Fusing together a design for sustained fuelling and tritium self-sufficiency
Ensuring tritium fuel self-sufficiency while maintaining continuous and high specification fuel flow to the tokamak via a low tritium inventory and controllable fuel cycle is a significant challenge to the STEP plant design. Effective and high-quality fuelling and exhausting is required to sustain and control a stable plasma, whereas fuel sufficiency is required to prevent depletion of available tritium supply. Concerns regarding the lack of tritium availability preventing continous tritium import are countered by breeding, where highly energetic neutrons from the core fusion reactions interact with lithium atoms suspended in the surrounding breeder blanket to produce tritium. The compact nature of STEP prohibits the integration of inboard breeder blankets posing a significant challenge for the design team looking to ensure more tritium is bred and made available than consumed within the core plasma.
This paper outlines how purposeful technology selection and system architecting has converged on the outline of a conceivable and tritium capable fuel cycle and breeder blanket concept design. Before introducing the STEP fuel cycle design concept and summarising the approach undertaken to address the challenges facing plasma fuelling, the many key aspects of fuel self-sufficiency are discussed. This includes outlining the design concept of a helium cooled liquid lithium breeder blanket and possible technology options for tritium extraction from lithium. Lastly, there is a brief process modelling overview which emphasises the central contribution of various employed modelling methods. Reflections on the presented fuel cycle design concept conclude that substantial development work is still required to realise a continous tritium fuel cycle design and moreover to overcome the major challenges posed by tritium and lithium handling. Nonetheless, a viable design concept baseline for a DT fusion powerplant fuel cycle is outlined. Reflections on the presented breeder blanket design concept conclude that while many substantial risks and blockers remain, no showstoppers have yet to be proven which preclude fuel self-sufficiency on a compact spherical tokamak configuration, nor preclude the application of liquid lithium as a breeder medium. Nonetheless, it is recognised that further consideration is required to ensure that the selection of liquid lithium as a breeder medium provides the overall simplest route to a self-sufficient and realisable design.