At the ITER facility in southern France, engineers are assembling the vacuum vessel of the world's largest fusion reactor from nine wedge-shaped sector modules, each weighing up to 1,200 tonnes — heavier than a fully loaded jumbo jet.
Each module is lowered into the tokamak assembly pit through a gap of well under a metre — ITER Organization's own figures put the usable margin at under 20 centimetres at the tightest point. It is a heavy lift and a nuclear-grade precision problem occurring simultaneously, with almost no room to correct mid-descent.
ITER's own account of the first sector module lift describes vertical clearance during transfer as not exceeding 20 centimetres, tightening further during final descent. At a scale where the module is the size of a five-storey building, that margin leaves essentially no tolerance for error.
A component the size of a five-storey building, lowered through a gap of less than 20 centimetres — nuclear-grade tolerance applied at heavy-lift scale.
The full vacuum vessel, once its nine sectors are welded together, will weigh approximately 5,200 tonnes; with in-vessel components installed, that rises to roughly 8,500 tonnes. ITER's own engineers describe the assembly as combining nuclear-grade quality constraints with clearances and component geometries that have no direct precedent elsewhere — lessons with relevance well beyond fusion energy.
For further reading, see Erection Engineering for Industrial Construction, which places confined-clearance precision lifting in a wider sector context.