Patent Number: 048470406
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a reactor protection building 1 enclosing a prestressed concrete pressure vessel 2. The pressure vessel defines a reactor cavity 3 clad with a metal liner 4, containing a gas cooled high temperature reactor 5. The core 6 of the reactor contains spherical fuel elements. The reactor cavity 3 also contains several steam generators 7 for operational heat removal, and at least two auxiliary heat exchangers 9 for removal of decay heat. The cooling gas, which flows downward through the reactor core is circulated in normal operation by blowers 8. Blowers 10 circulate the cooling gas in the decay heat removal mode. A shutdown system 11 is provided for control and shutdown of the high temperature reactor 5. The shutdown may include a plurality of absorber rods insertable into the reactor core 6. A thermal protection system made up of a thermal insulating layer 12 and a liner cooling system 13 is arranged on the inside of the prestressed concrete pressure vessel 2. The liner cooling system contains a plurality of cooling pipes 14. Water flows through the cooling pipes which are part of a closed intermediate cooling loop 15 (shown in subsequent figures). The liner cooling system 13 and the intermediate cooling loop 15 are laid out in a manner and have sufficient capacity such that they are capable of removing all of th decay heat in case of a failure of the auxiliary heat exchanger 9. FIG. 2 shows the prestressed concrete pressure vessel 2 with the (highly schematically drawn) liner cooling system 13 and the intermediate cooling loop 15, connected by a forward or feed line V and a return line R connect the intermediate cooling loop 15 to the liner cooling system 13. The intermediate loop 15 further includes a plurality of intermediate heat exchangers 16 each with a corresponding cooling water pump 17 (only one of each is shown in the figures). The intermediate heat exchanger 16 is located in a position elevated above the upper edge of the prestressed concrete pressure vessel 2 in order to have available a sufficiently large driving pressure difference. The driving pressure is a function of the height H. A bypass line 18 is provided for every cooling water pump 17. A check valve 19 is located in the bypass line. The check valve may be actively controlled by the speed of the pump. The valve may be set so that it opens at a number of rotational speed equal to or less than 100 rpm. A supplemental pump 20 may be connected in parallel with every cooling water pump 17 in order to assure an adequate driving pressure difference even in case of a power failure (resulting in the deactivation of the cooling water pump 17). The supplemental pump may have a significantly lower capacity than the main pump and is connected to the emergency power system. The liner cooling system 13 is divided into several cooling pipe zones of differing elevations available for the creation of a driving pressure difference within the protective reactor building. At least one cooling pipe zone in a lower height location is connected to at least one zone in a higher location. Each of FIG. 3 and 4 shows an alternative of the interconnection of different cooling pipe zones. The liner cooling system is divided into individual zones. In order to facilitate natural convection through the various zones it is important that zones of differing elevations be connected serially rather than in parallel. As can be seen in FIG. 2 the height differential between a roof reflector zone and the heat exchanger may be minimal due to required layout within the pressure vessel. By connecting the roof reflector zone to a zone of lower elevation, such as a side reflector zone, the effective height differential is greatly increased (to H) thereby increasing the driving pressure to the roof zone and facilitating natural convection when needed. Driving pressure and thus natural convection is further facilitated by a vertical "hot-strand". If the height of the "hot-strand" is insufficient there will not be an adequate driving pressure. For this reason at least the horizontal roof reflector zones are serially coupled to lateral wall zones thus establishing a sufficient "hot-strand" to drive natural convection. During normal operation of the facility the parallel zone layout enhances performance by favorable impacting design considerations, i.e. redundancy, reliability, lowering pressure differential etc. FIG. 3 shows direct coupling of individual cooling pipe zones, i.e. the interconnection of the cooling pipe zones has been effected in the course of the layout of the liner cooling system 13. As seen in FIG. 3, the liner cooling system 13 is divided into six zones I . . . VI, of which zones IV and V impact the roof with a lower .DELTA.P.sub.tr, the cooling pipe zones IV and are coupled with the cooling pipe zones II and III. In this embodiment the check valve 19' of the intermediate cooling loop 15 is controlled passively by the pressure difference applied to the cooling water pump 17. For the purpose, the check valve 19' is moved into its closed position upon the start-up of the cooling water pump 17 by a single electromagnetic impulse (for example the starting current) and remains closed due to the aforementioned pressure difference in normal operation. In case of a failure of the cooling water pump 17, the pressure difference is reduced approximately to 0, and the check valve 19' is opened without any additional energy by gravitational force due to its own weight only. FIG. 4 shows an embodiment where cooling pipe zones I . . . VI are interconnected by valves 22 and 22' accessible from the outside. A passively controlled check valve 19" is provided in the intermediate cooling loop 15. This check valve may be closed by an electromagnetic impulse and remains in this position under the effect of the pressure difference applied to the cooling water pump 17. However, the check valve 19" is opened here in case of the failure of the cooling water pump 17 not by gravity, but by a tension spring 21, which is stressed when the valve is close and is released when the pressure difference is eliminated. In the embodiment shown in FIG. 4 the liner cooling system 13 is laid out in the planning stage so that it is possible to short-circuit the feed lines V1, V2 . . . V5 and the return lines R1, R2 . . . R5 of the cooling pipe zones concerned in case of an accident, by means of the valves 22, 22'. Thus for example the cooling pipe zones II and V or III and IV, which in normal operation are supplied individually, may be interconnected by valve actuation in the following manner: interconnection of cooling pipe zones II and V by connecting the return R2 and the forward line V5 and interconnection of the cooling pipe zone III and IV by connecting R3 with V4. During normal operation all valves 22 are open and valves 22' are closed. In order to establish natural convection valves 22 are closed and 22' are opened thereby establishing serial connection between zones II and V, and zones II and IV. FIG. 5 shows an installation for decay heat removal by natural convection, in addition to the liner cooling system 13 with the intermediate loop 15. The natural convection installation is actuated when the intermediate heat exchangers fail. The installation is made up of a water reservoir 23 with a nitrogen cushion, connected with the forward line V of the liner cooling system 13, a vertical boiling tube 24 located in the intermediate cooling loop 15 and being placed geodesically higher than the liner cooling system 13 and connected to the return line R of the liner cooling system 13, a water separator 25 with a nitrogen cushion, also located in the intermediate cooling loop 15 and connected to the vertical boiling tube 24, and a safety blow-off valve 26 mounted on the water separator 25. The nitrogen cushion in the water reservoir 23 and the water separator 25 has a pressure of-.gtoreq.1.5 bar. An embodiment of the water or steam separator 25 is illustrated in FIG. 6. The steam separator is made up of a horizontally aligned water-steam drum 40. One or more vertical boiler tubes 24 are connected to the bottom of the drum. The tube diameter is large enough to remove the quantity of steam generated in the liner cooling system. Baffles or diverting plates 41 may be built into the drum to prevent escape of water through steam outlet 42. In order to maintain appropriate pressure in the steam separator (and the water reservoir 23) a volume of nitrogen, referred to as a cushion is present in the freespace above the water surface. The installation operates in the following manner: The safety blow-off valve 26 is set for example at 2 bar. In normal operation the nitrogen cushion of .gtoreq.1.5 bar and the setting of the safety blow-off valve 26 establish a pressure of between 1.5 bar and 2 bar at the uppermost point of the intermediate cooling loop 15 (water separator 25). The water separator 25 prevents the release of a water and steam mixture by the blow-off valve 26 upon the occurrence of bubble boiling and thus the rapid emptying of the intermediate cooling loop 15 without the complete utilization of the heat of evaporation. Natural convection in the intermediate cooling loop initially takes place in the single phase zone. Up to a temperature of approx. 111.degree. C. and a saturation pressure of 1.5 bar, the pressure in the water supply of the loop 15 is controlled by th nitrogen cushion at 1.5 bar. If the cooling water outlet temperature continues to rise, the corresponding saturation pressure determines the pressure in the water separator 25 (a check valve 28 provided on the water separator 25 for the N.sub.2 supply, closes). At a water outlet temperature of 120.degree. C.=2.0 bar saturation pressure boiling begins on the surface in the water separator 25. At a continued rise of the water outlet temperature to approx. 130.degree. C., the onset of boiling is shifted to the lower end of the vertical boiling tube 24, which has a height of about 5 to 10 m. If the amount of heat supplied by the liner cooling system 13 is larger than the amount of heat discharged in the boiling tube 24 by evaporation, the onset of boiling is also displaced to the geodesically lower areas of the liner cooling system 13. To increase the evaporation energy contained in the intermediate cooling loop 15, the water reservoir 23 is provided. The latter may have an open or closed configuration. FIG. 5 shows a closed water reservoir 23. In this case the reservoir must be brought to the pressure of the water separator 25 by a connecting line 27. The geodesic height of the water reservoir 23 must be such that the water levels in the water separator 25 and the water reservoir 23 may establish themselves at the same height. In the downward direction, an extension of the water reservoir 23 to the lowest point of the liner cooling system 13 is useful and appropriate. A battery of standing pipes is used conveniently as the water reservoir. While FIG. 5 illustrates a vertical cylinder for a reservoir 23, any configuration may be used as long as the water level in the reservoir 23 is the same as that in the water or steam separator 25. If the water reservoir has an open configuration, it must be at a geodesic height such that in case of a rise of the pressure in the intermediate cooling loop 15 to the actuating pressure of the safety blow-off valve 26, the blowing of the intermediate cooling loop 15 though the open water reservoir is prevented (not shown). With an actuating pressure of the safety blow-off valve 26 of for example 2 bar, the water level in the water reservoir must therefore be at least 10 m above the water level of the water separator 25. If these requirements relative to the geodesic height of a water reservoir cannot be satisfied (for example because reactor protective building is too low), a closed water reservoir must be used.