Characterization of tritium transport in the PAV for the EU DEMO reactor

One of the main challenges for the EU DEMO reactor is to achieve tritium self-sufficiency by means of the Tritium Extraction and Removal System (TERS). The Permeator Against Vacuum (PAV) is a candidate to become the tritium extractor for the TERS: it has a shell-and-tube configuration, where tritium is dissolved in the carrier liquid metal (PbLi) and is forced to cross a metallic (niobium) membrane by pumping vacuum on the secondary side. However, currently there are huge uncertainties in the tritium transport properties, particularly regarding solubility values, which pose a significant challenges for the modeling of tritium permeation. This work aims at comprehensively understand tritium transport within the PAV. This involved several steps: the development of a CFD model using the software OpenFOAM, whose flexibility allowed to craft a set of custom boundary conditions and a solver that are not available in commercial CFD software; the validation against Brasimone's research center (ENEA, Italy) experimental data; a sensitivity analysis on key physical parameters; and, finally, the development of a correlation for the mass transfer coefficient. The model has shown high reliability in the validation step, where both experimental and numerical errors were taken into consideration, by factoring in uncertainties from several input parameters. The sensitivity analysis study identified the Liquid Limited Regime (LLR) as the limiting transport mechanism, contrary to what is commonly found in literature, where diffusion or surface effects were assumed dominant. This result emphasizes the need to focus on fluid transport, leading to the computation of the mass transfer coefficient of hydrogen in the liquid metal. The latter is the key parameter to properly estimate the PAV's extraction efficiency, and future work foresee to develop a correlation for it, advancing our understanding of tritium transport for the DEMO extractor.