# Status, Features, and Future Development of the LIFUS5/Mod4 Experimental Facility Design
## 1 Introduction  
_Scope of the Work_  
In the framework of the development of fusion technology, the Water-Cooled Lithium-Lead (WCLL) is one of the most prominent candidates to address the issue of tritium breeding and power conversion. This technology will be implemented and tested in one of the two Test Breeding Modules (TBMs) that will be installed in the ITER reactor and the DEMO EU reactor [1, 2, 3]. The concept of the WCLL TBM/BB is based on the simultaneous presence in the reactor wall of flowing Pb-Li eutectic alloy, which is used for neutron multiplication and tritium breeding, and water, which serves as a coolant for the system. This coexistence of water and molten alloy poses several questions for the safety of the technology, especially in the case of the worst possible postulated accidental scenario, an in-box LOCA, in which one of the TBM water-cooling pipes is expected to rupture [4, 5, 6]. To address these issues, several experiments have been and are being performed, to deepen the understanding of the underlying phenomena that occur in this scenario, both from the physical and chemical points of view [7, 8, 9]. Previous experiments performed at ENEA Brasimone Research Centre on the LIFUS5/Mod3 facility focused on the separate-effect study of the PbLi-water interaction, isolating it from the structure response, tohighlight the thermochemical interactions between the two substances. These experiments allowed the collection of a set of data that lead to a deep understanding of the phenomena and the implementation and validation of chemical reaction models inside dedicated numerical codes [10; 11; 12]. However, these experiments did not investigate the response of the structures to the strong pressure variations and temperature changes induced by the two fluids' interaction. For this reason, a new integral-effect facility is being constructed at the Brasimone RC, based on the previous expertise acquired on the existing experimental plants. The new facility, namely the LIFUS5/Mod4, aims at studying the actual response of the WCLL-TBS both during nominal operation and in-box LOCA incidental scenarios. It will be composed of a 1:1 reproduction of the ITER WCLL-TBS PbLi loop and it will try to mimic and reproduce its hydraulic behavior as closely as possible, aligning itself to the main objectives of the FP9 Horizon Europe EUROfusion WPBB [13]. The main expected scientific outcomes and technological advancements of this facility will include:  
* Generating data applicable to the full-scale ITER WCLL Test Blanket System (TBS) conditions that may be directly used for the safety analysis;
* Providing integral test facility data for the understanding and the study of the \"in-box Loss Of Coolant Accident\" transient progression in the WCLL BB of DEMO;
* Supporting the development and demonstrating the reliability (i.e., validation and qualification) of computer codes, coupling techniques, and procedures for code use, when applied for simulating the behavior of the \"in-box Loss Of Coolant Accident\" at a system level. To simulate the exact conditions that the components will encounter during the actual reactor operation, the LIFUS4/Mod4 will be connected to a second and much larger facility, named Water Loop (WL). This second facility will be composed of a loop that will mimic the ITER WCLL Water Cooling System (WCS). Inside it, water will be circulated at PWR-like conditions, i.e., 155 bar and 295-328 \({}^{\circ}\)C. These conditions will be reached through a 1 MW electrical heater that simulates the power transfer from the reactor breeding blanket to the primary water loop [14]. By coupling the WL and LIFUS5/Mod4, the facilities will be able to simulate system operation and in-box LOCA events maintaining the representativeness of the thermodynamical phenomena, hydraulic conditions, and dynamical behavior of the loops.