The European PASSAM project: R&D outcomes towards enhanced severe accident source term mitigation

The European PASSAM project (Passive and Active Systems on Severe Accident source term Mitigation) involved nine partners from six countries during four year (2013-2016): IRSN (project coordinator), EDF and University of Lorraine (France); CIEMAT and CSIC (Spain); PSI (Switzerland); RSE (Italy); VTT (Finland) and AREVA GmbH (Germany). It was mainly of an R&D experimental nature and aimed at investigating phenomena that might enhance source term mitigation in case of a severe accident in a LWR. Both already existing systems and innovative ones were experimentally studied. This paper presents the main outcomes of this project, including experimental results, understanding of phenomena and corresponding models and correlations with some preliminary analyses for potential use in severe accident management strategies, taking into account the passive or non-passive nature of the systems studied. Pool scrubbing represented the most studied domain of the PASSAM project. As an example of results, it was shown that gas hydrodynamics, at least in some relevant scenarios, is significantly different from what is nowadays encapsulated in severe accident analysis codes, particularly at high velocities and, that in the long run, maintaining an alkaline pH in the scrubber solution is absolutely necessary for preventing a delayed iodine release. Regarding sand bed filters plus metallic pre-filters, implemented on all French nuclear power plants, filtration efficiency for gaseous molecular and organic iodine was checked. Other experiments showed that under severe accident conditions, cesium iodide aerosols trapped in the sand filter are unstable and may constitute a delayed source term, which is not the case for CsI particles trapped on the metallic pre-filter. As innovative processes, both acoustic agglomeration and high pressure spray systems were studied mainly in the aim of leading to bigger particles upstream of filtered containment venting systems (FCVS), and so enhancing the filtration efficiency. An increase of the particle size by ultrasonic fields was experimentally observed and, as a secondary effect, aerosol mass concentration was decreased. As for high pressure spray, the increase in particle size could not be really measured, but the system showed a good efficiency: it allowed reducing the airborne particle concentration much more efficiently than low pressure sprays. Experimental studies for trapping gaseous molecular and organic iodine using wet electrostatic precipitators (WESP) confirmed the importance of optimizing the WESP design and the utility of different process steps (e.g. oxidation of I₂ or CH₃I into iodine oxide particles) for a good trapping efficiency. The influence of several parameters, as steam content, was also studied. Extensive testing of zeolites as regards their capability for trapping gaseous molecular and organic iodine was performed, showing very good trapping efficiencies. Preselected zeolites were compared in various conditions: silver Faujasite-Y zeolite gave the best results. The global stability of trapped iodine under irradiation and steam conditions has also been checked. The combination of a wet scrubber followed by a zeolite filtration stage was extensively studied in representative severe accident conditions and showed its ability to reach a very good retention for gaseous organic iodides.