OFFICIAL USE ONLY eventually filled with aggregate (obtained from the same island complex) after the container was placed in an area of relatively high exposure rate (125 uR/hr). A compartment was located in the center of the structure in which a survey meter was placed to measure exposure rates without aggregate, with 6 inches of aggregate on bottom, and with walls of aggregate. Results of this experiment are as follows: Background 125 uR/hr Six inch aggregate bottom 75 uR/hr Six inch aggregate bottom 45 uR/hr plus 6-inch walls Similar results were seen when exposure rates were measured inside and outside of a drained concrete cistern in the camp area which was constructed with the same aggregate. The average exposure rate of 10 uR/hr outside of the cistern was reduced to 3 uR/hr at waist height in the center of the cistern, From these observations it is expected that concrete houses made from this aggregate may reduce the exposure rates by a factor of approximately 3. 7. SUMMARY AND RECOMMENDATIONS The predominant radionuclides in the terrestrial organisms in Bikini Atoll are 137¢s and 90sr, whereas in marine organisms 60Co and 55Fe predominate. The range in the amount of a radionuclide found in the same tissue from the same species at the same island is wide. When detectable amounts of radionuclides are present, the minimum and maximum values often differ by factors of four or five and sometimes by a factor of ten. External radiation levels were measured on all islands of Bikini Atoll as part of the clean-up program. The highest exposure rate was 800 uR/hr measured on Eneman in a low lying algae covered area. The maximum exposure rate encountered on the islands scheduled for rehabitation, Bikini and Eneu, was about 120 uR/hr in the interior of Bikini. Other islands exhibiting exposure rates greater than those found on Bikini were: Enidrik - 300 uR/hr Lukoj - 180 uR/hr Jelete - 150 uR/hr Nam - 500 uR/hr Lomilik ~ 500 uR/hr Analyses of soil samples taken on Bikini indicated that more than 95 percent of the exposure rate was due to 137¢cs, Thus, the reduction in exposure rate can be assumed to closely follow the decay of 137Cs. 19 OFFICIAL USE ONLY Mixing