Patent Number: 047956070
Section: description

Referring now to the FIGURE of the drawing, there is shown a cylindrical core 1 of a gas-cooled high-temperature nuclear reactor formed of a non-illustrated bed of numerous spherical fuel assemblies, which enter into the core 1 through a loading tube 2 and leave the core 1 through a discharge tube 3. The core 1 is surrounded initially by an inner layer of numerous graphite reflector blocks 4 which have a circular-sector or ring-segment shape or also a trapezoidal cross section and are surrounded by an outer layer of insulating stacked carbon blocks 5 which likewise have a circular-sector or ring-segment shaped cross section and are held together by a steel receptacle or jacket 6. The steel receptacle 6 is cooled by numerous vertical U-shaped tubes 7 which are uniformly distributed around the circumference thereof. In the two legs of the respective U-tubes 7, natural circulation is produced due to the different temperatures; this circulation leads the cooling water to an otherwise non-illustrated heat exchanger which is arranged outside the primary system of the reactor. The hot leg of these U-tubes is connected in heat transfer relationship to the steel receptacle 6 in order to absorb the after-heat from the reactor core. The graphite blocks 4 of the side or lateral reflector contain, in the vicinity of the core, a first set of several vertical channels 8, uniformly distributed over the circumference, for containing non-illustrated control rods, which can be inserted into the reflector from the top through guide tubes 9. These guide tubes 9 are spaced from one another by a ring plate 10. The ceiling of the reactor is formed, like in the AVR, of several layers of graphite or carbon blocks which are arranged on top of one another, have a sector-shaped cross section and are supported by the graphite blocks 4 and the carbon blocks 5 of the side or lateral wall. The uppermost blocks 11 of the reactor ceiling or cover are formed of carbon blocks and are closed except for holes provided for the absorber rod guides 9. The blocks 12, 13 and 14 of graphite which are located underneath are formed not only with a second set of channels 15 and the first channels 8 already provided in the reflector blocks 4, for receiving control rods therein, but also with several radial slots 16, through which the cooling gas enters into the core space. The blocks 11, 12 and 13 respectively support a rotary part 17 of graphite which is disposed in the longitudinal axis of the reactor and in which the loading tube 2 is guided. In the bottom of the reactor, the discharge tube 3 is surrounded by several bottom blocks 18 and 19 which form a funnel-shaped bottom and are formed with numerous vertical holes through which the hot coolant flows off through an intermediate space supported by columns 20 and through an annular space 21 surrounding the discharge tube 3. The receptacle 6 is provided with a base plate 22 which is formed with several holes 23 uniformly distributed over the circumference thereof and a central opening 24 which is protected from the emerging hot gas by insulation 25, which is not otherwise described in detail. The gas entering through the holes 23 flows through an second channels 15 vertically upwardly, then through an intermediate space between the blocks 11 and 12 and through the slots 16 in the blocks 12, 13 and 14 down into the core 1. In normal operation, the side or lateral and ceiling or cover reflector are cooled. In the normal shutdown procedure of this reactor, the control rods are inserted and the blower power is throttled; in the process, the temperature in the core rises and the core becomes subcritical due both to the high negative temperature coefficient of the reactivity and to the inserted rods. Thereafter, the reactor can be cooled down with reduced blower power to the cold, subcritical state. If the blowers fail, the reactor goes into the hot subcritical state due to the rising temperature, even without the control rods having been inserted. The heat stored in the fuel elements and the after-heat newly produced in them is thus distributed over the core and the reflector which subsequently gives off the heat through the insulation of carbon blocks 5 to the U-tubes 7. As long as the operating pressure in the primary loop of, for example, 50 bar is maintained, the heat exchange in the core is aided by internal convection. If the pressure drops, i.e. for example below 10 bar, this convection is no longer of great importance, so that the entire heat must and can be relinquished through conduction and radiation from the core, via the reflector and the insulation, to the U-tubes 7. The following table contains the main design data of the illustrated reactor. ______________________________________ Thermal power output MW 125 Height of core m 6.0 Diameter of core m 3.0 Average power density of the core MW/m.sup.3 2.94 Reflector thickness m 0.75 Single-zone core OTTO(once through, then out)loading Type of cycle Uranium/Plutonium Heavy-metal loading/fuel element g 7 Burn-up GWd/t 40 Average exit temperature of coolant .degree.C. 750 Average entrance temperature of coolant .degree.C. 250 Number of reflector rods 20 Thickness of reflector rods cm 8 Reactivity data: Temperature effect (20.degree. C. to 750.degree. C.) % 6 Xenon effect % 3 Maximum water break-in % 1 Effectiveness of reflector rods % 12 Effectiveness of one reflector rod % 0.5 Initial enrichment % 5 Temperature coefficient: Equilibrium core k/.degree.C. -7 .multidot. 10.sup.-5 Xenon-free core k/.degree.C. -10 .multidot. 10.sup.-5 Fuel element dwelling time d 500 Conversion rate 0.4 Radial temperature difference K 120 Maximum fuel element temperature in the .degree.C. 850 equilibrium core Maximum fuel element temperature after shutdown: Reactor at pressure .degree.C. 1200 Loss of pressure accident .degree.C. 1400 Thickness of carbon block insulation m 0.25 ______________________________________