| Accurate calculation of operational and shutdown radiation fields is an important content of nuclear design and safety analysis for fusion reactors.The Monte Carlo method is widely used to simulate radiation fields of fusion reactors because of its high-fidelity geometric modeling ability.Compared to fission reactors,fusion reactors have large radiation attenuation gradients and complex geometric structures.In the calculation of the operational radiation field,the deep penetration shielding problem will cause the Monte Carlo particle transport calculation to cost a lot of computing time and computing resources.High resolution "space-energy"distribution calculation of shutdown dose rate for fusion reactors requires a fine mesh division of the entire reactor,resulting in significant memory consumption.And the structured mesh often leads to a large error in dose rate results.For the efficiency and memory bottleneck issues of full reactor high-resolution calculation for fusion reactors,this dissertation carries out the research and program development of an efficient variance reduction algorithm,adaptive mesh algorithm,hybrid parallel algorithm,and unstructured mesh based shutdown dose rate calculation algorithm,which can improve the calculation accuracy of operational and shutdown radiation fields for fusion reactors.The main research contents include:(1)For the deep penetration shielding problem of the full-space radiation field calculation,the weight window variance reduction algorithm based on the response matrix method is studied.The importance distribution solution is obtained through the Monte Carlo forward transport calculation to guide the setting of weight window parameters.The weight window iteration process of the response matrix method is improved to accelerate the convergence speed of the weight window.And the memory allocation of the importance solver is optimized,which reduces the memory consumption of the solver and improves the scale of parallel computation.(2)For the complex geometry and the large material difference of fusion reactors,the multilevel adaptive mesh algorithm is studied.The resolution of the mesh is adjusted according to the geometric material.In order to realize the accurate location of the particles,the recursive algorithm is used to process the mesh mapping between different levels of the tree mesh structure.According to the inheritance of neighbor relationships between different levels,the rapid construction of unmatched neighbor relationships between meshes is realized,and a response matrix algorithm based on adaptive meshes is developed to reduce the number of weight window meshes while ensuring calculation accuracy.(3)For the memory consumption and load imbalance problems of high resolution large-scale parallel computing for radiation fields,a hybrid parallel algorithm based on MPI and OpenMP is developed in the Monte Carlo program.With the memory sharing between OpenMP threads,the memory consumption problem is overcome.Dynamic load adjustment between OpenMP threads alleviates the load imbalance caused by the long history problem.(4)For shutdown dose rate calculation under complex geometry,based on the rigorous two-step method,a shutdown dose rate calculation program is developed through a two-way coupling of particle transport calculation and activation calculation.In addition,an unstructured mesh based shutdown dose rate calculation algorithm is established,which implements Monte Carlo mesh tally,decay photon source sampling,and activation calculation based on unstructured meshes,improving the calculation accuracy of the shutdown dose rate.The above method is implemented based on the Monte Carlo particle transport program cosRMC,and the developed program is applied to the full reactor radiation field calculation of the China Fusion Engineering Test Reactor(CFETR)and the shutdown dose rate calculation of the fusion experimental device with Frascati Neutron Generator(FNG).The results show that:(1)The response matrix method can produce high-quality weight window distributions.Compared to the weight window generation method based on flux distribution,the total time required for convergence of statistical results under the response matrix method is reduced by about 76%,and higher computational accuracy is achieved.The optimization of the weight window iteration process reduces the weight window convergence time by about 25%,further improving computational efficiency.(2)The adaptive mesh algorithm can provide a more reasonable mesh division scheme for the weight window method.In the variance reduction calculation based on the response matrix method,the computational efficiency of the adaptive mesh is 53.3%higher than that of the uniform mesh with the same number of meshes.(3)The hybrid parallel algorithm can effectively overcome the load imbalance caused by the long history problem,and the computational efficiency is improved by a maximum of 40.8%compared to pure MPI parallel without affecting the weight window performance.The memory footprint of hybrid parallel using dual threads is also reduced by 44%compared to pure MPI parallel,effectively overcoming the memory pressure in large-scale parallel computing.(4)The reliability of the shutdown dose rate calculation algorithm based on the unstructured mesh is verified through the calculation of the FNG ITER shutdown dose rate experiment.Compared to structured mesh,the maximum error between the shutdown dose rate calculation results and experimental values under unstructured mesh is decreased from 30.06%to 10.1%.The new methods developed in this dissertation improve the computational efficiency of the Monte Carlo particle transport calculation in the deep penetration problem of fusion reactors,effectively overcome the memory consumption problem of high-resolution calculations for the full reactor,improve the accuracy of shutdown dose rate calculations under complex geometry,and provide efficient computational methods and tools for nuclear design and safety analysis of fusion reactors. |
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