Scientists who were engaged in the atomic program before the end of 1943 had pretty well outlined most of the types of reactors which are being discussed today. These outlines were usually quite general and merely sketched the theoretical possibility of building a particular type of reactor. Many of these discussions were not recorded, and the records of others are hard to find today since they are buried in the Project literature and have not been properly indexed. However, a careful search of reports such as the minutes of the Project Council and the weekly and monthly reports of the various projects would unearth them. Those who remember that period will recall that reactors were divided into various categories, such as thermal, intermediate, and fast reactors; homogeneous and heterogeneous reactors; and lightwater, heavy-water, graphite, beryllium, molten- salt, and molten-metal reactors. Most of these were in the dream or speculative stage. It was obvious to everyone that the first reactors would have to be thermal reactors using natural uranium because, until enriched materials could be obtained and the pressing war needs could be satisfied, little time could be devoted to the other types of reactors
