Fallout formation

This paper deals mainly with the condensation of trace concentrations of radioactive vapor onto spherical particles of a substrate. For this situation the relation between the engineering approach, the molecular approach, and the fluid-dynamic approach are illustrated for several different cases of rate limitation. From these considerations criteria are derived for the use of basic physical and chemical parameters to predict the rate-controlling step or steps. Finally, the effect of changing temperature is considered and the groundwork is thereby laid for a kinetic approach to predicting fallout formation. The relation of these approaches to the escape of fission products from reactor fuel and to the deposition of radon and thoron daughters on dust particles in a uranium mine is indicated. 

Because the subject is vast, the presentation is limited to a discussion of the uptake of a tracer from the vapor phase by spherical particles. This is the viewpoint of one concerned with fallout formation. The reverse process—escape from spherical particles—is the viewpoint of one concerned with reactor fuels. For the idealized case the treatment is exactly the same for the two situations. The fact that we deal with trace quantities and concentration means that we can neglect changes in the particle properties as the reaction proceeds and that we need not be concerned with surface nucleation. 

Recent work at NRDL (1) has established the rate-controlling steps for several cases of interest, and the time is now ripe to lay the foundation for a realistic approach to fallout formation. 

Of course, Equations la and lb can be reversed to consider fractional uptakes in an atmosphere that at equilibrium would lead to a concentration C0 throughout the particle. This amounts to only a change in initial conditions, and the fractional uptakes are given by F — 1 — C(t)/C0. This form of Equations la and lb is more useful in calculations of fission product fractionation during fallout formation as discussed later. 

Calculation of Isotope Fractionation During Fallout Formation... 

Fractionation of fission products during fallout formation was recognized by Freiling (4) in early studies of fallout particles. He also recognized that this phenomenon involved the volatility of the fission products. In an attempt to describe fractionation quantitatively, Miller (9) devised... 

The concepts presented in this paper emphasize the importance of diffusional phenomena during fallout formation and HTGR fission product retention. While other phenomena may be of considerable importance, these studies present a worthwhile position from which to view these processes. The advances resulting from those conceptual points of view have been considerable and it is believed are far from exhausted. 

Tables II through V give the final selected and recalibrated data for Small Boy in alphabetical order of the performing laboratory. In a number of cases, data from similarly behaving nuclides are grouped and averaged in the belief that little or no information is lost in the process. Inspection of these tables indicates a gratifyingly high precision in many cases. The data should form a fruitful basis for subsequent analysis of the fallout-formation phenomena occurring in Small Boy. The relation of sample designation to sample source and treatment is as follows ...

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). 

Rayleigh distillation is a process in which the condensate is immediately removed from the vapor after formation (by fallout of rain and snow in the meteorological case) and leads to a higher... 

Touring the formation of radioactive fallout particles, one of the most important processes is the uptake, in the cooling nuclear fireball, of the vaporized radioactive fission products by particles of molten soil or other environmental materials. Owing to the differences in the chemical nature of the various radioactive elements, their rates of uptake vary, depending upon temperature, pressure, and substrate and vapor-phase composition. These varying rates of uptake, combined with different residence times of the substrate particles in the fireball, result in radiochemical fractionation of the fallout. This fractionation has a considerable effect on the final partition of radioactivity, exposure rate, and radionuclides between the ground surface and the atmosphere. 

The half-life of 244Pu (8.2 X 107 years) is short compared with the age of the earth (4.5 X 109 years), and hence this nuclide is now extinct. However, the time interval (a) between the element synthesis in stars and formation of the solar system may have been comparable with the half-life of 244Pu. It has been found recently in this laboratory that various meteorites contain excess amounts of heavy xenon isotopes, which appear to be the spontaneous fission decay products of 244Pu. The value of H calculated from the experimental data range between 1 to 3 X 108 years. The process of formation of the solar system from the debris of supernova is somewhat analogous to the formation of fallout particles from a nuclear explosion. 

Table I. Time Interval (S) Between Nuclear Detonation and Formation of Single Fallout Particles...

Table II summarizes some of the features of the radioactive fallout processes in geophysical and astronomical settings. It seems that similarities do exist between the processes of formation of single particles from nuclear explosions and formation of the solar system from the debris of supernova explosion. We may be able to learn much more about the origin of the earth, by further investigating the process of radioactive fallout from the nuclear weapons tests.

As in the case of the land surface burst, complete characterization of the particle population requires only that particle mass, a volatile species, and a refractory species distribution with particle size be determined. All other isotopic distributions may be deduced from the istotope partition calculations described above. In the subsurface detonation, the earliest aerial cloud sample was obtained in the cloud 15 minutes after detonation. The early sample was, therefore, completely representative of the aerial cloud particle population. In Figure 5 the results of the size analysis on a weight basis are shown. Included for comparison is a size distribution for the early, local fallout material. The local fallout population and the aerial cloud population are separated completely from the time of their formation. 

It has been known for many years that the fission products observed in the field or in the laboratory some time after the event are in fact not usually the species produced in fission at all but the result of one or several consecutive beta disintegrations of shorter lived isobaric precursors which are formed directly in the fission process. From the chemist s point of view this is important because the f -decay process is an actual transmutation of elements, and the time scale involved is frequently comparable with that for the formation of fallout particles. 

Fractionation correlation techniques have been applied to cloud, fallout, and ground-filter samples from the Transient Nuclear Test of January 1965. Although safety analysts do not consider fractionation effects to be of operational importance for this type of event, analysis of such data provides insight into the mechanisms of debris formation. The results show many similarities to the correlations observed for fallout. Those dissimilarities found indicate the importance of escape processes to the formation mechanisms for this type of debris. 

Although the transient test was orders of magnitude below a nuclear weapon in regard to energy release and temperature achieved, the debris showed many similarities to fallout. These included not only the size and appearance of the particles but also the correlation properties of various radionuclides. Dissimilarities in the correlations and the variation of specific activity with particle type confirm expectations of the importance of escape processes to the formation mechanisms for this type of debris. This study shows that data-correlation techniques developed for fallout characterization are also useful in studying reactor debris. 

FALLOUT (Radioactive . The term fallout generally has been used to refer to particulate mutter that is thrown into the atmosphere by a nuclear process of short time duration. Primary examples are nuclear weapon debris and effluents from a nuclear reactor excursion. The name fallout is applied both to matter that is aloll and to matter that has been deposited on the surface of the earfh. Depending on the conditions of formation, this material ranges in texture from an aerosol to granules uf considerable size. The aerodynamic principles governing tls deposition are the same as for any Other material of comparable physical nature that is thrown into the air. such as volcanic ash or particles from chimneys. Therefore, many of the principles learned in. studies of fallout from nuclear weapons can be applied lo studies of other particulate pollution in the atmosphere. 

The radioactive isotope cesium-137 was produced in large amounts in fallout from the 1985 nuclear power-plant disaster at Chernobyl, Ukraine. Write the symbol for this isotope in standard format. 

Figure 3. The d 4g and d O values of dissolved sulphate of Gorleben groundwaters GoHy 201, 611 and 2227 together with regions of fallout sulphate, terrestrial sulphate and sulphate of different geological formations at the Asse salt mine.

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