Thermal and mechanical analysis of the NET vacuum vessel

The vacuum vessel of the NET machine is a 'D' shaped toroidal structure assembled from 32 segments each weighing over 100 tonnes. It is subjected to five major types of loading: Gravity, Thermal, Pressure, Electromagnetic and Earthquake. These loadings have been combined to represent quasi-static load cases at various times during the maintenance, normal operation, abnormal operation and fault periods. Each load case has been analysed for two different scheme designs using the general purpose finite element program ANSYS. The vacuum, shielding and maintenance requirements have lead to designs in which up to five layers of stainless steel plates, from 25 mm to 230 mm thick, form the segments which are bolted together. The position, number and size of the bolts is partially determined by plate thickness and access considerations and also by the requirement to keep the joint at the vacuum seal closed under all loading conditions. The analysis has shown the importance of ensuring that the inner plates are always warmer than the outer plates of the vessel to encourage the gaps on the internal surface to remain closed. The most significant load cases are the high internal pressure, bakeout and plasma disruption load cases. (author). 4 refs.; 7 figs.; 1 tab