EVALUATION OF RADIOACTIVE FALL-OUT I. INTRODUTION The purpose of this report is to assess the magnitude and extent of the hazard to human life imposed by radioactive debris deposited in the human environment by large numbers of nuclear weapons. ‘The large amount of data which has been accumulated from field tests of nuclear weapons, laboratory findings, and field research is so voluminous that an evaluation of these is indicated. The evaluation is made in the light of known facts of nature, nuclear weapon test experience, and pertinent theoretical considerations. All explosive nuclear devices Imown to have been constructed or envisioned thus far utilize either wholly or in part a nuclear process | known as fission to achieve the energy release desired. The energy yield of a pure fission device is limited by the fact that the quantity of fissionable material that can be assembled in a given configuration cannot sefely exceed a particular amount, called a critical mass. This amount can be increased indefinitely by expanding or dispersing the configuration; however, a practical limit is soon reached because the size of the configuration becomes cumbersome and the problem of assem- bly at the desired instant of detonation becomes more and more difficult. A companion process to the fission process is that of fusion. Al- though the unit energy yield is less, there is no criticality problem for fusionable materials; thus large quantities of fusionable material can be engineered into weapon designs with no attendant nuclear safety problem. "Boosted" finsion weapons and all thermonuclear devices make some use of this process- Since very high temperatures are required to initiate the fusion reaction, a fission "trigger" or primary is a necessary component of the fusion devices successfully built thus far. The fission process is a phenomenon whereby radioactive nuclides are formed from the splitting of large atoms, with concurrent release of large amounts of energy. Under the proper physical conditions, 1