Waters, isotopic, properties

Table 5.9 IE s on some of the thermodynamic properties of isotopic waters (Jancso, G. and Van ...

Answer 3.4 Similar chemical and isotopic compositions of the waters of wells I and II support the hypothesis that these wells are hydraulically interconnected. Differences in the water properties of wells III and IV support the hypothesis that these wells are hydraulically separated. 

The difference in molecular composition and weight of the isotopic water molecules gives rise to slight differences in their chemical and physical properties. These differences, even though very small, are not insignificant and provide the basis for their separation and analysis. They are also useful for investigation of chemical and biological isotope effects and the structure of molecules. Important properties of H2O, D2O, and T2O are compiled in Table 2. 

Table 2 Physical properties of isotopic waters (Continued)...

Nielsen, D. R., Reichardt, K., and Wierenga, P. J. (1983). Characterization of field-measured soil-water properties. In Isotope and Radiation Techniques in Soil Physics and Irrigation Studies 1983. pp. 55-78. Report lAEA-SM-267/40, International Atomic Energy Agency, Vienna. 

One of the most significant sources of change in isotope ratios is caused by the small mass differences between isotopes and their effects on the physical properties of elements and compounds. For example, ordinary water (mostly Ej O) has a lower density, lower boiling point, and higher vapor pressure than does heavy water (mostly H2 0). Other major changes can occur through exchange processes. Such physical and kinetic differences lead to natural local fractionation of isotopes. Artificial fractionation (enrichment or depletion) of uranium isotopes is the basis for construction of atomic bombs, nuclear power reactors, and depleted uranium weapons. 

The small differences in physical properties of substances containing elements with isotopes are manifested through mea.surement of isotope ratios. When water evaporates, the vapor is richer in its lighter isotopes ( Hj O) than the heavier one ( Hj O). Such differences in vapor pressures vary with temperature and have been used, for example, to estimate sea temperatures of 10,000 years ago (see Chapter 47). 

The primary issue is to prevent groundwater from becoming radioactively contaminated. Thus, the property of concern of the long-lived radioactive species is their solubility in water. The long-lived actinides such as plutonium are metallic and insoluble even if water were to penetrate into the repository. Certain fission-product isotopes such as iodine-129 and technicium-99 are soluble, however, and therefore represent the principal although very low level hazard. Studies of Yucca Mountain, Nevada, tentatively chosen as the site for the spent fuel and high level waste repository, are underway (44). 

Thermodynamic Properties. Ordinary water contains three isotopes of hydrogen [1333-74-0] (qv), ie, H, H, and H, and three of oxygen [7782 4-7] (qv), ie, O, and The bulk of water is composed of and O. Tritium [15086-10-9] H, and are present only in extremely minute concentrations, but there is about 200-ppm deuterium [16873-17-9], H, and 1000-ppm in water and steam (see Deuterium and tritium). The thermodynamic properties of heavy water are subtly different from those of ordinary water. lAPWS has special formulations for heavy water. The properties given herein are for ordinary water having the usual mix of isotopes. 

The recognition in 1940 that deuterium as heavy water [7789-20-0] has nuclear properties that make it a highly desirable moderator and coolant for nuclear reactors (qv) (8,9) fueled by uranium (qv) of natural isotopic composition stimulated the development of industrial processes for the manufacture of heavy water. Between 1940 and 1945 four heavy water production plants were operated by the United States Government, one in Canada at Trail,... 

The only large-scale use of deuterium in industry is as a moderator, in the form of D2O, for nuclear reactors. Because of its favorable slowing-down properties and its small capture cross section for neutrons, deuterium moderation permits the use of uranium containing the natural abundance of uranium-235, thus avoiding an isotope enrichment step in the preparation of reactor fuel. Heavy water-moderated thermal neutron reactors fueled with uranium-233 and surrounded with a natural thorium blanket offer the prospect of successful fuel breeding, ie, production of greater amounts of (by neutron capture in thorium) than are consumed by nuclear fission in the operation of the reactor. The advantages of heavy water-moderated reactors are difficult to assess. 

Properties of T2O. Some important physical properties of T2O are Hsted in Table 2. Tritium oxide [14940-65-9] can be prepared by catalytic oxidation of T2 or by reduction of copper oxide using tritium gas. T2O, even of low (2—19% T) isotopic abundance, undergoes radiation decomposition to form HT and O2. Decomposition continues, even at 77 K, when the water is fro2en. Pure tritiated water irradiates itself at the rate of 10 MGy/d (10 rad/d). A stationary concentration of tritium peroxide, T2O2, is always present (9). AH of these factors must be taken into account in evaluating the physical constants of a particular sample of T2O. 

D2O and the tritium analogue T2O (p. 41). The high bp is notable (cf. H2S, etc.) as is the temperature of maximum density and its marked dependence on the isotopic composition of water. The high dielectric constant and measurable ionic dissociation equilibrium are also unusual and important properties. The ionic mobilities of [H30] and [OH] in water are abnormally high (350 X 10 " and 192 x 10 cms per V cm... 

Magnesium. Mg, at wt 24.312, at no 12, valence 2. Isotopes 24 (77.4%), 25 (11.5%) 26 (11.1%). Physical properties of 99.9% pure Mg are (riven in the fnllnwino tsKle fRef 10 n 6791 Mg is very abundant in nature, occurring in substantial amounts in many rock-forming minerals such as dolomite, magnesite, olivine, and serpentine. In addition, it is also found in sea water, subterranean brines, and salt beds. 

Isotope distribution among the different phases of water are generally assumed to mostly depend on the behavior of their respective parent nuclides, in particular on their sorption/solubility properties. For instance, according to Sarin et al. (1990), covariation between ratios and Th/U ratios in Himalayan rivers reflect the preferential... 

The role of radionuclides as tracer of the chemical transport in river is also reinforced by the fact that each of the U-Th-Ra elements has several isotopes of very different half-lives belonging to the U-Th radioactive series. Thus, these series permit comparison of the behavior of isotopes of the same element which are supposed to have the same chemical properties, but very different lifetimes. These comparisons should be very helpful in constraining time scales of transport in rivers. This was illustrated by Porcelli et al. (2001) who compared ( " Th/ U) and ( °Th/ U) ratios in Kalix river waters and estimated a transit time for Th of 15 10 days in this watershed. The development of such studies in the future should lead to an important progress in understanding and quantifying of transport parameters in surface waters. This information could be crucial for a correct use of U-series radioactive disequilibria measured in river waters to establish weathering budgets at the scale of a watershed. 

As described above, it is probably adequately clear that the vibrational spectroscopy of water is complicated indeed One can simplify the situation considerably by considering dilute isotopic mixtures. Thus one common system is dilute HOD in D2O. The large frequency mismatch between OH and OD stretches now effectively decouples the OH stretch from all other vibrations in the problem, meaning that the OH stretch functions as an isolated chromophore. Of course the liquid is now primarily D2O instead of H2O, which has slightly different structural and dynamical properties, but that is a small price to pay for the substantial simplification this modification brings to the problem. 

IE s on some of the other properties of water are shown in Table 5.9. Many properties (like the enthalpies of phase change, triple points, etc.) are closely related to VP and can be interpreted similarly. Molar volume isotope effects are interesting and are discussed in Chapters 12 and 13. In the low temperature liquids... 

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