Patent Number: 041860497
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to nuclear reactors of the molten combustible salt type, and more specifically of the type having a primary integrated circuit. In such reactors the core and its reflector are located in a first vessel called the "reactor skirt" which is itself contained in its tower with pumps and primary exchangers in a second vessel called the main vessel. These reactors use a liquid fuel heated to a high temperature of at least about 600.degree. C. by nuclear fission in the core, whereby said fuel is generally constituted by uranium or plutonium fluoride or a mixture of uranium fluoride and thorium dissolved in lithium fluorides and beryllium so that the mixture has a relatively low melting point, a suitable fluidity and a low vapour tension. When neutrons have to be thermalised the core of such reactors contains a suitable moderator mass such as graphite, whereby there are discharge channels for the combustible salt which then exchanges the calories obtained on passing through the core in a primary heat exchanger with another molten salt called the buffer salt, for example sodium fluoborate. In turn this buffer salt exchanges its calories in a secondary circuit comprising a steam generator, the steam being finally expanded in an electricity production plant. In such integrated reactors the molten combustible salt is contained in a metal vessel called the main vessel whose walls are protected against chemical corrosion and high temperatures by carbonaceous materials. For example, this is the case with the nuclear reactor forming the object of French Patent Application EN 7442767 of 24.12.1974 entitled "Molten combustible salt nuclear reactor". In said reactor the main vessel is almost entirely filled with said carbonaceous materials and the only cavities provided in said carbonaceous material mass are those containing the reactor core, the heat exchangers, the circulating pumps for the molten salt and the different galleries ensuring the hydraulic connection between the components specified hereinbefore. In theory, this design of the integrated primary circuit in a main metal vessel permits the insulation of said confining structure relative to the molten combustible salt contained therein. Unfortunately, experience has shown that although interesting in theory these constructions give rise in practice to a certain number of shortcomings due in particular to the great difference existing between the thermal expansion coefficients of the vessel material on the one hand and the carbonaceous filling material on the other. As a result, it is necessary to provide a better protection of the main vessel relative to the molten salts. A device for protecting a main vessel of this type forms the object of French Patent Application EN 7517939 filed on June 9, 1975 entitled "Process and apparatus for protecting the vessel of a molten salt nuclear reactor". A per se known device of this type will be described in greater detail with reference to FIG. 1. FIG. 1 shows in the form of an axial half-section a molten salt reactor having an integrated primary circuit designated by the general reference numeral 1. In a protective concrete enclosure 2 said reactor comprises a main vessel 3 containing the reactor core 4 itself located in the reactor skirt 5 and the exchangers and circulating pumps for the molten salt, whereby the location of the single exchanger 6 is clearly visible in FIG. 1. The inner wall of said main vessel 3 as well as its internal volume not occupied by the reactor elements are lined with a per se known carbonaceous filling material designated by 7 in FIG. 1. In this construction a second outer vessel 8 is placed around the main vessel 3, whereby the space between vessels 3 and 8 is filled with a climatisation or air-conditioning fluid 9. The temperature of this fluid is controlled, for example by at least one submerged circulating fluid exchanger 10, the water entering at 11 and leaving in the form of steam at 12. Firstly, the main vessel is heated to a temperature of, for example, 400.degree. C. by acting on the air-conditioning fluid 9 and exchangers 10 leading to the formation of an empty space 13 as a result of the differential expansion occurring between carbon 7 and the steel of vessel 3. Vessel 3 is then first filled with an auxiliary salt which contains no fissile or fertile material and whose melting point is below the temperature to which vessel 3 has been heated, i.e. 400.degree. C. in the case described herein. This salt can be of different types and advantageously it is constituted by the eutectic of lithium fluorides and beryllium whose melting point is 350.degree. C. It is absolutely necessary for it to be chemically compatible with the actual combustible salt. This auxiliary salt which is neutral from a nuclear standpoint fills the said empty space 13 and the interstices located in the carbonaceous lining mass 7. In a second phase which follows the first the temperature of the main vessel 3 is lowered to below the melting point of the auxiliary salt used by means of the air-conditioning fluid 9 and the exchangers 10 which causes the agglomeration or solidification of that part thereof which has filled space 13 created between the main vessel 3 and the carbonaceous filling material mass 7 and to a certain depth the interstices emerging at the periphery of the carbonaceous mass 7. In the present example where the eutectic of lithium fluorides and beryllium has a melting point of 350.degree. C. the air-conditioning temperature of fluid 9 is, for example, lowered to 300.degree. C. When all the auxiliary salt in space 13 has solidified and this solidification has also penetrated a certain depth into the interstices emerging in said area 13, the reactor is loaded with the final combustible salt. At the end of these operations vessel 3 is definitively maintained at 300.degree. C. and the reactor is ready for operation. The crust of auxiliary salt in the space 13 between vessel 3 and the carbonaceous filling mass 7 substantially has no contact with the combustible salt. Thus, there is no need to fear a nuclear reaction in said crust of neutral salt which is thus maintained at an essentially constant temperature and can effectively fulfil its function of providing corrosion protection for the main vessel 3. The present invention applies to the molten salt reactor of the type described with reference to FIG. 1 and the invention in fact aims at improving the heat exchangers of reactors of this type. In such reactors the primary heat exchange system has hitherto been constituted by an integrated system in the main vessel and has comprised the arrangement in alternate manner of an exchanger and a pump in the carbonaceous filling material, whereby said components are distributed over the entire periphery of the structure. In the known constructions, the pumps circulate the molten salt in the reactor skirt from bottom to top and the hot salt descends in countercurrent flow by forced circulation in the adjacent exchangers which it thus traverses from top to bottom. A transverse hydraulic connection is then necessary between the base of each primary exchanger and the adjacent pump shafts in order to ensure the return of the cold salt to the reactor core resulting in significant expansion problems in operation. BRIEF SUMMARY OF THE INVENTION The present invention relates more particularly to a heat exchanger integrated into the main vessel of a nuclear reactor of the molten combustible salt type which obviates this serious disadvantage in a simple and effective manner. This heat exchanger integrated into the main vessel of a molten combustible salt reactor comprises a reactor skirt containing the active core, a main vessel surrounding the reactor skirt, pumps and primary exchangers, an outer vessel which doubles the main vessel, a thermostatic coolant between the main and outer vessels maintaining the main vessel wall at a temperature below the melting temperature of a crust of salt which is inactive from a nuclear standpoint and which forms a coating of solid salt protecting the inner surface of said main vessel, wherein the calories are extracted from the core by means of autonomous heat transfer modules each comprising a primary exchanger and a pump, whereby each module is suspended in the intermediate space between the main vessel and the reactor skirt and supported by a bearing surface whose base is located on a cooperating bearing surface provided around an opening made in the wall of a supporting ferrule fixed close to the bottom of the reactor skirt and over the entire circumference of the latter, said ferrule extending from the skirt to the vicinity of the main vessel in the solid protective salt crust. According to a preferred embodiment of the invention, the bearing surface of the module has a spherical profile and cooperates with a planar supporting surface. According to a special feature of the invention, each autonomous module is suspended elastically on a structure which is generally made from concrete and forming the upper slab of the reactor in such a way as to permit a limited angular displacement of the assembly comprising an exchanger, a pump and optionally a flow regulating valve. According to another feature of the invention, the supporting ferrule has a conical surface fixed by one side to the reactor skirt and whose opposite end towards the main vessel is left free. On the reactor core periphery, said ferrule has openings which cooperate with the lower part of each autonomous module and permit the forcing back of the cold salt within the reactor skirt. Moreover, the periphery of each of these openings has a planar bearing surface cooperating with the homologous spherical bearing surface of each autonomous module which ensures both the support and sealing at the contact area between said module and said ferrule. On the basis of the above features, two special embodiments can be envisaged. According to a first embodiment, each opening made in the wall of the ferrule sealingly cooperates with the lower part of the core by means of a delivery pipe which serves to pipe the cold salt from the pump to the base of the reactor, the end of the ferrule being free in the inner area of the main vessel in the solid protective salt zone. In a construction of this type a delivery pipe serves to pipe the cold salt from the base of each autonomous module to a return opening in the reactor skirt, whereby said delivery pipe is tightly sealed. The lower part of the main vessel of the reactor cannot therefore be subject to the delivery pressure of the cold salt. As a result, the sealing between the main vessel and the ferrule is unnecessary at the inner periphery of the latter. This variant is of interest for the latter reason and due to the fact that the delivery pipe is in principle made from the same material as the vessels and ferrule and is at the same temperature so that there is no longer any danger from the thermal stresses occurring in the prior art devices where a transverse hydraulic pipe was necessary for ensuring the delivery of the cold salt from the base of each exchanger to the base of each pump. However, differential expansion problems may possibly occur during accidental transient operating cycles in which the various elements constituting the ferrule and delivery pipe may not have the same thermal response time and may consequently be heated to different temperatures. This disadvantage is completely eliminated with a second embodiment of the invention in which each opening in the ferrule wall communicates directly with the space in the lower area between the main vessel and the reactor skirt, whereby the base of the latter has direct passages for the return of the cold salt into the reactor core. On the side of the main vessel this space is sealed by the solid salt crust in which is arranged the free end of the conical ferrule. The essential advantage of said second variant is that the solid salt crust which is hypothetically provided along the inner wall of the main vessel results in an adequate sealing between the free floating end of the ferrule and the main vessel. This sealing can be made even more reliable in accordance with a special arrangement provided by the invention whereby the tapered end of the ferrule cooperates with an annular boss on the main vessel, said ferrule engaging beneath said boss in the solid salt crust area. During operation this arrangement permits all relative radial and heightwise expansions between the end of the ferrule and the main vessel without there being any break in the sealing because the boss and the end of the ferrule are always contained in the zone filled by the internal crust of salt which is neutral from the nuclear standpoint. According to this second embodiment of the invention, all the lower annular part between the reactor skirt, the main vessel and the ferrule is filled by cold salt delivered by the pumps of the autonomous heat transfer modules and consequently this space is subject to the pressure of said pumps which does not constitute a disadvantage. The return of the cold salt to the inside of the reactor skirt is ensured by openings made in the lower part of the reactor skirt beneath the ferrule. Therefore, the most important feature of the heat exchanger according to the invention is that it permits by simple means the elimination of the thermal stresses occurring in the prior art as a result of the spatial division of the functions of the exchanges and the pumps whilst ensuring for each autonomous module a tightly sealed support which permits the displacements made necessary by the differential thermal expansion of each of said autonomous modules. Moreover, as said tight support is provided by simple cooperation between a spherical bearing surface and a conical planar bearing surface, the fitting and/or removal of each autonomous module is made possible by a simple vertical translation without it being necessary to disassemble any member in the vicinity of the reactor core.