Call number: LD2668 .T4 1966 M678 / Master of Science

Thesis (M.Sc.(Agric, Science and Tech) North-West University, Mafikeng Campus, 2012

No description available.

The application of multi-concentration group mathematical modelling to the leaching of radionuclide waste-forms which have continuous porous matrix such as cemented waste form is described. The modelling is illustrated analysing a hypothetical of some transport mechanisms such as molecular diffusion, ionic migration and convective flow for release of interest radionuclide from a solidified waste form which contains discrete particles of radioactive Sr-85 nuclides into the aqueous environment. The group parameters are derived from the classical electrochemistry concept of ion transport in dilute electrolytic solution. The numerical analysis is based on the Crank-Nicolson Implicit Methods which assures the stability of the solution at a practical time step. The results show that, for a short-time period of leaching in demineralized water the leaching behavior follows the predominating diffusion mechanism. After this point, the role of other processes apparent and continue until all radionuclides in the cement waste are leached out when compared to the Semi-Infinite Diffusion model which is based on pure diffusion mechanism. The multi-concentration group model can also be applied to long-term prediction of complicated release mechanisms of the radionuclides in the waste form of a particular disposal environment, unless the variables of interest such as the corrosion rate, the chemical reaction, erosion rate and etc. are determined by experiment or theoretical hypothesis. The appropriate differential equation then can be solved by the same general numerical approach. Also, the results of the modelling indicate which parameters should be measured experimentally in order to provide a quantitative test of the hypothetical release mechanism.

No description available.

No description available.

In this thesis, I discuss coherent manipulation of a trapped ytterbium ion using long wavelength radiation and the results of experiments towards the performance of a two-ion entanglement gate using a static magnetic field gradient of 23.3(6) T/m to create coupling between an ion's internal state and its motion. After using these experiments to explain the requirements for high-fidelity entanglement operations, I continue by examining existing methods for creating this gradient, the current limiting factor in producing the highest fidelity operations. This includes a full characterisation of the gradients produced by symmetric scheme permanent magnets and buried current carrying wires including development of scaling laws in order to create optimum gradients for a given trap geometry. I continue by proposing a new method by which extremely high gradients over 100 T/m can be created for planar chip traps with minimal modification to an existing experiment. These gradients are tailored for axial as well as radial entanglement schemes and aim to show that the technology exists in order to produce a two-qubit gate over the fault tolerant threshold. Subsequently, I discuss the implementation of this new scheme in an experiment before constructing the apparatus to accurately align a chip with these magnets and documenting their installation into two new experimental setups. This includes a preliminary measurement of the gradient produced by an imperfect setup outside of vacuum which verifies those simulated at ~ 110 T/m. Lastly, I discuss the prospects of on-chip magnetic materials and propose a new method which when sufficiently developed should allow for high magnetic field gradients to be produced on-chip at higher ion heights than when solely using current carrying wires. Additionally this scheme should allow for switchable gradients with maximised stability in geometries previously not possible to create.

Measurements of the radioactive content of environmental samples are potentially very costly, especially when these are made ex situ in a laboratory. A less expensive alternative is to acquire in situ measurements in the field. Both measurement types are subject to uncertainties, some of which arise from different sources depending on the measurement method used. Surveys on radioactively contaminated land found that in situ measurements produced results that were as useful in satisfying the typical objectives of such surveys as ex situ measurements. The random component of analytical uncertainty estimated from duplicated in situ measurements was 2-4 times higher than would have been expected from Poisson statistics, however the sampling uncertainty (0-10 %) was found to be much lower than that for ex situ measurements (44-73 %). This resulted from the combined effects of high heterogeneity of the target radionuclide (137Cs) in the ground, and the comparatively large primary sample mass associated with in situ measurements of gamma-emitting radionuclides. A large sampling mass also means that in situ measurements have an advantage in finding small hotspots of activity, although they may not provide sufficient resolution for spatially mapping lateral distributions of contaminants for remediation purposes. The degree of resolution can be readily changed in the field, however, by the simple expedient of changing the detector height. Experiments with an in situ detector close to the ground surface enabled the position of a small hotspot to be determined to within a few centimetres. To evaluate activity concentrations in the soil, assumptions need to be made about the dimensions of the measured sample, and the distributions of activity within it. This requires some information that might be best obtained from ex situ measurements of excavated samples. However, well planned in situ surveys have the potential to significantly reduce the requirement for these expensive laboratory measurements. A new method of optimising the design of in situ surveys has been developed, based on a generic model for predicting the detector response to small particles of activity at different positions relative to the detector. The new mathematical model used by this method compares well with field measurements, and also with predictions made using a commercially available calibration program.

Typewritten copy / 1 v. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, 1953

In order to determine the physical location of contaminants in soil, two solidified soil "thin" sections, which preserve the undisturbed structural characteristics of the original soil, containing weapons-grade plutonium from the Rocky Flats Environmental Test Site were prepared. Two autoradiographic methods were used in radionuclide mapping: contact autoradiography using CR-39�� plastic alpha track detectors and neutron-induced autoradiography that produced fission fragment tracks in Lexan�� plastic detectors. The combination of the two autoradiographic methods distinguished alpha-emitting particles of natural U, from ������������������Pu and non-fissile alpha-emitters. The locations of 990 alpha "stars" caused by plutonium "hot particles" in two soil sections were recorded, particles were sized, their size-frequency and depth distributions were analyzed. Several large colloidal conglomerates of "hot particles" were found in soil profiles. One such conglomerate with a geometric size of about 500 ��m contained over 94% of the total recorded contaminant alpha activity in the sample. It was found that the upper 6.5 cm of soil contained 20% of all recorded particles (mean equivalent size 0.35 ��m). The deeper portion of the surface 6.5 cm soil layer contained 80% of the particles (with mean equivalent size 0.25 ��m). The average specific activity (SA) for 989 hot particles (with the conglomerate of the particles excluded) with equivalent diameters over 80 nm was found to be greater than 23.9 Bq g����� (about 90% of the overall average SA). For dissolved and defragmented (below 80 nm of equivalent diameter) actinide particles, SA was found smaller than 2.9 Bq g�����. Over 99% of the total actinide contaminant activity in the analyzed soil sample (with the conglomerate of the particles included) was found in particles with equivalent diameters over 0.08 ��m. it suggests that larger particles of plutonium oxide moved down more slowly than smaller ones and no significant breakup of plutonium oxide particles occurred since the original plutonium soil deposition. Both profiles of the depth contaminant activity and number of actinide particles have distinctive peaks at the same depth, about 10 cm. Independent from nuclear track analysis, this pattern of the actinides depth distribution was observed in the measured gamma activity depth profile of �������Am (daughter product of �������Pu beta decay) in solidified soil blocks. / Graduation date: 2001