Considerable experimental and analytical work has been performed in recent years which has led to a broader and deeper understanding of phenomena related to reactivity-initiated accidents (RIAs). Further, newly designed fuels – such as mixed-oxide (MOX) fuel and rods with new cladding – have been introduced which might behave differently than those used previously, both under normal operating conditions and during transients. Compared with 20 years ago, fuel burn-up has been significantly increased. These and other factors have led the NEA Committee on the Safety of Nuclear Installations (CSNI) and its Working Group on Fuel Safety to produce this state-of-the-art report. The report should be of particular interest to nuclear safety regulators, nuclear plant operators and fuel researchers.