| Large amounts of low and intermediate level radioactive waste (LILW) will beproduced and piled up with the nuclear power industry development and the increasingwidespread application of radioisotopes. Currently, a large number of radioactive wastegenerated in the operation of nuclear power plant are stored in the temporary repository,and it may affect the safe operation of nuclear power if the nuclear waste can not gettimely disposal. In addition, the safe disposal of radioactive waste from national defenseindustry and decommissioning of nuclear facilities is also an urgent problem. Thedistribution of the radionuclide composition and activity must be accurately measuredbefore the final disposal of LILW to provide a scientific basis for its storage,transportation and final disposal.Due to the particularity of LILW, non-destructive assay (NDA) method iscommonly used to measure the waste drums. The γ-ray analysis method is based on theγ-ray emitted by the sample itself and does not produce the secondary radioactive waste,so it is one of the most widely-used NDA method. The γ-ray scanning technique forwaste drums has undergone two stages: Segmented Gamma Scanning (SGS) andTomographic Gamma Scanning (TGS). The SGS measurement process in simple and fast,but with a large measurement error; TGS can obtain the distribution of the material andradionuclides in the waste drum accurately, but it is difficult to meet the requirements oflarge-scale detection because of its complexity and long time of the measurement process.The research of improved SGS (ISGS) and TGS method is carried out in this paper,aiming at the more rapid and accurate detection technique for LILW. The main contentsand results are as follows:(1) The establishment of the theory and measurement system of ISGSThe traditional SGS assume that the matrix and radioactive sources are distributeduniformly in each vertical segment, resulting in a large error in measuring thenon-uniform sample. The ISGS proposed in this paper assume the uniform matrix in eachsegment, but the radioactive sources exist in the form of an equivalent ring source, andthe equivalent radius is analyzed by the count rates of two detectors at different positions.The theoretical basis of the ISGS is described in detail by formula derivation, and theactivity reconstruction algorithm and measurement system layout are also presented.(2) Simulation and experimental verification of ISGSThe simulation measurements of the cases of single source in homogeneous matrix,multiple sources in homogeneous matrix and single source in heterogeneous matrix arecarried out by using MCNP code. For the homogeneous matrix, in the most extreme caseof a single point source, the relative errors of reconstruction activity results of ISGS arein the range of±10%at the density of0.3g/cm3,±15%at the density of0.6g/cm3,±20%at the density of1.0g/cm3, and±25%at the density of1.5g/cm3. Meanwhile, theexperimental measurement of a heterogeneous sample is also carried out. Thereconstruction results of simulation measurement data and experimental data both showthat the ISGS has a higher accuracy than SGS.(3) Study of TGS image reconstruction using dynamic gridsIn order to solve the problem of coarse grids and low resolution in TGS, usingdynamic grids in TGS image reconstruction is proposed. A adaptive grid refinementstrategy which is suitable for TGS reconstruction is developed in order to locate smallsize grids in the vicinity of the point source for accurate positioning of the ‘hot spots’ and locate big size grids in the area of no or low radioactivity. The simulation measurementsof a heterogeneous matrix with point sources are carried out by using MCNP code. Thereconstruction results of simulation measurement data demonstrate that dynamic grids inemission reconstruction outperform the fixed grids in terms of the accuracy and ’hotspots’ positioning with fewer grids in most cases.(4) Study on the optimization of TGS detection by using dynamic grids in theemission reconstructionReducing the scan times and using dynamic grids in emission reconstruction isproposed to solve the problem of the long measurement time of TGS. To verify thefeasibility of the solution and find the optimal scan scheme, we set five differentsimplified scan schemes which reduce the rotation or (and) translation number based onthe original scan scheme of24rotating times and4translation times. The simulationmeasurements of a homogeneous sample and a heterogeneous sample with point sourcesare carried out by using MCNP code, and the transmission reconstruction is implementedby fixed grids, the emission reconstruction is implemented by fixed grids and dynamicgrids respectively to compare the activity reconstruction error of all the scan schemes.The statistical results of the reconstruction error of216single point sources and100multiple point sources indicate that simplification of TGS scan process from24rotatingtimes and4translation times to12rotating times and4translation times will lead to littlechange of the emission reconstruction error if dynamic grids are applied, but thissimplification will lead to a obvious reduction of TGS scan time. |
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