Some fast reactors use oxide fuels, but those made with metallic fuel are even faster, since oxygen is a fairly light atom. (whatisnuclear.com)
Unlike the current generation of light - water nuclear reactors, PRISM uses metallic fuel, such as an alloy of zirconium, uranium, and plutonium, and PRISM's fuel rods sit in a bath of a liquid metal — sodium — at atmospheric pressure, which ensures that the transfer of heat from the metal fuel to the liquid sodium coolant is extremely efficient. (gehitachiprism.com)
Therefore, it will cost less to manufacture PRISM metallic fuel and the design requires a relatively small plant size to disposition the plutonium. (gehitachiprism.com)