Investigation On Neutron Analysis Method For Diagnostic System Of Tokamak Device

The tokamak diagnostic system is used to obtain plasma parameters by detecting electromagnetic waves and particle radiation actively or passively.It is usually installed close to the plasma chamber and is an important part of the tokamak device.With the development of magnetic confinement fusion physics and engineering technology,and driven by the demand for stable output clean energy,plasma combustion will become the focus of experimental research on tokamak devices.In the case of high neutron flux of combustion plasma discharge,the diagnostic system will face an extremely harsh nuclear radiation environment,which requires a large number of neutron analysis for shield design and radiation level assessment.Neutron analysis is very important for optimal design,integration and reliable and stable operation of diagnostic system in nuclear environment.With the advancement of ITER diagnostic design and integration research,the computational power and accuracy of the neutronics have been greatly improved,but there are still some challenges in several aspects involving neutron analysis methods,such as accurate plasma neutron sources,high-fidelity geometric models of complex heterogeneous objects,neutron transport calculation methods,and efficient parallel computing of super-large clusters.This paper focuses on the research of neutron analysis methods,including plasma neutron sources,global neutronics modeling methods and Monte Carlo transport calculation methods,aiming at the requirements of neutron analysis and the challenges of analysis methods in the diagnostic system.By making full use of the experimental foundation of the EAST device,the calculated results of neutron analysis method are compared with the measured data of corresponding neutron diagnosis,so as to verify the reliability and accuracy of the neutron analysis method.In addition,the neutron analysis method has been widely applied in the EAST device,which provides important support for the design optimization and physical understanding of neutron diagnosis.Plasma neutron source is the input source for neutron analysis of tokamak device and diagnostic system,and its accuracy directly affects the simulation results.At the same time,the research of neutron source is also conducive to the analysis and understanding of neutron diagnostic system data.In order to describe the spatial distribution of plasma neutron sources more accurately,the plasma neutron source model(NSM-PS)based on plasma discharge configuration is proposed,and the corresponding neutron source generator program is written to obtain the neutron emissivity and energy spectrum.The differences in neutron emissivity distribution,neutron energy spectrum and neutron wall load between the NSM-PS model and the neutron source model described by the magnetic surface approximate equation are compared and analyzed.Finally,the accuracy of the NSM-PS model is verified by the experimental results of the neutron flux monitor.The plasma neutron source model(NSM-NBI)suitable for NBI heating is established according to the non-maxwell characteristics of ion velocity distribution under NBI heating.The corresponding neutron source is obtained by combining with the EAST discharge experiment parameter configuration.The influence of some factors on the neutron source is studied through a variety of calculation cases,and the simulation results are self-consistent.The neutron source is verified by using the experimental data of a variety of neutron diagnostic systems,including neutron yield,plasma energy storage,neutron energy spectrum and neutron emissivity distribution.The neutron source calculated by the NSM-NBI model is accurate and reliable within the acceptable error range.The neutronics model is the geometric structure and material property setting for neutron analysis in the Monte Carlo transport program.In order to calculate some neutronics parameters(such as neutron flux,energy spectrum,etc.)more accurately,the model should be as close to the actual physical geometry as possible.However,the plasma diagnostic system’s neutronics model includes tokamak device and diagnostic main component,which has the characteristics of complex geometric structure,heterogeneous materials and many kinds,and huge modeling project.By referring to domestic and foreign experience in the development of local and global neutronics models for fusion devices,the process of the establishment of neutronics models is analyzed in detail.Aiming at some difficult problems,an extended filling space method is proposed to reduce the difficulty of the modeling and improve the efficiency of the modeling.This method is applied to the EAST device,and a detailed global neutronics model of the EAST host and a detailed neutronics model of the EAST experimental hall are established,which are integrated into an EAST Full reactor neutronics Model(EFnM).The EFnM is composed of 13,760 cells and 15,338 surfaces,and its particle loss rate is 4.4×10-8,which meets the requirements of Monte Carlo simulation program operation.The neutron transport calculation is carried out based on the EFnM,and the fusion neutron and secondary photon flux distributions of the EAST device are obtained.Diagnostic systems for combustion plasma experiments are often integrated into multiple layers of thick port plug(e.g.ITER)to reduce radiation loss and penetration leakage from fusion neutrons,but greatly increase the difficulty of neutron analysis calculations.Some features and difficulties of neutron analysis in diagnostic window are analyzed in detail.Aiming at the frequent calculation requirements in the initial stage of nuclear shield design of diagnostic system,the Local Monte Carlo method(LMC)based on SSW/SSR features is established to improve the efficiency of simulation calculation greatly and reduce the design cycle.This method is characterized by two steps of simulation calculation.Because the second step involves a simplified calculation model,the calculation time is effectively reduced.In addition,the AntiForward Local Variance Reduction method for Multi-Object Counting(AFLVR-MOC)is developed for the diagnosis system with multi-aperture shield collimation,where the result of neutron transport calculation is difficult to converge.The characteristics of some variance reduction methods are analyzed by weight window gradient.The two methods are applied to the neutron transport calculation of the Radial Neutron Camera(RNC)on the EAST,and the accuracy and effectiveness of the method are verified.The response matrix of RNC is calculated,and the position optimization of the RNC is studied,which provides important support for the physical analysis based on the data of the RNC.