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In this paper, an efficient and accurate imaging algorithm is presented for Ground-Based Synthetic Aperture Radar (GB-SAR) or other radar systems that could be formed by a physical or synthetic linear aperture. The imaging algorithm is based on the fractional Fourier transform (FrFT) for the azimuth compression. A mathematical framework is derived according to the projection of a sample reflectivity image onto the pseudopolar coordinate, and its implementation was presented. With the data acquisition geometry and the pseudopolar imaging coordinate, the phase of a point target can be expressed as a quadratic phase exponential. It makes that only 1-D FrFT is needed for the azimuth compression of the time-domain backscatter data for the GB-SAR imaging problem. By further research, the optimal transformation order that represents the spatial frequency changes by the FrFT was given subsequently. Taking advantage of this optimal representation, the proposed approach avoids the large calculation that occurs in the time-domain back projection (TDBP). Comparing to the far-field pseudopolar format algorithm (FPFA), the accuracy of the proposed algorithm is much improved. Meanwhile, the proposed approach holds almost the same computational cost and complexity as the FPFA. The proposed approach keeps the advantages of the imaging quality of the TDBP and the computational cost of the FPFA that are two important aspects of the GB-SAR applications. Both the numerical simulation and the field GB-SAR experiment show that the algorithm is more suitable for the high-precision GB-SAR imaging, especially for the near field. [ABSTRACT FROM AUTHOR]

The reliable simulation of radionuclide migration away from the near field of a geological repository for safety analyses requires a fundamental understanding of processes in the near field. We focus this study on investigating the behavior of UK MW25 waste glass (a nonradioactive simulant of U.K. Magnox glass) and other near‐field components in a geological repository over long periods of time. The approach used involved a two‐step process: waste glass dissolution measurements and modeling in potential repository groundwaters, and reactive transport simulation of radionuclide release and transport in the near field. The Stage 1 rate of MW25 measured in lower‐strength‐sedimentary‐rock groundwater was found to be slightly higher than that measured in higher‐strength‐rock groundwater. Experimental results show the critical role of groundwater chemistry on waste glass dissolution kinetics. Reactive transport model parameterization was accomplished by coupling direct laboratory measurements and literature data with transport processes. The results revealed that 79Se, 99Tc, and 237Np are the radionuclides that will have the greatest impact on the environment at long times. The performance assessment results show that the improvement afforded by the addition of a copper canister in the waste package, as opposed to having only a steel canister, is marginal and higher strength rock would be a more effective barrier at long time than lower strength sedimentary rock in terms of restricting the transport of the radionuclides. Our model provides deep insights into the contribution of waste glass durability and other near‐field components in the containment and retardation of radionuclide transport. Key Points: The dissolution rate of MW25 waste glass was found to be slightly higher in LSSR groundwater than in HSR groundwaterThe waste package was effective in retarding the radionuclide transport, as only ~1.25% of the total activity in the initial inventory was releasedWaste glass dissolution rate is more important than diffusive transport in the release and transport of radionuclides in the near field [ABSTRACT FROM AUTHOR]