Patent Number: 039430367
Section: summary

The invention relates to a fast breeder nuclear reactor having fluid cooling, preferably liquid metal cooling and canless fuel elements in the fissionable fuel zone or core thereof. The breeder reactor differs from a normal or ordinary nuclear reactor in that the breeder reactor is constructed of at least two regions or zones which differ basically in their function. The core zone is arrayed with fissionable fuel elements in which there occurs a controlled chain reaction for splitting U.sub.235 or plutonium atoms, with the development of very great quantities of heat. A breeder material zone or breeder mantle is located outside the fissionable fuel core zone and has a construction similar to that of the core zone, but contains, however, in the elements thereof i.e. the breeder elements, no nuclear fuel but rather so-called breeder material such as U.sub.238. Due to neutron capture from the reactor core proper, the U.sub.238 atoms are transferred or transmuted into plutonium atoms and thereby into fissionable fuel. Heat production in this zone is relatively small compared to the heat produced in the nuclear fuel zone, and the differences with respect to the heat power per volume element are even greater. The same coolant such as, for example, steam (vapor), gas or primarily liquid metal like sodium, potassium, etc., is normally employed for cooling both zones. Since the efficiency of the transformation of the produced heat to electrical energy depends very greatly on the final temperature of the coolant, measures must be taken to have the outlet temperature of the coolant overall uniform i.e. over the entire cross section of the breeder reactor. To attain this objective, it has been proposed heretofore to surround the individual fuel elements and breeder material elements with a metallic mantle so that the inner space of the entire reactor is formed of a multiplicity of such coolant channels subdividing the flow. It is accordingly possible to so shape the core zone of the breeder reactor, wherein the fuel elements are disposed, that in addition to the reduction of the so-called void coefficients, the temperature release is the same from all of the fuel elements. In the breeder mantle, however, this uniformity of the thermal release is no longer present because the power output of the individual breeder elements is no longer constant with time but rather, in the course of the operating time increases in proportion to the growth of plutonium. The coolant throughput through this zone must therefore first be throttled so that the outlet temperature is virtually the same as that from the core zone. Moreover, the heretofore required throttling devices must be adjustable from the outside in order to do justice to the change in the power release due to the increasing plutonium production. Another possibility is to provide within the breeder material zone various regions with different coolant throughput and to transform the breeder material element continuously within these regions in accordance with the plutonium formation therein. This construction of the fuel and breeder material zones has the disadvantage that a considerably pressure difference occurs between the inner space of the fuel and breeder material elements, on the one hand, and the gap therebetween, on the other hand. Since there is virtually no flow in the gaps between the fuel elements, the foot thereof between the mantles, boxes or cases of these elements is at coolant outlet pressure. This pressure difference, which can be of the order of magnitude of 4 atmospheres absolute, for example, has an effect upon the fuel element mantle or casing, and these must therefore be given a suitable wall thickness in order to avoid mechanical deformations and bends or distortions. The additional structural material costs moreover impairs the breeding rate of the entire reactor; also the mechanical long-term characteristics of these casing or cans at the high neutron flux density are not to be overlooked. It should also be noted that in addition to the pressure-loading, a strong bending of the casing or can in the order of magnitude of centimeters occurs due to the radially decreasing irradiation intensity by fast neutrons and the threshold effect of the structural material required thereby. Mechanical prevention of such bending, for example by tension or clamping members, is impossible. The question was therefore than raised if it were possible to construct the fuel elements without any outer mantle or casing as for light water reactors i.e. as so-called open or canless fuel elements. No close approach could be made, however, to this idea because the disadvantages with respect to the efficiency that were associated therewith, no possibility of controlling the coolant flow, were too difficult to overcome. It is accordingly an object of the invention to provide a fast breeder reactor which avoids the foregoing disadvantages of the heretofore known devices of this general type. More specifically, it is a further object of my invention to provide a fast breeder reactor wherein a uniform outlet temperature of the coolant over the entire reactor cross section is afforded by employing canless fuel and breeder material elements. With the foregoing and other obejcts in view, I provide in accordance with my invention fluid-cooled fast breeder reactor comprising an outer cylindrical boundary wall, a plurality of fuel elements and breeder material elements of canless construction received within the boundary wall and being in an array therein forming a fissionable fuel zone and a breeder material zone coaxially surrounding the fissionable fuel zone, a coolant supply system for applying fluid coolant at uniform pressure to the entire cross section within the cylindrical boundary wall, and flow guide devices extending substantially horizontally and disposed one above the other within the breeder material zone which coaxially surrounds the fissionable fuel zone, the flow guide devices, respectively, being alternately elastically secured to the boundary wall and spaced by an annular gap therefrom. With this arrangement of guide devices, the coolant flow streaming through the breeder mantle is increased in length and the flow resistance is thereby increased so that for considerably reduced flow quantities, in this section the same outlet temperatures as in the fuel region are attained. The solution for this problem is achieved thus with a forced transverse flow in the breeder regions. An apparently even simpler solution, namely the introduction of a partition between the core zone and the breeder mantle must be replaced about every three years due to the radiation load and the reduction in the mechanical properties associated therewith, a fact which ought to lead to no inconsiderable difficulties with the large diameter. Another apparently simple solution would be the increasing of the flow resistance within the breeder mantle due to reduction of the spacings between the individual fuel rods. This is not technically feasible because the spacings must become small, so that for the slow flow rates, depositions would not be avoidable and would clog the element. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in fast breeder reactor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.