Under normal operating conditions, cladding and core structural materials operate around 300 degrees Celsius (C), and fuel pellets experience peak temperatures below 2,000 degrees C at the pellet centerline. At these temperatures, fuel cladding integrity is maintained, and fission products are contained within the fuel rods. However, under some abnormal conditions, higher temperatures and other conditions significantly alter the behavior of these materials. These conditions can threaten core coolability and lead to fission product release. This NUREG report considers the following two types of accident conditions: (1) reactivity-initiated accidents and (2) loss-of-coolant accidents. This report describes the fuel behavior of each accident condition from basic concepts to the current state of the art. It also mentions safety criteria, references the classic experimental work in each of these areas, and presents equations and figures that permit some quantitative evaluations.