The phenomenology in the neutrino sector requires physics beyond the Standard
Model. One possibility is the existence of new massive leptonic states which
could be probed at the high intensity machines. The present results on heavy
neutral leptons from the study of kaon decays in flight with the NA48/2 and
NA62 experiments are presented and the future prospects for such searches at
CERN SPS are discussed.Comment: Talk presented at NuPhys2018 (London, 19-21 December 2018). 8 pages,
6 figure