dr. ramsey: we eventually shifted over to the plutonium model. >> this required science and engineering that when an angel -- that would enable the explosive material of plutonium. the plutonium weapon would have to be tested. it would be many months before the material would be delivered. the four that could happen, there were many questions which came down to the central problem, how to make the uraniumble serial of 235 or the plutonium 239 to release their energy efficiently in a casing that an airplane could deliver? one of the biggest problems of p-238 was u-235 or another location. thousands of miles of piping was remove the metal from the uranium-rich gas. method was used to produce and refine the material. required thousands of workers. there was also a method for extracting plutonium. hanford, washington was selected for the location of refining the plutonium. dr. seaborg: we had been working with what you called tracer amounts, invisible amounts detected by its radioactivity, but we couldn't deduce the properties with certainty that way. we needed to work with actual w eighable amo