Comparison of the performance of heterogeneous variational nodal method and 2D/1D-MOC on pin-resolved neutron transport calculations of PWR

Heterogeneous Variational Nodal Method (HVNM) and the 2D/1D Method of Characteristics (2D/1D-MOC) are two promising methods for high-fidelity neutronics calculation. HVNM starts from even-parity form of neutron transport equation and employs iso-parameteric finite elements, orthogonal polynomials and piece-wise constants for spatial discretization. Additionally, spherical harmonics are applied for the angular expansion. In terms of 2D/1D-MOC, it starts from first-order transport equation and splits the three-dimensional transport equation into one 2D and one 1D equations which are coupled by the radial and axial leakage terms. Then space and angle are discretized by characteristic lines in radial plane and 1D Sn calculation is employed in axial direction. Both HVNM and 2D/1D-MOC have the capability of dealing with pin-resolved geometry and strong transport effect in PWR core problem. Thus, we developed two codes PANX and NECP-X which based on the methods of HVNM and MOC respectively. In this paper, to compare their performance on pin-resolved problems in PWR, the KAIST problem, the NuScale problem and the Beavrs problem are calculated. Both computational accuracy and efficiency are evaluated. The numerical results shown that both PANX and 2D/1D-MOC can obtain accurate eigenvalues with the error less than 100 pcm. However, PANX obtains more accurate pin power distribution and achieves higher computational efficiency than 2D/1D-MOC.