Patent Number: 047987006
Section: summary

BACKGROUND OF THE PRESENT INVENTION The invention relates to ceramic installations and more particularly to ceramic reflectors for use in nuclear reactors. Ceramic installations are utilized in gas-cooled, high temperature reactors as side, bottom and roof reflectors intended to reduce losses caused by the migrating of neutrons. At least part of the neutrons moving to the outside are to be reflected back into the fission zone of the reactor, the so-called reactor core. The neutron flux increased at the edge of the fission zone by the reflector effect raises the output produced by unit mass of the fissionable material, leading to the improved utilization and more economical consumption of the nuclear fuel. Highly purified graphite is used in gas-cooled, high temperature nuclear reactors as the material for ceramic installations. It is relatively inexpensive, has adequate strength and may be processed mechanically. It is refractory and has good thermal conductivity. A disadvantage is the change in its crystalline structure caused by neutron and gamma radiation and expressed by changes in mechanical strength and in volume. Under the effect of temperature and high neutron fluxes, graphite expands at first negatively, but then positively beginning at a point of reversal, with increasing fluxes, which go beyond the original dimensions of the graphite body. The process is displaced with rising temperatures toward lower fluxes. The differences in expansion within the structural part--in keeping with the flux distribution, at the onset of the irradiation the block layers close to the surface of the front facing the core tend to become shorter to a relatively greater extent due to shrinkage than the deeper layers--are the cause of the generation of residual stresses. To reduce these residual stresses, it is necessary to provide relief by expansion equalization. This may be obtained advantageously by slit surface structures which are equivalent to a dimensional reduction of parts of the graphite block. In more recent developments of gas-cooled, high temperature nuclear reactors, in particular those of lower capacity (approximately 100 MWel) and with correspondingly smaller core diameters, in place of absorber rods inserted directly into the pile of spherical fuel elements, small absorber elements in the spherical form are provided for the shutdown of the reactor and are introduced into corresponding cavities of the reflectors. In a manner similar to the AVR nuclear power plant in Julich, in the core of which nose-like projections with vertical cavities, the so-called nose stones, are provided to receive the control and shutdown rods, the newer gas-cooled high temperature nuclear reactors presently in the planning stage are equipped with nose stones of this type, but here they are intended for shutdown rods only. The nose stones are ashlar-shaped graphite blocks extending radially from the side reflector, to which they are physically joined, over the entire height of the reactor core into which they are projecting. In view of the above-mentioned volume variations and the residual stress state caused by them in the irradiated graphite blocks, the surfaces facing the core of the latter are provided with vertical and horizontal surface slits, representing a resolution of the original large surface into small individual rills. To control stresses in the nose stones, the cavities provided for the introduction of the absorber elements are connected with the core by means of gaplike, vertically disposed continuous openings. These openings reduce the residual stresses in the nose stones to tolerable levels. However, the afore-mentioned expansions lead in the course of the operation to a widening of the openings to such an extent that the separation of the absorber material and the fuel elements is no longer assured and the absorber elements are able to exit from the cavities and fuel elements can enter them. Based on this state of the art, it is the object of the invention to provide measures for the design of ceramic installations which are simply and cost effectively realized and which prevent in a highly reliable manner the aforementioned deficiencies, in particular the passage of absorber and/or fuel elements through the continuous opening. SUMMARY OF THE PRESENT INVENTION In accordance with the present invention, the innermost gap width, i.e., the distance of the gap surfaces defining the gap from each other on the innermost side adjacent the cavity is determined such that it will correspond at most to one-half of the size of the minimum dimension of an absorber element present in the cavity. This insures that no absorber element can become jammed in or pass through the gap and hinder the deformation equalization for which the gap or continuous opening has been provided. Simultaneously, this determination also takes into account the fact that the deformations caused by neutron irradiation are essentially confined to a zone close to the surface of the graphite nose stone facing the core, so that the width of the gap varies only slightly inside the cavity and that therefore said deformations are not hindered and additional stresses are avoided. In addition, the individual absorber elements are appreciably smaller than the fuel elements, so that by means of the spalt geometry described above, the passage of both the absorber elements and the fuel elements is safely prevented. In a further embodiment of the invention the continuous opening is positioned on the front side of the nose stone and provided with an inserted graphite blocking member. This blocking member is guided positively in grooves molded opposingly into the surfaces of the opening. It is set loosely and transversely in the opening and assures the uniform absorption of neutron radiation by the absorber material inserted into the cavity, without preferential treatment being given to any spatial zone due to the absence of reflecting graphite. Another embodiment of the invention takes this condition into account by the fact that the gap opening comprises a continuation of a slit-like groove on the face of the nose stone projecting into the core. Here again a direct impact on absorber material of neutron radiation is almost entirely excluded. It has been found to be advantageous in this context to curve the opening away from the central portion of the core thereby obtaining a stronger reflection of the neutrons entering the gap and preventing the irradiation of the inner graphite areas. Further advantageous embodiments of the invention concern the design of the gap geometry with parallel gap surfaces or with surfaces expanding from the inside out. The latter characteristic takes into consideration the aforementioned deformation, which appears primarily on the surface of the nose stone facing the core and which in the case of extended irradiation times, i.e., over the entire period of operation, is manifested by a volume increase.