In this research the burnup performance of (1) boron nitride (BN) and (2) boron nitride-boron (B), hybrid coated urania (UO2) and urania-gadolinia (Gd2O3) fuels were studied. The behavior of fuel burnup, depletion of BN and B, the effect of coating thickness and also Gd2O3 content on the burnup performances of the fuels were found by using the code WIMS-D/5 for pressurized water reactor (PWR) and boiling water reactor (BWR) fuel assemblies. The optimum thickness ratio of B to BN was found as 4 and their thicknesses were chosen as 4 and 1 mum, respectively. Based on this observation, calculations were performed for the standard fuel assemblies with and without burnable poison and similar assemblies with BN-B hybrid coating with the desired BN-B thicknesses. Results are discussed to make an assessment of the effect of such hybrid coating on fuel cycle characteristics. (C) 2001 Elsevier Science B.V. All rights reserved.