Structural integrity assessment and manufacturability-oriented design improvement of the HCCP TBM shield

The Helium Cooled Ceramic Pebble (HCCP) TBM shield is a safety-important component (SIC-1) serving as a vacuum boundary and the first confinement barrier, and a design with a simplified internal structure has been proposed to improve its manufacturability. In this study, a structural integrity assessment of the simplified HCCP TBM shield was performed in accordance with RCC-MRx 2018, considering 15 single loads and 17 load combinations. The assessment covers all operating states, namely the normal operation state (NOS), the baking state, and the tritium outgassing state (TOS), as well as the electromagnetic and seismic loads. While sufficient margins were confirmed for the accidental loads, negative margins were found in the load combinations governed by the pressure in the baking state, identifying the baking state as the design driver of the TBM shield. The cause was analyzed to be the combination of the reduction in the allowable stress intensity at the elevated temperature of 240 °C and the increased bending stresses of the walls caused by the enlarged spacing between the internal plates. In the design improvement for the margin recovery, manufacturability was consistently applied as a decision criterion. Parameter studies on the side wall thickness, the front plate thickness, and the pipe casing layout were performed for an improved geometry in which the width of the shield block containing the pipe casing bends was increased, and it was confirmed that the combination of a 40 mm side wall and a front plate of 60 mm or thicker recovers the structural integrity in all shield blocks. In addition, it was quantitatively shown that the pipe casings act as structural stiffeners and that their reinforcing effect increases with the symmetry of the casing layout. These results provide a design solution reconciling the structural integrity with the manufacturability without reintroducing the internal plates, and a quantitative basis for the geometry optimization in the final design phase.