| Uranium, plutonium, fission nuclides and activation products and other radioactive waste will be produced in the process of whole nuclear industry, such as manufacture of nuclear fuel, decommissioning of nuclear facilities, disposing of spent fuel. Qualitative and quantitative analysis of nuclear waste must be done, when nuclear waste is disposed. Some can be recycled. Thus, some should be recycled by classification. Generally, destructive assay (DA) for nuclear waste is very difficult and high cost. Especially, high level radioactive waste can’t be analyzed by DA. The most ideal analysis technique for high level waste is nondestructive analysis (NDA). While keep the physical and chemical form of samples unchanged, NDA can analyze categories and contents of radionuclides in samples precisely. NDA is low cost and quick. Moreover, NDA doesn’t add any other nuclear waste.Tomographic gamma scanner (TGS) technique is one of the most advanced NDA techniques. TGS can determine quantitatively with high accuracy transuranic nuclides in heterogeneously distributed media with medium and high density. Thus, it is widely used to assay the location and quantity of selected radioisotopes in scraps and wastes within sealed containers. TGS technique draws on computed scanning imaging technique and gamma-ray spectrometry, and corrects the attenuation due to heterogenous distributed of sample medium in the process of emission scanning. Thus, TGS can greatly improve the analysis accuracy on inhomogeneous radioactive samples.In an attempt to meet with requirements of American Department of Energy (DOE) about safeguards and nuclear waste management, Los Alamos National Laboratory (LANL) had started the work on developing TGS since the later 1980s with funding from office of safeguards and security. Subsequently, other countries, such as Switzerland and Argentina, also started the work on TGS. So far, TGS is already a matured technology in America with various types of TGS setups for determining radioisotopes in the samples with medium and high density heterogeneous drummed scraps and wastes. And TGS has been lot manufactured and detected. However, TGS is a high-tech integration, which involves in nuclear physics, nuclear radiation measurement methods, radiation image, mechanical design, automatic control, computer technique, etc... So, scholars abroad never stop researching TGS. They are still exploring and optimizing TGS, including the analysis time, continuous scanning, total uncertainty analysis, versatility, and miniaturization, etc... In our country, the exploration of TGS is very late. Therefore, research on TGS is very lack. Scholars just carry on preliminary study by now.According to the needs of nuclear security and waste management, and on the basis of absorbing the depth studies of domestic and foreign predecessors, this work focus on the key points of transmission reconstruction technique and emission reconstruction technique for TGS, including efficiency calibration, calculation of track length and algorithm for reconstruction, and utilizes the simple TGS device independently developed by ourselves to carry on the research on reconstruction techniques of TGS by Monte Carlo simulation and experiment. This work is supported by National Natural Science Foundation of China (No.41274109).The main contents and results of the work are as follows:(1) Monte Carlo simulation of TGS systemMonte Carlo was used to simulate TGS measurement system, including radioactive sources, waste drum, collimator and HPGe detector. Based on the error of the detection efficiency of simulation and experiment, the thickness of dead layer and the radius of cooling finger were determined with combination Monte Carlo simulation and experiment method. Finally, the thickness of dead layer and the radius of cooling finger is determined as 2.2mm and 4.05mm respectively. In this condition, the error of the simulation detection efficiency and the experiment efficiency was less than 4%. It is within the limits of experiment error.(2) Efficiency calibration by MCStructure and scanning model of TGS determines that the TGS efficiency calibration by experimental method and analytic calculation method is undesirable. In this work, Monte Carlo was used to simulate TGS measurement system and study detection efficiency calibration. The results show that the error of efficiency value by simulation and experiment is less than 5%. It is within the limits of experiment error.(3) Calculation of track lengthTrack length of gamma ray through the voxel is used in TGS transmission reconstruction and emission reconstruction. A new algorithm which is on the basis of particle transport theory is proposed to calculate the track length. Compared with the "point to point" model and "average value" model, this method is more accurate. The relative error between calculated value and reference value is less than 4%.(4) Transmission reconstruction based on OSEM algorithm Ordered subsets expectation maximum (OSEM) algorithm was applied to reconstruct the attenuation coefficient values for transmission measurement. The relative deviation between simulation value and reference value is less than 4%. Experiment was carried on to validate OSEM algorithm. The relative deviation between experiment value and reference value was about 6%.(5) Emission reconstruction based on OSEM algorithmIn order to achieve fast reconstruction of TGS emission measurement, "split-run" and "pre-calculation" were applied to sort out emission data. OSEM algorithm was applied to calculate radioactivity of emission measurement. The relative deviation between calculated value and reference value is 5.36%. Experiment was carried on to validate OSEM algorithm based on transmission measurement data. The relative deviation between experiment value and reference value was 12.58%.In conclusion, this work carried out simulation and experiment research, and ultimately resolved efficiency calibration, calculation of track length and algorithm for reconstruction. Besides, there are certain methods and technology innovation. However, I deeply recognized that a large number of experiments need to be carry out; and that research on TGS is not enough comprehensive; and that factors affecting the accuracy of TGS have not been fully taken into account. This work accumulated a lot of experience for studying TGS comprehensively, and it has much reference value. |
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