Preload Analysis of Screw Bolt Joints on the First Wall Graphite Tiles in EAST

The first wall in the Experimental Advanced Superconducting Tokamak (EAST) uses graphite tiles facing the plasma. All of the graphite tiles have been mounted on a heat sink using screw bolts, which have been preloaded to produce a clamp force. The clamp force is very important to keep the graphite tiles tightly on the surface of the heat sink so that the heat flux crosses this contacting surface in a small thermal resistor. Without the clamp force, the small gap between the graphite tiles and the heat sink will make it impossible for thermal power to be carried away by cooling water. Some bolts may even fall off with the loss of clamp force. From the mathematical models, the process of the loss of clamp force has been studied. Research results explain how the different thermal expansion of three members of the screw joint makes the clamp force decrease to zero under temperature rise and external force, and how the stiffness affects the relation between the clamp force and temperature. The research also gives the critical temperature at which the clamp force can remain above zero. Research results indicate that the current screw joints are almost destined to lose their clamp force during the running time of EAST, so the bolt joints should be redesigned in order to improve its reliability.