Dottorato in ENERGIA E AMBIENTE

Titolo della tesi: Experimental characterization of tritium extraction systems and tritium anti-permeation barriers in heavy liquid metal systems.

This PhD thesis is divided into two parts which are joined together by the common theoretical background of tritium transport. The first part aims at supporting the development of tritium extraction technologies for the Water Cooled Lithium-Lead Breeding Blanket (WCLL BB). The experimental facility TRIEX-II, located at ENEA Brasimone R.C., has been refurbished and intensively operated to contribute to the characterization of the Permeator Against Vacuum (PAV) and Gas-Liquid Contactor (GLC) technologies. The PAV has been often considered as the reference technology to extract tritium from LiPb, but it has never been tested in relevant conditions, nor a sizeable mock-up has ever been completed before. In this thesis, the engineering design and fabrication of a niobium-based PAV are shown, with particular attention paid on the manufacturing processes and on the selected equipment. The mock-up has been integrated in TRIEX-II and tested at different hydrogen partial pressures and LiPb temperatures, demonstrating that a PAV can be manufactured and operated at relevant conditions. The experiments showed that the permeated flux was in the order of 10−9 mol/s at 350 ◦C and increased, at 450 ◦C, to about 10−8 mol/s. The activity on PAV is complemented by data on the compatibility of niobium in flowing LiPb, obtained by exposing samples at 0.5 m/s and 500 ◦C in IELLLO facility for about 4000 hours. Vanadium, the other PAV candidate material, was also exposed in the same campaign. The samples of both materials withstood the aggressive environment but a tendency was noted to form ternary compounds with nickel, already dissolved in LiPb. A complementary experimental campaign aimed at providing new data on the GLC operation and showed low extraction efficiencies, caused by the strong corrosion of the packings. The lessons learnt in this campaign on instrumentation and packings were useful for the PAV experiments and are being used to prepare a new GLC campaign that will be performed during Fall 2022. The second part of the thesis deals with the fusion-fission cross-cutting subject of alumina coatings, which have the double task of reducing corrosion of structural materials and minimizing tritium permeation towards the coolant in the WCLL BB and in the primary loop of ALFRED lead fast reactor. In this field, the main achievement has been the design and construction of an innovative experimental apparatus, named APRIL, that is capable of testing permeation from a gas phase towards water in relevant conditions for fusion and fission reactors. This facility has been used to measure the Permeation Reduction Factor (PRF) of 3 μm alumina coatings made by Pulsed Laser Deposition (PLD) with deuterium concentration of 0.5% and water at ALFRED conditions. The evaluated PRF is about 13.5. A second activity on coatings has been devoted to test their effectiveness in reducing the corrosion rate of EUROFER and their resistance in flowing LiPb by exposing samples in the same IELLLO campaign mentioned above. While PLD-made coatings protected the EUROFER samples and resisted to the LiPb corrosive attack, those made by Atomic Layer Deposition showed a tendency to detach from the substrate, leaving EUROFER exposed to the LiPb flow. Finally, both coatings revealed a grainy appearance related to the formation of LiAl-oxides.