Tracer Scale Experiments

Although the outline of a chemical separation process could be obtained by tracer-scale investigations, the process could not be defined with certainty until study of it was possible at the actual separation plants. Therefore, the question in the summer of 1942, was as follows How could any separations process be tested at the concentration of plutonium that would exist several years later in the production plants when, at this time, there was not even a microgram of plutonium available This problem was solved through an unprecedented series of experiments encompassing two major objectives. First, it was decided to attempt the production... 

Prior to experiments with SHE, systematic studies with homologues at the tracer scale have to be carried out to select the experimental conditions (solvent extraction, ion exchanger, aqueous media,. ..). On line experiments with short lived isotopes of homologues are also necessary to improve the setup, e.g. to evaluate the eventual impact of edge effects (sorption,. ..). Furthermore, on line experiments involving SHE may preferably be performed with homologues since it is the only way to ensure strictly identical experimental conditions for all present elements. 

Experimentally, limitations are mainly imposed by the one-atom-at-a-time concept since the time devoted to the collection of data may be important. The collection of experimental data must also include effects of the media and the temperature (if used). Prior to experiments to be carried out at the level of a single atom, the absence of edge effects must be checked carefully at the tracer scale. 

Figure 3 Some horizontal mixing estimates as a function of space scale. Results are from a small-scale float deployment (triangles, Stommel, 1949), the tracer release experiment (squares, Ledwell et al., 1998), and some larger-scale advection-diffusion balances (circles, see later sections) and estimates from radium isotopes (diamonds,...

We have shown that phytic acid readily hydrolyzes to produce phosphate with a projected lifetime of 100-150 years in the absence of microbiological effects, that actinide-phytate compounds are insoluble, and that europium and uranyl phytates are converted to phosphates within a month at 85 °C. Thorium solubility, on the other hand, is controlled by hydroxide or oxide species. Furthermore, the solubilities of radiotracer europium and uranyl are reduced by phosphate dosing of a simulated groundwater solution, even in the presence of citric acid. In the same systems, neptunium(V) solubility is only affected by 0.01 M phosphate at pH greater than 7. The results of these tracer-scale immobilization experiments indicate that phosphate mineral formation from representative deposits is under thermodynamic control. 

First, the trivalent actinide and lanthanide elements are separated from the other elements in the waste. In the second step, americium and curium are then separated from the lanthanide elements. Experimental studies have largely been laboratory-scale in which synthetic waste solutions and tracer levels of radioactivity were utilized. A few laboratory-scale experiments were made in hot cells on the coextraction of trivalent actinides and lanthanides. The two most promising methods investigated for co-removal of trivalent actinides and lanthanides are ... 

Information obtained from field-scale experiments conducted during the last decade on mobile water-tracer chemicals is not encouraging. Extensive vertical and lateral variability of water and chemical transport and retention parameters has created difficulties in testing laboratory scale models (11). Furthermore, a... 

To simulate recovery of uranium and thorium from irradiated 6 percent uranium, 94 percent thorium fuel from the first loading of Consolidated Edison Company s Indian Point 1 nuclear power plant, Oak Ridge National Laboratory [R3] made small-scale experiments on application of the acid Thorex process to fuel containing the appropriate amounts of uranium and thorium, with tracer quantities of the principal fission products. Spent uranium-thorium fuel from the Indian Point 1 plant was subsequently processed by Nuclear Fuel Services, Inc., at West Valley, New York, for recovery of uranium, but without separation of thorium from fission products. No account of this separation has been published. 

Sulfur hexafluoride is very insoluble for small-scale experiments it can be dissolved by presaturating drums or tanks of water with the gas. However, the practical limit for the amount which can be injected in this way is a few moles, sufficient for tracer experiments on the 10-100 km scale only. For large open ocean releases, we designed an injection package which releases liquid SFg into water by pumping it through fine orifices at high pressure, so that it breaks into an emulsion of fine droplets on contact... 

Several further applications of the tracer technique are presently under way. Two large scale experiments in the open ocean are being actively monitored, in the Greenland Sea and the Brazil Basin. Numerous useful subsurface experiments can be imagined. However, because of the conflict between such subsurface release experiments and the use of SFg as a transient tracer, there is a need to establish a forum by which the wider oceanographic community can have input into the planning of prospective release experiments. 

In an unpublished work Hansen (13) actually compared the behavior of long-lived Eu, Tb, and In activable tracers with the fiuorescent dye Rhodamine B in a small (0.5-mi long) stream. Hansen showed the conservative nature of the tracers over this distance, but he did not demonstrate the economic viability of these tracers or their use in large-scale experiments. In a preliminary study Schmitt (14) showed that the... 

Morell et al. (1985) as well as Hellmann et al. (1991) reported on such technical-scale experiments using an experimental setup which is schematically shown in Fig. 6.6. From a high-pressure vessel, water at 310 °C, 10 MPa and doped with an appropriate tracer is supplied to a IS mm diameter orifice simulating the break in the transducer line, from where it expands into a SOm vessel initially kept at ambient temperature (which in the course of the experiment increased to about 100 °C) and pressure this volume roughly corresponds to that of a transducer compartment. The steam produced in the flash evaporation process is carried off... 

According to Eq. 3, a decrease in the concentration of the species E and E2 by a factor 10 " increases the reaction half-time by 10". A reaction that is fast at macroscopic concentrations becomes too slow to be observed at tracer level. Thus, reactions between two micro-components cannot be observed at the time scale of laboratory experiments. Since the probability of encounter between two species at tracer scale is very low, they coexist in the same state as when they were introduced into the solution. Disproportionation and polymerization reactions are therefore excluded. 

Experiments on the scale of 1 to 1 are often used to study the local ventilation around an operator s workplace. Tracer gas is used to simulate the contaminant transport, and a high concentration level of the model tracer gas makes it possible to work with a convenient level of concentration for the measurements. Figure 12.31 shows an enclosure with an emission source S and a laboratory. setup with a model source 5,. The dimensionless concentration c/cg is... 

Ross (R2) measured liquid-phase holdup and residence-time distribution by a tracer-pulse technique. Experiments were carried out for cocurrent flow in model columns of 2- and 4-in. diameter with air and water as fluid media, as well as in pilot-scale and industrial-scale reactors of 2-in. and 6.5-ft diameters used for the catalytic hydrogenation of petroleum fractions. The columns were packed with commercial cylindrical catalyst pellets of -in. diameter and length. The liquid holdup was from 40 to 50% of total bed volume for nominal liquid velocities from 8 to 200 ft/hr in the model reactors, from 26 to 32% of volume for nominal liquid velocities from 6 to 10.5 ft/hr in the pilot unit, and from 20 to 27 % for nominal liquid velocities from 27.9 to 68.6 ft/hr in the industrial unit. In that work, a few sets of results of residence-time distribution experiments are reported in graphical form, as tracer-response curves. 

As mentioned in Section 11.3, fluidized-bed reactors are difficult to scale. One approach is to build a cold-flow model of the process. This is a unit in which the solids are fluidized to simulate the proposed plant, but at ambient temperature and with plain air as the fluidizing gas. The objective is to determine the gas and solid flow patterns. Experiments using both adsorbed and nonadsorbed tracers can be used in this determination. The nonadsorbed tracer determines the gas-phase residence time using the methods of Chapter 15. The adsorbed tracer also measures time spent on the solid surface, from which the contact time distribution can be estimated. See Section 15.4.2. 

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