A new approach to core-reflector boundary conditions for nodal reactor computations

New core-reflector boundary conditions designed to replace the explicit representation of the reflector in nodal computations are developed taking into account the transverse leakage in the reflector region. Two approximations are introduced for the transverse leakage in the reflector region: exponential approximation for the slab reflector and quadratic polynomial and exponential approximation for the L-shaped reflector. Core-reflector boundary conditions that relate net current with flux at the core-reflector interfaces are then derived by solving the transverse integrated neutron diffusion equation with transverse leakage approximations in the reflector region. To test the usefulness of new core-reflector boundary conditions, nodal expansion method computations with and without explicit representation of reflectors are performed for the core power distribution and criticality of Zion-1 and YGN-1 pressurized water reactors. It is demonstrated that core power and criticality computations with new boundary conditions agree very well with those with the reflector included explicitly in computational nodes