183 Radiation Safety and Cleanup Preparations RADIOLOGICAL ENVIRONMENT The nuclear testing at Enewetak Atoll dispersed radioactive materials in varying quantities over most of the northern islands. The decay of these materials produces ionizing radiation in the forms of alpba and beta particles and gammarays. As a result of the Enewetak Radiological Survey of 1973 and some subsequentfield surveys, the residual radioactivity had been quite well characterized with regard to the types of isotopes present, the levels, and the pattern of distribution. In general, the residual radioactivity could be grouped into three categories, based on its source: (1) unfissioned nuclear fuel—the device material not consumed in fissioning during detonation; (2) fission products—-the radioactive elements created when the nuclear fuel fissioned; and (3) induced radioactivity—materials that became radioactive through the capture of neutrons released as a result of the detonation. The most important of these categories from the standpoint of the cleanup was the unfissioned nuclear fuel. The principal radioisotope was plutonium-239 (Pu-239), which has a half-life (the time required for a given element to lose half of its radioactivity) of approximately 24,000 years. In addition, varying amounts of Pu-238, -240, and -241, along with Am-24l, were present. These elements, collectively termed transuranic elements because they are above uranium on the atomic numberscale of elements, were spread in forms ranging from microscopic- to centimetersized particles. The predominant decay method of transuranics is by emission of alpha particles; however, somebeta particles and gammarays are emitted also. (Indeed, the gamma rays produced from the radiological decay of Am-241 were of particular interest during the cleanup, as described in this chapter and Chapter 7.) While the transuranics constituted little problem in their undisturbed state, they would be a potential hazard once cleanup began. Although the detonation offission devices produces hundredsoffission products, the vast majority have very short half-lives and decay very rapidly. Only two fission product elements that had been deposited on the islands remained in sufficient quantity to be of concern. These were strontium-90, which has a half-life of about 27 years and decays by emission ofbeta particles, and cesium-137 (Cs-137), which has a half-life of about 30 years and decays by emission of both beta particles and gamma rays. The induced radioisotopes resulted when various elements in the immediate proximities of the GZ captured neutronsthat had beenreleased at the instant of detonation. The capture of a neutron by the nucleus of the element creates an unstable condition (i.e., the element becomes -— A