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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]

A modified α − k power iteration method is presented for the prediction of time-eigenvalue( α ) of the neutron transport equation. By developing a direct relationship between K-eigenvalue and α -eigenvalue, a new formula is introduced to estimate the value of α . Compared with the conventional method, it is not required to provide the initial values of α for the modified method. Since it is always difficult to guess the suitable initial values, the modified method is more convenient for solving time-eigenvalue problems. Computational experiences show that the accuracy of the modified method is the same as the conventional method. [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 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]