ETD

• ASTEC
• FeCrAl alloys
• Severe Accident

The present work investigates the impact of FeCrAl cladding on the progression of a Severe Accident (SA) in a reference BWR-4 Mark-I nuclear power plant under station blackout conditions. The analysis is performed using the integral SA code ASTEC (Accident Source Term Evaluation Code), adopting a best-estimate approach. Both deterministic simulations and an Uncertainty and Sensitivity Analysis (UaSA) are carried out to assess the influence of key modelling parameters on the main figures of merit.

A comparison between conventional Zircaloy cladding and FeCrAl Advanced Technology Fuel (ATF) is conducted, with particular focus on the in-vessel phase of the accident. The results show that FeCrAl significantly delays early accident progression due to its slower high-temperature oxidation kinetics, leading to postponed temperature escalation and cladding degradation. Consequently, hydrogen production is reduced by approximately 40%, and its onset is significantly delayed.

The analysis of the late in-vessel and ex-vessel phases highlights more complex behaviour. Nevertheless, after cladding failure, late phase model rely, at same extent, on Zr-based models which introduce limitations in the treatment of molten materials. As a result, the predictions in these phases are affected by higher uncertainties. Differences in corium configuration and heat transfer mechanisms are observed, leading, in the FeCrAl case, to earlier failure of the reactor pressure vessel lower head and to a more rapid progression of the ex-vessel phase.

The UaSA focuses on two key uncertain parameters governing FeCrAl oxidation: the Arrhenius pre-exponential factor and the transition temperature between oxidation regimes. Results indicate that the switching temperature is the dominant parameter affecting accident progression and hydrogen generation, while the pre-exponential factor plays a secondary role.

Overall, the study demonstrates the potential advantages of FeCrAl in mitigating early-phase SA phenomena, particularly hydrogen generation. However, its impact on later accident stages remains uncertain and requires further investigation.