By the time World War II broke out in Europe in 1939, Urey was recognized as a world expert on isotope separation. Thus far, separation had involved only the light elements. In 1939 and 1940, Urey published two papers on the separation of heavier isotopes in which he proposed centrifugal separation. This assumed great importance due to speculation by Niels Bohr that uranium 235 was fissile. Because it was considered "very doubtful whether a chain reaction can be established without separating 235 from the rest of the uranium," Urey began intensive studies of how uranium enrichment might be achieved. Apart from centrifugal separation, George Kistiakowsky suggested that gaseous diffusion might be a possible method. A third possibility was thermal diffusion. Urey coordinated all isotope separation research efforts, including the effort to produce heavy water, which could be used as a neutron moderator in nuclear reactors.