Direct post-accident sampling system for containment atmospheres (Di-PAS systems)

In order to reduce the residual risk associated with hypothetical severe nuclear accidents, nuclear power plants in Germany have been backfitted amongst other things with supplementary systems such as containment venting systems. In conjunction with these measures the German Reactor Safety Commission (RSK) imposed the additional requirement that provisions be made for representative post-accident sampling of the containment atmosphere for the purpose of obtaining information on the condition of the core and on potential hazards to the environment. To meet this requirement, enveloping conditions for relevant accident scenarios were defined, such as an enveloping aerosol with an aerodynamic diameter in the range of 0.1 to 10 μm, widely varying iodine concentrations, and humidity, pressure and temperature conditions. In addition to these basic requirements it is also considered appropriate to provided a possibility for measurement of the gas composition, e.g., CO, CO2, H2 and O2. An investigation of the existing options for piping system sampling with pipe lengths of up to 50 m based on this requirement revealed that, despite the use of special piping materials and heat tracing equipment, etc. considerably high pipe factors for iodine and aerosols as well as significant memory effects would have to be expected under the various accident conditions in question, including impurity effects. Consequently, new sampling systems based on sample transport via piping systems and sample collection outside the containment, as well as systems for direct post-accident sampling inside the containment were designed and the characteristics of the two approaches compared. Finally, in order to minimize cost and effort and to provide exclusively passive components inside the containment, an in-situ scrubber sampling unit was developed