To support the activities carried out by the ENEA Brasimone Research Center on the Lead-cooled European Advanced Demonstration Reactor (LEADER) project, University of Pisa has improved the RELAP5/Mod3.3 code introducing the Lead and Lead-Bismuth Eutectic (LBE) as coolant fluids. This new version of the STH code was applied to perform several analyses for the steam generator HERO (Heavy liquid mEtal pRessurized water-cOoled tubes) installed in the CIRCE (CIRColazione Eutettico) pool-type facility that uses LBE as working fluid. HERO consists of a bundle of seven Double-Wall Bayonet Tubes (DWBT) with a leakage monitor system, and it was tested in the CIRCE facility to support the development of the steam generator proposed for ALFRED (Advanced Lead Fast Reactor European Demonstrator).

A preliminary analysis was performed comparing the RELAP5/Mod3.3 results against available experimental data. In particular, an isothermal test of the CIRCE-HERO facility allowed to set-up the RELAP5/Mod3.3 nodalization. As a second analysis, a steady state at full power was investigated. This steady-state condition was employed to perform a sensitivity study on the influence of the thermal conductivity of the stainless steel powder characterizing the HERO steam generator. Finally, the analysis of unprotected and protected loss of flow transients, and the analysis of a protected loss of flow with a simultaneous partial Loss of Heat Sink (LOHS) were performed. These last transients were investigated to assess the behavior of the HERO steam generator, especially to verify the possibility to establish a natural circulation capable to safely remove the “decay heat”.

The results of the first analysis (isothermal hydraulic characterization) demonstrated the good capabilities of improved RELAP5/Mod3.3 code to reproduce the forced circulation conditions. The results of the second analysis (steady-state at full power) highlighted the strong influence of the stainless steel powder thermal conductivity on the achieved steady-state conditions. The simulations of the last tests provided the time evolution of the LBE mass flow rate and of the temperatures in different regions inside the pool during unprotected and protected loss of flow transients. In the paper, a critical discussion of the obtained results is performed.

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