The first batch of canned slugs to be irradiated was received on December 20, 1943, allowing the first plutonium to be produced in early 1944. The slugs used pure metallic natural uranium, in air-tight aluminum cans long and in diameter. Each channel was loaded with between 24 and 54 fuel slugs. The reactor went critical with of slugs, but in its later life was operated with as much as . To load a channel, the radiation-absorbing shield plug was removed, and the slugs inserted manually in the front (east) end with long rods. To unload them, they were pushed all the way through to the far (west) end, where they fell onto a neoprene slab and fell down a chute into a pool of water that acted as a radiation shield. Following weeks of underwater storage to allow for decay in radioactivity, the slugs were delivered to the chemical separation building.