Quasi-reflected interface conditions for variational nodal lattice calculations

Quasi-reflected interface conditions are formulated to partially decouple periodic lattice effects from the pin-cell to pin-cell flux variation in the finite subelement form of the variational nodal code VARIANT. With fuel-coolant homogenization eliminated, the interface variables that couple pin-cell sized nodes are divided into low-order and high-order spherical harmonic terms, and reflected interface conditions are applied to the high-order terms. This approach dramatically reduces the dimension of the resulting response matrices and leads to sharply reduced memory and CPU requirements for the solution of the resulting response matrix equations. The method is applied to a two-dimensional OECD/NEA PWR benchmark containing MOX and UO₂ fuel assemblies. Results indicate that the quasi-reflected interface conditions result in very little loss of accuracy relative to the corresponding full spherical harmonics expansion.