Patent Number: 047770120
Section: summary

The invention concerns a gas cooled high temperature reactor charge with spherical fuel elements with a reflector surrounding the pile of fuel elements on all sides with a hot gas collector compartment adjoining the bottom part (bottom reflector) of said reflector consisting of graphite columns, the hot gas collector compartment being bounded in the downward direction by the bottom layers of graphite blocks of the high temperature reactor with a charging means and a removal means for the spherical fuel elements wherein the removal means comprises at least one ceramic pebble removal tube passing through the bottom reflector and the bottom layers and a metal pebble removal tube ajoining the ceramic pebble removal tube underneath the bottom layers. High temperature reactors having a core formed by a pile of spherical fuel elements that are continuously added to the core and removed after their burnup from the core are part of the state-of-the-art. Nuclear reactors of this type have very high gas temperatures in their lower parts, wherefore, only high heat resistant materials may be used in the bottom reflector, the bottom layers and the part of the pebble removal installation located within the core structure. These materials are thus made of a ceramic material, such as graphite, but graphite cannot be exposed to high tensile and bending stresses. The forces of the reactor core are thus transmitted to a thermal shield made of a metal, preferably a cast metal, which is surrounding the entire reflector. In the removal installation for the spherical fuel elements only the part of the removal tubes located within the space enclosed by the thermal shield is made of a ceramic material. The continuing pebble removal tubes connecting the ceramic removal tubes with a scrap separator are made of metal for the sake of higher strength. It has now been found that even the metal pebble removal tubes are exposed to high temperatures in the areas adjoining the ceramic removal tubes. This is the result of the fact that the fuel elements standing in front of the ceramic removal tubes and located in them are not completely burned up and are thus still generating heat. The temperature in the above-described area may thus be reduced only by eliminating or limiting the flow of neutrons in the ceramic pebble removal tubes. It is known in the prior art (West German Offenlegungsshrift No. 23 47 817) to provide neutron absorbing materials in a nuclear reactor with spherical fuel elements and a single passage of the fuel elements through the reactor core in the roof reflector and/or the upper part of the side reflector in order to reduce the flow of fast neutrons in the areas exposed to doses of radiation of the reflector, thereby preventing damage to the graphite of the reflector. There is also known a process for the operation of a high temperature reactor of the above-described structural type (West German Offenlegungsschrift No. 22 41 873) whereby the output distribution in the reactor core may be affected so that the axial output density declines only slightly in the downward direction. Such a mode of operation permits a higher load on the fuel elements which is utilized either for an improvement in the economy of the nuclear reactor or to increase its safety. A series of measures have been proposed to obtain this result, among others the addition of combustible poisons to the loading charge, whereby the generation of power output is transferred to the lower reaches of the reactor core. The combustible poisons may also be provided in the roof, bottom and/or side reflectors. It is further known (West German Offenlegungsschrift No. 23 65 531) to improve the shutdown efficiency of absorber rods directly insertable in a nuclear reactor with spherical fuel elements and directly insertable absorber rods by a special measure without increasing the number of absorber rods or their depth of insertion. This measure consists of doping the graphite of the bottom reflector and the lower part of the side reflector with neutron absorbing substances. This thus represents a quasi-homogeneous poisoning of the lower range of the reflector. It has the disadvantage that the neutron absorbing substances are burning up in a very short period of time and their effectiveness is, therefore, inadequate for the life time of a nuclear reactor estimated at approximately 30 to 40 years. Based on this state-of-the-art, it is the object of the invention to design a gas cooled high temperature reactor of the above-described type so that the temperature is sufficiently lowered from the core bottom to the metal pebble removal tubes so that the thermal exposure of the last-mentioned removal tubes remains within permissible limits. This object is attained according to the invention by that each ceramic pebble removal tube within the area of the bottom reflector and the hot gas collector compartment is surrounded directly by a ring of graphite columns, wherein a plurality of recesses, a number of boron bodies is arranged. The boron bodies reduce the neutron flux in the area affected to the extent that there is no more output by the fuel elements. A further number of boron bodies may be provided within the area of the bottom layers of the high temperature reactor to reinforce the absorption of neutrons. The boron bodies are arranged preferably directly outside each ceramic pebble removal tube. As a solution extremely favorable from an economic standpoint, the boron bodies are designed in the graphite columns in the form of rods and inserted into vertical bores adapted to their shape and size. Advantageously, the length of the boron rods is determined so that the graphite columns are almost entirely penetrated by the boron rods. Preferably, the boron rods are arranged in at least two rows around each ceramic pebble removal tube and the boron rods belonging to different rows are staggered so that they always face a gap. The boron bodies located in the area of the bottom layers may advantageously consist of solid plates arranged so that each ceramic pebble removal tube in this area is completely or almost completely surrounded by them. The form of the boron bodies chosen and the mode of their location effect a gradual decline of the temperature in the direction of the metal pebble removal tubes, so that no or only slight thermal stresses are generated in the structural parts of the high temperature reactor. Furthermore, the boron rods and boron plates are readily built into the corresponding graphite structural parts (graphite columns or bottom layers). The realization of the invention thus does not require a substantial effort.