## 4 RELAP5/Mod.3.3 Nodalization  
The RELAP5 nodalization of the CIRCE-HERO facility is Figure 5. It is worth mentioning the presence of RELAP5 default structures called ANNULUS--used in this case to simulate the steam generator components--which are similar to pipes, but require that all the water must be in a film on the wall (no drops) in the annular mist flow regime (ANNULUS structure [5, 6]). Details concerning the implemented components are reported in the Appendix A. The nodalization of the HERO-loop is designed according to the following configuration:  
* The feeding conduit is modelled with PIPE 20 which is connected to the downstream mixing zone of the FPS (PIPE 50) by means of a Single Junction, (SNGLJUN [5, 6]). * The FPS region is modelled with PIPES 50, 60, and 70 where the active length of the FPS (1000 mm active region) is modelled with PIPE 60. The FPS is modelled using a heat structure inside the pipe 60 with a source term representing the power generated by the 37 electrical pins. The upstream mixing zone of the FPS (PIPE 70) is connected to the fitting volume. * The fitting volume is represented via PIPE 80 and BRANCH 90 [5, 6] and it is connected to the riser by means of a SNGLJUN. * At the inlet of the riser (PIPE 130) argon gas is injected to enhance the LBE circulation. This argon injection system is modelled with Time Dependent Volume 3, (TMDPVOL [5, 6]). * After the riser, the separator is modelled by BRANCH 132. Furthermore, it is connected to the argon cover gas volume (TMDPVOL 151) through the BRANCH 150. * Connected to the separator there is the PIPE 172 which represents the primary side (LBE side) of the SGBT. LBE is released into the pool by means of PIPE 174. Figure 4: Feedwater mass flow rate time trend. Figure 3: Lead-Bismuth Eutectic (LBE) mass flow rate time trend. The primary side of the SGBT is connected to the secondary side (steam-water) by means of a heat structure. The secondary side is modelled as follow:  
* The feedwater manifold is modelled with TMDPVOL 410. * The feedwater flow rate is supplied to the system via the TMDPJUN (Time Dependent Junction, [5, 6]) connecting TMDPVOL 410 to the BRANCH 418, which models the feedwater inlet header to the BTs. * The central BT (PIPES 432, 420 and 424 for the feedwater inlet, and ANNULUS 442, and 454 for the steam outlet). * The 6 lateral BTs (PIPES 532, 520 and 524 for the feedwater inlet, and ANNULUS 542 and 554 for the steam outlet). * The outlet steam plenum (BRANCH 460) connected to the steam chamber (TMDPVOL 470). Figure 5: Nodalization of CIRColazione Eutettico—Heavy liquid mEtal pRessurized water cOuled tubes (CIRCE-HERO). The pool is modelled via a group of pipes, i.e., PIPES 203, 213, 223 and 253. These pipes are vertically connected via branches, i.e., BRANCHES 200, 210, 220, 250. The upper and the lower plenum are modelled by means of BRANCHES 150 and 260, respectively. The upper plenum is connected to BRANCH 200 through BRANCH 138. The lower plenum is connected to the feeding circuit of the HERO-loop through a SNGLJUN. Concerning the heat structures, the active length of the FPS region is modelled as a power source embedded inside the thermal structure of PIPE 60. The power source is defined by a power time trend card and the Ushakov heat transfer correlation [10] is used for the triangular lattice fuel bundle region. Furthermore, the following heat structures and wall boundary conditions were applied:  
* Heat structures between the primary and the secondary side of the BTs of the SGBT. Both ANNULUS 442 (central BT) and ANNULUS 542 (6 lateral BTs) are connected to PIPE 172. This heat structures model the heat transfer between LBE and steam-water flowing inside BTs. * Heat structure in the secondary side of the central BT, which is connected from one side to the water (pipe 432), and the other side to steam (ANNULUS 442). As well as for the 6 lateral BTs the heat structure links the water side PIPE 532 to the steam ANNULUS 542.