Unlocking maintenance-architecting STEP for maintenance and realizing remountable magnet joints

Abstract

The architecture of the Spherical Tokamak for Energy Production (STEP) has been developed to enable a hybrid maintenance approach using ports in the vacuum vessel for a limited list of tasks that must be performed shortly after shutdown, and larger openings to simplify and speed up major refits. Robotic handling systems in zero-human entry facilities will prevent workers from being exposed to the most hazardous environments. While the approach is largely grounded in existing technologies, the scale and environment of STEP will require significant technology development. Notably, programmes have been established to develop service connections and in-vessel robotic technologies. The engineering integration of the maintenance strategy into the tokamak remains a priority, as does ongoing work to simplify and reduce the cost of the buildings required to facilitate maintenance. Remountable magnet joints are critical to ensuring life-limited magnet components can be replaced during the STEP lifetime and realizing the STEP maintenance strategy. It is a high-risk endeavour owing to the low technology maturity of the potential solutions and owing to the tough and intertwined technical challenges and constraints imposed by both the fundamental physics and the STEP requirements and architecture. An integrated design approach has been taken to balance many competing factors and integrate with interfacing systems, and a multi-faceted technology development programme has been established to address technical risk and to inform, verify and validate the STEP remountable magnet design. This article is part of the theme issue 'Delivering Fusion Energy - The Spherical Tokamak for Energy Production (STEP)'.

Keywords: STEP; remote maintenance; remountable magnet joints.

Figure 1.

Main components of the STEP with expected failure modes.

Figure 2.

TF coil with three remountable joints (orange) with a fixed fourth joint (blue).

Figure 3.

Competing factors in the maintenance of STEP.

Figure 4.

Tokamak vertical maintenance segmentation (left) and port-based capability proposals (right).

Figure 5.

Tokamak complex including the tokamak hall and TLC with the tokamak highlighted in red.

Figure 6.

Diagram showing current flow across a remountable magnet joint.

Figure 7.

Remountable magnet joint major challenges.

Figure 8.

STEP centre rod with compact inboard magnets and remountable joints that must fit within the bore of the tokamak to enable vertical maintenance.

Figure 9.

Remountable magnet joint decision hierarchy map.

Figure 10.

Architectural options, selections and principal rationales.

Figure 11.

Plan view of remountable joint electrical connection region, with a detailed view of edge-to-edge connector pair.

Figure 12.

Workflow incorporating modelling with testing to reduce the burden of full-scale tests.