Results 1 - 10 of 1868
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[en] The major goal of this project was to develop and demonstrate a novel experimental approach to access the dynamic regime of radiation damage formation in nuclear materials. In particular, the project exploited a pulsed-ion-beam method in order to gain insight into defect interaction dynamics by measuring effective defect interaction time constants and defect diffusion lengths. This project had the following four major objectives: (i) the demonstration of the pulsed ion beam method for a prototypical nuclear ceramic material, SiC; (ii) the evaluation of the robustness of the pulsed beam method from studies of defect generation rate effects; (iii) the measurement of the temperature dependence of defect dynamics and thermally activated defect-interaction processes by pulsed ion beam techniques; and (iv) the demonstration of alternative characterization techniques to study defect dynamics. As we describe below, all these objectives have been met.
[en] Here we consider an unsteady detonation with diffusion included. This introduces an interaction between the reaction length scales and diffusion length scales. Detailed kinetics introduce multiple length scales as shown though the spatial eigenvalue analysis of hydrogen-oxygen system; the smallest length scale is ∼ 107 m and the largest ∼ 10-2 m; away from equilibrium, the breadth can be larger. In this paper, we consider a simpler set of model equations, similar to the inviscid reactive compressible fluid equations, but include diffusion (in the form of thermal/energy, momentum, and mass diffusion). We will seek to reveal how the complex dynamics already discovered in one-step systems in the inviscid limit changes with the addition of diffusion.
[en] The approach of SKB in providing Kd values for transport calculations for SR 97 has been to provide what is termed a reasonable estimate for the sorption of each radioelement on granitic rock under saline and non-saline groundwater conditions, as well as a corresponding uncertainty range based on available experimental data. Typical treatments of input data for Kd values in the past have been to provide 'realistic' and 'conservative' estimates of Kd values, but the precise meaning of 'conservative' has often been vague and inconsistent. Thus, the approach, to provide a reasonable estimate supported by an uncertainty range for saline and non-saline conditions, is a different but reasonable methodology for the treatment of Kd values. Carbol and Engkvist have updated the review of sorption data and associated recommendations by Albinsson, provided for the SKB 91 safety assessment. The update consisted of an examination of additional experimental sorption data published since the initial review in 1991, with modification of the recommendations of Albinsson where warranted. The new data are not presented, although references are provided. Bruno and Duro reviewed the recommendations of Carbol and Engkvist, and were able to provide additional experimental data from the literature, although no experimental details were provided concerning new sorption data cited. Based on their review, Bruno and Duro made some reasonable recommendations. Although not taken into account in the database in Andersson, the effect of such recommendations, particularly for Sr, will be covered by uncertainty range calculations. Given the importance of Kd as a retardation parameter, the uncertainty ranges appear relatively narrow. There is a danger that the ranges will be treated as absolute, when frequently they are based generally on a small number of experimental data or even a lack of experimental data for the rock water systems being studied. There is always likely to be a lack of experimental data to support what is effectively expert judgement. Therefore, the best management approach is to focus on those elements for which no data exist (to confirm or update current recommendations, as appropriate), or for radionuclides which appear as key contributors to dose. In the latter case, variation in Kd values can be explored by sensitivity analysis and, where a factor of 10 reduction is significant in terms of a radionuclide's contribution to total dose, experimental work should be carried out. Thus, Andersson's recommendation for probabilistic cases should ensure that any Kd -limiting cases are identified. Radionuclide transport through the bentonite buffer is taken into account via diffusion, with sorption where merited. The input parameters required, therefore, are effective diffusion coefficient, De, distribution coefficient, Kd , and porosity, ε. Andersson notes that the code used by SKB to model migration through the near field can use only one value of porosity, which leads to logistical problems regarding data input. In particular, Kd values must be 'manipulated' to yield a De (effective diffusion coefficient) value representative of the transport porosity and the true sorption capacity for that radionuclide. In this regard, comparison of Kd values with other programmes must be treated with caution. For input to the work of Yu and Neretnieks, who provided recommendations on relevant Kd values for sorption on compacted bentonite, the lack of a reference porewater seems a major omission. The omission may well have been influenced by the lack of a consensus in how to establish a reference porewater composition. It is now well established that Kd values obtained from batch sorption experiments must be treated with caution when applying such values to sorption on compacted bentonite. In particular, measurements using disaggregated bentonite samples effectively ignore the partially mobile fraction. Consequently, the erroneously high (non-conservative) batch Kd values result in calculated values for De , for bulk intact bentonite which are, in turn, too high. In terms of experimental work on sorption of radioelements on compacted bentonite, the need for a database of well-documented diffusion (apparent and intrinsic) and sorption data was recognized at a recent Workshop. Ideally, effective (intrinsic) and apparent diffusion coefficients should be carried out on the same compacted bentonite sample. Any variation of this approach should be interpreted with caution. The topic of surface diffusion in compacted bentonite continues to generate heated debate, which has still not been resolved by recent experimental or theoretical methods. In this context, the comment by Ochs, that Kd values cannot and should not be evaluated independently of the diffusion model used (i.e. with or without surface diffusion), is an important statement. Given that the near-field release code COMP23 applies a traditional diffusion model, the data input should be consistent with this model. In justifying the selection of Kd value recommendations, Andersson makes a series of assumptions and arguments which are clearly stated and sensible. He takes account of all the relevant comments from the review of Ochs as well as the modelling calculations performed by Bruno et al. on bentonite-groundwater interactions and bentonite porewater evolution
[en] A theory of the thermopower is developed with consideration for the nonequilibrium charge produced in a p-type semiconductor and metal contacts. It is shown that the thermopower is generated due to redistribution of the nonequilibrium charge between the metal contacts and semiconductor via transport of nonequilibrium electrons from the metal to the semiconductor through one of the surfaces and from the semiconductor to the metal through the other surface. In a p-type semiconductor sample with thickness smaller than the diffusion length, at certain surface parameters, the thermopower nonlinearly depends on the temperature difference.
[en] We study the spin-transfer torque acting on the magnetisation when injecting polarised conduction electrons into a magnetic system. The spin accumulation is calculated self-consistently and naturally includes the adiabatic and non-adiabatic contributions which depend on the rate of change of magnetisation in relation to the spin diffusion length. As an example we consider a system where a spin-polarised current is injected into a structure containing a domain wall. We calculate the spin torque and related parameters corresponding to the adiabatic and non-adiabatic terms directly from the spin accumulation, and find that the dynamic micromagnetic approach based on adiabatic and non-adiabatic terms with constant coefficients is valid only for systems with slowly spatially varying magnetisation. (paper)
[en] Traditionally, the Euclidean lines, circles and spheres have served as the basis of the intuitive understanding of the geometry of nature. Recently, the concept of fractals has caught the imagination of scientists in many fields. This paper is to continue our previous work on position annihilation near fractal surfaces to demonstrate that the concept of fractals provides a powerful tool for understanding the structure and properties of defects and rough surfaces in relation to positron annihilation studies. Some problems on Berry geometrical phase have also been discussed. (author). 15 refs, fig., 1 tab
[en] This report describes measurements of the diffusion area of neutrons in a solid graphite exponential stack, and in a stack containing cylindrical air channels of 4.5 in. diameter, arranged on a square lattice of 8 in. pitch. The resulting diffusion area ratios are compared with the theoretical predictions of a number of authors. The diffusion area ratios deduced from a pair of experiments in which the orientation of the air channels with respect to the source-plane is changed are found to be in agreement with those deduced from experiments in which the stack size is changed but a constant air channel orientation maintained. (author)
[en] The temperature coefficients of reactivity of the BEPO, Windscale and Calder reactors are calculated, using the revised methods given by Lockey et al. (1956) and by Campbell and Symonds (1962). The results are compared with experimental values. (author)