[en] Core flow orificing is important because fuel lifetime (or burnup) is strongly dependent upon the peak cladding temperature. The peak cladding temperature in different assemblies can be made nearly equal by properly allocating the flow to each assembly through flow orificing. In the method described each orifice zone has the same peak cladding temperature at some time in life. This method is especially important for large heterogeneous LMFBR (parfait) cores where interassembly heat transfer effects become more important than for the corresponding homogeneous LMFBR. The method was applied to orifice the flow in both homogeneous and heterogeneous cores