Patent Number: 043354670
Section: summary

The present invention relates to liquid metal cooled nuclear reactors. More specifically, the invention relates to such a reactor of the type with cooling loops. It is well known that there exist two main categories of fast neutron reactors (or fast reactors), viz. those with loops, on the one hand, and those of the so-called "integrated" type, on the other hand. In the latter category, the so-called main vessel contains an inner vessel, or primary vessel, the principal function of which is to separate the hot liquid metal issuing from the core from the cold liquid metal issuing from the pumps and exchangers and fed into said core. Said reactors are said to be "integrated", in view of the fact that the core neutronic protective means, the primary pumps and the primary exchangers are all contained in the vessel. A second liquid metal circuit (secondary circuit) is adapted to transfer the heat of the liquid metal primary circuit to a water-steam circuit for feeding the electric power generating turbines. It will be readily understood that, in such a case, the main vessel must be bulky, which substantially increases the manufacturing cost as well as the cost of the liquid metal forming the coolant material. In the first category, i.e. the reactors with cooling loop, the vessel, in practice morely contains the core and the liquid metal for cooling same, whereas the pumps, the primary exchangers and the connecting ducts are all outside said vessel. Each heat-exchanger is connected to the vessel by means of piping for feeding cold liquid metal at the bottom of the core, on the one hand, and for the exhaust of the hot liquid metal from said core. It can be easily understood that, in said first category, the vessel may be much smaller, which decreases the weight thereof as well as the requested amount of liquid metal, but, on the other hand, since the exchangers and the primary pumps are outside the vessel, it is necessary to resort to pipes provided with curved portions allowing their free expansion under thermal stresses, which renders the whole installation more cumbersome. The present invention relates to said first category of reactors and, more specifically, to the problem of connecting the vessel with a heat-exchanger via the outlet primary duct with a view to restricting the mechanical stresses due to temperature and to decreasing the length of the ducts in order to render the whole installation less cumbersome. Still preferably, but by no means exclusively, the present invention relates to the portion of the nuclear installation in a specific type of reactors with loops, viz. the so-called semi-integrated reactors. More precisely, the latter reactors are fast reactors, the heat-exchangers and pumps of which are outside the vessel, but in which (as in reactors of the "integrated" type), a main vessel contains a primary inner vessel for separating the cold liquid metal from the hot liquid metal, said inner vessel being submitted but to a restricted irradiation in view of the fact that the core is surrounded by neutronic protection means. According to a still more particular embodiment the present invention relates to a semi-integrated nuclear reactor devoid of liquid metal secondary circuit. In other words, in the primary exhangers, the primary liquid metal issuing from the core is in direct heat-exchange with the water/steam fluid that feeds the turbines. These exchangers are, of course, of a very special type so as to meet the safety requirements linked to the direct heat-exchange between the primary liquid metal and the water/steam fluid. Embodiments of such exchangers are described in French Patent application No. 77 03192 of Feb. 4, 1977 for a "Pump-exchanger assembly", filed in the name of the applicant. In addition, in such exchangers, the pump is integrated to the exchanger so as to eliminate the connecting duct between the two devices. In addition to the problem of thermal stresses in the connections between the vessel and the exchangers, account has also to be taken of the necessity to balance said exchangers so that they can withstand earthquake shocks of reasonable amplitude in situ. In the prior art, several approaches to the problem of thermal stresses were made, e.g. in British Pat. No. 985,463, except however as regards the consequences of earth tremors. In the above patent, indeed, in which is described a liquid metal cooled reactor with loops, the connection between the vessel and the heat-exchangers is ensured by coaxial whereas said vessel and conduits are supported in the horizontal plane containing the common axis of said conduits. The support of the tanks containing the exchangers is obtained via annular supporting members, resting on roller bearings and allowing movements in the direction of the common axis of the conduits, whenever a thermal expansion occurs. Such an arrangement, however, has the severe drawback of solving only the problem of the absorption of the movements of the exchangers in the direction of the coaxial conduits, the risk of earth-tremors not being contemplated; on the other hand, said coaxial conduits do not permit, along the liquid metal path, to mount safety valves that would allow to regulate the flow-rates and, in some cases of emergency, to isolate one of the exchangers without stopping the reactor operation. The object of the present invention is to provide a nuclear reactor capable of solving all the above problems, while affording a safer operation, in particular by providing means for supporting the exchangers in such a manner that the movements of the latter be restricted. Said reactor is mainly characterized in that it comprises: a vessel adapted to receive the reactor core and said liquid metal; at least one heat-exchanger adapted to ensure a heat-exchange between said liquid metal and a second fluid, a first duct adapted to connect said vessel with the inlet of the, or of one of the, heat-exchangers, said duct opening into said vessel above the core; a second duct adapted to connect said vessel with the outlet of said, or of one of said, heat-exchangers, said vessel being provided, along the outer periphery thereof, with so-called first supporting means situated at a determined level, said first duct being substantially rectilinear and horizontal and opening into said vessel above, and in the vicinity of, said first supporting means, the, or each, heat-exchanger being provided with so-called second supporting means at a level that it only slightly differs from said determined level and so selected as to minimize the stresses resulting from differential expansions of the ducts, said second supporting means being such as to allow said exchanger to move in the horizontal direction of said first duct, said second supporting means further comprising means for restricting the movements of said exchanger in the two directions perpendicular to the direction of said duct, said two directions being at right angles to each other. It will be readily understood that the exchanger is thus allowed to move freely so as to absorb any thermal expansion of the hot liquid metal outlet duct and of the vessel, between the filling temperature of the vessel (viz. about 150.degree. C.) and the temperature of the hot liquid metal (viz. about 530.degree. l C.), under normal operation. Moreover, in order to restrain the exchanger from unacceptably moving, for instance in case of earth tremors, said second supporting means comprise means for restricting the movements of said exchanger along two directions at right angles to that of said duct, said two directions being at right angles to each other. Finally, due to the use, of two separate ducts, viz. one for the "cold" metal and the other for the hot metal, at two different levels, to connect the vessel to each exchanger, the problems of expansion can be more readily controlled. In such a case, the relative position of the exchanger and the vessel is determined by the hot duct exclusively and, this time, the exchanger is supported at a level different from that at which the vessel is supported and calculated in such manner that the torque exerted on said exchanger by the combination of the two ducts be minimized. In addition, since two separate ducts are provided, they can be different from each other: most frequently, it is preferable that the hot conduit be rectilinear and that the "cold" conduit be provided with elbows; finally, safety valves as well as insulating valves are mounted on said conduits, so that, if necessary, each exchanger may operate at its own rate, and even be momentarily isolated from the reactor, which is a substantial advantage, as regards the reactor safety, with respect to the prior art methods, such as disclosed, e.g., in British Pat. No. 985,463. Conveniently, the centers of gravity of the assembly formed by the reactor, the main vessel and the contents thereof, and of the exchanger-pump assembly are situated lower than their supporting levels, which permits to improve the stability of each of said assemblies. Preferably, the nuclear reactor is of the semi-integrated type and it is characterized in that said vessel comprises a so-called main outer vessel provided with said first supporting means, a so-called primary inner vessel, coaxial with the outer vessel, said inner vessel containing the core and being provided with so-called third supporting means constituted by a ring integral with the main vessel inner wall and supporting a further ring integral with the primary vessel outer wall, said third supporting means being at a determined level, in that said first duct opens into said primary vessel above said core and passes through the annular space between said main vessel and said primary vessel, and in that said second duct opens into said annular space under a horizontal partition provided in said annular space. Still preferably, the or each of the heat-exchangers comprises integrated means for causing said liquid metal to circulate in the exchanger, in said ducts and through said core. According to a preferred embodiment, the means for supporting the heat-exchanger are constituted by at least two supporting members diametrically opposed with respect to the vertical axis of the exchanger outer cover, each of said supporting members comprising a horizontal fixed support plate, a horizontal backing plate integral with said exchanger and situated above said support plate, and a first plurality of rollers between said plates, the axes of revolution of said rollers being at right angles to the direction of said tubing.