A Thermal Striping Evaluation Methodology for Natrium™ Internal Components Using Frequency Response Function Solutions

Abstract

Abstract Liquid sodium is used as the coolant in the Natrium (a TerraPower and GE-Hitachi technology) reactor design because of its outstanding thermal properties. Sodium becomes liquid at above 98°C and remains in liquid state up to a very high temperature of 883°C at atmospheric pressure. This feature allows the reactor to have a high operating temperature while maintaining a sufficient margin to avoid boiling of the coolant under all design basis events, without any pressurization as is typically utilized in a light water reactor (LWR) design. On the other hand, there are certain critical structural mechanics issues for the components in contact with high temperature liquid sodium. Besides the design basis thermal transients, the primary coolant streams flowing out of the fuel assemblies and control assemblies may experience incomplete mixing of hot and cold jet flows, causing a special thermal hydraulics phenomenon called thermal striping. The thermal striping may have a temperature fluctuation over 50°C at frequencies of 0.1 to 10 Hz. The temperature fluctuation in the flow may also be caused by thermal stratification at the interface of the horizontal relatively hot and cold streams. Due to the high frequency temperature fluctuation in the liquid sodium, the structures in contact with the flow are subjected to nontrivial cyclic thermal stresses which may cause high cycle fatigue (HCF) damage to the structures. The structures containing both liquid sodium and cover gas may also experience temperature fluctuation due to free surface level fluctuation at the liquid/gas interface where the cover gas and the liquid sodium have very different thermal conductivities. This paper presents frequency response function (FRF) solutions to evaluate the thermal striping limits of the Natrium internal components against the fatigue crack propagation prevention criteria while considering the cyclic range of thermal stresses and stress intensity factor due to the thermal striping on one side of a component, and various boundary conditions on the other side.