The design of the reactors for plutonium production involved several problems, not just in nuclear physics but in engineering and construction. Issues such as the long-term effect of radiation on materials received considerable attention from the Metallurgical Laboratory. Two types of reactors were considered: homogeneous, in which the moderator and fuel were mixed together, and heterogeneous, in which the moderator and fuel were arranged in a lattice configuration. By late 1941, mathematical analysis had shown that the lattice design had advantages over the homogeneous type, and so it was chosen for CP-1, and for the later production reactors as well. For a neutron moderator, graphite was chosen on the basis of its availability compared with beryllium or heavy water.