  Context. The intrinsic TeV emission of some BL Lacs are characterized by a
hard spectrum (the hard-TeV spectrum) after correcting for the extragalactic
background light. The hard-TeV spectra pose a challenge to conventional
one-zone models, including the leptonic model, the photohadronic model, the
proton synchrotron model, etc.
  Aims. In this work, we study if the one-zone hadronuclear (pp) model can be
used to interpret the hard-TeV spectra of BL Lacs without introducing extreme
parameters.
  Methods. We give analytical calculations to study if there is a parameter
space and the charge neutrality condition of jet can be satisfied when
interpreting the hard-TeV spectra of BL Lacs without introducing a
super-Eddington jet power.
  Results. We find that in a sample of hard-TeV BL Lacs collected by Xue et al.
(2019a), only the hard-TeV spectrum of 1ES 0229+200 could be explained by
gamma-ray from pi-0 decay produced in the pp interactions, but at the cost of
setting a small radius of the radiation region that comparable to the
Schwarzschild radius of the central black hole. Combining with previous studies
of other one-zone models, we suggest that the hard-TeV spectra of BL Lacs
cannot be explained by any one-zone models without introducing extreme
parameters, and should originate from the multiple radiation regions.
