NEVADA TEST SITE FALLOUT ATOM RATIOS: 240pu/239pu and 241py/239pu Harry G. Hicks Lawrence Livermore National Laboratory Livermore, CA 94550 Donald W. Barr Los Alamos National Laboratory Los Alamos, NM 87545 The exposure of the population in Utah to external gamma radiation from the fallout from nuclear weapons tests carried out between 1951 and 1958 at the Nevada Test Site (NTS) has been reconstructed from recent measurements of 137¢5 and plutonium in soil.l The fraction of !37Cs in the fallout from NTS events was calculated from the total plutonium and the 240pu/239pu ratios measured in the soil, using the values of 0.180 = 0.006 and 0.032 + 0.003 for that ratio in global fallout and NTS fallout, respectively. The total population exposure from NIS events was then calculated on the basis of exposure rates resulting from short-lived radionuclides associated with the 137cs at the time of deposition. While the 240py/239pu ratio is constant in global fallout, this ratio varies greatly in the fallout from individual events. While the composition of fall- out on Utah from NTS events is rather uniform, the Off-Site Radiation Exposure Review Project is currently reconstructing radiation exposures for locations close to NTS where the fallout may be predominantly from one event. Therefore, the authors compiled the pertinent ratios in order to provide information concerning the exposure resulting from any individual event. The plutonium ratios measured at 30 days postshot, shown in Table 1, were compiled from unpublished values in the archives of the Nuclear Chemistry Division of LLNL and INC-11 of LANL. These ratios are pertinent to fallout data. Dates for each event were taken from a publication by the Nevada Operations Office of the Department of Energy.2 This work was funded by the Off-Site Radiation Exposure Review Project.