Development of a lumped multi-physics model for Tritium Extraction Systems based on the Permeator Against Vacuum Technique for the next fusion machines

In the framework of next fusion machines, the tritium cycle is of particular interest since its central role in fusion reaction as a fuel element. The high permeability of Tritium, as well as its decay time, requires an “in situ” production. The Tritum Extraction and Removal System is the system aimed to manage the tritium cycle inside the reactor and the Permeator Against Vacuum (PAV) is one of the technologies proposed for the TERS of the Water-Cooled Lithium Lead Breeding Blanket. The aim of this work is to develop a lumped model for the tritium extraction based on the PAV. This work is carried on in collaboration with the research centre of ENEA-Brasimone (Camugnano, BO) where a PAV mock-up has been installed and is under test in the TRIEX-II facility. The lumped multi-physics 1D model is developed in Modelica grouping unit elements called “Radiatubes”, based on separate models of the different physical aspects characterizing the tubes in the PAV. The driving phenomena of the PAV technology is the tritium permeation and diffusion, due to the pressure difference between the tritium in the PbLi flowing into the bank of tubes and the vacuum inside the PAV. The permeation model is described considering a diffusion limited permeation regime and its governing equations, moreover, a sensitivity analysis on the solubility constant is done to deal with some uncertainties found in literature. A detailed 3D thermal-hydraulic model using the commercial software STAR-CCM+ allowed the computation of the hydraulic characteristic of the U-tubes, used as input to the Radiatubes. Beside the advection, the lumped model accounts also for the radiative effects present in the detailed 3D thermal model, in the form of view factors between the Radiatubes and the envelope. Eventually, a suitable coupling of the different Radiatubes to inlet and outlet manifolds leads to the assembly of the whole PAV lumped model. A first benchmark of the lumped model is performed reproducing the global behaviour of the 3D thermal-hydraulic model, and cross-checking the permeation results against the ones obtained using a consolidated MATLAB model.