Patent Number: 
Section: claims

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; andresponsive 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. 2. The method in claim 1, wherein the lattice eigenvalue is initially determined based upon user defined reflective boundary conditions. 3. The method in claim 2, wherein the user defined reflective boundary conditions vary along the plurality of boundary segments. 4. The method in claim 1, wherein the lattice eigenvalue is produced using a stochastic method. 5. The method in claim 4, wherein the stochastic method is a Monte Carlo method. 6. The method in claim 1, wherein the lattice eigenvalue is produced using a deterministic method. 7. The method in claim 1, wherein the at least one of the reflective boundary conditions is adjusted based upon a difference between the reactor eigenvalue and the lattice eigenvalue. 8. The method in claim 7, wherein adjustment of the at least one of the reflective boundary conditions varies along the plurality of boundary segments. 9. The method in claim 1, wherein the reflective boundary conditions are defined in terms of at least one reflection coefficient.