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The neutron transport equation is usually solved against a stationary background medium. When the background material is moving, the transport equation will need to be modified. Solving the transport equation with moving material is quite complicated, especially for the curved coordinate system because of the double angular redistributions. In this study, the discretization method of the simplified transport equation considering the moving-material effect is implemented in three-dimensional cylindrical geometry. Directly solving this modified transport equation with the standard solution technique is problematic since the advection term introduced by moving material may render the transport solver numerically unstable. The speed ratio λ is defined for stability analysis. A forced-stable method is proposed in this study to achieve good numerical stability for any material speeds and time-step sizes. The accuracy of this new method is verified using manufactured solutions. Steady numerical results demonstrate that the effects introduced by background motion cannot be neglected as the material speed starts to approach one-tenth of the neutron speed. Moreover, transient analysis indicates that the moving background has a considerable impact on the criticality of a system. [ABSTRACT FROM AUTHOR]

• The transport equation considering background motions is studied. • Analytic forms of the coupled transport equation are derived. • Methods of transforming equations between different phase spaces are discussed. The neutron transport equation is usually solved against stationary background materials. When the background material is moving, the transport equation will need to be modified. Two historical approaches, "effective" cross-sections and "co-moving" frame, have been used in the past decades. The neutron transport community prefers to use the second method. However, the modified transport equation in the "co-moving" frame is extremely complicated. In this study, we discuss these modifications for non-relativistic neutrons. Transport equations considering background motions have been derived under the three-dimensional (3-D) Cartesian, cylindrical, and spherical coordinates, which can be conveniently reduced to lower-dimensional systems. Moreover, methods of transforming the coupled equation between different phase spaces have been studied. [ABSTRACT FROM AUTHOR]

Pinhole imaging is an important test technique to obtain information on the spatial distribution of the radiation field in the target region and has been widely used in nuclear physics and inertial confinement fusion (ICF). Coded-aperture, able to maintain good resolution as well as enhancing signal strength, has become a more frequently used method than pinhole imaging in experimental studies. Thus, implementing Monte Carlo simulations of coded-aperture imaging will improve coded-aperture design, image reconstruction, and other related works. However, the current international mainstream Monte Carlo transport simulation programs do not possess the ability to directly simulate coded-aperture imaging. This paper develops a relatively complete coded-aperture imaging simulation function on the Neutron Photon Transport System code based on the next-event estimation method. With the application of Monte Carlo simulation techniques, such as variance reduction and rejection sampling, it is capable of simulating coded-aperture accurately, flexibly, and efficiently, including problems of multiple shapes and even irregular geometry. The results are consistent with combined pinhole imaging, and the computational efficiency has been improved significantly. [ABSTRACT FROM AUTHOR]

The deterministic methods are efficient for solving the neutron transport equation (NTE), but suffer from discretization errors. The increasingly complex geometric models make spatial discretization errors the primary source of discretization errors. Considering that spatial discretization errors come from inaccurate geometric shape descriptions and low-accuracy numerical schemes, this paper develops a Discontinuous Galerkin Finite Element Method for the NTE on unstructured polygonal meshes to reduce spatial discretization errors. In this method, the physical modal basis is adopted to handle the polygonal mesh and to achieve high-order accuracy in a uniform and efficient way. The numerical results of various fixed-source and k-eigenvalue benchmarks demonstrate that the method developed in this paper can give accurate solutions on polygonal meshes with high convergence rates. [ABSTRACT FROM AUTHOR]

• The detailed derivations of the DSA algorithm in the r - θ - z geometry is given. • A parallel DSA algorithm is implemented in the frame of the KBA algorithm. • Based on the red-black sweep, a half-reduced PCG method has been proposed. • The effectiveness and efficiency of this parallel DSA algorithm has been verified. • The HRB(m)-PCG algorithm has well parallel scalability. A parallel DSA algorithm based on the red-black sweep PCG method is proposed to accelerate the neutron transport calculation in the 3-D cylindrical geometry. This parallel acceleration technique shares the same spatial domain decomposition strategy with the KBA method. The red-black sweep iteration is formed by moving the r - and θ -axial components of the diffusion operator into the right hand side of the diffusion equation. Using the initial values refreshed by several red-black sweep iterations simplifies the matrix-vector multiplication in the CG procedure. Through the homogeneous problems, how the times of the red-black sweep effects the DSA algorithm is analyzed. A wide range of problems including FBR benchmark, PWR Pin-by-pin calculation and PWR shielding calculation have been performed to test the effectiveness and efficiency of the new method. The solution time is reduced by 3 ~ 10 times in different problems. This new acceleration algorithm has well parallel scalability. [ABSTRACT FROM AUTHOR]

• Next event estimation for neutron transport in moving objects. • Directional emission probabilities based on nuclear data in different frames. • The effect of the motion of the pellet material on the pinhole imaging. The next event estimation is one of the important algorithms employed in Monte Carlo simulations for particle transport. It finds wide applications in point flux, image flux, and variance reduction techniques. With the advancement of transport simulations for moving objects, the next event estimation for moving target nuclei has yet to be realized. The next event estimation requires the calculation of directional emission probabilities based on the outgoing distribution provided by nuclear data. When the target nucleus is in motion, determining the outgoing direction in the coordinate system prescribed by nuclear data and calculating the emission probability in that direction becomes more intricate. In this study, regarding the outgoing distributions in different coordinate systems, we present a systematic approach to calculate the next event estimation for moving targets. The accuracy of this method is verified through the energy and angle distributions of surface current tallies in the test model. Applying the new method, we conducted pinhole imaging simulations for a moving radiator, quantitatively comparing the differences in simulation results with and without considering motion. [ABSTRACT FROM AUTHOR]

Monte Carlo programs are increasingly considered the most accurate tool for simulating particle transport problems. The neutron-photon transport simulation capabilities of domestic particle transport Monte Carlo softwares are relatively advanced, while the electron transport simulation functions have not been well developed yet. This paper independently developed the functions of electron transport and photon-electron transport in the CH (condensed history technique) scheme based on the NPTS (Neutron Photon Transport System) program, and implemented the multi-coupling transport simulation of neutrons, photons, and electrons, which can meet the needs of actual applications. Several test models were run on both NPTS and the popular programs such as EGS and RMC to verify the correctness of NPTS. In addition, an energy deposition tally is developed. It is shown both theoretically and experimentally that the energy deposition calculation by NPTS are more accurate than RMC. [ABSTRACT FROM AUTHOR]