Considerations on the DEMO pellet fuelling system

Abstract

The Demonstration Fusion Power Reactor DEMO is the step foreseen to bridge the gap between ITER and the first commercial fusion power plant. One key element in the European work plan for DEMO is the elaboration of a conceptual design for a suitable core particle fuelling system. First considerations for such a system are presented in this contribution. Following the well-considered ITER solution, most analysis performed in this study assumes conventional pellet technology will be used for the fuelling system. However, taking advantage of the less compressed time frame for the DEMO project, several other techniques thought to bear potential for advanced fuelling performance are considered as well. In a first, basic analysis all actuation parameters at hand and their implications on the fuelling performance were considered. Tentative transport modelling of a reference scenario strongly indicates only particles deposited inside the plasma pedestal allow for efficient fuelling. Shallow edge fuelling results in an unbearable burden on the fuel cycle. Sufficiently deep particle deposition seems technically achievable, provided pellets are launched from the torus inboard at sufficient speed. All components required for a DEMO pellet system capable for high speed inboard pellet launch are already available or can be developed in due time with reasonable efforts. Furthermore, steps to integrate this solution into the EU DEMO model are taken.

Core Fueling of DEMO by Direct Line Injection of High-Speed Pellets From the HFS

“…According to a commonly accepted conceptual scheme, the pellet trajectory inside a curved guide tube is thought of as a polygonal track, with the pellet bouncing off the wall of the guide tube [2]. If the curvature radius is large enough, we can also imagine that the pellet slides inside the guide along the outboard face of the track contour.…”

“…As a consequence, the higher the pellet speed, the larger is the curvature radius required to preserve pellet integrity. An empirical scaling law, calibrated on the AUG experiment for cubic shaped pellets [2], is commonly adopted to estimate the maximum speed V inj (m/s) that pellets can withstand, given the curvature radius R (m) of the guide tube and the pellet size L (m):…”

“…Consequently, injection from the Low Field Side (LFS), where the B is unfavourable, is inadequate, whereas effective core fuelling can be achieved launching pellets from the High Field Side (HFS) at speeds  ~1 km/s. These speeds are necessary to ensure that pellets are ablated underneath the plasma pedestal, where the drift effect can contribute to improve the fuel deposition depth [2]. Vertical injection may be effective only if pellets are injected at relatively higher speed from a radial position ≤ 8 m from the axis of the reactor; this distance, however, depends on specific plasma scenarios.…”

Consequences of the technology survey and gap analysis on the EU DEMO R&D programme in tritium, matter injection and vacuum

“…R&D in the matter injection area will be defined by the results of pellet transport and ablation modelling studies which translate the DEMO plasma scenario in pellet injector engineering requirements (parameters such as e.g. launch location and according injection velocity, optimized pellet mass flow for according pellet size and rate) (R&D topic 10) [42]. Potential needs are outlined in the following sections, separately for the three sub-systems of a typical pellet launching system.…”

Operation of JET with an ITER-like Wall

“…-Re-installation of the JET ITER-like Antenna, with the goal of providing more central electron heating; and -Relocation of the High Frequency Pellet Injector so as to allow efficient delivery of pellets for ELM pacing (in a geometry relevant for future fusion devices [23]). …”