As an important improvement in the light water nuclear reactor operations, the nuclear fuel burnup rate is increased in recent decades and this increase causes heavier duty for the nuclear fuel. Since the high burnup fuel is exposed to very high thermal and mechanical stresses and since it operates in an environment with high radiation for about 18 month cycles, it carries the risk of losing its integrity. In this study; it is aimed to determine the effects of pellet cladding gap thickness and gap pressure on reliability of high burnup nuclear fuel in Pressurized Water Reactors (PWRs) under steady state operation conditions and suggest optimum values for the examined parameters only and validate these suggestions for a transient condition. In the presented study, fuel performance was analyzed by examining the effects of pellet cladding gap thickness and gap pressure on the integrity of high burnup fuels. This work is carried out for a typical Westinghouse type PWR fuel. The steady state conditions were modeled and simulated with FRAP-CON-3.4a steady state fuel performance code and the FRAPTRAN-1.4 fuel transient code was used to calculate transient fuel behavior. The analysis included the changes in the important nuclear fuel design limitations such as the center-line temperature, cladding stress, strain and oxidation with the change in pellet cladding gap thickness and initial pellet cladding gap gas pressure. In addition, the analysis were performed to observe whether the changes in pellet cladding gap thickness and the gap gas pressure can substitute each other or not when it comes to design changes in nuclear fuel design to increase burnup. Also, the verification of the suggestions for pellet cladding gap thickness was performed under transient conditions. The results of these analyses showed the favorable design change for the pellet cladding gap for heavier fuel duty conditions. (C) 2013 Elsevier Ltd. All rights reserved.