Patent Number: 048266523
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As seen in FIG. 1, an underground cylindrical pressure vessel 1 of reinforced concrete encases a cavity 2. A removable cover 3 closes off the opening of the pressure vessel. A nuclear reactor 4 with a stationary cores (also referred to as piles or pebble-beds) of spherical fuel elements 6 is housed in the cavity 2. The fuel elements 6, which are manufactured by the hot or cold press method, comprise a heavy metal load, enabling extended retention time in the core. The core 5 is enclosed on all sides by a graphite reflector 7. The reflector includes a large proportion of spherical pure graphite elements 8 arranged in a pile. The graphite elements 8 may have the same diameter as the spherical fuel elements 6. The graphite reflector 7 has a solid, outer reflector jacket 29, an inner portion 10 comprising the graphite pebbles 8, a bottom reflector 9 of graphite pebbles 8, and a roof reflector 11, also made of graphite pebbles 8. The roof reflector graphite pebbles 8 rest directly on the fuel elements 6. A free space 12 is located between the roof reflector 11 and the cover 3. Another free space 13 is provided between the bottom reflector 9 and the bottom of the pressure vessel 1. This space contains a metal support structure 14 for supporting the nuclear reactor 4 on the bottom of the pressure vessel. A cooling gas, preferably helium, flows downward through pile 5 of fuel elements. The gas is circulated by a blower 15. The blower 15 is mounted in a centered position on the bottom side of the cover 3 with its rotor located in the free space 12. The drive motor 19 for the blower 15 is installed in a passage 16 of the cover 3, said passage being equipped with an external closure part 17. This facilitates the maintenance of the motor and the blower. The fuel element pile 5 is enclosed laterally and on the bottom by a metal core vessel 18, which also houses the entire poured graphite reflector 7, i.e., the solid outer jacket 29 is located outside the core vessel 18. The core vessel 18 has the configuration of a cage open at the top, the cylindrical side wall and bottom of which consist of lattice work or perforated sheet metal. It is capable of supporting the entire weight of the graphite pebbles 8 and the fuel elements 6. The mesh of the lattice or the holes of the sheet metal are dimensioned so that no graphite pebbles 8 may leave the cage. The bottom of the core vessel 18 exhibits a closeable opening 28, through which the fuel elements 6 and the graphite pebbles 8 may be removed. A plurality of uniformly distributed cylindrical sleeves 20 may be mounted on the inner wall of the core vessel 18. The sleeves are made of the same mesh or perforated sheet metal, extending approximately over the entire height of the core vessel 18 and serving as vertical channels for the absorber rods 21. The absorber rods 21 are located displaceably in the sleeves 20. The driving means 22 for the absorber rods 21 are located in passages 23 of the cover 3. The passages 23 are closed off by covers 25. The absorber rods 21, which thus are located inside the side reflector 10, are intended only for trimming or shutdown purposes. The reactor output regulates exclusively the speed of the blower 15 and the secondary flow of a cooling system (described below), utilizing the stabilizing property of the negative temperature coefficient. Active controls by the absorber rods 21 may therefore be eliminated. The trim and shutdown rods 21 in combination with burnable neutron poisons (for example, gadolinium) serve to bind any excessive initial reactivity. The variations of excess reactivity occurring during the operation of the reactor are compensated by the displacement of the trim and shutdown rods 21. In the process, the trim and shutdown rods 21 are displaced gradually and intermittently manually or by hand. No regulation or automation is necessary for these slow reactivity variations. Short term fluctuations of the fuel element temperatures are tolerated in view of the high temperature strength of the ceramic elements 6 over a relatively wide range without difficulty. The core vessel 18 together with the graphite reflector 7 (with the exception of the solid outer jacket 29), the fuel element 6, and the absorbers 21 may be removed following the removal of the cover 3, in the upward direction. A shielding bell may be used in the process. The absorber rods 21 assure the subcriticality of the pile 5 in the course of the installation and removal process. The core vessel 18 is removed when the fuel elements 6 are sufficiently burned up. A cooling system 24 is mounted over the entire inside of the pressure vessel 1. The system may preferably comprise pipes through which cooling water flows which are laid out so that the heat generated by the pile 5 can be safely removed, both in power operation and when removing the decay heat. The gas pressure in the primary loop is higher than the pressure of the medium in the cooling system 24 in order to prevent intrusion of water into the primary loop. A gas conduction jacket 26 is provided in the free space 12 to separate the suction and compression side of the blower 15. It is connected with the upper end of the core vessel 18. The reactor output is regulated exclusively by the speed of the blower 15 and the secondary flow of the cooling system 24, wherein the negative temperature coefficient inherent in the pebble-bed reactor is utilized. The blower 15 suctions the cooling gas, the pressure of which, in normal operation, is approximately 8 to 10 bar, from the free space 12 and transports it into the pile 5. In the course of the coolant gas flow through the pile, the temperature of the gas increases from approximately 300.degree. C. to 500.degree. C. The heated cooling gas enters the free space 13 through the bottom reflector 9, where it is distributed and conducted into an annular space between the inside of the pressure vessel and the core vessel 18. From here the cooled gas flows along the outside of the gas conduction jacket 26 to the blower 15. The absence of water carrying components in the primary loop and the fact that no fuel elements 6 are added in the course of the operation, together with the condition that no impurities are passed in the primary loop in any other manner, enable the elimination of gas purification installations. Furthermore, no charging apparatus, reactor protection systems or active control systems are required for the nuclear reactor 4 and therefore are not provided. Consequently, the nuclear reactor 4 has very low energy production costs and the required maintenance effort is low. The decay heat developed in the nuclear reactor 4 may be removed securely even in case of accidents. In case of a failure of the blower 15, the decay heat is removed by natural convection to the cooling system 24. The direction of the flow of cooling gas in the fuel element pile is reversed. This does not, however, result in a risk of overheating the blower 15 and its drive motor 22. A potential pressure rise in the primary loop may either be taken into account in the layout of the primary loop or compensated for by the flow of the cooling gas into gas reservoirs. The cooling system 24 is laid out so that a volume of the cooling medium sufficient for the removal of the decay heat is circulating through the pipes of the cooling system 24. In the case of a pressure release accident the decay heat is again transferred to the cooling system 24 by heat conduction through the graphite reflector 7 and by thermal radiation by the graphite reflector 7 to the cooling system 24. Here again the temperatures prevailing in the core of the reactor 4 during normal operation are not significantly exceeded. Even if the cooling system 24 fails, the decay heat is safety removed without damage to the fuel elements or the release of activity from the fuel elements 6. Decay heat removal in this case is effected by conduction through the pressure vessel 1 into the surrounding ground and into the atmosphere. If a steel or prestressed concrete vessel is used as the pressure vessel 1, the conduction of heat may be favorably affected by the specific layout of the steel reinforcements. The simple configuration renders only a very slight surveillance effort necessary. FIG. 2 shows the entire nuclear reactor plant with the pressure vessel 1 resting on a foundation 31 arranged underground in a cavity 40 with a concrete cover 32 and a hall 33 built of light structural components. The concrete cover 32 protects the nuclear reactor 4 in combination with its placement underground against external effects. The hall 33 comprises a door 36 and is divided into a workshop and operating room 37 and a room 38 for the installation and removal of the core vessel 18. Rails 35 upon which a crane 34 runs are provided in room 38 for this purpose. The cavity 40 is lined with concrete. The intermediate space 39 between the wall of the cavity and the pressure vessel 1 is monitored for leakage and activity. A slightly reduced pressure may be established optionally with respect to the environment. Potential leakages are removed discontinuously in a programmed manner.