One such proposal calls for a gas-cooled fast reactor that is fueled primarily by plutonium and americium. The neutronic properties of americium make it difficult to use in any critical reactor, because it tends to make the moderator temperature coefficient more positive, decreasing stability. The inherent safety of an ADS, however, would allow americium to be safely burned. These materials also have good neutron economy, allowing the pitch-to-diameter ratio to be large, which allows for improved natural circulation and economics.