But we're excited by the ambition of the project. Gates said "The idea is to be very low cost, very safe and generate very little waste," and classified it as a traveling wave reactor. It's a theoretical kind of reactor, under study by Terrapower that requires no enriched uranium, but actually uses depleted uranium, which means hardly any nuclear waste. They can also run for, theoretically, decades without refueling. The name comes from the idea that fission begins in a specific part of the core and moves outward, in a traveling wave.