Tritium water diffusion

Diffusion measurements169 with tritium-labelled water shows an increase of the water diffusion coefficient with water content. It reaches a plateau at 11 mole water/ mole lipid. Above 21 mole water/mole lipid D increases strongly162,169. ... 

A number of isotopically different forms of water can be prepared, which greatly facilitates experimental studies. Replacing both of the usual hydrogen atoms with deuterium (2H) results in heavy water, or deuterium oxide, with a molecular weight of 20. The role of water in chemical reactions can then be studied by analyzing the deuterium content of substances involved as reactants or products. Tritium (3H), a radioactive isotope with a half-life of 12.4 years, can also be incorporated into water. Tritiated water has been used to measure water diffusion in plant tissues. Another alternative for tracing the pathway of water is to replace the usual 160 isotope with lsO. This labeling of water with lsO helped determine that the O2 evolved in photosynthesis comes from H20 and not from CO2 (Chapter 5, Section 5.5A). 

Tritium water HjO, denoted as HTO, is non-adsorptive, so it is appropriate to check the applicability of our analysis. The diffusion coefficient of HTO in free water is reported as 2.44x10" cm /s=769.48cm/year (Klitzsche et al, 1976). 

We assume that the profile of diffusivity of tritium water HTO is the same as the normal water shown in Fig. 9.12 however, its diffusivity in the bulk water is specified by experiments as 2.44 x 10 cm /s at 25°C (Klitzsche et al. 1976), therefore the maximum value in Fig. 9.12 is replaced by this experimental value. The diffusivity in the clay mineral is, of course, nuU. 

Radioactive or stable isotopes of noble gases are also used to determine vertical turbulent diffusion in natural water bodies. For instance, the decay of tritium (3H)— either produced by cosmic rays in the atmosphere or introduced into the hydrosphere by anthropogenic sources—causes the natural stable isotope ratio of helium, 3He/ 4He, to increase. Only if water contacts the atmosphere can the helium ratio be set back to its atmospheric equilibrium value. Thus the combined measurement of the 3H-concentration and the 3He/4He ratio yields information on the so-called water age, that is, the time since the analyzed water was last exposed to the atmosphere (Aeschbach-Hertig et al., 1996). The vertical distribution of water age in lakes and oceans allows us to quantify vertical mixing. 

Impurities in the water and water activation products also contribute to the radioactivity of the coolant water. Tritium is produced as a low yield ( 0.01%) fission product that can diffuse out of the fuel, by activation of boron or fiLi impurities in PWRs. 24Na and 38C1 are produced by neutron activation of water impurities. In BWRs, the primary source of radiation fields in the coolant and steam systems during normal operations is 7.1s 16N. This nuclide is produced by 160(n, p)16N reactions from fast neutrons interacting with the coolant water. This 16N activity can exist as N07, NO in the coolant and NHj in the steam. 

Convection, diffusion, and dispersion can only describe part of the processes occurring during transport. Only the transport of species that do not react at all with the solid, liquid or gaseous phase (ideal tracers) can be described adequately by the simplified transport equation (Eq. 94). Tritium as well as chloride and bromide can be called ideal tracers in that sense. Their transport can be modeled by the general transport equation as long as no double-porosity aquifers are modeled. Almost all other species in water somehow react with other species or a solid phase. These reactions can be subdivided into the following groups, some of which have already been considered in the previous part of the book. 

The tritium diffuses to the grain boundaries, migrates through the grain boundaries to the particle surface, and then percolates through the packed bed to low pressure helium purge channels. Heat is removed by pressurized water flowing in a stainless steel tube network in the packed bed. 

Because of greatly contrasting low tritium levels before thermonuclear tests and because of the distinct peak tritium levels that occurred in the atmosphere during 1962-1965, tritium has been used as an environmental tracer in the studies on surface water budgets, groundwater age and flow velocities, groundwater recharge, and dispersion and diffusion in aquifers. A 1989 study detailed the distribution of tritium... 

It is evident from inspection of Equation (25) that the mixing of water parcels with differing tritium concentrations will yield a disproportionate weighting toward the water with higher tritium. This can be quantified by considering the coupled tritium and He advection-diffusion equations ... 

Tritium, however, was largely released directly to the stratosphere, where it is oxidized to tritiated water vapor. It is subseqently mixed back into the troposphere, predominantly at mid-latimdes and rapidly rained out as tritiated rainfall (e.g., see Weiss and Roether, 1980). Thus, there is a far greater asymmetry between the two hemispheres, and characteristically different deposition histories. Figure 7 shows the hrst two EOFs of the tritium in precipitation as analyzed by Doney (Doney et ai, 1992) from IAEA data. The pattern decomposes into two dominant factors a northern impulse (the largest component) and a smaller, more diffuse southern component. 

Budget methods. Vertical exchange rates and turbulent diffusivities Kz can be calculated from the heat balance or the mass balance of tracers for which transformation rates are known. Assuming horizontal homogeneity, the temporal change of tracer mass below a given depth z must be the sum of the net vertical mass flux through the cross-section at z and all sources and sinks of tracer mass below z. In the case of conservative tracers sources and sinks below z must be mass fluxes across the sediment-water interface. In the case of H, radioactive decay is an additional sink. In the case of He, tritium decay represents a source. If the increase of mass due to all sources and sinks, Sm, is known, the net mass flux can be calculated ... 

Robbins PE, Jenkins WJ (1998) Observations of temporal changes of tritium-He-3 age in the eastern North Atlantic thermocline Evidence for changes in ventilation J Mar Res 56 1125-1161 Robbins PE, Price JF, Owens WB, Jenkins WJ (2000) The importance of lateral diffusion for the ventilation of the lower thermocline in the subtropical North Atlantic. J Phys Oceanogr 30 67-89 Roether W, Fuchs G (1988) Water mass transport and ventilation in the Northeast Atlantic derived from tracer data. Philos Trans R Soc London, Ser. A 325 63-69 Roether W, Kromer B (1978) Field determination of air-sea gas-exchange by continuous measurement of Rn-222. Pure Appl Geophysics 116 476-485... 

Tritium present in an oxide layer can be removed by acid dissolution of the oxide or more gently by isotopic exchange with normal water or activated hydrogen gas (plasma). Because diffusion of oxide- or bulk-dissolved tritium back to the surface of a material undergoing decontamination is often slow, exchange at an elevated temperature may be advantageous. 

Several factors influence the half-time of tritium loss from the film tubes (Table 1). As size of the air space above the scintillator (and therefore the total exchange surface area of the tube) increases, the rate of loss of THO increases. This indicates that THO not only diffuses out of the tube directly from the solvent phase but from the gas phase above the scintillator as well. Solvent systems which increase the water vapor pressure in the enclosed tube by decreasing the soliiiility coefficient of water, micelle stability or micelle surface area to volume ratio would be eiqjected to decrease the half-time for THO loss. This would e q)lain the difference in half-times between the toluene and xylene derivative based scintillation solutions. 

Schematic of the tritiated water prototype electrolyser (1) H20 feed tank (2) electrolyser (3) tritiated water level probe (4) hollow thimble-shaped Pd-25%Ag membrane (5) argon circuit (6) upstream condenser (7) catalyst reactor (8) downstream condenser (9) zeolite container for storage and re-injection (10) sampling point for mass spectrometry (11) container for tritium gas storage after diffusion, (12) container for tritide storage (13) instrumentation for diffusion calculations (14) mass spectrometry (T) valve. 

The absolute rise in shown here occurred in only three of the ten studies but in each case there was a marked divergence between the concentration of and tritium. The mean rates of change for each isotope could be determined from regression lines through the experimental points. Summary data are shown on the next slide. The rate for water (-0.72 %/min) is strikingly faster than that for urate (-0.18 %/tnin) or inulin (-0.08 %/inin). They could only occur as a result of diffusion away from the injection site. It is this process of differential diffusion which concentrates total urate in this experimental model and which I would like to implicate in the pathogenesis of podagra. 

Under normal operating conditions, the water-steam circuit (or secondary circuit) of pressurized water nuclear power reactors is completely free of radionuclides. Activated corrosion products and tritium, which have been reported in very low activity concentrations from the water-steam circuits of some high-temperature reactors and which are caused by the neutron Held reaching into the steam generator or by diffusion through intact steam generator heating tubes, do not appear at the PWR secondary side. 

In a similar study, Brewis et al prepared sandwiches of a nylon-epoxy adhesive between aluminum adherends and exposed them to water vapor labeled with tritium for up to 6 weeks at 50°C. The bonds were then cut into small rectangular sections and the fractional water uptake measured by liquid scintillation counting. The values were compared with those calculated for diffusion and were found to agree well for the outer pieces. For the inner pieces the measured uptake was less than the predicted value. The adhesive contained a carrier and the results seem to imply that this hinders the passage of water into the adhesive bond. These results show... 

Cleland RE (1975) Auxin-induced hydrogen ion excretion correlation with growth, and control by external pH and water stress. Planta 127 233-242 Cleland RE (1976) Kinetics of hormone-induced H excretion. Plant Physiol 58 210-213 Cleland RE, Prins H, Harper R, Higinbotham N (1977) Rapid hormone-induced hyperpolarization of the oat coleoptile transmembrane potential. Plant Physiol 59 395-397 Clor MA (1967) Translocation of tritium-labelled gibberellic acid in pea stem segments and potato tuber cylinders. Nature 214 1263-1264 Cohen D, Robinson JB, Paleg LG (1966) Decapitated peas and diffusible gibberellins. Aust J Biol Sci 19 535-543... 

A potential application of the WGS reaction carried out in an MR is represented by the tritium recovery process from tritiated water from breeder blanket fluids in fusion reactor systems. The hydrogen isotopes separation at low concentration in gaseous mixtures is a typical problem of the fusion reactor fuel cycle. In fact, the tritium produced in the breeder needs a proper extraction process to reach the required purity level. Yoshida et al. (1984) carried out experimental and theoretical studies of a catalytic reduction method which allows tritium recovery from tritiated water with a high conversion value (> 99.99%) at a relatively low temperature, while Hsu and Buxbaum (1986) studied a palladium-catalysed oxidative diffusion... 

Example 5.5-1 Self-diffusion in water Estimate the diffusion at 25 °C of a trace of tritium-labeled water in regular water. Water molecules are about 0.26 nm in diameter, separated by 0.30 nm. 

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