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The European Passam project: R&D outcomes towards enhanced severeaccident source term mitigation

pubblicazioni - Memoria

The European Passam project: R&D outcomes towards enhanced severeaccident source term mitigation

Questo documento presenta i principali risultati sperimentali di questo progetto, oltre che la comprensione di fenomeni,modelli e correlazioni corrispondenti ed alcune analisi preliminari per il potenziale uso nelle strategie di gestione degliincidenti severi, tenendo conto della natura passiva o non passiva dei sistemi studiati.

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), EDFand 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 enhancesource term mitigation in case of a severe accident in a LWR. Both already existing systems and innovativeones were experimentally studied.

This paper presents the main outcomes of this project, ncludingexperimental results, understanding of phenomena and corresponding models and correlations with somepreliminary analyses for potential use in severe accident management strategies, taking into account thepassive or non-passive nature of the systems studied.Pool scrubbing represented the most studied domain of the PASSAM project. As an example of results, itwas shown that gas hydrodynamics, at least in some relevant scenarios, is significantly different from what isnowadays encapsulated in severe accident analysis codes, particularly at high velocities and, that in the longrun, maintaining an alkaline pH in the scrubber solution is absolutely necessary for preventing a delayediodine release. Regarding sand bed filters plus metallic pre-filters, implemented on all French nuclear powerplants, filtration efficiency for gaseous molecular and organic iodine was checked. Other experimentsshowed that under severe accident conditions, cesium iodide aerosols trapped in the sand filter are unstableand may constitute a delayed source term, which is not the case for CsI particles trapped on the metallic prefilter.As innovative processes, both acoustic agglomeration and high pressure spray systems were studiedmainly 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 wasexperimentally observed and, as a secondary effect, aerosol mass concentration was decreased. As for highpressure spray, the increase in particle size could not be really measured, but the system showed a goodefficiency: it allowed reducing the airborne particle concentration much more efficiently than lowpressure 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 ofseveral parameters, as steam content, was also studied.

Extensive testing of zeolites as regards theircapability for trapping gaseous molecular and organic iodine was performed, showing very good trappingefficiencies. Preselected zeolites were compared in various conditions: silver Faujasite-Y zeolite gave thebest results. The global stability of trapped iodine under irradiation and steam conditions has also beenchecked. The combination of a wet scrubber followed by a zeolite filtration stage was extensively studied inrepresentative severe accident conditions and showed its ability to reach a very good retention for gaseousorganic iodides.

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