Patent Number: 048470406
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a nuclear power plant with a gas cooled high temperature reactor, and more particularly to a reactor with a spherical fuel element core and a prestressed concrete pressure vessel surrounding the high temperature reactor. The reactor cavity is clad with a liner and contains a plurality of heat exchangers, preferably steam generators for operational heat removal. At least two auxiliary heat exchangers for decay heat removal are also arranged in the reactor cavity. The prestressed concrete pressure vessel has a thermal protection system including a thermal insulating layer and a liner cooling system comprising a plurality of cooling pipes through which water flows and which, together with intermediate heat exchangers and cooling water pumps, a closed intermediate cooling loop used for the removal of the decay heat in case of an auxiliary heat exchanger failure. 2. Description of the Related Technology U.S. Pat. No. 4,554,129 shows a gas cooled nuclear reactor installed in the cavity of a prestressed concrete pressure vessel. The heat generated in the reactor core is transferred to several heat exchangers located in the cavity above the nuclear reactor. No auxiliary heat exchangers are provided for the removal of the decay heat. Decay heat is removed by a liner cooling system made up of cooling pipes welded to the cavity liner inlet and return lines connected to the cooling pipes and a plurality of circulating pumps. Heat is transported from the reactor core to the liner cooling system by central pipes provided inside the heat exchanger and by shut-off valves closed in normal operation in a annular space bordering on the liner. A natural downward directed flow of heat is established in the annular space, so that all areas of the liner are exposed to the cooling gas. DE-OS No. 33 35 268 shows a high temperature reactor with spherical fuel elements. Steam generators and blowers for operational removal of heat and auxiliary cooling systems for the decay heat removal in case of accidents are located together with the other components of the primary loop in the cavity of the prestressed concrete pressure vessel. The cavity is provided with a liner exhibiting a cooling system. This liner cooling system may be used for the removal of the decay heat, should the auxiliary cooling systems fail. It has also been proposed to conduct the decay heat to the liner cooling system by natural convection without coolant loss in case of accidents. DE-OS No. 31 21 377 shows a liner cooling system for a prestressed concrete pressure vessel intended to house a nuclear reactor and containing an interior thermal insulating layer. The liner cooling system is redundant, i.e. it consists of several mutually independent water circulating loops. SUMMARY OF THE INVENTION It is an object of the invention to provide a nuclear power plant with an adequate mass flow of water in the liner cooling system for removal of decay heat in any situation. According to the invention, this object is attained by the following characteristics: (a) Arranging intermediate heat exchangers above the upper edge of the prestressed concrete pressure vessel (b) Dividing the liner cooling system into several cooling pipe zones according to vertical or height positions wherein at least one cooling pipe zone in an upper zone range (by height) is always connected to a zone in a lower location; (c) Arranging a supplemental pump connected to an emergency power system in parallel with each primary cooling water pump, where the supplemental pumps may have a significantly lower capacity; (d) Providing every cooling water pump with a bypass line with an actively or passively controlled check valve, where the check valve opens if the cooling water pump is inoperative and; (e) Providing an installation for removal of the decay heat by natural convection in case of a failure of the intermediate heat exchangers connected to the liner cooling system. The natural convection decay heat removal system includes: a water reservoir with a nitrogen cushion connected to the forward or feed line of the liner cooling system, a vertical boiling tube, placed in the intermediate loop, connected to the liner cooling system return line and located geodesically higher than the liner cooling system, a water separator located in the intermediate cooling loop, connected to the vertical boiling tube and including a nitrogen cushion, and a safety blow-off valve connected to the water separator. Adequate flow through the liner cooling system requires an appropriate pressure forcing the water through the system. The amount of pressure is referred to as a "driving pressure difference" or .DELTA.P.sub.tr. The decisive factor for the presence of an adequately high water mass flow in the liner cooling system without the use of active aggregates, such as pumps or blowers, is a sufficiently large driving pressure difference .DELTA.P.sub.tr in the intermediate cooling loop. The driving pressure difference is proportional to the product .DELTA..rho..g.H (.DELTA..rho.=difference in density, H=height). According to invention, the necessary driving pressure difference .DELTA.P.sub.tr is provided by acting on the factor H, i.e. by the geometric layout of the intermediate heat exchangers, and by an appropriate interconnection of cooling pipe zones of differing heights. The latter measure is required, as the reactor protection building enclosing the prestressed concrete pressure vessel only has a limited height available for the intermediate heat exchangers. According to the invention, in case the cooling water pumps are not operating due to a power failure, the necessary driving pressure difference .DELTA.P.sub.tr is assured by supplemental pumps connected in parallel to the cooling water pumps and operated with emergency power. No problems arise from the connection to the emergency power system as the supplemental pumps have capacities lower by one to two orders of magnitude than the normal operation cooling water pumps in view of a permissibly higher cooling water heating range and correspondingly lower water flow rates. The reduction of pressure losses in the intermediate cooling loops leads to an increased water flow rate. High pressure losses can occur in case of a failure of a the cooling water pumps; that is exactly a situation where an adequate flow of water is absolutely necessary since heat may be removed from the liner cooling system only by natural convection. High pressure losses of inactive cooling water pumps are reduced to a minimum by the proposed bypass lines with check valves. In case the intermediate heat exchangers of the intermediate cooling loop fail, a device connected to the liner cooling system is activated, whereby the decay heat is removed by the heat of evaporation contained in a water reservoir through a safety blow-off valve. The advantage of the invention lies in that decay heat can be removed safely even in case of a failure of the auxiliary heat exchangers and the intermediate heat exchangers and also in the case of a power failure, so that any impermissible heating of the primary loop components is prevented and the risk of the release of activities (radioactivity) reduced. The higher density differences of cooling water present in the liner cooling system may be utilized advantageously to increase the water flow rate by natural convection in the rare case of accidents, as a result of the high, acceptable, concrete and liner temperatures during removal of decay heat. An increase in temperature of 200.degree.-300.degree. C. over normal operating temperature due the thermal capacity. The temperature rise will result in a considerable rise in density difference .DELTA.P.sub.tr of the cooling water above the normal operation level. The driving pressure difference .DELTA.P.sub.tr is also raised in the process. In a single phase heat removal process in the liner cooling system the temperature of the cooling water may be raised to slightly below the boiling point. A further increase of .DELTA..rho. may be obtained by two-phase heat removal (bubble boiling) in the liner cooling system. An increase in the flow of water in the liner cooling system may be achieved by reducing pressure losses in the intermediate cooling loop. Pressure losses in the operation of decay heat removal is defined by: ##EQU1## wherein N=nominal operation NWA=decay heat removal operation PA1 .DELTA..delta.=heating range of the cooling water PA1 Q=volume of heat removed by the liner cooling system .DELTA.P.sub.P =pressure loss of the inactive cooling water pumps. It follows from this relationship that it is advantageous to choose a high heating range for the cooling water (to the boiling temperature or slightly thereunder). The ratio of the volumes of heat removed in nominal operation and in a decay heat removal operations are determined by the temperatures generated. The nominal pressure losses .DELTA.P.sub.N may be reduced by the appropriate layout of the liner cooling system and choice of the components of the intermediate cooling loop. The following measures may be effected to reduce nominal pressure loss extensive equalization of pressure losses in the individual cooling pipes, in particular reduction of peak values, or selection of intermediate heat exchangers with low pressure losses on the liner cooling system side. The composition of different cooling pipe zones of the liner cooling system may be established advantageously by direct coupling during the layout of the liner cooling system. Alternatively, the cooling pipe zones may be connected to each other in the decay heat removal operation by short circuiting the forward and return lines of the zones by externally accessible valves. The valves may be manually actuated. This so-called external coupling is taken into consideration in the layout of the liner cooling system. Check valves located in the cooling water pump bypass lines may be actuated in a number of ways: actively, controlled by the speed of the cooling water pump involved (for example, the check valve concerned remains closed at pump speed of equal to or higher than 100 rpm and opens if the velocity drops below that rate), or passively by the pressure difference applied to the cooling water pump involved. In the latter case the check valve concerned, which in normal operation is closed, may be opened by its own weight or the release of a spring. According to a further embodiment of the invention the decay heat removal capacity of the liner cooling system by natural and forced convection must be optimized against the thermal resistance of the thermal insulating layer so that the maximum permissible liner, fuel element and reactor installation temperature is not exceeded. Heat resistance corresponding to insulation thickness of the thermal insulating layer decisively affects a rise in temperature of structural parts located within the prestressed concrete pressure vessel in case of failure of decay heat removal by the auxiliary heat exchangers and the liner cooling system. The aforementioned component temperatures are calculated as a function of the thermal resistivity of the thermal insulating layer during layout of the thermal insulating layer for the case of a "failure of the decay heat removal installation". The optimum is achieved when the same safety margin from the maximally permissible limiting temperature is established for all of the structural components. The two extreme cases described below serve to demonstrate this condition in case of failure of the auxiliary heat exchangers and the removal of the decay heat through natural convection in the liner cooling system (this is also valid for forced convection, decay heat removal by the liner cooling system. If the insulating layer thickness is inadequate: Excessive heat is transported to the liner and in the liner cooling system and leads to exceeding the maximum permissible liner and concrete temperatures, however, in view of the effective removal of heat from the primary loop through the liner cooling system the temperature of the fuel elements and of the reactor installations remain far below its limiting value. If the thickness of the thermal insulating layer is too large: Inadequate decay heat removal from the primary loop results in fuel element and the reactor installation temperatures exceeding their failure limits due to an excessive cooling gas temperature while the liner temperature remains far below the permissible limiting value (and the heat removal capacity of the liner cooling system remains unutilized). According to a feature of the invention an optimization is effected for the liner cooling system alone (i.e. not for the entire thermal protection system) relative to the maximum removal of decay heat by natural convection and of a minimalization of the liner temperature respectively. The parameters used in for this optimization are the cooling pipe diameter and the cooling pipe spacing. The flow of cooling water and thus the amount of heat that may be removed in a natural convection liner cooling system depends on the choice of the cooling pipe diameter and the existing cooling pipe spacing. The flow of cooling water in a liner cooling system (flow per m.sup.2 of cooling water) is predetermined by the heating range. In order to increase the natural convection flow of the cooling pipe cross section must be enlarged proportionally to the spacing of the cooling pipes. This measure is limited by the maximum temperature of the liner which rises with increasing cooling pipe spacing. A variation of the cooling pipe cross section and the cooling pipe spacing leads to optimalization of minimizing the liner temperature and to maximizing the cooling water flow and liner cooling system natural convection heat removal capacity resulting in a reduction of the residual risk in case of a decay heat removal failure. This optimalization leads to a reduction in liner cooling pipe spacing (while maintaining cooling pipe diameter) or an increase in cooling pipe diameter and a correspondingly larger cooling pipe spacing or a combination of the two measures. Advantageously, the installation for the removal of decay heat, in case of a failure of the intermediate heat exchangers, the pressure of the nitrogen cushion in the water separator and the actuating pressure of the safety blow-off valve may be correlated in a manner such that heat removal from the intermediate cooling loop to a predetermined cooling water temperature is effected initially in a single phase range and passes into a two-phase range only when the saturation pressure in the water separator attains the actuating pressure the safety blow-off valve. Additionally the water reservoir may have an open configuration and be located geodesically high enough so that upon a rise of the pressure in the intermediate cooling loop to the actuating pressure of the safety blow-off valve a blow-off through the open water reservoir is prevented. Alternatively the water container may have a closed configuration and is pressure connects by a connecting line to the water separator and that its geodesic height is determined so that the same water level may be established in the water separator and the water reservoir.