Dedicated thermo-mechanical analyses in support of DEMO HCPB BB design

Within the framework of the EUROfusion Work Package Breeding Blanket for the fusion reactor DEMO, several concepts of Breeding Blanket (BB) are considered. One of the concepts being investigated is the Helium Cooled Pebble Bed (HCPB) BB. The present study focuses on the assessment of this component from the steady-state thermal and thermo-mechanical point of view for Normal Operation conditions, with Finite Element Method Analysis and according to RCC-MRx criteria. First design assessment revealed that the criteria were exceeded in three main areas. Design improvements in terms of mechanical performance are thus proposed and assessed. The first area is the Side Wall and radius fillet. The reduction of the flow cross section, combined with the inward shifting of the First Wall cooling channels, allows compliance with the allowable stress (the maximal Eurofer temperature increase but remains below the limit of 550 °C). With regards to the allowable stress under irradiation, the suggested modifications do not enable the admissible value to be achieved and the wall thickness should be increased in the Side Wall area. With regards to the second area, the manifold walls, the initial thickness of 10 mm enables the allowable stress to be satisfied, and this thickness should not be reduced. Increasing the manifold wall thickness does not allow to comply with the allowable stress under irradiation. A possible solution could be to rearrange the manifolds in order to achieve a more homogeneous temperature field and a reduction in thermomechanical stresses. Concerning the pressure tube, increasing its thickness allows significant improvements and it would appear that a thickness of 10 mm is required to achieve the allowable stress under irradiation. However, increasing the pressure tube thickness will reduce the volume of the neutron multiplier and this could impact the Tritium Breeding Ratio.