Jan 10, · Neutron HDPEelding materials ntain polymer-based mposites, high density ncrete, heavy metals, paraffin, and other neutron HDPEelding materials with additives, such as hexagonal boron nitride (h-BN), boron carbide (B 4 C), boric acid, lemanite, cadmium (Cd), gadolinium (Gd), and samarium oxide (Sm 2 O 3) fillers.