Patent Number: 043354670
Section: description

The invention will be described hereinafter with respect to a typical variant of a semi-integrated fast reactor. It is, however, to be noted that the features concerning the connection between the reactor vessel and the, or each, exchanger could be applied as well to fast or thermal neutron reactors with cooling loops, since, in the latter case, the same expansion problems concerning the outlet duct ant the vessel are raised and similar solutions might be adopted. FIG. 1 diagrammatically represents the main elements of the reactor. Inside a concrete enclosure 2 are to be found the reactor main vessel 4 with its safety shell and a heat-exchanger 6 with its shell for the retention of sodium leaks. It is, of course, to be understood that the reactor might comprise several cooling loops, comprising each a heat-exchanger 6 associated to vessel 4. In the specific example disclosed, heat-exchanger 6 ensures the heat-exchange directly between the liquid metal issuing from vessel 4 and the water-steam fluid. Quite obviously it would not be going beyond the scope of the invention to use exchanger 6 for ensuring a heat-exchange between the sodium issuing from the vessel (viz. the primary sodium, in the present instance) and secondary sodium. Moreover, with a view to rendering the whole plant less cumbersome, pump 6' for the circulation of liquid metal is integrated to the heat-exchanger. The so-called main vessel 4 is supported by supporting peripheral flanges or brackets such as 12 resting on supporting members 14 connected to concrete structure 2. The vessel is closed by a roof slab 4a resting on the upper peripheral flange of main vessel 4. The latter contains a primary vessel 16 coaxial therewith provided with a bottom portion 18. Said primary vessel 16 is provided, along its periphery, with supporting members 20 adapted to cooperate with supporting members 22 integral with the main vessel inner surface. It is to be noted that the supporting members of the main vessel and of the primary vessel are substantially in the same horizontal plane. Inside primary vessel 16 are to be found core 24 resting on diagrid 19, together with the neutronic lateral protective means 26. The liquid metal circulation between vessel 4 and exchanger 6 is achieved by means of an outlet conduit 28 adapted to connect the inside of primary vessel 16 with the inlet of exchanger 6, and of a liquid metal inlet duct 30 adapted to connect the heat-exchanger integrated pump outlet with the annular space 32 defined by the primary vessel and the main vessel. It should be noted that duct 28, in which flows a hot liquid metal that has passed through the core, passes through annular space 32. The fuel handling devices and the reactor control devices are of known types and, therefore, are not shown in the diagrammatic view. These various component parts are such that the reactor center of gravity be at a lower level than supporting means 12. According to the invention, and as shown in FIG. 1 in simplified form, steam generator 6 is supported by supporting means 40 integral with concrete enclosure 10. These supporting means 40 allow the generator to move freely in the direction of the axis of duct 28 due to the expansion of said duct and of the vessel. Of course, in addition to ducts 28 and 30 connecting heat-exchanger 6 with vessel 4, ducts relating to the exchanger secondary circuit, and diagrammatically shown at 42 and 44, are also provided. According to the preferred embodiment, the latter ducts (42,44) are water-vapour ducts. In a conventional exchanger, these ducts would be used for the circulation of the secondary liquid metal. It is important to note that, in all cases, it is indeed in duct 28 for hot active sodium that it is intended to minimize the stresses resulting from expansion or generated by earth tremors. Indeed, in return duct 30 flows a liquid metal that is "colder" by about 200.degree. C., which limits the temperature differentials between standstill and normal operation and, in addition, allows the metal to be submitted to higher stresses. It is to be noted that duct 30 is provided with elbows 30a, 30b giving thereto the requested flexibility. As regards ducts 42 and 44, the fluids they carry, whether it is secondary liquid metal or steam water, are not radioactive, so that the problems of expansion can be solved by the conventional methods, and all the more easily as said ducts are, along a major portion of their length, outside the radioactive protection zone. It is also to be noted that the level of the supporting-plane of the pump-exchanger assembly 6 determined by supporting members 40 is only slightly different from the levels of the supporting planes of the main vessel and of the primary vessel determined by supporting elements 14 and 22, the spacing between said levels being appropriately selected by calculation so as to minimize the stresses generated by the reactions of the "hot" duct and of the "cold" duct that open into the vessel as well as to the pump-exchanger assemblies at different levels. Indeed, as disclosed in French Pat. No. 78 18823 in the name of the applicant, tubing 28 is attached to the main vessel, whereas tubing 28 is extended between the main vessel and the primary vessel by separate fittings. In other words, the difference between the displacements resulting from thermal expansions in the vessel portion between supporting member 22 and the junction of tubing 28 with vessel 4, on the one hand, and in the shell portion between the extremity of the junction of tubing 28 with the exchanger inlet and supporting device 40, on the other hand, is negligible since the heights of said vessel portion and of said shell portion are rather small. Accordingly, the stresses resulting from the reactions of the piping on the main vessel and on the pump-exchanger assembly will readily assume acceptable values for the various temperature distributions. As explained in the above-mentioned patent, the main vessel, in that zone, is cooled by colder liquid metal. As will be explained later on, the exchanger containing the primary pump is supported by a member that is rather cold with respect to the liquid metal flowing in the exchanger. The effect of expansion variations in the vertical direction can thus be rendered substantially negligible. As shown in FIG. 1, damping jacks 46 integral with the concrete wall are attached to the lower portion of the exchanger outer shell. The function of these jacks is to damp out the oscillations likely tube imparted to the exchanger in the case of earth tremors, the exchanger center of gravity being, of course, at a lower level than the exchanger supporting plane. With reference to FIGS. 2 to 5, the following description will explain in more detail how the supporting device of the, or of each, exchanger is operated to allow the movements of said exchanger or exchangers in the direction of the axis of duct 28. The upper portion of the heat-exchanger outer shell 50 is surrounded by a sleeve 52, the upper extremity of which is fixed to outer shell 50. Said sleeve 52 is capable of withstanding the overall weight of the exchanger. Said sleeve 52 is, of course, at a temperature substantially lower than that of shell 50, the latter being provided with an outer heat-insulator (not shown). In the example described, the exchanger support is obtained through two supporting members, both of which are designated by reference numeral 40; of course, a portion 40a of each supporting member is integral with sleeve 52 (and, accordingly, with the exchanger), whereas another portion 40b is integral with concrete block 2. A notch 51 permits duct 28 to pass through sleeve 52. Portion 40a comprises a lug 60 arranged along a radius of sleeve 52 and welded or bolted to said sleeve. A so-called horizontal "backing plate" is rigidly fixed to its free end. Portion 40b comprises a horizontal supporting plate 64 anchored to concrete block 10, said plate 64 being situated under plate 62. Between these two plates is mounted a first series of cylindrical rollers 66 having horizontal axes. More specifically, the horizontal axes of rollers 66 are at right angles to the axis of duct 28. The extremities 68 of said rollers are conventionally retained in a cage 70, so that the various rollers form a solid assembly. It can thus be easily understood that, when backing-plate 62 moves horizontally with respect to supporting plate 64 as a result of the expansion undergone by the main vessel, duct 28 and the exchanger shell 50, a rolling movement of rollers 66 with respect to both plane surfaces is obtained. Thus, the free expansion of the various parts as a result of thermal stresses is not accompanied by any substantial mechanical stress. In fact, two sets of rollers (66a, 66b) are provided, which increases the resistance to the stresses applied. By way of example, it can be mentiond that, in the variant under examination, the normal temperature of the hot liquid metal is in the vicinity of 530.degree. C., whereas the temperature of the liquid metal at the moment of filling the vessel is in the vicinity of 150.degree. C., which, as regards the 1200 MW reactor described, results in a displacement of the pump-exchanger assembly of about 125 mm with respect to its position at the mounting temperature. Such a displacement, in the absence of the appropriate device, would lead to high stresses in duct 28. In addition, each supporting member is provided with means adapted to prevent any movement of the exchanger, e.g. in case of earth tremors. In this respect, it is to be noted that the pump-exchanger assembly 6 weighs about 265 metric tons, so that, under such conditions as earth tremors, the amount of energy involved might be considerable and, thus, lead to unacceptable movements and stresses. With a view to counteracting the consequences of earth tremors, each supporting member comprises a second series of rollers 72 with their horizontal axes in parallel relationship with the axes of rollers 66. These rollers 72 are bound to supporting plate 64 and so mounted that, in normal operation, there be no contact between the upper surface of plate 62 and said rollers 72. The latter are put in action only in case of earth tremors, for counteracting any substantial tilting or rising movements of the heat-exchanger. In fact, a horizontal upper plate 73 is made integral with the supporting plate, and, should the exchanger be unexpectedly caused to tilt, thus raising plate 62, rollers 72 would be brought into contact with backing plate 62 and with upper plate 73. Thus, stresses are absorbed by the whole cross-section of the rollers and not merely by the stub-axles at both ends of said rollers. Similarly, a third series of rollers 74 with vertical axes are pivotally mounted in part 76 integral with supporting plate 64. In normal operation, there is a clearance between rollers 74 and the vertical surface 62a of plate 62. In case of earth tremors, the function of said rollers is to restrict any horizontal movement at right angles to the axis of duct 28 to an acceptable value. Thus is obtained a system for supporting the movable pump-exchanger assembly, said system providing the mobility required for absorbing expansions in the direction of duct 28, the latter being itself so designed as to withstand any transient stresses resulting from components of the maximum earthquake contemplated for the site of the plant. Such a system permits to minimize the stresses applied to the connecting ducts between the vessel and the exchanger, while ensuring the stability of said exchanger in case of earth-tremors. As already specified, such a supporting system could be applied both to the primary or intermediate exchangers of a reactor with loops, and to the pumps, when the latter are not integrated to the corresponding exchanger, as is usual in reactors with loops.