130 Health Physics Table 2. Estimatedeffective half-lives (y) of '*’Cs ordered (top to bottom) by increasing annual average rainfall. Estimated effective Atoll or island half-life (y) Utirik Rongelap Atoll: Rongelap Island Rongerik Atoll: Eniwetak Island Ailinginae Taka Ailuk Mejit Wotho Jemo Erikub Ujelang Wotje Likiep Kwajalein (northern half of atoll) Ujae Kwajalein (southern half of atoll) Lae Maloelap Lib Aur Namu Alinglapalap Majuro Arno Jaluit Knox Namorik Kili Ebon Mili 16 16 16 20 15 15 14 15 14 14 20 14 15 14 14 14 14 14 13 14 13 13 12 12 12 11 11 12 12 11 effective half-life of 12 y is still somewhat smaller than the values we estimate for the northern atolls, the Robisonet al. (2003) estimates are only for a few sites on Bikini and Enewetak Atolls. Our analyses, based not only on '°’Cs to “Sr ratios, but also on a comparison of '’Cs measured in soils years apart, indicated that the degree of environmental loss varied considerably from island to island on each atoll, as well as from atoll to atoll, depending on the depth of the soil layer and type of vegetation. The effective half-life generally decreases with increasing average annual rainfall rates, as is to be expected because of precipitation-driven downward migration of the radioactivity in the soil. We estimate that the uncertainty in our estimates of effective half-life ranges from +1 to +2 y in the northern Marshall Islands, where the estimated half-lives are based on '*’Cs to ”’Sr soil measurements and comparisonsof '*’Csinventories in different years, to +2 to +3 y in the southern Marshall Islands, where the estimates are based primarily on relative precipitation rates. While the environmental half-life strongly depends on long-term precipitation rates, it is not solely a function of average annualrainfall since the depth of the soil layer varies among islands, depending on their size and stage of vegetative development. Our estimated uncertainty in August 2010, Volume 99, Number 2 the effective half-life for '*’Cs in soils of the northern Marshall Islands is based on the variations in the loss rates calculated from '°*’Cs to *’Sr ratios at atolls with similar rainfall. We assigned a larger uncertainty to the effective half-life at southern atolls where '*’Cs to ”’Sr ratios were not measured and the estimate of effective half-life was based on extrapolation. Average precipita- tion rates in the Marshall Islands are knownto increase from north to south with some references suggesting they level off just south of Majuro, consistent with the observed remaining inventories of '*’Cs in soil at atolls south of Majuro. Influence of fractionation The radionuclides created during the explosion of a nuclear device are usually classified as either refractory (R) or volatile (V) according to whether their melting point is higher or lower than 1,500°C (Hicks 1982). For example, isotopes of iodine and cesium are volatile and isotopes of zirconium and cerium are refractory. The mixture of radionuclides in the fallout cloud can change with time after the test (due to factors other than radioactive decay) as part of a phenomenon termed fractionation. A radionuclide mix is fractionated when the activity ratio of the refractory to the volatile radionuclides in deposited fallout differs from what would be expected from the composition in the initial debris cloud. Fractionation is due to the tendency of refractory radionuclides to be distributed throughout fallout particles and volatile nuclides to be distributed preferentially on the surface of particles. This phenomenongivesthe ratio of the refractory to the volatile radionuclides a dependence on particle size, with smaller particles, because of their relatively larger surface to volumeratio, typically enriched in volatile nuclides and larger particles typically enriched in refractory nuclides. Large particles, 1.e., those greater than ~50 um in diameter, deposit quickly because of their higher gravitational settling velocity. They also tend to be enriched in refractory nuclides because those nuclides have higher volatilization temperatures and condense more quickly as the fireball cools. Conversely, the volatile nuclides have lower volatilization temperatures and tend to remain gaseous longer and are adsorbed onto smaller particles that remain aloft longer. The so-called “unfractionated” radionuclide composition varies only slightly from test to test according to the fissionable material and construction characteristics of the nuclear device. Fractionation is very sensitive to the explosive yield, type of soil, height of burst and other factors (Freiling 1961, 1962, 1963; Freiling et al. 1965). On average, the unfractionated activity ratio of the refractory to the volatile radionuclides in fallout is about 1.6 at H+12.