  In Newtonian fluid dynamics simulations in which composition has been tracked
by a nuclear reaction network, energy generation due to composition changes has
generally been handled as a separate source term in the energy equation. A
relativistic equation in conservative form for total fluid energy, obtained
from the spacetime divergence of the stress-energy tensor, in principle
encompasses such energy generation; but it is not explicitly manifest. An
alternative relativistic energy equation in conservative form---in which the
nuclear energy generation appears explicitly, and that reduces directly to the
Newtonian internal + kinetic energy in the appropriate limit---emerges
naturally and self-consistently from the difference of the equation for total
fluid energy and the equation for baryon number conservation multiplied by the
average baryon mass $m$, when $m$ is expressed in terms of contributions from
the nuclear species in the fluid, and allowed to be mutable.
