12 The emphasis on transuranics in the cleanup operation also was influenced by the fact that Pu-239 has a half-life of 24,000 years In the whereas Sr-90 and Ce-137 have half-lives of about 30 years. near future, however, the fission products must be of great concern because of their rapid rate of movement through the soil and their very active incorporation into the food chain. Since there was virtually no contamination on the southern islands, it was planned that only these islands would be settled at the beginning. Occupation of the northern islands after cleanup was to be postponed until radioactive decay brought the concentrations of Sr-90 and Ce-137 to acceptable levels. 3.5 Location of the Contamination Before work could begin it was necessary to find out which islands were significantly contaminated and to identify the specific places where, remedial work would be required. As has been explained, the cleanup was concerned with the transuranics, mainly plutonium, but _ plutonium emits only an alpha particle accompanied by a very low-energy xeray so it is not practicable to measure it in the field. However, the plutonium is associated with americium, which has a sufficiently penetrating gamma ray for detection through several inches of soil. Aerial surveys and in-situ monitoring detected the significantly contaminated islands and specially designed detection equipment mounted on a tracked vehicle then was used for a detailed survey. Readings were taken at every intersection of a SO-meter grid. Soil samples at various depths were taken at each intersection for laboratory analysis to determine the plutonium/americium ratio. In areas of high contamination, samples were taken at 25=, 12.5=, and 6.25-meter intervals. This work provided the basis for radiation contour maps that would be used by the cleanup crews. 3.6 Nature of the problem The cleanup problem was not confined to surface soil contaminated with transuranics. During the weapons testing program, debris from tests frequently was cleared from a site and dumped in an old crater in preparation for reuse of the site for additional tests. All detectable dumps, crypts, and burial sites were excavated and any radioactive contents transferred to the Cactus Crater. Old block- houses, sunken barges, and landing craft in less than 15 feet of water and other miscellaneous debris were collected, monitored, and disposed of in the containment structure, if contaminated, or in deep parts of the lagoon if not contaminated. Retrieval from dumps often was difficult and a crypt on Aomon Island was so extensive that a year was required to plan and complete its excavation. About 16,000 items from World War II (unexploded artillery and mortar shells, hand grenades, small arms, and ammunition) also were detected, dug up, and detonated or removed by Navy Explosive Ordnance Disposal Teams. After the rubbish was removed, the top 6 inches of soil was scraped off, loaded into barges, and transported to Runit Island. 2006329