Stratospheric fallout

The topographic distribulion of fallout is divided into three categories called (I) local (or close-in) (2) tropospheric (or intermediate) and 3i stratospheric or worldwide) fallout. No distinct boundaries exist between these categories. The distinction between local and tropospheric fallout is a function of dislance from source to point of deposit The primary dislinclion between tropospheric and stratospheric fallout is the place of injection of the dehris into the atmosphere, above or below the iropopause. Whether the radioactive debris from a nuclear weapon becomes tropospheric or stratospheric fallout depends on yield, height, and lalilude of burst (the height of the iropopause is a function of latitude). 

For a nuclear weapon hurst in air. all materials in the fireball are vaporized. Condensation of fission products and other bomb materials is then governed by the saturation vapor pressures of the most abundant constituents. Primary debris can combine w ilh naturally-occurring aerosols, and almost all of (he fallout becomes tropospheric or stratospheric. If the weapon detonation takes place within a few hundred Icet of (either above or below) a land or water surface, large quaniilies of surface materials are drawn up or thrown into the air above Ihe place ol detonation. Condensation of radioactive nuclides in this material then leads in considerable quantities of local fallout, but some of the radioactivity still goes into tropospheric and stratospheric fallout. If the hurst occurs sufficiently fur underground, the surface is not bruken and no fallout results. 

The concentration of atmospheric radionuclides has a special distribution which depends on latitudes and altitudes. Cosmogenic radionuclides have higher production rates in the stratosphere than in the troposphere, because of a higher intensity of cosmic rays in the stratosphere. Fallout nuclides have higher concentrations in mid-latitude of the Northern Hemisphere, because most atmospheric nuclear explosion experiments were made there. 

VOLCHOK Resuspension has not been extensively studied for strontium 90. The only thing that I ve seen was by the English at Harwell. They are doing some, studies of resuspension of the surface ocean layer, and seeing some small amount of re-emission in that way. But from land I don t think there have been any studies. We would probably not be able to sense it as it would undoubtedly be a small fraction of the stratospheric fallout. 

On the other hand, sediments from the Ob and Yenisey Rivers show the presence of weapons-grade Pu originating from nuclear fuel reprocessing. The data are distinctly different from those of northern-hemisphere stratospheric fallout (see references in Haque and Nakanishi 1999). 

Fallout plutonium arrives in natural waters either by direct atmospheric deposition or by erosion and/or dissolution from the land. Although in the past, this plutonium was considered to be in a refractory form due to formation within the fire ball, it seems more likely that most of the plutonium originated in the stratosphere by the decay of 239Np (from 239U formed during the detonation)(4). Deposition occurs predominantly with one or a few atoms incorporated in a raindrop. Investigations by Fukai indicate that collected rain contains soluble plutonium which has oxidation states that are almost totally Pu(V+VI)05). 

Denmark 1.5 days after the explosion. Air samples collected at Roskilde, Denmark on April 27-28, contained a mean air concentration of 241Am of 5.2 pBq/m3 (0.14 fCi/m3). In May 1986, the mean concentration was 11 pBq/m3 (0.30 fCi/m3) (Aarkrog 1988). Whereas debris from nuclear weapons testing is injected into the stratosphere, debris from Chernobyl was injected into the troposphere. As the mean residence time in the troposphere is 20-40 days, it would appear that the fallout would have decreased to very low levels by the end of 1986. However, from the levels of other radioactive elements, this was not the case. Sequential extraction studies were performed on aerosols collected in Lithuania after dust storms in September 1992 carried radioactive aerosols to the region from contaminated areas of the Ukraine and Belarus. The fraction distribution of241 Am in the aerosol samples was approximately (fraction, percent) organically-bound, 18% oxide-bound, 10% acid-soluble, 36% and residual, 32% (Lujaniene et al. 1999). Very little americium was found in the more readily extractable exchangeable and water soluble and specifically adsorbed fractions. 

Mechanisms and rates of transport of nuclear test debris in the upper and lower atmosphere are considered. For the lower thermosphere vertical eddy diffusion coefficients of 3-6 X 106 cm.2 sec. 1 are estimated from twilight lithium enhancement observations. Radiochemical evidence for samples from 23 to 37 km. altitude at 31° N indicate pole-ward mean motion in this layer. Large increases in stratospheric debris in the southern hemisphere in 1963 and 1964 are attributed to debris from Soviet tests, transported via the mesosphere and the Antarctic stratosphere. Most of the carbon-14 remains behind in the Arctic stratosphere. 210Bi/ 210Pb ratios indicate aerosol residence times of only a few days at tropospheric levels and only several weeks in the lower stratosphere. Implications for the inventory and distribution of radioactive fallout are discussed. 

It was pointed out above that the 18r W tracer observations, the excess 210Pb in the tropical stratosphere, and the radiochemical evidence from fallout samples collected by balloon can be reconciled with the slow mean motion of air upward across the equatorial tropopause and within the tropical stratosphere and outward toward higher latitudes at stratospheric levels up to about 35 km. Above 23 km. altitude, such mean motions poleward can explain the lack of significant equatorwards transport of fallout debris from Novaya Zemlya. In the stratosphere, below 23 km., meridional eddy mixing obscures the pattern of slow mean motions. However, even in this layer, little Soviet test debris mixes south of about 30 °N, suggesting that poleward mean motions restrict equator-wards transport by eddy mixing within the lower stratosphere between the equator and 30 °N. 

Drevinsky, P.J. Pecci, J. (1965) Size and vertical distributions of stratospheric radioactive aerosols. In Radioactive Fallout from Nuclear Weapons Tests, ed. A.W. Element Jr. CONF765. U.S. Dept of Commerce. N.T.I.S. Springfield, Va. 

The long half-life of the two end products makes them especially dangerous. In an atmospheric nuclear explosion, the tertiary fission products are formed in the stratosphere and gradually come down to earth. Every spring about one-half to two-thirds of the fission products in the stratosphere come down and are eventually deposited by precipitation. Figure 11-6 gives a schematic outline of the pathways through which the fallout may reach us. 

I would like to congratulate Dr. Crutzen on one of the most impressive papers that I have heard in the last 43 years. With regard to its policy implications, it stands certainly in a class all by itself. I would simply underscore Dr. Crutzen s emphasis on the conservativeness of his estimates. Having looked at thousands of vertical temperature profiles, it is clear, I believe, that the residence time will be of about an order of magnitude difference between the stratosphere rather than the troposphere, and also the lapse rates, would no longer apply. So as you say, it is just an entirely different atmosphere. This would inhibit the precipitation and the fallout, so that I just really wanted to underscore the conservative nature of your calculations. 

The partitioning of radioactive debris in the troposphere and stratospheric regions is determined by the total explosive yield and the height and latitude of the burst. The total yield is the sum of the fission and fusion yields of the device. The production of important fallout radionuclides is determined by the fission yield of the weapon. Smaller yield nuclear explosions are produced by fission reactions, while larger yield explosions result from boosted fission or thermonuclear fusion reactions. Of the total yield of all atmospheric tests of440 Mt, an estimated 182 Mt, or about 40% of the total, was fission yield and the remainder was fusion yield. The contributions of countries conducting atmospheric tests to the total fission yield is shown in Table 10.3. 

Not all of the radioactive debris produced in nuclear tests is carried into the troposphere and stratosphere and dispersed as global fallout. For tests conducted on the ground or water surface, an estimated 50% of the debris remains in the local vicinity of the test site. Many tests conducted by the United States were surface explosions. Other... 

From ratio of l44Ce to 137Cs fission products in surface air and 90Sr in stratosphere and fallout... 

Fabian, P., W. F. Libby, and C. E. Palmer (1968). Stratospheric residence time and interhemis-pheric mixing of Strontium 90 from fallout in rain. J. Geophys. Res. 73, 3611-3616. 

The contribution of radiotherapy and the use of radioactive substances for diagnostic purposes to overall exposure to radiation may be very great in individual cases, but in relation to the population as a whole it is comparatively small. It is estimated at only a few percent in the form of X-rays. Surface explosions of nuclear weapons released mainly dust-bound fission and activation products about half of these were conveyed into the stratosphere, the other half caused local fallout. About three quarters of the total amount released have already decayed, the remaining external dose is estimated at approximately 0.27 p,Sv in mid-latitudes. It is due primarily to the decay of long-living Cs. 

PSCs therefore provide the surface on which the heterogeneous reactions occur. If the PSC is a solid particle, then its surface area is the relevant quantity for the reaction if a supercooled liquid, then the volume of the entire particle is accessible for the reaction. In either event, the heterogeneous reaction is fast, and, furthermore, the exact chemical composition of the PSC does not appear to exert an important influence on the reaction rate. However, the abundance of PSCs, their location and persistence, is likely a function of PSC composition. As noted above, denitrification is a necessary component of the heterogeneous catalytic cycle that is, the HNO3 must be removed from the system. Denitrification is accomplished if the PSC particles are sufficiently large to fall out of the stratosphere, carrying the absorbed HNO with them. The extent to which such fallout actually occurs is still uncertain. 

Plutonium does not undergo transformation processes in the air beyond those related to radioactive decay. Radioactive decay will be important for the short-lived isotopes with half-lives less than the average residence time in the troposphere of approximately 60 days. For example, plutonium-237 has a half-life of 46 days and undergoes electron capture to form neptunium-237 which has a half-life of 2.1x10 years (Nero 1979). Therefore, neptunium-237 may form in the stratosphere prior to deposition of plutonium-237 on the earth s surface as fallout. 

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