ITER Limiter First Wall 3-D Thermo-Mechanical Analysis

This paper describes, in part, the design activity related to the ITER limiter first wall (FW). The limiter is needed to protect the reactor FW during plasma startup and shutdown. Steady state heat fluxes of 0.5 MW/m² are expected with short duration excursions to 5 MW/m² during startup/shutdown. A 3-D finite element model has been created to represent the beryllium-copper-steel layered construction of the limiter FW. The model takes advantage of the design symmetry, and the large aspect ratio of the limiter which helps in optimizing the finite element model by assuming infinite extent in the poloidal direction. Different options with various boundary conditions are investigated to optimize the limiter FW design and to simulate as close as possible, actual conditions in the limiter. The model is that of a 10 mm diameter hole running poloidally in a Cu block made of GlidCop A125 which is 1.9 cm thick, and the spacing between the hole centers is 2.2 cm in the toroidal direction. The Cu block has a 1 cm thick castellated layer of Be facing the plasma and itself is attached to a cooled SS backing. Each block is discrete with a 1 mm groove separating it from the adjacent block. The interface between the various layers assumes no inter-layer compositions and thus has a singularity due to different material properties. For this preliminary analysis the value of 3.0 MW/m² heat flux is chosen for reference case. Furthermore, the analysis is elastic, not allowing any plastic deformation. These two rather severe assumptions tend to give higher stresses at the Cu/Be interface. One of the aspects investigated is the depth of the groove in the Cu between the coolant tube blocks. Analysis has shown that when this groove is deeper than 6 mm, the additional effect on the stress at the Cu/Be interface is negligible, the maximum stress in the Cu is reduced, leveling off at a depth of 13 mm. The maximum Be temperature is 552° C and 866° C at the 3 MW/m² and 5 MW/m² heat fluxes, respectively. The maximum von Mises stresses at the Cu/Be interface corners are 354 MPa and 679 MPa for the 3 MW/m² and 5 MW/m² heat fluxes respectively. These stresses are superficially high due to the stress singularity at the interface and the assumption of no plastic deformation.