid flow in the annular space 3, a local narrowing 18 of said space to the right of the slots makes it possible, if necessary, to improve the vacuum effect. Following the removal of the vessel 2, FIG. 3 shows two radial pipes 16, in perspective and with one broken away, on the wall 6 of the external annular compartment. Arrow F indicates the reentry path into the core 4 of the water from the complementary annular space 8 through the radial pipes 16 and the slots 17. FIG. 4 is used for illustrating the second embodiment of the means for increasing the vacuum in the annular space 8. On the ferrule 6 or the vessel 2 and below the lower part of the exchanger 9, a narrowing 19 is created in order to increase the speed of the fluid descending into the space 3. Towards the location where the speed is highest, a slot 20 in an annular chamber 22 and passing round the ferrule 6 permits the communication between the annular space 8 and the annular space 3. Obviously, the communication 12 is partially blocked by means 30. The annular space 22 communicates by the orifices 24 with the complementary annular space 8. The communication 11 between the annular space 8 and the upper plenum 10 can be in the form of a series of holes. An optional option for improving the system by creating an overpressure is for said communications to be constituted by short pipes, whereof one side is fixed to the aforementioned holes and whose other end is on the side of the upper plenum 10 with the orifice turned facing the main flow direction in the plenum 10. To aid natural convection, the exchanger 9 is positioned as high as possible with respect to the core 4. The communication 11 must be above the exchanger 9, but below the pipes 1 and 5, because the latter determine the minimum water level in the vessel.