Patent Number: 041837851
Section: summary

This invention relates to a moderator structure for the core of a molten-salt reactor. It is known that in reactors of the molten-salt type, a salt containing fertile and fissile materials is circulated within the reactor core, this molten salt being therefore intended to perform the functions both of nuclear fuel and of primary fluid. In primary heat-exchangers, the molten salt exchanges its heat with a second fluid which again preferably consists of a salt. This second salt passes through a steam generator, the steam thus produced being finally caused to expand in an installation for the production or electric power. The molten fuel salts preferably consist of plutonium or uranium fluoride or else of a mixture of thorium or uranium fluoride dissolved in lithium and beryllium fluorides in order to obtain a mixture having a relatively low melting point, a suitable fluidity and a low vapor pressure. The advantage of these reactors lies in the fact that they do not entail complicated refuelling operations since the fuel is provided in liquid form and not in the form of rods as in reactors of conventional types. Depending on the type of molten fuel employed and on the nature of the moderator which is stored in the reactor core region, these reactors are capable of operating with a thermal-energy neutron flux or a fast-neutron flux. This invention is concerned with the case of reactors which employ a thermal-energy neutron flux. By reason of the fact that the fuel is in the liquid state, it is necessary to employ a moderator in the solid state in order to achieve the heterogeneity which is essential to good neutronic efficiency. This moderator is graphite which is employed in the form of moderator assemblies in conventional designs. In known reactors of this type, the moderator structure is constituted in most cases by elements having a length equal to the height of the reactor core. The graphite which constitutes these moderator elements has low mechanical performances from the point of view of tensile strength and these performances are further impaired after irradiation since graphite undergoes a structural change which results in swelling after incipient shrinkage. In consequence, the moderator elements must be periodically withdrawn prior to swelling and excessive deformation since the existence of a gradient in the neutron field causes differential deformations in the moderator elements and results in bowing of these latter. Handling of these elements is necessarily carried out by gripping the upper ends, with the result that the material works in tension. Furthermore, bowing of these elements of substantial length gives rise to two major disadvantages: spontaneous crack formation under the action of internal stresses, PA1 jamming which occurs before a prohibitive value of swelling has been attained. PA1 maximum fractionation of the moderator in order to reduce bowing effects as far as possible, PA1 handling of the moderator elements without subjecting these latter to tensile stresses. The present invention is precisely directed to a moderator structure and a method for handling said structure which overcome the disadvantages mentioned in the foregoing since the two following requirements are satisfied: In accordance with the invention, the moderator structure for the core of a molten-salt reactor is essentially constituted within a space limited by a bottom wall, a top wall and a lateral reflector by a stack of separate nodules applied in mutual contact in the three dimensions which prevent relative displacements of said nodules and so arranged as to form a lattice of interstices through which the circulation of the molten salt takes place. In accordance with a first embodiment, each nodule has the shape of a sphere and all the spheres have the same size. In accordance with a second embodiment, the entire structure is formed by nodules having the shape of prismatic blocks having vertical lateral faces and a height in the vertical direction which is considerably smaller than the total height of the moderator structure. Preferably, each nodule has a hexagonal cross-section and is provided with a plurality of vertical channels for the passage of the molten salts. In accordance with a further characteristic feature of this embodiment, the structure is formed by an assembly of juxtaposed vertical columns, each column being constituted by a stack of nodules, the channels of nodules constituting any one column being located in the line of extension of each other. In accordance with a third embodiment, the structure is distinguished by the fact that the reactor core comprises an inner core and a lateral outer core, that the inner core is constituted by columns obtained by stacking of prismatic nodules and that the lateral outer core is constituted by spherical nodules. The present invention is also connected with a method for positioning and extracting the moderator structure a number of different alternative forms of the method being contemplated according to the type of moderator structure. In the case of spherical nodules, the method essentially consists in feeding in bulk the quantity of spherical nodules required for filling the core region which is limited by a bottom wall, a top wall and a lateral reflector and in making use of hydraulic or pneumatic transfer means for the extraction operation. Preferably, packing of the spherical nodules is perfected by subjecting the entire assembly to vibrations. In the case of prismatic nodules, the entire moderator structure is first assembled on a temporary supporting rig outside the reactor and then introduced into said reactor, the different nodules constituting said structure being then disengaged from said supporting rig. Preferably, said temporary supporting rig is constituted by a series of hollow rods on which are threaded the central channels of the columns of nodules, said hollow rods being rigidly fixed to a top support plate and each column of nodules being locked in position at its base on the corresponding hollow rod by means of a temporary locking member. In this same case, the extraction of the moderator structure essentially consists in introducing in place of the fuel salt within the space which limits said structure a liquid having a higher density than that of the constituent material of the nodules in order to cause said nodules to float on said liquid and in extracting them from said liquid by means of a handling device designed to grip only the nodules aforesaid.