The risk relevance of meltdown SGTR scenarios lies in the potential direct release of fission products to the environment from a degrading core without passing through containment (i.e., containment bypass). Given the lack of knowledge a decade ago, no credit has been traditionally given to any retention at the secondary side of the steam generator. Nonetheless, more than 10 years of research, through projects like EU-SGTR, ARTIST and ARTIST-II, have demonstrated that retention would occur even in the worst case, when water level is below the tube breach and no fission product can be scrubbed by water.

The ARI3SG model was developed to estimate the aerosol retention around the breach nearby (i.e., break stage). Based on a semi-empirical filter approach, ARI3SG was validated against an ad-hoc database and a theoretical correlation depending on particle Reynolds and Stokes non-dimensional numbers, was derived. By implementing such a correlation through the filter model in MELCOR 2.1 and by using a control function to describe the particle retention efficiency in the break stage, an analysis of a meltdown SGTR sequence has been conducted and the results compared to those obtained for the same postulated scenario when ARI3SG contribution is not considered.

This paper presents the results of such analysis in terms of the mass fraction released to the environment and discusses difficulties found when using the MELCOR filter component to introduce the ARI3SG correlation. All in all, the outcome from this study is the confirmation that a substantial retention might be achieved at the break stage (up to around 70% of incoming mass), whenever particle aggregates entering the secondary side of the steam generator have densities around or higher than 2000 kg/m3 and their cohesive forces among primary particles prevent their massive fragmentation when colliding with tubes and/or undergo shear stress.

This work is framed in the collaboration agreement with CSN for investigation on severe accidents (CSNAS).

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