Current Status of Cr-coated Claddings in Severe Accident Codes and Evaluation within the Frame of the QUENCH-ATF-1 Benchmark of OECD Project QUENCH-ATF

Abstract

Zirconium-based claddings have been widely used for decades due to their good mechanical properties, corrosion resistance, and low neutron absorption under operational conditions. However, at elevated temperatures, these claddings exhibit negative characteristics, such as exothermic oxidation reactions in steam or air environments. Research and development of conventional cladding and furthermore Accident Tolerant Fuel materials has been motivated by the need to establish an additional safety element in nuclear power plants. These new materials should exhibit significantly slower oxidation kinetics compared to the Zr-based alloys. Among others, the coating of claddings with is being intensively investigated and tested for increased accident tolerance. Its oxidation rate is several orders of magnitude smaller than that of typical zirconium-based cladding, assuming the coating is in the protective state. Afterwards, the kinetics approaches Zr-alloy behavior. In addition to experiments, code modelling capabilities for DBA and DEC analyses must be improved for the analysis of systems that include such materials. Suitable oxidation models must be implemented to predict the ATF behavior under DBA and DEC conditions. Additionally, different material properties like density and thermal conductivity or heat capacity have to be considered. The ability to model Cr-coating varies among severe accident codes, including AC²/ATHLET-CD, ASTEC, MAAP, SAMPSON, and MELCOR. This will be described in detail in the paper. To validate the performance of integral codes that model Cr-coated cladding behavior, the benchmark on the experiment QUENCH-ATF-1 within OECD/NEA QUENCH-ATF project is considered. The boundary condition of the sequence is an extended DBA scenario, with a PCT close to the eutectic temperature of the Cr/Zr binary system. In the paper the results concerning thermal evolution and hydrogen generation will be discussed and the model basis for Cr-coated cladding will be evaluated.