  In order to extract the precise physical information encoded in the
gravitational and electromagnetic signals from powerful neutron-star merger
events, we need to include as much of the relevant physics as possible in our
numerical simulations. This presents a severe challenge, given that many of the
involved parameters are poorly constrained. In this paper we focus on the role
of nuclear reactions. Combining a theoretical discussion with an analysis
connecting to state-of-the-art simulations, we outline multiple arguments that
lead to a reactive system being described in terms of a bulk viscosity. The
results demonstrate that in order to properly account for nuclear reactions,
future simulations must be able to handle different regimes where rather
different assumptions/approximations are appropriate. We also touch upon the
link to models based on the large-eddy-strategy required to capture turbulence.
