Construction commenced on the pilot separation plant before a chemical process for separating plutonium from uranium had been selected. Not until May 1943 would DuPont managers decide to use the bismuth phosphate process in preference to one using lanthanum fluoride. The bismuth phosphate process was devised by Stanley G. Thompson at the University of California. Plutonium had two oxidation states; a tetravalent (+4) state, and hexavalent (+6) state, with different chemical properties. Bismuth phosphate () was similar in its crystalline structure to plutonium phosphate, and plutonium would be carried with bismuth phosphate in a solution while other elements, including uranium, would be precipitated. The plutonium could be switched from being in solution to being precipitated by toggling its oxidation state. The plant consisted of six cells, separated from each other and the control room by thick concrete walls. The equipment was operated from the control room by remote control due to the radioactivity produced by fission products. Work was completed on November 26, 1943, but the plant could not operate until the reactor started producing irradiated uranium slugs.