Study On The Method For Determination Of Hazard Elements Inlow And Intermediate Radioactive Waste

In last 30 years, the development of the nuclear energy satisfied the more and more needs of the energy and also the increasing of the economy. But the use of the nuclear energy produced a lot of radioactive waste which will threaten the environment and people's life. So IAEA and many governments paid a lot of attention to the management of the radioactive waste. Before the disposal of the waste, the information of the radioactive nuclide and toxic elements should be determined to satisfy the safety regulation. After the 60-year's development, the NDA (Non-destructive Assay) method has been the best way to detect the hazard elements in the radioactive waste.In the NDA method, normally the GS (Gamma Scanning) and NAA (Neutron Activation Analysis) are used to determine the radioactive nuclide and the toxic element, respectively. Based on the features, the assumption and theoretical analyses, the two methods are studied in this thesis.First, the application of the NDA method in the determination of the hazard elements in the radioactive waste is studied.The analysis and the comparison of the SGS (Segmented Gamma Scanning) and TGS (Tomographic Gamma Scanning) showed that the basis of the SGS method is the rotation measurement of the each segment of the drum. Because of the unknown distribution of the matrix, the different assumption of the attenuation coefficient will produce different errors to the determination. Comparing to the SGS method, TGS method could determine the distribution of the attenuation coefficient in the drum and give better results. But the enhance of the accuracy could not counteract the consuming working time, that is why TGS is not popular in the routine detection.The study of the method to determine the toxic elements shows that the traditional method could give good determination to the small sample, but it still needs the typical sample and will destroy the integrity of the drum. But usually the radioactive waste should not be break. To the toxic elements in large sample, it is very hard to get the typical sample which could represent the average density. Because of the high penetrating property of the fast neutrons and the high cross section of the toxic elements to the thermal neutrons, the PGNAA (Prompt Gamma Neutron Activation Analysis) method may get good result from the analysis to the large sample.Secondly, the method to determine the radioactive elements is studied.From the analysis of the SGS system, the assumption that the matrix in the drum could be separated into two layers and the radioactive source is point source is studied and an improved SGS method is developed to determine the activity of the nuclide and the count rate distribution with the rotation of the drum is also studied to get the information of the source. To verify the improved SGS method, the detection of single and double point sources in the drum is simulated. The determination shows the improved method may get better results than the traditional method, and the average relative error is 0±29%, while the one of the traditional method is 83%±14%.Thirdly, the PGNAA method is studied to analyze the toxic elements in large samples.Based on the theory basis of the PGNAA method and the property of the matrix and the toxic elements in the drum, a PGNAA system is developed. The analysis of the simulation executed by MCNP-5 for the transportation of the neutron in the drum and graphite irradiation chamber and also the gamma ray in the drum and the detector shows that the composition determination of large sample by PGNAA is a non-linear problem.To fix the non-linear problem, the fixed point iteration method is use to determine the concentration of Cd in the drum. A numerical experiment is analyzed by the fixed point iteration method, and the result shows this method works well, but the iteration needs several MCNP-5 calculations which are time consuming. So the fixed point iteration is difficult to be used in routine application.Instead of the fixed point iteration, a direct determination method based on the basic equation of PGNAA is developed to calculate the thermal neutron flux in the drum. This method found the relation between the thermal neutron flux and the count rates of the important elements in the sample. The deviation of the fitting equation from the database calculated by MCNP-5 code is acceptable.Not only the fixed point iteration, but also the direct determination method is based on the analysis of the homogeneous sample. The difference of the heterogeneous and homogeneous sample which may influence the accuracy of the two methods is studied. The result shows the spatial distribution of the toxic elements will influence the average thermal neutron flux in the sample and then the fitting equation.All the studies in this thesis focus on the detection of the hazard elements in the radioactive waste drums. The improved SGS method could get more accurate result in the detection of the radioactive elements than the traditional SGS method and the PGNAA method works well in the determination of the toxic elements in large samples. The research results of the two methods in the determination of the radioactive waste drums may provide the theoretical basis and technical support for the radioactive waste management and also the development of the NDA method.