Patent Number: 047599014
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a nuclear reactor installation arranged in the cavity of a pressure vessel, with a nuclear reactor, the core, surrounded by a thermal side shield, which is being traversed from top to bottom by a cooling gas, with a plurality of main loops, each containing within the pressure vessel a heat exchanger and a blower, together with two gas conduits to connect the said components with the reactor core, and with a plurality of auxiliary loops for the removal of decay heat, wherein each auxiliary loop is connected by means of two gas conduits with the reactor core. In nuclear reactor installations, rapid shutdowns (insertion of absorber rods) are followed in the core by the development of decay heat, which initially amounts to approximately 4% of the nominal thermal capacity. This decay heat development declines rapidly at first and then slower and in its later variation approaches a value of zero asymptotically. In order to prevent an excessive rise in temperature in the reactor core, the decay heat must be removed from the core. 2. Description of the Prior Art It is part of the state of the art to equip nuclear reactor installations of higher capacities with auxiliary loops for the removal of decay heat, each of which contains auxiliary heat exchangers and auxiliary blowers. The auxiliary heat exchangers are connected by means of gas conduits with the hot gas and cold gas collector spaces of the reactor. During normal operation, the auxiliary heat exchangers are closed, for example by gravity actuated butterfly valves on the auxiliary blowers, which causes a slight backflow of cold gas over the auxiliary heat exchangers. No hot gas can therefore pass by free convection in the auxiliary heat exchangers. In high temperature reactors at least two auxiliary loops are provided for the removal of decay heat; they may be operated independently of each other and independently of the main loops. Because these systems are important from the standpoint of safety engineering, there are very high requirements concerning their operating availability and safety. In German Offenlegungsschrift No. 31 41 734 and 32 26 300, and in German P No. 33 44 527.3 high temperature reactors equipped with such auxiliary loops are described. Every installation for the removal of decay heat comprises: (1) an intermediate cooling water loop, wherein by means of a circulating pump, water is circulated through the auxiliary heat exchangers; (2) an intermediate cooler, in which the water transfers its heat to a service cooling water loop; and (3) the auxiliary loops with the auxiliary heat exchangers and the auxiliary blowers for the cooling gas. In the intermediate cooling water loop, a further circulating pump and a cooling tower are arranged. The intermediate cooler, the circulating pumps and the service cooling water loops are located outside the reactor pressure vessel. One disadvantage of the known decay heat removal installations is that it is composed of several active components, such as pumps. A second disadvantage is if a tube fractures in the auxiliary water exchangers, which must be anticipated, a large volume of water (up to several m.sup.3) may enter the reactor core. Furthermore, the auxiliary heat exchanger involved in no longer available for the removal of decay heat because it must be shut off as soon as possible. SUMMARY OF THE INVENTION In view of the above described state of the art, it is the object of the invention to develop a nuclear reactor installation of the above described structural type so that the disadvantages of the known decay heat removal facilities may be avoided. According to the invention, this object is attained in that each auxiliary loop comprises a bundle of independent, parallel heat pipes, together with a cooling gas blower, with the heat absorbing part of the heat pipes being arranged in an interruptable flow of cooling, and by an external cooling water system operated by means of a circulating pump which is provided as a heat sink for each bundle of heat pipes, with a cooling tower being located in a known manner in each loop. Compared to the known nuclear reactor installations, the plant according to the invention has the advantage that the installations for the removal of decay heat have a high degree of availability. First, if a tube fractures, which must be anticipated, only a limited, small amount of water may enter the reactor core. Thus, an additional expensive leakage monitoring system becomes superflous. Second, if there is a leak in a bundle of heat pipes, this would affect only one heat pipe and therefore, the removal of the decay heat is not affected because a bundle consisting of independent individual heat pipes provides a high degree of redundancy. In contrast, in case of a leak in a heat exchanger, the entire auxiliary loop is affected. The high availability of the decay heat removing installations is further enhanced by the fact that they are capable of operating with few active components, such as circulating pumps, since one cooling water loop per installation has been eliminated. The heat transmitting part of the heat pipes of each bundle terminates preferably in a reservoir filled with water, located above the pressure vessel, to which the cooling water system involved is connected. According to an advantageous further development of the invention, the reservoirs are equipped with at least one evaporator line for the water and are sufficiently large enough to be capable of removing the decay heat by evaporation for a certain period of time in case of a failure of the external active cooling water loop. In an emergency therefore the removal of the decay heat is possible without the use of active components. If in the nuclear reactor installation, the heat exchangers of the main loops are located in an annular space formed by the wall of the cavity and the thermal side shield, it is advantageous to arrange the heat absorbing part of the heat pipes also in this annular space. The part of the heat pipes, which transports the heat absorbed to the heat transfer part, is installed in bundles in a vertical passage located in the roof of the pressure vessel.