On the thermomechanical behavior of the European target assembly design of IFMIF-EVEDA lithium test loop under start-up transient scenarios

Uncoupled thermomechanical transient analyses have been carried out to investigate the behavior of IFMIF-EVEDA lithium test loop bayonet backplate target assembly under two selected start-up transient operational scenarios. The first transient scenario considered foresees that the target assembly, starting from the initial uniform temperature of 50 °C, is heated up uniquely by convective heat transfer with lithium, flowing from inlet to outlet nozzle at its reference nominal temperature and pressure, until its nominal steady state thermal field distribution is reached. The second transient scenario foresees, more realistically, that the target assembly, starting from the uniform temperature of 50 °C, is initially warmed-up by electric heaters mounted onto its main accessible surfaces and, subsequently, by convective heat transfer with lithium reference flow, until nominal steady state conditions are reached. Heaters have been supposed to operate in an on/off stepwise mode, resulting to be alternatively switched on and off in order to allow the target assembly thermal field to grow up minimizing thermal gradients. To this purpose, a parametric analysis has been performed to realistically assess, for each electric heater, its heat flux and duty-cycle. Numerical results obtained are presented and critically discussed.