Abstract
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), then the planes are coupled axially through a 1D NEM-P$_3$ calculation.
This iterative calculation is then accelerated using the Coarse Mesh Finite
Difference method.
One problem that arises frequently when using the 2D/1D method is that of
control rod cusping. 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, causing an error
known as cusping. The most straightforward way of resolving 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.
