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INVESTIGATING FALLOUT FROM NUCLEAR TESTING HOT PARTICLES & THE

BY PIER ROBERTO DANESI

ntering the next century, stratospheric fallout. The first more than 2000 nuclear consists of that are not Etest explosions of various carried across the tropopause sizes and varieties will have been and that deposit with a mean recorded. Nearly all of them of up to 30 days. were conducted during the Cold During this time, the residues War period ending in the . become dispersed in the latitude Atmospheric nuclear tests band of injection, following dispersed radioactive residues trajectories governed by wind into the environment. They are patterns. Stratospheric fallout partitioned between the local arises from particles that later ground (or water surface) and give rise to widespread global the tropospheric and fallout, most of which is in the stratospheric regions, depending hemisphere where the nuclear on the type of test, location, and test was conducted. It accounts yield. The subsequent for most of the residues of long- composition precipitation carrying the lived fission products. between the time of a nuclear residues leads to both local and results in the detonation and the time global fallout. Concentrations exposure of people to samples are collected for 43 of certain can radioactivity through internal radiochemical analysis. A result in formation of “hot (due to inhalation process called fractionation particles” — tiny bits of of radioactive materials in air causes samples of radioactive materials containing radioactive or ingestion of contaminated residues to be unrepresentative chemical elements. food) and external irradiation of the detonation products. Local fallout includes large (by radioactive materials Fractionation begins with radioactive aerosols, particles present in surface air or the condensation of radioactive which are generally deposited deposited on the ground). and inert material from the within about 100 kilometers of Extensive studies in these areas fireball. The mixture may the test site. Local radioactive have been done. begin to separate while contamination at nuclear- In the case of nuclear testing, condensation is still in progress weapon test sites additionally is evaluations of the nature of the because of the influence of attributed to safety trials of primary event typically include wind, gravity and the nuclear devices that often analyses of radioactive material turbulence of the atomic dispersed . This deposited on the ground. Such cloud. Further separation of material is released in various studies are problematical, the condensate occurs through forms, including however, because there can be various processes, including vapour, plutonium aerosols of significant alteration of contact of the residues with various sizes, plutonium oxide inert material. , plutonium-coated Photos: A marker at the former When the atomic cloud is particles, and sizeable lumps of nuclear test site in , formed, the processes of plutonium-contaminated southern Australia. At the structural material destroyed Mururoa test site in French Mr. Danesi is Director of the by the test explosion. Polynesia, an IAEA team IAEA Seibersdorf Laboratories Global fallout encompasses collected soil samples for analysis near the Agency’s headquarters in both tropospheric and of hot particles. (Credit: Danesi/IAEA) Vienna, Austria. IAEA BULLETIN, 40/4/1998 Doc-11.qxd 1/21/99 10:25 AM Page 44

cooling, condensation, Information on the physico- STUDIES IN SOUTH coagulation, mixing, and chemical form is limited. AUSTRALIA separation take place After deposition, particles are From 1953-63, the United simultaneously but to different subject to weathering and the Kingdom conducted a pro- extents in different regions of associated radionuclides are gramme of nuclear-weapon the cloud. Furthermore, the mobilised over time. The particle testing at Maralinga and Emu initial radioactive products composition, possible structural in South Australia. At these change in elemental form changes, and the chemical sites, now completely rehabili- through processes of conditions at the deposition site tated, nine nuclear explosions . will influence the weathering and several hundred smaller- Hot particles. Scientific rate. Moreover, the mobilised scale trials were performed. understanding of the radionuclides can also interact While the major explosions fractionation phenomena is with soil and sediments. had a yield between one and important for interpreting Most models assessing the 27 kilotons, the minor trials global fallout and the nuclear transfer and consequences of only involved the burning and chemistry of the detonation radionuclide contamination explosive dispersal of , process. It is also useful for assume that the radionuclides plutonium, and short-lived evaluating contamination and are present as ionic or low radionuclides. The ingestion hazards. molecular weight species. This environmental consequences of Variations in size and type of may easily lead to an these trials were extensively particles are accompanied by overestimation of the short- studied by the Australian variations in radiochemical term consequences of the Laboratory (ARL) composition, according to radiological contamination. and have been the subject of a fractionation patterns. The On the other hand, if the number of publications. nature and concentrations of particles are rather inert, as The most significant radionuclides, together with the often the case, the transfer of plutonium contamination at 44 size and shape of particles, are in radionuclides will be delayed Maralinga resulted from a turn the factors determining the until weathering occurs. series of twelve safety trials in inhalation or ingestion hazard. Consequently, the assessment which 22 kg of plutonium Other important scientific of the long-term consequences (and a similar quantity of information concerns the of radiological contamination uranium-235) were released chemical form in which can be underestimated. into the environment. The radionuclides are present in the It is then apparent that material was dispersed by particles. The radionuclides unless the role of hot particles conventional explosives and present in hot particles are in is taken into account, model little or no nuclear reactions general relatively inert compared predictions can be affected by took place. Plutonium was to , atoms and low significant uncertainties. ejected vertically to altitudes molecular mass species that are The presence of hot particles up to 800 meters and was more mobile and easily available. may also render invalid some dispersed by the prevailing To assess short- and long- assumptions made when winds over considerable term consequences of dealing with soil and sediment distances. This caused the atmospheric fallout — and in contamination. This includes contamination of considerable particular the leakage of the frequent assumption that portions of land. Plutonium radionuclides from hot the scaling of surface or mass could be found many particles — detailed physico- activity concentrations to units kilometers away from the chemical information on this of various magnitudes (for detonation points. source term is essential. example, from Bq/cm2 to ARL identified plutonium in Usually, characteristics of Bq/m2 or Bq/km2 and vice basically three forms — as source terms have been versa) is a legitimate operation. superficially coated on materials restricted to inventory The following examples such as pieces of metals, plastic, estimates, activity levels, or illustrate specific approaches wire, and lead bricks that were activity concentrations of for studying hot particles at part of the experimental IAEA BULLETIN, 40/4/1998 deposited radionuclides. nuclear-weapon test sites. assemblies; as tiny fragments or Doc-11.qxd 1/21/99 10:25 AM Page 45

particles, not always visible to the 241) and for biological uptake. plutonium-239 in the form of eye but easily detectable by a Five sub-millimeter particles finely divided plutonium and gamma monitor; and as very with activities ranging from 30 plutonium oxide. Although finely dispersed material Bq to 5 kBq were also analysed extensive cleanup operations consisting of contaminated soil by proton-induced X-ray were done in 1982-87, particles and recondensed emission spectroscopy to gain plutonium hot particles were left plutonium particles in the same information on their elemental on the surface of the Colette size range as the soil itself. composition and homogeneity. region and in the adjacent sand Samples of this material were In these particles, which were bank in the lagoon. collected and separated by several hundreds of microns in As part of the international sieving down to a diameter of diameter, plutonium and Study, the residual 45 micrometers to determine uranium were found contamination in the terrestrial mass and activity homogeneously distributed on environment of Mururoa and concentrations. Results showed the surface. The major Fangataufa, including the that the greatest mass was elements identified were Colette region, was assessed by generally associated with the aluminum (1.8%), potassium a team of scientists. A sampling 250-500 micrometers fraction; (2.3%), (1%), iron campaign was organized and however, most of the activity (23%), lead (1.9% to 35%), coordinated by the IAEA’s (about 41%) was concentrated uranium (2.9% to 0.8%) and Seiberdorf Laboratories and in the fraction below 45 plutonium (19%). Dissolution conducted in the summer of micrometers, which contained studies in simulated lung fluid 1996. This was followed by only 5% of the total mass. indicated that the particles had extensive radiochemical The lowest fraction was no significant solubility. measurements on about 300 further fractionated into seven samples. (See article, page 24.) aerodynamic sizes ranging STUDIES IN The analysis of the residual from 45 micrometers to less FRENCH contamination in the Colette than three micrometers. In this POLYNESIA region identified the presence 45 way, the inhalable fraction — Other investigations of hot of plutonium-containing hot defined as that with a size less particles were conducted in particles. Twenty relatively than seven micrometers — was French Polynesia by an expert large particles, ranging in size also identified. team as part of the interna- from 200 micrometers to one Particles were also identified tional Study on the millimeter, were separated in an 800-gram sample of soil Radiological Situation at the from debris and crushed (having an activity of 25 Bq of Atolls of Mururoa and coral rocks. They were americium-241), broken down Fangataufa, completed in measured by high resolution by a process of binary 1998. From July 1966 to gamma spectrometry to separations into discrete September 1974, forty-one evaluate their activity and the particles. All the sample’s atmospheric nuclear tests were ratio of plutonium-239 to activity was found to be conducted there. In addition, americium-241. The activity of contained in the separated 54 five safety trials were con- plutonium-239 was found to hot particles. The activity of ducted on the surface at the fall in the range 5 to 300 kBq, the individual particles was northern tip of the Mururoa although a particle with a level estimated to be in the range atoll, in areas generally referred of about one MBq was also 0.1 to 2.0 Bq of americium- to as the Colette region. found. The americium-241 241 and the average diameter The safety trials were activity in the particles was in was about 20 micrometers. conducted to investigate the the range 0.2 to 5.6 kBq. Six of The study also identified a behaviour of the core of nuclear these hot particles, with large number of sub-millimeter devices under simulated faulty diameters ranging from 200 to hot particles which were detonation conditions. The core 500 micrometers, were also analysed by high resolution was destroyed by a conventional studied by optical microscopy gamma-ray spectrometry (to explosive detonation with the and micro X-ray fluorescence. determine the ratio of consequential dispersion during The observations revealed that plutonium-239 to americium- each test of about 3.5 kg of some particles had a smooth, IAEA BULLETIN, 40/4/1998 Doc-11.qxd 1/21/99 10:25 AM Page 46

glassy compact nature while FUTURE NEEDS in 1968, the Cosmos others appeared rough and The investigation of hot parti- crash in Canada in 1978, and could even be conglomerates of cles is highly relevant to correct the accident in 1986. smaller particles. evaluations of radiation hazards Hot particles have also been In addition to plutonium, at sites which were contaminated released to the environment at the particles were found to by nuclear-weapon testing. nuclear facilities contributing contain other elements. They Studies so far, however, have to the production of fissile included uranium and led to more questions than material for nuclear-weapon neptunium at concentrations final answers. They indicate programmes. 10 to 100 times lower than that the information on hot In general, any facility for plutonium; calcium (indicating particles and particle fractiona- the processing of nuclear they had a calcium carbonate tion obtained through radio- material is known to release matrix); iron (indicating their chemical, chemical and physi- small, but detectable, amounts metallic nature); chlorine (most cal analyses of debris is still of radioactive and non- likely from the sea salt); and too small and too scattered radioactive isotopes to the traces of manganese, nickel, when viewed against the diver- immediate environment. These chromium, , and titanium sified nature of particles pro- releases, which are often (probably reflecting the duced and the number of vari- insignificant from the radiation composition of the ables requiring investigation. hazard point of view, can be in containers of the fissile material Therefore, the complex phe- the form of waste streams, used for the safety trials). nomena which control the for- aerosols, or particles and can To estimate the distribution mation, the chemical and be found at some distance of plutonium in coral debris, a radiochemical composition, downwind or downstream of 1053-gram sample was also and the physical and morpho- the point of release. sieved in seven size fractions. logical properties of hot parti- The same modern analytical The activity of plutonium and cles and their behaviour in the techniques used to investigate 46 americium in the various natural environment are still hot particles from nuclear testing fractions were measured by not fully understood. can be applied to studying other high resolution gamma It is apparent that the sources of environmental spectrometry. The results phenomena leading to the radioactivity. They enable showed that 99.9% of the mass formation of hot particles and scientists to measure extremely and 95.8% of the activity were their behaviour in the ecosystems small amounts of chemical present in particles larger than are complex. Thus, any elements and isotopes present in 250 micrometers. generalization must be advanced released radioactive materials, Nevertheless it must be with caution. It is believed that thereby providing information pointed out that the study did further progress in this area will about the process which formed not exclude the presence of require multi-disciplinary teams them. particles smaller than 10 of scientists. They should include Hopefully in years ahead, micrometers (containing the physical chemists, specialists in more studies on hot particles will inhalable fraction) with non-destructive microanalytical be carried out at sites that have plutonium-239 activities of techniques, radiochemists, been contaminated by nuclear- several hundred becquerels. In nuclear physicists, and health weapon testing and various types vitro dissolution studies in physicists. of nuclear accidents, and at sites serum simulant of six hot It also must be pointed out near nuclear installations. The particles showed that the that hot particles of various sizes work will benefit from advances quantity of plutonium and composition containing in instrumental analytical solubilized was in all cases less actinides, fission or activation techniques. It will further lead to than 0.07% of that initially products are released to the greater understanding of the role present in the particles. This environment from other sources that hot particles play in indicated dissolution besides nuclear testing. For evaluating radiation hazards, and characteristics similar to those example, they were produced by in providing information on the of the particles from the the Windscale pile fire in 1957, type and purpose of the facilities IAEA BULLETIN, 40/4/1998 Maralinga nuclear test site. the B-52 aircraft crash at Thule that generated them. ❐