data will be used in dose assessments for Enewetak and the 13 atolls of the NMIS and will be published during 1981; these assessments will provide the initial dose assessment for the atolls and a dose assessment will not be required again for another 5 years. In order to ensure that the expected reduction in exposure at the atolls is occurring, and to provide external dose data for assessments, and to help evaluate the environmental residence time of 1376) external gamma " measurements should be made every 4 or 5 years beginning within the next 4 years at the residence islands and one agriculture island at each of the atolls. Measurements would be made using either hand held scintillation detectors or detectors. vehicle mounted scintillation or high resolution solid state No aerial surveys would be required. Once an effective half-time of 137¢, at the atolls has been determined, the frequency of external gamma measurements could be reduced or perhaps even eliminated. The first external radiation reassessment program, assuming one atoll per year, would begin in the next 4 years and the second reassessment, in the same sequence, would begin 5 years later. A decision on whether to continue the program at this frequency could be made at this point. C. Research Requirements 1.) Environmental Residence Times for 137 Cs, 90 Sr, 239,240, U and eu an. A key parameter in any dose assessment model is the effective half-time of the radionuclides in the environment being evaluated. The effective half-time is a combination of the radiological (physical) half-life of a radionuclide and the residence half-time of the radionuclide in the environment. 34