Containment In-Situ PASS and Emission Monitoring System Design and Implementation

In order to reduce the residual risk associated with hypothetical severe nuclear accidents, nuclear power plants in Europe have been back-fitted with supplementary systems such as containment venting systems and hydrogen control systems. The potential hazard posed by radioactive releases to the environment in the event of an accident in a nuclear power plant largely depends on the airborne material in the containment atmosphere, like aerosol-bound and non aerosol-bound radionuclides. In conjunction with this the German Reactor Safety Commission (RSK) imposed the additional recommendations that provisions be made for: - 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 and - Emission monitoring for containment venting. Containment In-situ PASS: Various systems for use in German PWRs and BWRs have been investigated for their capability to take a representative probe of the radioactive nuclides in the containment atmosphere. Investigations showed that systems already installed in power plants - e.g. with sampling line lengths of 30 to 50 m and extraction pipe systems equipped with ball valves for sampling, exhibit considerable problems due to deposition inside the pipes. Deposition rates determined from various experiments resulted, when extrapolated for iodine and aerosols in sampling systems with sampling line lengths of 30 to 80 m, in deposition loss factors of 10 to 100 and more. As these problems cannot be solved satisfactorily with conventional sample piping systems, a new in-situ sampling technique has been developed together with the German utilities and the first full-scale prototype In-Situ PASS (Post-Accident Sampling System) for Germany's nuclear power plant unit Neckarwestheiml has been tested under all relevant containment conditions. The thermal-hydraulic performance as well as retention and deposition properties of this equipment were investigated in prototype tests using 80-m-long sample piping. The results showed that aerosol and iodine losses during transport by this method are minimized to values of approximately 10%, with the length of the piping playing almost no role. The tests were performed using a fully automated system and test parameters of 1 to 6 bar and 20 to 150 deg. C. Since the samples can be transported through long pipe runs with practically no losses, it is possible for two power plant units to share a single sampling system. On basis of the results of these analyses the fission product activity concentration primary of the containment atmosphere but also from the containment sump as well as from the condensation pool of BWR can be considered for further accident management. For this purpose measuring results have to be available to the plant crises team soon after onset of an accident and have to be frequently delivered also in the long term phase of an accident. Implementation of In-Situ PASS have already been completed for the first six NPPs, systems for other units are under construction. Emissions monitoring system The emissions monitoring system provides monitoring of radioactive materials discharged during filtered containment venting, broken down into the following nuclide groups: - Aerosol-bound radionuclides, - Non-aerosol-bound(gaseous) iodine isotopes, and - Radioactive noble gases. The data on emission levels determined in the event of containment venting are mainly intended to serve as a basis for implementing emergency response actions and as accurate accident documentation. The system is designed for the maximum activity concentration at the time early containment venting is commenced in the event of a core melt accident, i.e.: - 1 El I Bq/m3 aerosol-bound radionuclides, - 5 El I Bq/m3 gaseous iodine isotopes, and - 2 El 5 Bq/m3 radioactive noble gases. The gas to be monitored is a mixture of air, water vapour and other gases with a temperature up to 130 C and a pressure up to 2 bar The system complies with the recommendation made by the German Commi ssion for Radiological Protection (SSK) To date, this emissions monitoring system has been installed and is operational at 11 nuclear power plants in Germany. (authors)