1. A method for determining a spatial and energy distribution of neutrons in a nuclear reactor lattice depletion, the method, when implemented by a computer, comprising the steps of: obtaining a reactor eigenvalue, the reactor eigenvalue being a specified ratio of actual neutron production to loss in a nuclear reactor; determining a lattice eigenvalue based upon reflective boundary conditions (α) of a lattice representing at least a portion of the nuclear reactor, the lattice eigenvalue being an estimated ratio of neutron production to loss in the lattice, the lattice including a lattice boundary comprising a plurality of boundary segments, the lattice boundary associated with the reflective boundary conditions along the plurality of boundary segments; adjusting at least one of the reflective boundary conditions (α) of the lattice to cause convergence of the lattice eigenvalue and the reactor eigenvalue, while maintaining the heterogeneity of the lattice; repeating the determining and adjusting steps, without homogenization of the lattice, until the lattice eigenvalue is within a preset limit of the reactor eigenvalue; and responsive to the lattice eigenvalue being within the preset limit of the reactor eigenvalue, providing the adjusted reflective boundary conditions for determination of the spatial and spectral distribution of neutrons.