Progress in characterizing 2D/1D accuracy in MPACT

The 2D/1D method is an important tool in LWR analysis that enables pin-resolved transport solutions for large, full-core simulations at relatively low computational cost compared to a true 3D transport method. To use the 2D/1D approximation, several assumptions are made about the nature of the axial (z) component of the true 3D transport solution. Generally, it is assumed that the axial variation of the solution is weak enough that the axial streaming term can be approximated as isotropic in angle, and uniform in space over a coarse pin cell, without unacceptable detriment to accuracy. Also, it is assumed that a homogenized 1D axial pin cell calculation will give the correct axial streaming magnitude and power shape. In some cases, however, we desire a more accurate transport solution; we can achieve this by implementing higher-fidelity solvers that avoid some of these coarse approximations, e.g. allowing angular dependence in the radial and axial leakage terms that couple the 2D and 1D solutions. Since the introduction of 2D/1D methods, there has been interest in developing and improving these more accurate approximations, with the ultimate goal being a 2D/1D method that limits to the 3D transport solution with spatial and angular refinement. In this paper, the 2D/1D code MPACT is used, with SN as the 1D axial solver. We evaluate some of the approximations made in MPACT, and derive a new (to MPACT) angle-dependent 2D to 1D total cross section homogenization. The new homogenization shows good results for C5G7-type problems and the full 3D C5G7 benchmarks compared to what was previously the 'most accurate' 2D/1D method in MPACT, which used angle-dependent leakages but only scalar flux homogenization of the total cross section. We also quantify the effects of applying a within-pin fine-mesh spatial shape to the axial transverse leakage, which is generally much smaller than the effect of angle-dependent homogenization. (authors)