  In Pop III stellar models convection-induced mixing between H- and He-rich
burning layers can induce a burst of nuclear energy and thereby substantially
alter the subsequent evolution and nucleosynthesis in the first massive stars.
We investigate H-He shell and core interactions in 26 stellar evolution
simulations with masses $15 - 140\,\mathrm{M}_{\odot}$, using five sets of
mixing assumptions. In 22 cases H-He interactions induce local nuclear energy
release in the range $ \sim 10^{9} - 10^{13.5}\,\mathrm{L}_{\odot}$. The
luminosities on the upper end of this range amount to a substantial fraction of
the layer's internal energy over a convective advection timescale, indicating a
dynamic stellar response that would violate 1D stellar evolution modelling
assumptions. We distinguish four types of H-He interactions depending on the
evolutionary phase and convective stability of the He-rich material. H-burning
conditions during H-He interactions give $^{12}\mathrm{C}/^{13}\mathrm{C}$
ratios between $\approx 1.5$ to $\sim 1000$ and [C/N] ratios from $\approx -2.3
$ to $\approx 3$ with a correlation that agrees well with observations of
CEMP-no stars. We also explore Ca production from hot CNO breakout and find the
simulations presented here likely cannot explain the observed Ca abundance in
the most Ca-poor CEMP-no star. We describe the evolution leading to H-He
interactions, which occur during or shortly after core-contraction phases.
Three simulations without a H-He interaction are computed to Fe-core infall and
a $140\,\mathrm{M}_{\odot}$ simulation becomes pair-unstable. We also discuss
present modelling limitations and the need for 3D hydrodynamic models to fully
understand these stellar evolutionary phases.
