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Description
STUMM-PROTO is a full-scale pressurised and instrumented vessel developed to validate thermal and instrumentation experiments for the STUMM programme. ESS-Bilbao led the mechanical design and fabrication control of the pressure vessel in accordance with RCC-MRx rules for an N3Rx component. The work included the development of three dedicated FEM sub-models (Container + Lower Attachment; Upper Attachment + Cover; Frame), the definition of representative thermal and vacuum load cases (operation: 2.3 bar(g), 320 °C; vacuum: −1 bar(g), 320 °C; maximum allowable/overpressure up to 3.15 bar(g); pressure test at 4 bar(g)), and the full verification of P-type (membrane and membrane plus bending), S-type and buckling criteria in compliance with RCC-MRx. The design addressed the significant manufacturing challenge posed by 2 mm wall thickness and continuous TIG welds required for N3Rx pressure retention.
A comprehensive and high-fidelity FEM assessment was carried out to rigorously demonstrate compliance with RCC-MRx requirements, with particular focus on the superimposed bolted square-flange assemblies located in the upper part of the component. The geometric configuration of these interfaces leads to non-uniform bolt load distribution and local stress superposition effects arising from flange interaction and preload redistribution. These phenomena were explicitly captured through detailed 3D modelling, bolt-load extraction and stress linearisation along critical paths. The results demonstrate that the combined membrane and membrane-plus-bending stresses remain within allowable limits, confirming full compliance with RCC-MRx requirements for an N2Rx pressure-retaining component, even under the most demanding load combinations.
To overcome severe manufacturing constraints, the design introduced an innovative production route that avoids the need for electro-discharge machining (EDM/wire-EDM) for key features. This was achieved through a joint design-for-manufacture process with the manufacturer, involving iterative design refinements (including groove-and-tongue reductions and bolt pattern adjustments) and the definition of welding and inspection specifications formalised through ITP/WPS documentation and change notices. This collaborative approach enabled conventional machining and welding processes to meet the component’s critical assembly tolerances — including bolted-joint concentricity and sealing surfaces — without resorting to EDM, thereby preserving material integrity and simplifying post-weld finishing operations.
Finally, the manufactured component underwent an extensive validation campaign including pressure testing, helium leak testing, vacuum testing and thermal cycles up to 320 °C. The experimental results were fully consistent with analytical predictions and confirmed the structural integrity, leak-tightness and thermal performance of the STUMM-PROTO vessel.