How does zinc affect fast neutrons? How does metallic zinc interact with nuclear fission?
Is it transparent for fast neutrons, or does it interact as a reflector (tamper), absorber, or moderator, or some other way yet?
I'm asking it in relation to the question about using zinc as propellant in NTR engines; the propellant would be pumped through the reactor and inevitably affect the reactor itself through its interaction with the neutrons. I wonder just how would it influence it.
 A: 
Here's a plot of some cross sections for neutrons on zinc from the Evaluated Nuclear Data File at the National Nuclear Data Center.  Blue is total cross section, $\rm Zn + n \to\text{anything}$.  Green is elastic scattering (including scattering to excited states in the zinc nucleus, whence the structure), red is inelastic scattering, and grey is capture with gamma-ray emission.  There are some modes missing from high-energy end, probably where the incident neutron knocks out another nucleon.
Here's a similar plot for hydrogen:

So for "fast" neutrons, 1--10 MeV, the cross section for interaction with zinc is a little higher than for hydrogen.  However for all lower energies a hydrogen nucleus is more likely to interact with a passing neutron than a zinc nucleus by about an order of magnitude.  (I'm doing some averaging-by-eye over the cross section structure in the keV-MeV energy range.)
Note that an important part of sustaining a reaction is slowing the neutrons down to thermal energies ($\rm 25\,meV = 25\times10^{-9}\,MeV$).  For this process, zinc is not very effective, but not negligible either.
A: The usual place I go look up neutron cross sections is over at the National Nuclear Data Center in the Evaluated Nuclear Data File section. There you can get a variety of cross sections vs energy for Zinc or specific isotopes. 
For Zinc overall, they have data from 10E-4 eV to 10 MeV, and there appears to be reasonable structure in the 10 keV to 1 MeV range, sith the cross section varying over several orders of magnitude in that range.
