Given the high radio-toxic nature of fission products for environment and population, their release should be avoided at all costs. This highlights the importance of relying on efficient mitigation systems capable of reducing as much as possible any accidental release. This overall statement becomes even truer after the accident in the Fukushima Daiichi NPP (March 2011).Current NPPs have safeguards based on the Design Basis Accident (DBA) and some extensions to cope with accidents beyond the design bases.

This applies, in particular, to mitigation systems. There are many mitigation systems within a NPP, both to accommodate the energy release and to deplete most of potential radioactive emission to the environment. Sprays, suppression pools, high efficiency particle filters, etc. are among those contributing to source term attenuation.The current knowledge of existing mitigation systems needs to be completed by analysis of aspects such as: performance under degraded conditions of operation (i.e., loss of electrical current, flooding; loss of ultimate heat sink; etc.); efficiency for different potential source term compositions; long term behaviour of the trapping system.

About innovative systems, there are many techniques those have the potential to effectively contribute to radioactive retention in case of an accident. Their operation could result in either a source term reduction or a source term conditioning in order to improve the removal efficiency of mitigation systems. Their capability and reliability would be experimentally demonstrated under the conditions of a severe accident.