Preliminary Research On The Tomographic Gamma Scanning Technique For Nuclear Waste Drums

In order to promote domestic development of nuclear energy and expand the international communication and cooperation,China joined the International Atomic Energy(IAEA)in 1984 and establish a comprehensive partnership with the IAEA.With the development of domestic nuclear industry,a lot of radioactive waste has been accumulated in the operation process of nuclear facilities and urgently need to be handled and storaged safely.As one technique of nondestructive assay(NDA)techniques,Tomographic Gamma Scanning(TGS)is used to characterize low-level and mediate-level(LL&ML)radioactive waste of nuclear stations and other nuclear facilities to strengthen radioactive waste management and realize the subsequent safe processing and disposal of nuclear wastes,in order to fulfill the requirements of nuclear safety and security(i.e.,nuclear security)and implement accounting management of nuclear material(i.e.,nuclear accounting).So far,foreign TGS technique is relatively mature after over twenty-years'development and perfection and is mainly used to assay items(e.g.radioactive waste drums)with matrix density up to 1.0 g/cm3.However,the study into domestic TGS technique is relatively late and is still in the stage of theoretically exploring.TGS technique divides the items(e.g.nuclear waste drums)into many vertical layers in which every vertical layer is subdivided into many voxels(volume elements)that are assumed to be of homogeneous matrix and activity distributed uniformly.TGS technique uses a separate external transmission source to carry out the transmission measurements of the nuclear waste drum in order to obtain the transmission reconstruction images(i.e.,liner attenuation coefficients distribution maps)of each layer,and the results of each layer are coupled to obtain the three-dimensional corresponding linear attenuation coefficient distribution images of the whole drum.After moving away the external transmission source,TGS technique starts to carry out the emission measurements of the nuclear waste drum in order to obtain the three-dimensional activity distribution of gamma radionuclides inside the drum,and during the process,transmission reconstruction results are applied to solve for emission gamma photons' attenuation and obtain the radionuclides distribution on a voxel by voxel basis.This paper has carried out a preliminary systematic study on the TGS technique in order to lay a solid theoretical foundation for the domestic process of the TGS equipment.This study has accomplished the complete theoretical research of TGS technique to assay nuclear waste drums,and the main contents of this paper are as follows:(1)Application of the irregular-shaped drum edge voxel treatment modeThe voxel division method of nuclear waste drum follows the most generally used one in the world,which is based on the rectangular coordinate system,instead of using recently domestic popular voxel division method based upon the polar coordinate system.The "pointto-point"(PP)model is commonly used as the transmission and emission path length computation model all over the world,and the drum edge voxels are traditionally regarded as regular-shaped cubic voxels.In this study,the peripheral irregular-shaped voxels of the waste drum are taken into account their actual shapes when computing corresponding transmission and emission path lengths based on the traditional "point-to-point" model as the transmission path length computation model,and Cyrus-Beck algorithm in Computer graphics is used to compute the path length that gamma rays pass through voxels in the assayed drum.In order to validate the feasibility of this irregular-shaped drum edge voxel treatment mode,MCNP(Monte Carlo N-Particle transport)code is used to establish the transmission measurement models of two series of nuclear waste drums with different matrix(i.e.,the nonradioactive materials inside the drum)distribution,i.e.,homogeneous drums and inhomogeneous(heterogeneous)drums.Algebraic Reconstruction Technique(ART)with the non-negativity constraint and Maximum Likelihood Expectation Maximization(MLEM)are respectively used as the transmission reconstruction algorithms.Transmission reconstruction results show that,irregular-shaped drum edge voxel treatment mode has greatly improved the quality of transmission reconstruction images no matter which algorithm is used.However,when adopting the irregular-shaped drum edge voxel treatment mode to assay high-density drums,the transmission results of homogeneous drums are ideal,but the corresponding results of heterogeneous drums are unsatisfactory and with larger errors.(2)Using "point-to-detector"(PD)model as the transmission and emission path length computation modelIn order to improve the accuracy of transmission reconstruction results for abovementioned high-density heterogeneous nuclear waste drums,"point-to-detector"(PD)model is proposed to compute transmission and emission path lengths based on the irregular-shaped drum edge voxel treatment mode instead of the traditional "point-to-point"(PP)model.Compared to the PP model,the PD model no longer treats the detector as a point detector,but divides the germanium crystal face of the HPGe detector into multiple grids.Results show that the PD model is obviously superior to the "point-to-point" model to transmission-reconstruct un-uniform heterogeneous waste drums of high densities and obtain corresponding transmission reconstruction images with higher quality.Based on the above conclusions of transmission reconstruction,the "point-to-detector"(PD)model is used to compute emission path lengths,with the application of irregular-shaped drum edge voxel treatment mode.Results show that the more grids the germanium crystal surface of the HPGe detector is divided into,the better the emission reconstruction results are.However,when the number of grids reaches a certain amount,it doesn't make much sense to the quality of emission reconstruction image to continue to increase the number of grids.(3)Simplification computation of detection efficiency calibration for the whole drum and the improvement of the corresponding detection efficiency calibration simplification resultsWhen using gamma-ray spectrometry to carry out the non-destructive assay of items(e.g.nuclear waste drums)with different shapes and sizes,the premise of accurate and quantitative measurement is to implement the efficiency calibration of the given NDA system.As a precondition of TGS technique that belongs to the gamma-ray spectrometry to assay nuclear waste drums to obtain emission reconstruction image,detection efficiency calibration is an essential task and its accuracy directly affects the final assayed results.The workload of efficiency calibration is found to be very heavy and it is impossible to use conventional methods,e.g.,source-based detection efficiency calibration method,analytical calculation method and semi-empirical formula method,to fulfill the whole efficiency calibration.Through careful observation and analysis,there are a lot of dependent detection efficiency matrix elements existing when analyzing the detection efficiency calibration matrix.It is found that when the TGS measurement geometries are analyzed using the point source model in detail from three aspects,i.e.,limited fields of view(FOVs)of the collimated HPGe detector,the spatial position overlap and the symmetry of inner-drum point sources relative to the collimated HPGe detector,with the aim of intending to obtain independent matrix elements to avoid redundant workload,the original detection efficiency calibration workload could be greatly reduced and the left part only accounts for about 0.08%of the original whole detection efficiency calibration workload.However,the detection efficiency calibration results computed by this way that strictly deterimines the number of independent matrix elements actually deviate from the real situation,especially for those drum voxels that are close to the collimated detector but out of corresponding FOVs.Based on this situation,two improvement methods are proposed,i.e.,horizontal full-perspective method to calibrate detection efficiency and value-truncted method to calibrate detection efficiency.These two improvement methods respectively reduce to 0.68%and 0.19%of the original workload under the premise of no accuracy loss of detection efficiency calibration.The detection efficiency calibration results computed by the horizontal full-perspective method is significantly more accurate than that computed by the other method.Thus,in order to validate the feasibility of these two efficiency calibration methods,the less accurate efficiency calibration results computed by the value truncated method are used.The emission measurement models of a 200-liter air drum are established by the MCNP code,with three gamma point sources of different types distributing at different layer heights and different axial positions.Results reveal that all point sources could be easily recognized and precisely located,and their own corresponding radio-activities fit standard setting values very well with relative errors?1.1%,verifying the effectiveness of the efficiency calibration methods proposed.(4)Accomplishment of three-dimensional emission image reconstruction for whole drumMCNP code is used to establish three different series of drums,which has the same three point sources arrangement but with different matrix distribution.Results reveal that emissionreconstructed results could be affected by many factors,but effect of the radial position of the voxel that gamma radionuclide is located on the emission-reconstructed results occupies a dominant position.The density dynamic range of waste drums that TGS technique is applied to assay can be extended to a certain extent by using the ART algorithm as the emission reconstruction algorithm and improved transmission and emission path lengths that are obtained by coupling the irregular drum edge voxel treatment mode with the "point-to-detector" model.