Global fallout

Information is available on the levels of241 Am in soil and sediment in areas affected only by global fallout, at DOE installations and other nuclear facilities, as well as sites of nuclear explosions and accidents (Alberts et al. 1989 Bennett 1979 Cooper et al. 1994 DOE 1980 Pattenden and McKay 1994 Robison et al. 1997a, 1997b Sanchez et al. 1996). 241Am levels in soil around nuclear power plants in the United States were indistinguishable from fallout background (EPRI 1981). 

Bunzl K, Kracke W. 1987. Soil to plant transfer of 239t240Pu, 238Pu,241 Am, 137Cs and 90Sr from global fallout in flour and bran from wheat, rye, barley and oats, as obtained by field measurements. Sci Total Environ 63 111-124. 

Hardy, E. P. 1974. Depth Distribution of Global Fallout of 90Sr,, 37Cs and 239240Pu in Sandy Loam Soil. Fallout Program Quarterly Summary Report HASL-286. US Atomic Energy Commission, New York. 

For the area with a 90Sr contamination density of 0.1 Ci/km2 (double the level of global fallout) the maximum length of the deposition track under the radioactive plume reached 300 km for the 90Sr contamination density of 2 Ci/km2 it reached 105 km, with a width of 8-9 km. ... 

The term global fallout appears to refer to the peak level reached in the early 1960s rather than to that in 1957. 

In some tests carried out at the Nevada Test Site in 1956-8, fission was incomplete and some plutonium was dispersed. Also some weapons were exploded chemically without nuclear fission in safety tests. Analysis of soil from an area extending about 500 km north-eastwards from the N.T.S. into the neighbouring state of Utah showed excess Pu over the amounts expected from global fallout (Hardy, 1976). The Pu from these low-level, low-yield tests was distinguished from global fallout by two criteria ... 

The total fallout from the low-yield trials in Nevada and Utah (outside the NTS boundary) was estimated by Hardy at 26 TBq (700 Ci), compared with about 9 TBq of global 239+240pu falling in the same area. The peak measured fallout, outside the N.T.S., was 600 Bq m-2 at Eureka, NV. This compares with mean global fallout of 60 Bq m-2 in the USA (Hardy, 1976) and 50 Bq m-2 (normalised to 1000 mm a-1 rainfall) in the UK (Peirson et al., 1982). Within the N.T.S., the deposition of Pu was much greater, ranging from 4 x 104 to 2 x 108 Bq m-2 (Martin Bloom, 1980). 

Cambray Eakins (1982) measured Pu in samples of soil taken at various distances inland from the Cumbrian coast near Sellafield. Near the coast the activity exceeded by a factor of about 10 that expected from global fallout, and the 238Pu/239+240Pu ratio was approximately equal to that of the cumulative Sellafield effluent. Going inland, the excess Pu in soil declined (Fig. 5.8), and the rate of decline was similar to the rate of decline of deposits of NaCl inland from coasts (Cambray Eakins, 1982). However, the Pu/Na ratio in the deposit was two orders of magnitude higher than the ratio in seawater, whereas the activity of 137Cs in the deposit was about as expected from the Cs/Na ratio in seawater. 

From Table 3 data it follows that in the course of the first 10 days after the submarine sinking radionuclide composition of bottom sediments was determined (99%) by " k -natural-origin radionuclide the remaining 1% was due to man-caused Cs which concentrations (3-5 Bq/kg) were attributed to global fallouts. That conclusion was confirmed by the results of radiation background measurements and independents surveys (see Reference [8]). 

Actinides found as environmental contamination in mosses collected from a bog in the eastern Italian Alps were analyzed after their chemical separation by extraction chromatography (deposited on steel targets) with respect to isotope ratios and their concentration was determined by LA-ICP-MS. Moss samples were contaminated with a variety of actinide isotopes. The detection limits for actinides were determined as 3.6-7.2 x 10 gg for " Am and respectively. The Pu/ Pu isotope ratio (0.212 0.003) was almost constant within experimental error for all samples investigated. Pu contamination in moss samples was mainly the result of global fallout after nuclear weapons tests. " Am was found at the 2x 10 " gg level. This example demonstrates that mosses can be used as bioindicators for environmental contamination. ... 

The important accidents involving commercial plants were the Three Mile Island Reactor partial meltdown accident, which did not breach the outer containment but totally ruined the plant, and the Chernobyl meltdown accident, which caused major releases of radioactivity to the atmosphere and global fallout. 

The isotopic ratios of the investigated samples are shown in Figure 3-6. Typical isotope ratios of reactor-grade plutonium (Reactor GP ), weapons-grade plutonium (Weapon GP ) and global fallout values of the Northern Hemisphere" are also indicated. Note that Pu/ Pu and " Am/ Pu ratios are highly dependant on the decay time, while isotope ratios in reactor-grade plutonium depend on reactor-type and bum-up. ... 

Samples from contaminated areas have significantly different isotopic composition than the typical global fallout values. The isotope ratios of Chernobyl (CHERMOSS) and Pripjaty (PRIPMOSS) moss samples agree with previouslv reported values. " Beside the high Pu, Pu and " Pu values, significant amount of was also observed in the... 

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... 

The atmospheric tests The radioactive material resulting from the atmospheric tests is essentially all residual material from the four barge tests there was very little local fallout from the explosion of any device suspended from a balloon, since the fireball did not touch the surface and radioactive material was drawn up into the atmosphere. Most of the residual material is plutonium found in the lagoon sediments immediately underneath where the barge tests were conducted, remains of the nuclear cores of the test devices. There is also present in the environment of the atolls global fallout from atmospheric nuclear tests conducted by other states, mainly in the Northern Hemisphere. 

Mururoa has been the subject of three independent scientific missions during the 1980s. These studies were not allowed complete freedom to take samples and the information available to them was limited. The general conclusions were that there were low concentrations of radioisotopes in the environment consistent with the levels expected from global fallout from atmospheric tests in both the northern and southern hemispheres (which ceased in 1980). There was insufficient evidence available for the missions to form any firm conclusions about the likelihood and rates of leakage of radioisotopes from the underground tests. 

Only five transuranic elements exist or are anticipated to be produced in amounts which could lead to significant environmental concentrations. These are neptunium (Np), plutonium (Pu), americium (Am), curium (Cm), and californium (Cf). Of these five, only two, plutonium and americium, have been detected and measured already in the marine environment as a result of global fallout of nuclear testing debris. The procedures described below were developed specifically to measure plutonium and americium. However, as will be expanded later, the techniques for measuring americium are also able to detect curium and californium should they be present in significant amounts in the future. 

---