Comparative analysis of accidental sequences with conventional and Accident Tolerant Fuels (ATF) in VVER-1000 reactors

• Cr-coated Zry and FeCrAl allow to extend the time available for several human actions. • Cr-coated Zry and FeCrAl avoids damage in several SBLOCA sequences with HPIS failure. • The time available increases more in FeCrAl than in Cr-coated Zry. A significant percentage of reactors in operation, under construction or recently commissioned are VVER reactors. In parallel, there is a growing interest in analyzing the behavior of Accident Tolerant Fuels (ATF), also known as Advanced Technology Fuels, under development in different nuclear reactor designs. Among the new ATF designs, the FeCrAl and Cr-coated Zircaloy claddings are one of the most currently studied. In addition to having a relatively high level of technology readiness, these evolutionary cladding materials offer an improved resistance to oxidation and hydrogen absorption, as well as improved mechanical strength. All these properties are essential in accident sequences where high core temperatures are reached. In the present study, core damage sequences have been analyzed with a model of a VVER-1000 reactor for the TRACE system code. The results indicate that for certain sequences, such as a Small Break Loss of Coolant Accident (SBLOCA) or an Station Blackout (SBO), these new cladding materials increase the reactor safety margins by extending the time available to recover the safety systems involved in core cooling and replenishment of the reactor coolant system inventory.