Natural radioelements

The ionization of nitrogen, oxygen and trace gas molecules in the air due to the presence of the natural radioelements in the soil and air and the cosmic radiation has a direct effect upon the electrical characteristics of the atmosphere. 

The energy produced by decay of natural radioelements in the earth is assumed to contribute considerably to the temperature of the earth. In particular, the relatively high temperature gradient of about 30 °C per 1 km depth observed up to several kilometres below the surface is explained by radioactive decay taking place in the minerals, e.g. in granite. 

The natural radioelements are listed in Table 14.1. Isotopes of these elements are members of the uranium, actinium and thorium families (Table 1.2, and Tables 4.1 to 4.3). In the ores of U and Th the concentrations of natural radioelements are relatively high and proportional to the half-life. The average concentration of U in the earth s crust is about 2.9 mg/kg (ppm) and that of Th about 11 mg/kg (ppm). The... 

All natural radioelements with atomic numbers Z = 84 to 89 and Z = 91 have been identified as decay products of U and Th, but the first isotope of astatine (from Greek unstable Z = 85) was obtained in 1940 by the nuclear reaction... 

Many natural radioelements are of great practical importance, in particular U with respect to its application as nuclear fuel, but also Th, Ra and Rn. Ra (T/2 = 1600 y) is found in many springs and the noble gas Rn is the main source of natural radioactivity in the air. 

For special geochemical investigations, isotope ratios of other elements, such as B, N, Si, K and Se are also determined. The measurement of the distribution of the natural radioelements U and Th and their daughter nuclides in minerals, sediments, oil, water and the air gives information about the genesis of the minerals, sediments and oils, and about the processes taking place in the lithosphere, the hydrosphere and the atmosphere. The nuclear methods of dating will be discussed in chapter 16. 

Specific nuclear reactions capable of producing noticeable quantities of noble gas daughters in the Earth ( He and Ne in particular) are initiated by alpha and fission activities of the natural radioelements. Helium-3 is produced through a neutron capture reaction involving Li (HUl, 1941), whereas Ne production occurs through a number of a-induced reactions (Wetherill, 1954). In the case of helium, the He/ He ratio produced is of the order 10 and primarily reflects the lithium abundance at the site of production (Mamyrin and Tolstikhin, 1984). Eor neon, the only conspicuous isotope produced is Ne due to its low natural abundance. The present-day Ne/ He production ratio in the mantle has been calculated at 4.5 X 10 (Yatsevich and Honda, 1997) (see Ballentine and Bumard, 2002 for discussion regarding calculation of this parameter). 

Andrews J. N., Ford D. J., Hussain N., Trivedi D., and Youngman M. J. (1989) Natural radioelement solution by circulating groundwaters in the Stripa granite. Geochim. Cosmochim. Acta 53, 1791-1802. 

Aerial gamma mapping allows us to bring into the evidence, apart from artificial gamma emitters (fission and activation products), the three natural radioelement families, K, U, and Th. 

Research into the aquatic chemistry of plutonium has produced information showing how this radioelement is mobilized and transported in the environment. Field studies revealed that the sorption of plutonium onto sediments is an equilibrium process which influences the concentration in natural waters. This equilibrium process is modified by the oxidation state of the soluble plutonium and by the presence of dissolved organic carbon (DOC). Higher concentrations of fallout plutonium in natural waters are associated with higher DOC. Laboratory experiments confirm the correlation. In waters low in DOC oxidized plutonium, Pu(V), is the dominant oxidation state while reduced plutonium, Pu(III+IV), is more prevalent where high concentrations of DOC exist. Laboratory and field experiments have provided some information on the possible chemical processes which lead to changes in the oxidation state of plutonium and to its complexation by natural ligands. 

Wollenberg, H.A., Naturally Occurring Radioelements and Terrestrial Gamma-ray Exposure Rates An Assessment Based on Recent Geochemical Data, Report LBL-18714, Lawrence Berkeley Laboratory, Berkeley CA (1984). 

Actinides, the chemical elements with atomic numbers ranging from 89 to 103, form the heaviest complete series in the Periodic Table. They are radioelements, either naturally occurring or synthesized by nuclear reactions. Their predominant practical application depends on the nuclear properties of their isotopes decay, spontaneous or induced fission. Their chemical and physical properties reflect a very complex electronic structure, and their study and understanding are a challenge to experimentalists and theoreticians. 

Radiochemistry is defined as the chemical study of radioactive elements, both natural and artificial, and their use in the study of chemical processes (Random House Dictionary, 1984). Operationally, radiochemistry is defined by the activities of radiochemists, that is, (a) nuclear analytical methods, (b) the application of radionuclides in areas outside of chemistry, such as medicine, (c) the physics and chemistry of the radioelements, (d) the physics and chemistry of high-activity-level matter, and (e) radiotracer studies. We have dealt with several of these topics in Chapters 4, 13, 15, and 16. In this chapter, we will discuss the basic principles behind radiochemical techniques and some details of their application. 

Traces of elements or compounds, respectively, of other elements with similar properties may serve as non-isotopic carriers for radioisotopes of stable elements as well as for isotopes of radioelements. The influence of non-isotopic carriers depends on the nature of the compounds and the chemical operation. For example, in precipitation reactions, non-isotopic carriers or hold-back carriers, respectively, may play a major role. 

Research in nuclear and radiochemistry comprises Study of radioactive matter in nature, investigation of radioactive transmutations and of nuclear reactions by chemical methods, hot atom chemistry (chemical effects of nuclear reactions) and influence of chemical bonding on nuclear properties, production of radionuclides and labelled compounds, and the chemistry of radioelements - which represent more than a quarter of all chemical elements. 

Two types of colloids are recognized in the literature. Intrinsic colloids (also called true colloids, type I colloids, precipitation colloids, or Eigencolloids ) consist of radioelements with very low solubility limits. Carrier colloids (also known as pseudocolloids, type II colloids or EremdkoIIoides ) consist of mineral or organic phases (in natural waters primarily organic complexes, silicates and oxides) to which radionuclides are sorbed. Both sparingly soluble and very soluble radionuclides can be associated with this type of colloid. In addition, radionuclides can be associated with microbial cells and be transported as biocolloids. 

Natural attenuation encompasses processes that lead to reduction of the mass, toxicity, mobility, or volume of contaminants without human intervention. The US EPA has recently published guidelines for the use of MNA for a variety of contaminated sites (US EPA, 1997). For inorganic constiments, the most potentially important processes include dispersion and immobilization (reversible and irreversible sorption, co-precipi-tation, and precipitation) (Brady et al, 1998). Studies of remediation options at UMTRA sites (Jove-Colon et al, 2001) and the Hanford Site (Kelley et al, 2002) have addressed the viability of adopting an MNA approach for uranium and strontium, respectively. As discussed below, different approaches are required to establish the viabihty of MNA for these radioelements. 

Osmond, J.K., Cowart, J.B. Humphreys, C.L. and Wagner, B.E., Radioelement migration in natural and mined phosphate terrains. Final Report PB-86-223765/XAB. Univ. Tallahassee, Florida State University, USA, 1987. 

The French Atomic Energy Commission (CEA), Valduc Centre, has developed an aerial system of gamma cartography named HELINBUC. This equipment enables, in a few hours, the establishment of a map of radioactivity over areas several dozen to several hundreds of hectares in size, by identifying radioelements present, with a sensitivity between the level of natural radioactivity and that of artificial radioactivity resulting from a large-scale accident. 

It is customary to express the stability of a radioelement in terms of its half-life, by which is meant the time that would be required for one half of a given mass of the element to undergo natural disintegration. Thus the period of half-life or half-change of radium is 1600 years. If therefore we have to-day a gram of radium, in 1600 years there will be only half a gram left, and in a further 1600 years the amount will have fallen to 0-25 gm. and so on. 

Decomposition of a radioelement, whether natural or induced by bombardment is invariably accompanied by liberation of energy. When, for example, a radium salt is confined in a thick lead vessel almost all the evolved energy is converted into heat, some 25 gm-calories per hour being produced per gram of radium. For this heat to serve any useful economic purpose we should require Vastly greater quantities of radium than we could ever hope to obtain. 

Figure 1. Schematic diagram of a gas reservoir, illustrating the different noble gas components which may occur in cmstal fluids. Atmosphere-derived noble gases (e.g., °Ne and Ar) are input into the gas phase on equilibration with the groundwater system containing dissolved atmosphere-derived noble gases. Radiogenic noble gases (e.g., " He, Ne and " °Ar) are produced by the natural decay of the radioelements U, Th and K in the crast, and are also incorporated into cmstal fluids. Within areas of continental extension or magmatic activity, noble gases derived from the mantle (e.g., He) may also be present in cmstal fluids. The distinct isotopic and elemental composition of these different noble gas components allows the extent of their contribution to any cmstal fluid to be quantitatively resolved and information about volumes, source and transport process of associated fluids to be identified.

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