Direct numerical simulations of tritium extraction in PbLi-based breeding blankets in the laminar–turbulent transition region

Abstract

In a lead-lithium (PbLi) based breeding blanket, the bred tritium remains dissolved in the PbLi, which flows towards the Tritium Extraction Unit (TEU). In a TEU, tritium must be extracted at a fast enough rate that guarantees the plant’s self-sufficiency and safety. Different TEU concepts exist, one of the most promising being Permeation Against Vacuum (PAV), based on the extraction of tritium from the PbLi through a highly permeable membrane.

Even in the so-called low velocity blanket concepts, PbLi is expected to flow at relatively high total mass flow rates. This means that the high tritium extraction efficiencies that safe operation requires must be obtained partitioning the flow into several extraction channels, but this solution increases both cost and complexity. However, less partition channels may be required should turbulence be induced in the flow. Since turbulence increases the flow mixing, it should favor tritium extraction. Hence, in the range of velocities where turbulent phenomena start —i.e., the transition region—, extraction efficiency is expected to grow rapidly with velocity due to turbulence acting as a new transport mechanism. Thus, a turbulent flow in the TEU may achieve the high extraction efficiencies required with a more moderate partitioning scheme.

In this work, a PbLi flow in a prototypical PAV channel was modeled using Direct Numerical Simulations (DNS). To trigger turbulence, two different methods were implemented, namely an instability-inducing oscillating boundary condition, and a physical turbulator consisting of a geometrical obstacle. Both methods proved successful as they resulted in greater extraction efficiencies than those seen in the analogous laminar regimes. In fact, up to a 15% increase in extraction efficiency or up to a 5-fold increase in total extraction rate were obtained.