Subplane-based control rod decusping techniques for the 2D/1D method in MPACT1

The MPACT transport code is being jointly developed by Oak Ridge National Laboratory and the University of Michigan to serve as the primary neutron transport code for the Virtual Environment for Reactor Applications Core Simulator. MPACT uses the 2D/1D method to solve the transport equation by decomposing the reactor model into a stack of 2D planes. A fine mesh flux distribution is calculated in each 2D plane using the method of characteristics (MOC), and then the planes are coupled axially by using the 1D P₃ approximation wrapped in a two-node nodal expansion method (NEM-P₃) solver. This iterative calculation is then accelerated using the coarse mesh finite difference method (CMFD). Control rod cusping is a problem that arises frequently when using the 2D/1D method. This occurs when the tip of a control rod falls between the boundaries of an MOC plane, requiring that the rodded and unrodded regions be axially homogenized for the 2D MOC calculations. Performing a volume homogenization does not properly preserve the reaction rates due to an error known as cusping. The most straightforward way to resolve this problem is by refining the axial mesh, but this can significantly increase the computational expense of the calculation. The other way of resolving the partially inserted rod is through the use of a decusping method. This paper presents new decusping methods implemented in MPACT that can dynamically correct the rod cusping behavior for a variety of problems. (authors)