Gas Bubble Nucleation and Tritium Permeation Through Metallic Walls in a Liquid 83Pb-17Li Blanket

Recent studies have given lower and lower values for the solubility of hydrogen isotopes in the eutectic 83Pb-17Li alloy, a candidate breeding material for the blanket of fusion machines. Therefore, thermodynamic stability for the gaseous phase under the high pressure reached at the bottom of the alloy containers can be achieved even for very low tritium concentrations in the liquid phase. A mathematical model to determine when tritium bubble nucleation occurs at an appreciable rate is presented. Considering the design parameters and the operating conditions of the Next European Torus project, it is foreseeable that the tritium generated in the blanket could evolve almost completely in the gaseous phase by forming bubbles at the top of the containers even if acceptable values of the tritium inventory (<100 g) and permeation (∼0,5 g·day⁻¹) are maintained. This situation can be achieved if the molten alloy wets the metallic surface poorly and if the fouling on the exchanger side in contact with the cooling water causes a moderate increase of the resistance to tritium diffusion through the walls. Due to a lack of experimental data, a few assumptions are adopted, but the usefulness of the results obtained is not affected.