Radionuclides may enter the atmosphere as gas, aerosol or particulate matter. The transport of suspended radionuclides is dependent on particle size; larger particles will settle and deposit faster than smaller particles. Following the release into the atmosphere, the dispersion of radionuclides is mainly controlled by meteorological conditions (i. e. winds, turbulence, advection and wet and dry precipitation), radioactive decay and diffusion.

For atmospheric nuclear weapons tests, transport of the radioactive debris will depend on the height and yield of the explosion, the nature of the debris, the location of the test site and prevailing meteorological conditions. Refractory radionuclides, such as plutonium, 95Zr and 144Ce, are released mainly in parti­culate form,5,48 and so will tend to be deposited more rapidly, and be less widely dispersed, than more volatile radionuclides, such as 137Cs and 131I.49 During testing, radioactive debris will be injected into the atmosphere at different heights, and this will depend primarily on the height of the test and the explosive yield; low yield tests will tend to release debris into the troposphere, with the quantity of radioactive material released into the stratosphere increasing with yield.50 For tests conducted near the surface, it is estimated that around 50% of the debris is deposited locally or regionally, with the remainder more widely dispersed.5,49 Debris released into the troposphere (the lowest level of the atmosphere) can be transported up to several thousand kilometres from the test site over 1-2 weeks, as a result of the turbulent air movements that occur there.49,50 Removal of particulate debris from the troposphere is mainly caused by precipitation but dry deposition of radionuclides can also occur.50 Radioactive debris released into the stratosphere remains in the atmosphere for much longer periods of time (> 1 year) than material released at lower altitudes, and so will be dispersed over a much greater area, with precipitation the main mechanism for deposition.6,10 As a result, global radioactive contamination arising from deposition of material from the stratosphere will consist of longer-lived radionuclides, compared to local and regional contamination.6,49 Simon et al. (2004)6 investigated the geo­graphical distribution in the USA of radionuclide fallout arising from tests at the Nevada Test site (NTS) and global fallout. The distribution of radioactive debris from the low yield tests at the NTS depended on the wind patterns and local rainfall events at the time of the test, but in general the highest levels of deposition were in the region immediately east of the site. With global fallout, higher levels were deposited in the eastern and mid-western regions than the south-western states, reflecting the relative levels of precipitation in these regions.

More localised atmospheric transport of radionuclides occurs with the use of DU weapons and uranium mining and milling. When a DU munitions hits its target, an estimated 10%-35% (maximum of 70%) of the DU mass is converted into aerosol, with most of the dust particles < 5 pm.26 The transport of DU particles will depend on particle properties (i. e., size and density) and on prevailing meteorological conditions.51 Surveys of the post-conflict zone in Kosovo and Bosnia and Herzegovina reported DU contamination up to 200 m away from the point of impact.52,53 Lloyd et al. (2009)54,55 investigated the dispersion of aerosols formed during the combustion of waste metal at a ura­nium and DU processing factory in Colonie (NY, USA). The distribution of the DU aerosol was controlled by prevailing winds, with DU contamination found up to 600 m from the factory. It has been estimated that at least 3.4 tonnes of uranium was deposited within 1 km of the factory.56 Resuspension of DU dust has also been found to occur by wind or human disturbance.

From uranium mining and milling, radon gas will be released into the atmosphere, but in arid climates, windborne dispersion of fine radioactive particulate wastes can also be a problem.16,57 Lottermoser and Ashley (2006)58 investigated the physical dispersion of radioactive waste from a rehabilitated uranium mine in South Australia. Under the semiarid conditions at this site, there had been significant wind dispersion of radioactive particulates from the site. Around the main tailings storage facility, tailings material up to 10 cm thick was spread up to 80 m from the source in the northeast and southeast sides, reflecting the prevailing wind directions at the site. Around this source an area of 1km2 had uranium concentrations >100 ppm, with another 2 km2 con­taminated with 10-100 ppm of uranium. Radon, generated in the subsurface or in waste materials, is mainly released into the atmosphere by diffusion, but advection caused by wind and changes in barometric pressure can also play a role and mining activities will enhance rates of release into the atmosphere.24,59

Radioactive materials released into the atmosphere from the accident at the Fukushima Nuclear plant were detected globally but at very low levels. Mon­itoring undertaken by the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organisation (http://www. ctbto. org/press-centre/ highlights/2011/fukushima-related-measurements-by-the-ctbto/fukushima-related- measurements-by-the-ctbto-page-1/) detected traces in eastern Russia on 14th March 2011, three days after the earthquake and tsunami that damaged the reactors. Radiation was detected on the west coast of USA by 16th March and all across the northern hemisphere 15 days after the accident. The equator acts as a dividing line between the northern and southern air masses, and so the dispersal of radioactive materials was initially limited to the northern hemisphere; however, by 13th April, radiation from Fukushima had spread to the southern hemisphere.