## 2 Experimental Facility  
CIRCE is an integral effect pool-type facility, designed and constructed at the ENEA Brasimone research center. It consists of a cylindrical main vessel of AISI316L partially filled with nearly 70 tons of molten LBE covered with argon at about 0.2 bar [7; 8]. The vessel is externally equipped with electrical heating cables installed at the bottom and on the lateral surface and operating in a temperature range of 200 \({}^{\circ}\)C-400 \({}^{\circ}\)C. The pool vessel parameters are summarized in Table 1. The HERO-loop test section is installed inside CIRCE from the top of the main vessel through a coupling flange; a schematic view of the CIRCE-HERO facility is reported in Figure 1.  
The HERO-loop consists of:  
* A Fuel Pin Simulator (FPS), an electrical pin bundle composed of 37 electrically heated pins with a nominal thermal power of \(\sim\)1 MW representing the heat source of the LBE. The design of the component aims to provide a coolant temperature gradient of 100 \({}^{\circ}\)C/m with an LBE average speed of 1 m/s and a power density of 500 W/cm\({}^{3}\).
* A fitting volume, a volume located above the FPS collecting the hot LBE rising from the FPS.
* A riser, in which the LBE flows upward up to the separator.
* A separator, a component sited on the top of the test section. It is the hot plenum and acts as an expansion tank accommodating the LBE volume variations.
* A Steam Generator Bayonet Tube (SGBT), the heat sink of the loop for heat removal. It is composed of 7 double wall bayonet tubes. Each tube represents in scale 1:1 the tube foreseen for the SGBT to be adopted in the Advanced Lead Fast Reactor European Demonstrator-super (ALFRED) [7]. This configuration guarantees the separation between the primary side (LBE) and the secondary side (steam-water) decreasing the probability of LBE-water interaction and facilitating the monitoring of leakages by pressurizing the annular separation region.  
\begin{table}
\begin{tabular}{l c} \hline \hline
**CIRCE Parameters** & **Value** \\ \hline Outside diameter [mm] & 1200 \\ High [mm] & 8500 \\ Wall thickness [mm] & 15 \\ Electrical heating [kW] & 47 \\ Operating pressure [kPa] & 15 (gauge) \\ Design pressure [kPa] & 450 (gauge) \\ Argon gas volumetric flow rate [N/s] & 5 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Pool vessel parameters.  
Figure 1: Schematic view of the CIRCE-HERO facility.  
* An argon injection device is located at the inlet section of the riser. It circulates the LBE by imposing a fixed argon mass flow rate.
* A dead volume, placed above the fitting volume and insulated from the pool, which encloses and maintains the wirings and electrical connections of the FPS.  
A feeding conduit equipped with an LBE Venturi flow meter and a differential pressure transmitter allows the measuring of the LBE flowing upwards through the FPS region where it is heated up by the 37 electrical pins. Then the LBE passes through the fitting volume and the riser (at this point the flow is enhanced by the argon gas injection at the top of the fitting volume) reaching the separator. From the separator, the LBE enters the shell side (primary side) of the HERO-SGBT flowing downwards. Here, the LBE is cooled by the feedwater flowing in the 7 Bayonet Tubes (BTs) (1 central tube and 6 lateral tubes allocated in a hexagonal arrangement) to then be discharged in the bottom zone of the pool. Further details regarding the facility are reported in [7,8].