In a typical Nuclear Power Plant (NPP), there are several safety barriers retaining the radionuclides within the NPP. A Zr-based nuclear fuel cladding constitutes the first barrier after the fuel matrix performing this crucial safety function, and many design acceptance criteria established bytheregulatorybodyareformulatedintermsofcladdingproperties. Decadesofexpertise have led to a high optimization of this technology, and exhaustive licensing processes have constantly demonstrated the capacity of Zr-based claddings to preserve its integrity in a wide range of conditions, including a spectrum of postulated accident, the so-called Design Basis Accidents (DBAs). However, harsher conditions than DBAs, like those that took place during Fukushima Daiichi’s non-postulated accident, eventually cause the fuel cladding failure. Consequently, post-Fukushima international efforts have accelerated the development of Accident Tolerant Fuels(ATF),whichaimatincreasingthesafetymarginsbyensuringtheintegrityofthe f irst barrier containing the radionuclides in more severe regimes than Design Basis Accidents (DBAs). Among the proposed ATF, the Cr-coated Zr-based cladding is one of the most widely explored and most feasible technologies to be implemented in the short-term due to its costeffectiveness and higher resistance to high temperature steam environments. Before full-scale ATF implementation in Nuclear Power Plants (NPPs), a thorough safety analysis evaluation of the ATF benefits with respect to standard fuel rods is required. This task demands simulations that encompass design basis conditions and Severe Accidents, thus codes with sound capabilities in both regions of interest are needed. However, these codes often lack dedicated models that can represent new ATF designs. To contribute to this purpose and facilitate the Cr-coated ATF deployment, an oxidation model in high temperature steam environments has been developed from the available experimental data. The model relies on the evolution of three parameters, namely the Cr-coated cladding temperature, the time at which Cr oxidation starts and the growth of the chromia layer. As the first step, the model was implemented in RELAP/SCDAPSIM/MOD3.4SystemandSevereAccident code and acomplete process of verification was performed. This ensured that the model worked consistently without interfering with other simultaneous phenomena such as hydrogen production. The next step will be to validate the model with experimental data, while upcoming studies will focus on prolonged accidental sequences in the boundary between Design Extension Conditions with and without core melt, where Cr-coated ATF has a greater potential to increase the safety margins.
