Separate and Integral Effect Experiments of Sodium Heat Pipes using High-Resolution X-ray Radiography and Application to Microreactors

Abstract

In very recent years, several research efforts have been dedicated to so-called microreactors. Several designs are being pursued by various companies such as Westinghouse, OKLO, X-energy, HolosGen, USNC, to name a few, with financial support from private industries, the US Department of Energy (DOE) and the US Department of Defense (DOD). With power production up to 20 MWe, these reactors are designed to be easily transported within ISO containers and either provide heat for industrial processes or produce electricity in remote locations, which often rely on Diesel generators (e.g. mining operations). Several of these microreactor concepts use sodium heat pipes for passive heat removal, which means no external electricity is needed. The use of alkali metal heat pipes enables the system to operate fully passively with high mobility. To optimize the design of heat pipe microreactors and assess their behaviour under accident scenarios, the heat removal performance of sodium heat pipes and their behaviour in normal operation and postulated accident scenarios need to be thoroughly investigated so that reliable models can be developed to predict the behaviour of heat pipes under various operating conditions and safety analyses of heat pipe microreactor concepts can be carried out. Past studies on alkali metal heat pipes are scarce and have been limited to the system-level behaviour, without information on the flow phenomena of the working fluid inside the heat pipe. In addition, significant uncertainties exist on the heat transfer characteristics across the full range of flow regimes including dryout conditions. The visualization of the working fluid phases within the heat pipe is essential to gain insights into the particular flow regime developing under the different steady-state and transient operating conditions of the heat pipe. This is because the flow regime has a strong impact on the heat transfer and therefore heat removal performance of the heat pipe. This thesis aims to provide high-resolution experimental data for sodium heat pipes under various operating conditions, including startup, shutdown and abnormal conditions. Two experimental facilities were designed and built. The first experimental facility, the MIchigan single SOdium Heat pipe separate-effect (MISOH1) test facility, allows the investigation of the behaviour of a single sodium heat pipe under well-controlled heating powers and boundary conditions. The facility allows the investigation of the effect of various parameters such as evaporator heating rate, cooling conditions in the condenser region, heat pipe orientation, and the sodium filling ratio within the heat pipe itself. The Michigan High-Resolution Tomographic Imaging (CHROMA) system, which allows for high-speed, high-resolution x-ray radiography imaging, is employed at the MISOH1 facility to measure the time-dependent two-phase vapor-liquid sodium structures within the heat pipe under different operating regimes. The second facility, the MIchigan SOdium Heat pipe bundle (MISOH2) test facility, has been specifically designed to simulate the thermal-hydraulic behaviour of a sodium heat pipe microreactor during normal operation and postulated accidents. Special attention is focused on the potential occurrence of “cascade failure”, which might be caused by the heat load redistribution on neighbouring heat pipes consequent to the failure of local heat pipes. The thesis provides the first-time separate effect experimental database synchronized with x-ray radiography measurement and first-time experiment on the integral effect of heat pipes bundle. Both experimental databases can be used for the development and validation of heat pipe models.