Patent Number: 047568720
Section: description

BRIEF DESCRIPTION OF THE DRAWING As shown in the figure, the reactor pressure vessel 2 of prestressed concrete secure against bursting with its internal liner 4, liner 1 cooling system 14 and insulation and containing the primary loop and its components, is surrounded by the reactor protection building 1, which collects the cooling gas leakage and conducts it through filters 21, 22 and a stack controlled into the environment. This is permissible in view of the slight cooling gas activity of high temperature reactors 5. For the same reason, it is not necessary to equip the reactor protection building with a liner, thereby appreciably reducing the cost of the installation. In the unlikely event of an extensive leakage of cooling gas from the reactor pressure vessel (for example in the case of the relief of the pressure of the reactor pressure vessel through a safety valve following a massive incident involving the primary cooling loop), the pressure relief devices 17, 18, 19, 20, and 23 combined with filters of the reactor protection building are actuated. It is, therefore, not necessary to design the reactor protection building for full pressure. The cross section of the stack is such that in the case of maximum outflow values the design pressure based on the static design dimensions of the reactor protection building 1 is not exceeded. Static design dimensions are such that the protection building would remain intact even in emergency situations such as the crash of an aircraft into the reactor protection building 1. All the components of importance from the standpoint of safety technology are thereby protected against external effects. Optionally, larger access locations (personnel and material doors) may be protected by outbuildings in front of them, while they themselves remain unprotected. As mentioned above, the temperature plays an essential role, in addition to pressure, in the assurance of the safe retention of activities in the case of accidents. Temperature increases in the core normally result from a disproportionality between the production and removal of heat. Decay heat is removed in normal operation by means of the operational cooling systems 7, 8, 9, i.e. the steam generators 7 of the principal loops and the secondary loop 9. In case of a failure of the operating cooling systems or of external effects leading to increased temperature loads on the cooling systems, the removal of the decay heat is effected according to the invention by auxiliary cooling systems 10, 11, 12, which are separate from the operating cooling systems; i.e. the operating and safety systems are not interconnected. Even in the case of incidents resulting in a loss of cooling gas and reducing the pressure in the core to the environmental pressure, the existing cooling gas density is still adequate to remove the decay heat safely by means of the auxiliary cooling systems. During normal operation of gas-cooled high temperature pebble bed reactors, only slow temperature changes take place. Thus, a relatively long period of time is available for the actuation of the countermeasures required to control any accidents, i.e. the shut-down system 13 of the reactor and the activation of the auxiliary cooling systems. This sluggish temperature-time behavior of the high temperature reactor renders it possible to further introduce measures to re-establish the removal of decay heat in case of a failure of the auxiliary cooling systems. These measures may include manual operations. Should the failure of the auxiliary cooling systems extend over a longer period of time (several hours), the decay heat is removed by the liner cooling system 14. No additional loads are thereby placed on the system, as its availability in view of its capacity as a vessel cooling system is adequately high. In the case of the removal of heat by means of the liner cooling system 14 alone, the maximum temperature of the fuel elements rises within a few hours (to approx. 1500.degree. C.). This temperature, however, does not damage the fuel elements. Advantageously, the pressure relief means of the reactor protective building may be connected with relief paths, 15, 16 which are automatically opened when a certain pressure is exceeded and closed when the pressure drops below the actuating pressure. This measure provides the assurance that the retention of activity by the reactor protection building is maintained even over extensive periods of time. For the removal of decay heat in case of a failure of the operating cooling systems (for a high temperature reactor within a capacity range of 300-600 MW.sub.el) two separate auxiliary cooling systems are appropriately provided. One system is adequate to remove the decay heat, without exceeding the design limits of the components for incidents involving no loss of coolant. If following a rapid relief of pressure and only one auxiliary cooling system is available, the decay heat is removed by the utilization of design reserves. Should both auxiliary cooling systems fail, the liner cooling system 14 is used to remove the decay heat. In case of an event without loss of coolant, natural convection is generated in the primary loop by the buoyancy effect, the convection opposing the normal direction of flow. This flow removes the heat from the core and transfers it to the liner cooling system 14. In the case of a pressure relief accident in a gas cooled reactor the decay heat may again be transferred to the liner cooling system 14. However, the larger portion of the heat to be removed is initially stored in the core 6 and transferred by radiation and conduction to the liner cooling system. This results in a slow rise in temperature in the core 6; but as damage to the fuel elements occurs only after the normal operating temperature has been exceeded considerably, there is sufficient time to reactivate by suitable counter measures the auxiliary cooling systems. These counter measures initially include automatically actuated actions on the basis of automatic detection. Subsequently, manual measures can be performed by operating personnel from the control room, after a realistic evaluation of the accident and the state of the installation has been established. Finally, emergency measures may be effected by the operators to repair or replace failed parts of the plant.