New data and interpretation on source term attenuation within the break stage during meltdown SGTR sequences

Highlights: • Some attenuation of source term in the secondary side of a steam generator should be expected, even under a total absence of water. • Deposition of aerosols in the vicinity of tube breach is highly dependent on particle nature (i.e., aggregation state). • Retention efficiency shows a sigmoid curve behavior as a function of the ratio of non-dimensional Stokes and particle Reynolds numbers. - Abstract: During SGTR meltdown sequences the containment bypass of the fission products reaching the secondary side of the steam generator makes this scenario be a significant risk contributor in PWRs. During years the EU-SGTR and the ARTIST (I and II) projects have investigated the potential attenuation of the source term in these scenarios. The present paper focuses on the aerosol retention that might occur right at the vicinity of the tube breach under a total absence of water in the secondary side. Through an 11 test experimental matrix conducted in a mock-up facility of the secondary side of the steam generator (CAAT2 experiments) and a comparison to previous data, the study confirms the scrubbing effect of the break stage and the enormous influence of particle nature (i.e., aggregation state) on the aerosol net retention. Inertial and turbulent removal mechanisms make micron single sphere particles deposit on tube surfaces much more efficiently (75–95%) than same size fluffy aggregates built-up by hundreds of particles (<30%). Both CAAT2 and CAAT data have been satisfactorily correlated through a sigmoid-type curve as a function of the ratio of non-dimensional Stokes and particle Reynolds numbers. Effects of other variables like, gas mass flow rate, breach shape and size have been also explored and discussed, but their effect on the net aerosol retention has been observed to be negligible