Low-cycle fatigue in a convolution bellow

The core central area of PEC is occupied by a loop; in this loop cooled by a separate circuit, it is possible to introduce an experimental fuel element. For safety reasons it is necessary to use double containment for the test loop and this requirement is being met by using double wall construction for both the inlet leg and the outlet test leg. Since the inside and outside walls of each leg are subjected to changing thermal environments along the axial length, there is differential thermal expansion between two walls. It is therefore necessary to install some bellows. During the normal operations these bellows must support only the differential thermal expansion between the two coaxial tubes and there is no pressure acting on the bellows. In an accidental condition, as a rupture in the internal pipe, the gap between the two tubes is invaded by sodium. In this condition the external bellow supports the loop coolant pump pressure and it is important for plant safety to investigate the endurance as a function of the number of deformation cycles supported by the bellows. A special machine has been designed and constructed in order to carry out the indicated tests. The results show that the burst pressure is strongly affected by the displacement amplitude. The energy absorbed by the bellows at the rupture is very low; before the rupture (due to internal pressure) there is a shape deformation and a convolution removal. The new shape assumed depends on the bellow instability and therefore on the internal pipe constraints; a different gap between the internal pipe and the bellow changes the results. (Auth.)