What elements are used to prevent losing neutrons?

The cores of nuclear plants and nuclear weapons (pits) are two different animals.

Nuclear fuel uses relatively low enrichment, typcially just less the 5% enriched these days in most modern LWRs, or slightly enriched in CANDU's, as compared to nuclear weapons, which are based on highly enriched Pu-239 (~94% Pu-239).

The fuel in the cores have been design with low absorbing Zr-based alloys, while the structural materials are typically stainless steels of the 300 series (e.g. 304, 316 . . .). The core is optimized geometrically, and the power distribution is then optimized to reduce in the assemblies on the exterior of the core in order to reduce neutron leakage (loss). Some older plants had steel pads designed to reflect fast neutrons back into the core. In some exotic core design Be and compounds of Be have been proposed, however Be is a very toxic compound.

In nuclear weapons, the pits are high enriched and then compressed to much greater densities very quickly in order to maximize yield (energy from the fissile reaction). There is no reflector for reflecting neutrons since nuclear weapons are on a very fast time scale based on prompt neutrons.

 

The best reflector has a high fast removal cross-section with a low absorption cross-section. I believe W-184 has the lowest absorption cross-section of the W isotopes, and is about 30.6% of natural tungsten. One needs to look at ENDF/B data or the Barn book.

One drawback is the density. Iron has also been used as a reflector in thermal (LWRs)reactors.

I'm assuming one is referring to thermal/epithermal spectrum.