Patent Number: 044774109
Section: description

DETAILED DESCRIPTION FIG. 1 shows the protective wall 1 of a fast fission nuclear reactor, to which the slab 2 covering the reactor vessel is fixed. The main vessel 3 is fixed to the slab in its upper part, while the safety vessel 4, covered with a layer of insulating material 5, is fixed to the protective wall 1. Up to the level 6, the main vessel 3 contains liquid sodium, in which the reactor core 7 is immersed, which rests on a bed 8 and a floor 9, itself resting on the bottom of the main vessel 3. The interior volume of the main vessel 3 is separated into two different zones by a double partition 10, which is supported by the floor 9 at the periphery of the core and consists of an annular wall 11 and a cylindro-conical wall 12. This double partition, also referred to as a step, makes it possible to isolate the sodium contained in the upper zone 14, containing the core 7, from the sodium contained in the annular lower zone 15. Intermediate heat exchangers and pumps, not shown, make it possible firstly to cool the hot sodium contained in the zone 14 in contact with the secondary sodium, to transfer this sodium from the zone 14 to the zone 15, and finally to inject the cold sodium removed from the zone 15, into the lower part of the core, in the bed 8. The cold sodium re-injected into the base of the core passes through the latter from bottom to top, heats up and then passes from the zone 14 to the zone 15 via the intermediate exchangers. In addition to the step, an external shell 16 and an internal shell 17 are arranged in the vessel and coaxially thereto, and those parts of these shells which are located above the zone 15 delimit between one another, and between the outer shell and the main vessel 3, two annular spaces 19 and 20, in which the cold sodium circulates. To permit this circulation of cold sodium, the shell 16 is extended so as to bring the space 19 into communication with the base of the core, under the bed 8, at the point where the cold sodium is injected. As shown by the arrows 21, a circulation of cold sodium is set up from bottom to top in the annular space 19 created between the external shell 16 and the main vessel 3. In the top part of the annular passage 19, which emerges, under the slab 2, in the space 24 created between the slab and the liquid sodium level 6 and filled with an inert gas, for example argon, the cold liquid sodium flows along the shell 16, inside the annular space 20, this sodium moving down again, by gravity, into the part 15 of the vessel, containing the cold sodium. The annular passage 20 is brought into communication with the zone 15 containing the cold sodium, via a calibrated orifice 25, making it possible to adjust the pressure drop during the circulation of the sodium. The circulation of the cold sodium in contact with the internal wall of the main vessel 3 makes it possible to cool the latter and to keep it at a temperature which is virtually constant and corresponds to the temperature of the sodium before it enters the core. The pumping of the liquid sodium and the pressure drop during its circulation make it possible to maintain a difference in level between the sodium filling the tube 19 and the sodium flowing into the tube 20. FIG. 2 shows the elements corresponding to those shown in FIG. 1, provided with the same reference numbers. In contrast to the embodiment of the cooling device shown in FIG. 1 and corresponding to the prior art, the annular space 19 is brought into communication by a series of calibrated openings 30, making it possible to ensure an adjusted pressure drop during the circulation of the sodium, with the annular zone 15 containing the cold sodium, whereas the annular space 20 is brought into communication, via tubes 31, with the zone of the vessel located underneath the bed 8, into which the cold sodium is injected. In this way, the cold sodium circulates in the direction indicated by the arrows 32. The cold sodium therefore circulates first inside the tubes 31, through which it reaches the annular space 20, through which it passes from bottom to top up to the level of the space between the slab and the liquid sodium level 6, where the upper end of the outer shell 16 is located. The cold sodium then flows, by gravity, into the annular space 19, along the external surface of the shell 16. A difference in sodium level in the annular spaces 20 and 19, respectively, is maintained, as previously, by virtue of pumping and by the pressure drop, in particular at the level of the openings 30. In the external annular space 19, the sodium flows downwards, by gravity, in contact with the internal surface of the main vessel 3, which it cools and keeps at the temperature of the cold sodium, i.e., at about 400.degree. C. The cold sodium returns to the zone 15 via the calibrated openings 30. It is seen that, compared with the device according to the prior art, shown in FIG. 1, the device according to the invention has the advantage of placing the external shell 16 under internal pressure, whereas this shell is under external pressure in the device of the prior art. Likewise, the lower part of the baffle, which extends the shell 16 down to the level of the bed 8, is now subjected only to an internal differential pressure of low amplitude. In this way, it is possible to reduce the thickness of the shell 16 and thus to make a substantial weight saving in the design of the reactor. The difference in level h between the sodium filling the annular space and the sodium flowing in the annular space 19 is of the order of two meters for the nuclear reactors currently being constructed, the vessel of which has a diameter of the order of twenty meters. The part of the main vessel 3 which is located over this height h, between the sodium level in the space 20 and the sodium level in the space 19, is not in contact with the sodium as in the case of the device of the prior art. However, this does not give rise to large temperature differences between the points of the vessel which are in contact with this sodium and the points of the vessel which are in contact with the gas on top of the sodium, because, in the first place, the safety vessel 4 is lagged, which prevents heat looses from the main vessel, and, in the second place, the heat supplied by the radiation from the external shell 16, at the level of the zone of height h, keeps the main vessel, in this zone, at a temperature close to the temperature of the remainder of the vessel. FIG. 3 shows a second embodiment of the cooling device according to the invention, in which the shell 17 is of reduced height and in which the space 20 is in communication, by its lower part, with the lower part of the core, via tubes, such as 35, passing through the central space in the partition 10, located between the two parts of the partition constituting the step. As in the case of the device shown in FIG. 2, the annular space 19 communicates with the zone 15 containing the cold sodium, by means of calibrated orifices such as 30. The operation of the device is virtually identical to the operation of the device shown in FIG. 2, a small part of the cold sodium injected under the bed 8 passing into the tube 35, and from there into the annular space 20 constituting a channel for the sodium flowing in the annular space 19, for the cooling of the main vessel 3 and and return of the sodium into the annular zone via the openings 30. This device has the same advantages as the device shown in FIG. 2. The invention is not limited to the embodiments which have been described; on the contrary, it includes all the variants thereof. Thus, it is possible to envisage other means for connecting the annular space 20, constituting a channel for the liquid sodium, to the lower part of the core. It is also possible to envisage any kind of connection between the space 19, created between the main vessel and the external shell, and the zone, such as 15, containing the cold sodium, for the recycling of the latter. The device according to the invention can apply irrespective of the shape and structure of the step, whether the latter consists of a single partition or a double partition. Finally, the device according to the invention applies in the case of all integrated-type fast fission reactors.