Particles, plutonium-contaminated

The plutonium contamination close-in at Taranaki is mainly in three forms, viz. as a fine dust, as small sub-millimetre particles, and as surface contamination on larger fragments (Bums et al., 1986). In the trials, the plutonium was dispersed in narrow plumes, the main ones extending to the west, north-west, north and north-east of Taranaki. The most extensive of these is the north-west plume, which can be detected crossing West Street between Fifth and Tenth Avenues (Fig. 10.10). In the central area... 

Cooper et al. (1994) have reported re-suspension studies on soils contaminated with plutonium during nuclear weapons tests by use of a mechanical dust-raising apparatus. Airborne dust was analysed in terms of mass and Am activities for particle sizes less than 7 pm. The AMAD was determined as 4.8-6 pm for re-suspended soil. Also, surface soil was characterised in the laboratory by means of sieving and microparticle classification, yielding mass and "Am activity distribution with respect to size. Data indicate the granularity of plutonium contamination at both major and minor trial sites. Depth profile analyses for undisturbed areas demonstrate that most (74% on average) of the americium and plutonium activity is found in the top 10 mm of soil. Plutonium and americium activities were found to be enhanced in the inhalable fraction over their values in the total soil, and the enhancement factors were similar in re-suspended dust and surface soil. Observed enhancement factors ranged from 3.7 to 32.5. 

Since soil-adsorbed plutonium contamination exists as discrete particles of various sizes, analysis of larger soil volumes (25 to 100 grams) is recommended (Bernhardt 1976). Commonly, soil samples with high amounts of carbonate are difficult to analyze. More rapid, efficient, and economical procedures are being developed to sequentially analyze a number of radioactive actinides (Flindman 1986). 

Viewed under a microscope, the incinerator ash of plutonium-contaminated waste sample consists of primarily dark crystallites 10-50 /im in size with a small amount of clear and orange particles. The primary interest in this study was to identify the plutonium species present and their relationship to other elements or molecular components in the sample. 

Plutonium has a much shorter half-life than uranium (24.000 years for Pu-239 6,500 years for Pu-240). Plutonium is most toxic if it is inhaled. The radioactive decay that plutonium undergoes (alpha decay) is of little external consequence, since the alpha particles are blocked by human skin and travel only a few inches. If inhaled, however, the soft tissue of the lungs will suffer an internal dose of radiation. Particles may also enter the blood stream and irradiate other parts of the body. The safest way to handle plutonium is in its plutonium dioxide (PuOj) form because PuOj is virtually insoluble inside the human body, gi eatly reducing the risk of internal contamination. 

Half-lives span a very wide range (Table 17.5). Consider strontium-90, for which the half-life is 28 a. This nuclide is present in nuclear fallout, the fine dust that settles from clouds of airborne particles after the explosion of a nuclear bomb, and may also be present in the accidental release of radioactive materials into the air. Because it is chemically very similar to calcium, strontium may accompany that element through the environment and become incorporated into bones once there, it continues to emit radiation for many years. About 10 half-lives (for strontium-90, 280 a) must pass before the activity of a sample has fallen to 1/1000 of its initial value. Iodine-131, which was released in the accidental fire at the Chernobyl nuclear power plant, has a half-life of only 8.05 d, but it accumulates in the thyroid gland. Several cases of thyroid cancer have been linked to iodine-131 exposure from the accident. Plutonium-239 has a half-life of 24 ka (24000 years). Consequently, very long term storage facilities are required for plutonium waste, and land contaminated with plutonium cannot be inhabited again for thousands of years without expensive remediation efforts. 

Harrison JD, Hodgson A, Haines JW, et al. 1993. The biokinetics of plutonium-239 and americium-241 in the rat after subcutaneous deposition of contaminated particles from the former nuclear weapons site at Maralinga Implications for human exposure. Hum Exp Toxicol 12 313-321. 

Radionuclide Exposure of the Embryo/Fetus (1998) Recommended Screening Limits for Contaminated Surface Soil and Review of Factors Relevant to Site-Specific Studies (1999) Biological Effects and Exposure Limits for Hot Particles (1999) Scientific Basis for Evaluating the Risks to Populations from Space Applications of Plutonium (2001)... 

Plutonium is so toxic that processing and fabrication are always done in sealed cells or glove boxes, but accidental dispersions of aerosol occur from time to time. Following combustion of Pu metal chips in a production area at Rocky Flats, Colorado, in 1964, airborne contamination was widespread. Alpha tracks from individual particles caught on membrane filters were detected on nuclear film, and the Pu content, and hence the particle size, was deduced (Fig. 5.2, curve E). The activity median diameter was 0.3 /urn (Mann Kirchner, 1967). The same method, used during normal operations in a production area at Los Alamos, gave activity median diameters in the range 0.15 to 0.65 /urn (Moss et al., 1961). However, when a spill occurred, followed by clean-up operations, the Pu particles were found to be associated with inert dust particles of mass median diameter 7 /urn. 

Resuspension of dust from the floor is an important source of indoor pollution. Jones Pond (1966) studied resuspension of plutonium from the floor of a laboratory which had been deliberately contaminated. Droplets were dispersed over the floor and allowed to dry out. Pu was present in the droplets either as a suspension of particles, mass median diameter 15 /mi, or as nitrate in solution. The activity per unit area was measured with a floor probe. After 16 h had elapsed, airborne Pu was measured while operators performed various tasks in the laboratory. 

Emitted by heavy atoms, such as uranium, radium, radon, and plutonium (to name a few), alpha particles are helium nuclei, making them the most massive kind of radiation. Alpha radiation can cause a great deal of damage to the living cells it encounters, but has such a short range in tissue (only a few microns) that external alpha radiation cannot penetrate the dead cells of the epidermis to irradiate the living cells beneath. If inhaled, swallowed, or introduced into open wounds, however, alpha radiation can be very damaging. In nature, alpha radiation is found in rocks and soils as part of the minerals, in air as radon gas, and dissolved in water as radium, uranium, or radon. Alpha emitters are also found in nuclear power plants, nuclear weapons, some luminous paints (radium may be used for this), smoke detectors, and some consumer products. Objects and patients exposed to alpha radiation may become contaminated, but they do not become radioactive. 

Weapons-grade plutonium, dispersed at military accidents such as Thule in 1968 or as non-fissioned weapon particles after detonation of a Pu-bomb can be characterized by high Pu content relative to the other Pu-isotopes, while accidentally dispersed Pu from the previously widely used nuclear-powered satellites are characterized by high Pu content." The ratio of americium-241 to plutonium isotopes (as " Am is formed by the decay of Pu) is proportional to the initial " Pu concentration, thus it can also be used as an indicator to assess the origin of contamination. However, in most cases, as several sources may contribute to the transuranics content in environmental samples, mixing models applying several isotope ratios are required to assess the origin of possible contamination sources. 

Somewhat different results were found in studies from the British Atomic Weapons Test Site at Maralinga, South Australia, where specific activities were noted to be greater in the soil size fractions >90 pm (Ellis and Wall, 1982). Presumably there are numerous factors that might influence the relationship of plutonium activity with soil particle size including the nature of the contaminating event, the degree of weathering since the contamination event, the chemical nature of the soil, and the particle size distribution of the soil. 

Some quantitative insight into the transfer through the soil may be obtained from the behavior of the fallout plutonium. This represents an analogy to an industrial contamination by high fired PuC particles of a grain size about 0.04 y. 

Plutonium is dangerous mainly on account of its radioactivity, in the case of an internal contamination. Pu has a very long half-life (2.44 x lO years), and it emits alpha-particles E = 5.15 MeV (72%), 5.13 MeV (17%), 5.10 MeV (11%)) with a soft gamma-ray component [22]. It is deposited particularly in the bones and it is excreted with the urine. It damages the liver, haemopoiesis and it also frequently induces osteosarcomas... 

Atmospheric testing of nuclear weapons has been the main source of plutonium dispersed in the environment. Accidents and routine releases from weapons production facilities are the primary sources of localized contamination. Consumer and medical devices containing plutonium are sealed and are not likely to be environmental sources of plutonium (WHO 1983). Plutonium released to the atmosphere reaches the earth s surface through wet and dry deposition to the soil and surface water. Once in these media, plutonium can sorb to soil and sediment particles or bioaccumulate in terrestrial and aquatic food chains. 

Terrorism by way of radioactive materials can come in several forms. If the radioactive isotope is a fissionable material (material that can be used to create a chain reaction that could result in a nuclear explosion), such as some plutonium and uranium isotopes, it is possible that it could be combined into a nuclear weapon. Otherwise the radioactive material might be dispersed by a conventional explosion or by airborne means such as a powder or spray from an aircraft. A building could be contaminated by placing finely ground radioactive particles or powder in the ventilation system. The material could also be spread by hand (more dangerous for the terrorist) or mailed in a package or envelope. 

An area of more than 200,000 km in Europe was contaminated with radiocesium (above 0.04 MBq of Cs/m ), of which 71% was in the three most affected countries (Belarus, the Russian Federation, and Ukraine). The deposition was highly heterogeneous it was strongly influenced by rain when the contaminated air masses passed. Most of the strontium and plutonium radioisotopes were deposited close (less than 100 km) to the reactor, due to their being contained within larger (hot) particles. 

All of the plutonium isotopes, with one exception, decay by alpha particle emission. The exception - " Pu beta-decays to " Am. It follows that uranium which has been reprocessed, and materials contaminated by it, may contain all of these transuranic nuclides. Because of their low gamma-ray emission probabilities, low levels of the plutonium isotopes are not easily measured by gamma spectrometry. 

Plutonium aerosols can be formed in various ways, including (a) oxidation or volatilisation of Pu metal, (b) oxidation or volatilisation of irradiated U or UO2, (c) droplet dispersion from aqueous solutions or suspensions of Pu, and (d) resuspension of soil or dust which has become contaminated with Pu. The particle size of Pu-aerosols is very variable, depending on the mode of formation. 

---