Numeric implementation of two-phase tritium transport models for natural helium nucleated bubbles in lead-lithium. Implications for HCLL breeding blanket channels

Tritium (T) self-sufficiency requirement is linked to high helium (He) production rates in a D-T fusion reactor breeding blanket (BB). In Liquid Metal (LM) BB concepts, large local He concentrations may result in bubble formation, which might have an enormous influence in the components performance. The present work states that such a possibility is not remote in a Helium Cooled Lithium Lead (HCLL) BB design. Bubbles could act as an effective T sink, reducing T partial pressure in the bulk LM and thus affecting T inventory control. Models for He nucleation, bubble growth and transport, along with T absorption and transport, have been implemented in the CFD code OpenFOAM®. Classical Nucleation Theory has been used for He nucleation. In the growth model, bubble growth is controlled by diffusion (it is assumed that bubbles are small enough); the mean radius approach has been implemented in order to save computational time. Tritium absorption is modelled using the Lewis-Whitman film theory. He and T concentration maps have been calculated for a HCLL single channel. Results show the effect of gas bubbles on T concentration. A pressure driven nucleation case have also been calculated. Work presented is a first step towards the quantification of the complex phenomena involved in He nucleation in LM and its effects on T inventory within a BB design.