Patent Number: 050283782
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a safety system for use in an extremely unlikely case of an accident in a gas cooled high temperature reactor, more particularly, for use in a reactor housed with a plurality of steam generators in a prestressed concrete pressure vessel. equipped with absorber rod shutdown means and a reactor protection system for actuation of emergency shut-down systems and decay heat removal. 2. Description of the Related Technology Nuclear reactors in general and gas cooled high temperature reactors in particular, are equipped with automatic regulating and control devices, in order to prevent or limit deviations from the planned operating behavior (operational control) and to place the plant automatically into a safe state if predetermined limiting values are exceeded. The latter is the purpose of the aforementioned reactor protection system actuating the emergency shut-down and an appropriate heat removal and aftercooling procedure. All of the cases of disturbance to be considered in the licensing procedure with a probability of occurrence higher than about 10.sup. -5/ year are controlled by the reactor protection system, i.e., the effects on the environment remain within the limits determined by the radiation protection regulations. Accidents less likely than the above are not an object to be considered in the layout of the reactor and in the licensing procedure, however, certain precautionary measures to limit such accidents are required. High temperature reactors already possess a high degree of safety in view of their location in a prestressed concrete pressure vessel and the properties specific to high temperature reactors, even in extremely unlikely accident situations, so that the extent of damage remains limited even in such cases. The extent of damage may be reduced even more by appropriate measures. DE-OS 32 12 322, expressly incorporated by reference herein, shows a process for the control of layout and hypothetical accidents in a high temperature reactor, wherein a plurality of successive steps are given, which are always initiated following the occurrence of a certain event (accident), automatically as a matter of principle (only if an intended measure is not initiated, are emergency measures carried out manually). The steps are designed so that each of them will prevent the failure of one of the four barriers present in a high temperature reactor for the retention of fission products. SUMMARY OF THE INVENTION It is an object of the invention to provide a safety system for use in extremely unlikely accidents in a gas cooled high temperature reactor. The reactor together with several steam generators may be housed in a prestressed concrete pressure vessel, and shut down by absorber rods. The installation may be equipped with a reactor protection system to actuate an emergency shut-down. Utilization of the specific properties of high temperature reactors further improves limiting damage in case of extremely unlikely accidents whereby the plant may be placed into a controlled state. This object may be attained by a reactor installation emergency shut down system characterized by the following elements: (a) an accident instrumentation unit located in the high temperature reactor to monitor certain characteristic process parameters, which PA1 (b) means for electronic evaluation of the measuring signals, which interrupt the supply of power to the cooling gas blowers, the feed water pumps and the absorber rod holding devices if predetermined limiting values are exceeded; PA1 (c) several temperature or pressure sensitive devices located in the primary loop, which physically interrupt the supply of power to the cooling gas blowers, the feed water pumps and the absorber rod holding devices if the predetermined limiting value of the corresponding process parameter is exceeded; and PA1 (d) a central emergency switch, which enables manual interruption of the supply of power to the cooling gas blowers, feed water pumps and absorber holding devices in case of a danger signal of the safety system. PA1 1. the absorber rods for the shut-down of the reactor do not become effective, i.e., cannot be inserted into the reactor or are even retracted (increase in activity); PA1 b 2. water feed pumps continue to operate, although the steam generator involved is defective and additional water is introduced into the reactor (increased reactivity, pressure buildup); PA1 3. cooling gas blowers are running, although the cooling system of the steam generators has failed, thereby additionally damaging the steam generators and reactor installations. PA1 Reduction of water intrusion PA1 Reduction of pressure buildup PA1 Reduction of corrosion PA1 Reduction of fission product release PA1 Reduction of reactivity effects PA1 Prevention of steam generator overheating PA1 Reduction of nuclear output (by automatic shutdown due to the negative temperature coefficient) PA1 Reduction of thermal loads in the primary loop PA1 Reduction of the pressure buildup PA1 Assurance of reactor shut-down PA1 Failure prevention of absorber rods in case of core overheating, prior to their insertion into the core. PA1 Guidance of roof reflector blocks upon a failure of their suspension. (1) are independent of the reactor protection system; and PA2 (2) the limiting values of the monitored characteristic process parameters are appreciably higher than those of the reactor protection system; and As shown by accident and risk analyses of recent years, all significant accidental events or combinations may be traced to total failure of active installations for removal of decay heat, wherein the situation may be further aggravated by malfunctioning of individual systems. A disturtance situation is aggravated if: The safety system according to the invention is designed so that even in extremely unlikely cases of accidents the shut-down of the cooling gas blowers and feed water pumps, and the insertion of the absorber rods is assured with a high degree of reliability. The safety system is actuated only if the limiting values of the "normal" reactor protection system are exceeded, i.e. if a safe state of the plant can no longer be attached by conventional safety measures. The improvements obtained by the safety system according to the invention in case of accident are tabulated as follows. Emergency shutdown/feed water pumps: Emergency shutdown/cooling gas blowers: Emergency shutdown/absorber rods (core and reflector rods) According to a further development of the invention, a device is provided whereby the pressure in the prestressed concrete pressure vessel may be reduced manually, as soon as a danger signal is emitted by the safety system. Thus, the pressure vessel may be equipped with a safety valve, which relieves the pressure into the storage reservoirs of the gas purification installation. A limitation of pressure in the primary loop is favorable in all conceivable extreme situations, as in this manner the failure under excessive pressure of the closures of the concrete pressure vessel and the connecting pipelines is avoided. It may be advantageous to further equip liner cooling systems present in all reactor pressure vessels with a manually operated device for cooling water, which is actuated only in case of a danger signal of the safety system. The operating ability of the liner cooling system may be maintained or restored, with the consequence that the reactor core and primary loop temperatures remain limited and the enclosure of the active core by the concrete pressure vessel is assured. A few characteristic process parameters suffice to detect an extreme disturbance in a high temperature reactor. These parameters, which are continuously monitored by the accident instrumentation, may be the hot gas temperature, the cold gas temperature and the pressure of the cooling gas. The data obtained is entered into an electronic evaluating device, which is actuated in the aforedescribed manner if predetermined limiting values are exceeded. The process parameters may be further physically evaluated by temperature or pressure sensitive devices located in the primary loop. Fuses or bimetallic strips may be used as the temperature sensitive devices which interrupt power supply to the cooling gas blowers, feed water pumps and absorber rod retaining devices if the predetermined limiting temperatures are exceeded. The fuses or bimetallic strips are preferably located above and under the reactor core. Disturbances may be recognized early by temperature changes occurring in these locations. Pressure transducers or pressure contacts may be used as pressure sensitive devices. If the predetermined limiting pressure is exceeded, the pressure transducers or contacts will interrupt power supply to the aforementioned assemblies.