Cladding design on lower head of the central measuring shroud in a fast reactor against thermal striping

Abstract

The central measuring shroud in a fast reactor has the function to support the in-vessel measuring equipment and control rods. It is necessary to keep its integrity for safe operation of the reactor. Since the lower head of central measuring shroud (LHCMS) is located above the core outlet, it bears thermal striping caused by mixture of cold and hot coolant flowing out of the core channels. Thermal striping is one of the factors to cause initial crack or even damage of the LHCMS. As it is a common phenomenon in thermal engineering, it has been studied by many researchers. In order to protect the LHCMS, it is necessary to determine the moderate thickness of cladding. In this process, they usually need to calculate the fluid temperature fluctuations or structural stress by numerical simulation. However, this process may occupy a lot of computing resources. A simplified procedure is expected to achieve rapid calculation of thermal striping fatigue and to ensure a conservative result.In the paper, a simplified procedure is proposed to assess thermal striping fatigue of the LHCMS, in which heat transfer equation, frequency response function and stress are evaluated with some simplified formulas. If temperature fluctuation curve is given, the surface’s temperature, von Mises stress and fatigue damage factors of the LHCMS can be obtained by the procedure. Meanwhile, FEM models for LHCMS without cladding and LHCMS with cladding are established respectively to calculate temperature and stress of the LHCMS. By comparing the computational results of the simplified procedure and those obtained by FEM, it is found that the deviation factors are both around 0.1, which indicates the fatigue assessment procedure can quickly and conservatively obtain fatigue damage factors of different cladding thicknesses. Therefore, it can quickly and conservatively design for the LHCMS when properly programmed.Using the simplified procedure, we analyzed the fatigue damage factors of the LHCMS in a fast reactor. It is shown that during the designed life period of 45 years the LHCMS without cladding will fail, while its integrity can be ensured if the cladding is added with the thickness of 1 mm.