Patent Number: 052727386
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a device for monitoring the atmosphere within the containment shell of a reactor plant, a sample removal means being provided in the containment shell, from which a gas mixture is led via a measuring line through a measuring zone and subsequently discarded. DISCUSSION OF BACKGROUND The atmosphere in the containment shell of a nuclear power plant consists as a rule of air, steam, hydrogen, CO.sub.2, noble gases, iodine and aerosols. In normal plant operation this mixture, having an activity of approx. 10.sup.3 Bq/m.sup.3, is led from the containment shell via a ventilation system directly into the stack. In the event of an accident with a small leakage in the primary system, in which the activity is between 10.sup.3 Bq/m.sup.3 and 10.sup.8 Bq/m.sup.3, the gas is also led off via the ventilation system directly into the stack. In the event of a major accident with, for example, core meltdown, the activity can become greater than 10.sup.14 Bq/m.sup.3. The ventilation system is shut off in the event of the major accident, with the result that the pressure in the containment shell increases. In order to avoid too high a pressure increase, the containment shell is relieved via a filter system. In this filter system (e.g. dry filters, wet filters) the activity of iodine and aerosols is reduced by at least a factor of 1000. Downstream of the filter system the activity of the gas is then determined in a measuring zone. The measuring equipment used for normal operation, such as balancing filters and aerosol cannot be used in the event of an accident because of the residual very high activity in the subsequent clean gas line, which leads to the stack, since the measuring range would be exceeded and handling of the balancing filter would not be guaranteed. Therefore, special instruments having a high measuring range and complex shielding, as well as complicated devices for handling the balancing filter are normally used. In the event of an accident, it is therefore useful also to monitor the atmosphere inside the containment shell, as long as the pressure relief is not yet in operation, in order to obtain a measure of the increasing activity, and the activity that can be expected. In this case, the activity of the sample gas is also too high for the evaluation instruments that are used in normal operation. SUMMARY OF THE INVENTION Accordingly, one object of the invention is to avoid these disadvantages. Its object is, with a plant of the aforementioned type, to operate the plant using the existing measuring instruments and equipment even in the event of an accident. According to the invention this is achieved by reducing the activity concentration of the gas upstream of the measuring zone in a dilution plant. The advantages of the invention can be seen, inter alia, in the elimination of the heavy shielding of the entire measuring zone, which has hitherto been conventional. The shielding for transport of the balancing filter, which has to be measured, is also eliminated, greatly reducing the hazard to the operating personnel.