Patent Number: 052672824
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a device for monitoring the stack exit air in a reactor installation, essentially consisting of a pressure relief line which connects the containment vessel to the stack and in which a filter unit is located, a sampling point being provided downstream of the filter, from where gas mixture is branched off via a sampling line, passed through a measurement section and then returned to the stack exit air. 2. Discussion of Background The atmosphere in the containment vessel of a nuclear power station consists as a rule of air, steam, hydrogen, CO.sub.2, rare gases, iodine and aerosols. In normal operation of the installation, this mixture, which has an activity of about 10.sup.3 Bq/m.sup.3, is discharged from the containment vessel via a venting unit directly into the stack. In the event of an accident with a small leakage in the primary system, during which the activity is between 10.sup.3 Bq/m.sup.3 and 10.sup.8 Bq/m.sup.3, the gas is likewise discharged via the venting unit 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. During a major accident, the venting unit is isolated, whereupon the pressure in the containment vessel rises. To avoid an unduly great pressure rise, the containment vessel is relieved via a filter unit. In this filter unit (for example a dry filter or wet filter), the activity of iodine and aerosols is reduced by a factor of at least 1,000. Downstream of the filter unit, the activity of the gas is then determined in a measurement section. Because of the remaining very high activity in the downstream pure-gas line leading to the stack, the measuring apparatus used for normal operation, such as balancing filters and aerosol monitors cannot be used, since the measuring range would be exceeded and handling of the balancing filter would not be ensured. For this reason, special instruments having a wider measuring range and involved screenings as well as complicated devices for handling the balancing filter are normally used. SUMMARY OF THE INVENTION The invention attempts to avoid these disadvantages. Accordingly, one object of this invention is to provide, in an installation of the type described at the outset, for operation of the installation with the existing measuring instruments and apparatus even in the event of an accident. According to the invention, this is achieved when the activity concentration of the gas is reduced in a dilution unit upstream of the measurement section. The advantages of the invention are to be seen, inter alia, in the elimination of the hitherto usual heavy screenings around the entire measurement section. Screening for the transport of the balancing filter to be measured is also eliminated, whereby the hazard for the operating personnel is greatly reduced. It is particularly advantageous, if the gas sample in the sampling line is protected from cooling by means of a heater, before it is diluted. This avoids, on the one hand, condensation of steam in the sampling line and measurement section and an associated erroneous measurement due to precipitation of iodine and aerosols.