  We investigate the potential of the 100 TeV future circular collider (FCC-hh)
to probe heavy neutrinos. We concentrate in particular on heavy neutrino
production via a $U(1)_{B-L}$ $Z'$ gauge boson and contrast the resulting
limits with that mediated by Standard Model weak currents. We consider heavy
neutrino decays to semi-leptonic as well as fully leptonic final states,
particularly with muon flavour, and we show the importance of considering
searches both in prompt and displaced decays of the heavy neutrinos. For prompt
final states, semi-leptonic modes are more promising due to smaller background
and larger yields, and TeV-scale heavy neutrinos with active-sterile mixing
compatible with light neutrino mass generation in a seesaw scenario can be
probed for a 5 TeV $Z'$ and gauge coupling as low as $g_{B-L} = 10^{-2}$.
Displaced vertex searches can extend this range to heavy neutrino masses as low
as 10 GeV.
