Research On Key Technologies Of Nuclear Waste Drum Tomographic Gamma Scanning

In the past decades, a large sum of nuclear waste has been produced in nuclear test base; besides, more nuclear waste would be produced for the government restart the nuclear power project. The accumulation of nuclear waste would slow down the development of nuclear industry and be threaten to nation security. Limited by historic situation and technical shortage, the many ingredient marks of nuclear waste have been lost or worn, which makes us difficult to recognize nuclides, contents and distribution. In order to dispose and handle nuclear waste in a safe and efficiency way, we can measure the quantity and compound of waste before disposal. Using chemical analysis methods to test the nuclear waste is hard to get typical sample, in addition, is of low accuracy. One of the most ideal analysis method so far is NDA (Non-Destructive Assay) technique (including source and non-source neutron analysis, source and non-source y-ray analysis and thermal analysis method) which is capable of determining the nuclides species and their quantities accurately without changing the physical and chemical form of the sample. TGS (Tomographic Gamma Scanner) technique is one of NDA technique that using gamma-ray to test the sample. To solve the problem of inaccurate attenuation correction caused by uneven distribution of sample medium, experiments of gamma ray transmission scanning were performed to construct the attenuation coefficient matrix of sample. Currently TGS technique is one of the most ideal one when NDA testing is used.TGS is presented in the 1980s, has been a mature technical now. A variety of models of TGS system have been developed and applied in accurate analysis of radionuclide content such as transuranic and non-uniform sample. Chinese researchers started late in study this technology and foreign countries’monopoly policy. There are no commercial TGS products in China. To meet the need of nuclear engineering works. It is high time for us to study on TGS technology.In this paper, we following the goals of The National Science Fund for Distinguished Young Scholars " The study of Nuclear geophysical instrument development and application (NO:41025015)" National Science Foundation "Key technology study on nondestructive quantitatative analysis of U/Pu nuclides in in homogeneous drummed nuclear waste (NO:41274109)", we studied the design of TGS experimental system, pre-processing of experimental data, reconstruction of nuclear waste drum transmission images and reconstruction of radioactive source emission images. This paper is focus on reconstruction of transmission images and reconstruction of emission images of TGS system and the study is based on some previous work. The main research contents and conclusions are as following:(1) TGS mechanical system designAccording to the principle of TGS system, the optimization source and detection system has been discussed. We developed some modules for TGS system includes shielding and collimator module, radioactive source elevator module, waste drum (sample) horizontal scroll and spin module, detector elevator module, detector horizontal scroll and spin module etc. This system is of a high level of automation.(2) TGS system collimator optimization using Monte-Carlo simulationWe built precise TGS detector model by performing experiments and Monte-Carlo simulation. We optimized the design of radioactive source collimator and detector collimator. The radioactive source collimator is a cylindrical which outer diameter is 25.04cm, straight hole diameter is 1.04cm and hole depth is 12cm; Best sectional geometry of detector collimator is circular a regular hexagon of outer diameter 3.1cm cut after 30° rotation.(3) Experiments and Data PreprocessingExperiments have been performed on self-designed TGS measurement system, including energy calibration, stability test, source regulation. Then translation invariant wavelet transform method was adopted to smooth the Gamma energy spectrum of raw data and improve the high quality factors. MC calculation and experiments have been combined to estimate Gauss standard deviation of y-ray full energy peak. Simplified and common used response function model of HPGe detector has been developed and applied to HPGe detectors with efficiency of 30%,50% and 70%. TGS attenuation coefficient measurement, Transmission scanning measurement and efficiency calibration have been performed.(4) Reconstruction of waste drum transmission image techniqueART (Algebraic reconstruction technique), Richardson iteration and EM (Expectation Maximizaxtion) image reconstruction algorithm were used to reconstruct the 3X3 transmission image. The results show that using EM algorithm is best among these them in matrix construction. EM iteration algorithm was used in this paper.We built the geometric coefficient matrix of transmission image in a novel way by using voxel centre model. By adjusting the sample voxel model material and geometric distribution, we studied the effects of the system matrix calculation method for the reconstruction results. The results show that:when voxel density is the lower and arrangement is disperse, the average track reconstruction method has best effect; when voxel contains the larger line attenuation coefficient voxel, voxel center method has best effect; "point to point" model shows no obvious relation with the voxel distribution and material. Through experiments, the transmission image reconstruction model was using EM iteration by adjusting the suitable relaxation factor.(5) Emission image reconstruction modelWe virtual calibrated the waste drum emission image using Monte-Carlo method; we corrected attenuation coefficient factor to reconstruct transmission image, multiple attenuation coefficient factor fitting method, such as 121.78keV and 244.69keV, was used, a new system matrix was proposed by centre of voxel; an OSEM (Order Subset Expectation Maximizaxtion OSEM) iteration algorithm was used to built emission image reconstruction model based on emission scanning stratified into subset principle was developed, then reconstruct the 3×3×3 model with source and non-source model. When the model contains attenuation material, the relative error of the reconstruction is 10.6%-18.4%; without attenuation material, the reconstruction relative error of a single point source and multi-source is 0-2.2%, most less than 0.5%. By using the multi-energy y-ray of emission sources, we have established a small metal ball model to correct reconstructed source activity self-absorption, the accuracy of the reconstruction was increased.The main innovation points as following:(1) Monte-Carlo simulation was used to design radioactive source collimator and detector collimator.(2) Translation invariant wavelet transform was applied to smooth the TGS measured data, and a simple, costom HPGe detector response function model was developed.(3) Transmission image model of TGS system and attenuation coefficient matrix model of source were calculated. Transmission image reconstruction model was developed using EM iteration and Emission image reconstruction model was developed using OSEM was developed based on emission scanning stratified into subset principle.