Study Of Fast Calculation For Radiation Field And Source Term Inversion Algorithm With Their Preliminary Application

Collective dose is one of the important indicators for assessing the safety of nuclear facilities.It is the key to describe the situation of radiation exposure at all stages of operation,maintenance and decommissioning of the nuclear facilities.And effectively reducing the collective dose is the top priorty of on-site radiation protection work at the nuclear facilities.Using virtual reality and computer simulation technology to carry out the study on the operation dose simulation and optimization at nuclear facilities has become one of the most dynamic research areas in radiation protection,and it can provide the technical support for reducing the collective dose at nuclear facility site.This study mainly focused on simulation and evaluation of the operation dose,and carried out a series of research work on the buildup factors,the fast calculation of the radiation field,the source term inversion based on radiation field data,estimation and optimization of the operation dose.The main results are as follows:(1)Based on MCNP program,a new database of gamma-ray buildup factors(15keV-15MeV)and the corresponding fitting parameters were calculated and set up by using the newer database of photon cross sections,the newer database of attenuation coefficient and mass energy absorption coefficient.In addition,the buildup factors in the typical condition were calculated,which is more realistic.Based on Geant4,the programs were developed to calculate the buildup factors for single and double-layer materials.(2)Using the technique of point-kernel integration and solid modeling,the study on the fast calculation method of gamma radiation field was carried out systematically.Three-dimensional geometric scenes were modeled by using CSG method,and the threedimensional radiation field were calculated by using point-kernel integration method.At the same time,the computational efficiency was optimized from the aspects of weight discretization for the body source,nonuniform meshing for the radiation field and multithread parallel computation.Finally,based on VTK software and Visual Studio platform,a fast calculation program of 3D radiation field was developed,and the program was validated and tested by the benchmark calculation,software verification and field measurement.Compared with the traditional point-kernel program,the calculation speed of this program increased by about 10 times,and because of the new buildup factors,the accuracy of the dose calculation results under some typical conditions was improved by about 10%.(3)Based on the calculation program of radiation field,the methods of source item inversion and dose evaluation of operation plan were studied.Combined with the leastsquares method,the iterative algorithm was used to calculate the source activity,which was taking account of the weight and constraint conditions,and the source inversion program was tested and applied in the actual measurement.Using the point-to-point to simulate the path,and through the combination of personnel,scene,path and source factor,the scenario was made and the dose evaluation of the scenario was realized in two ways.One was using program directly and the other one was calculated through radiation field inerpolation.Finally,through the comparison and analysis of the dose information between the different scenarios,the optimization was completed,which can reduce the dose of personnel as much as possible.