TRACE Analysis for Transient Thermal-hydraulics of A Heavy Liquid Metal Cooled System

Abstract: Heavy liquid metal (HLM - lead or lead bismuth eutectic) is considered as a candidate coolant for next-generation fast reactor and accelerate-driven systems (ADS), due to its favorable chemical, thermo-physical and neutronic properties in comparison with sodium which has been used as coolant in fast breeder reactors (FBRs). To perform design-base-accident analysis for the HLM-cooled reactors, the well-known transient thermal-hydraulic analysis codes (e.g., RELAP5 and TRACE) are being applied to the reactors with the new coolant. Since these codes were originally developed for light water reactors (LWRs), validations are necessary to ensure the codes to count all influences of the thermo-physical properties of HLM. In this thesis, the TRACE code is employed to simulate the transients performed on the TALL test facility which is a medium-size loop for thermal-hydraulic study of lead bismuth eutectic (LBE). The objectives of the present work are two-fold: i) to interpret the transients performed on the test facility; ii) to qualify the capabilities of the TRACE code for HLM-cooled system by using the experimental data and then perform separate-effect study beyond the experimental conditions. The transients related to safety issues such as loss of heat sink, loss of primary pump, loss of both primary/secondary pumps, overpower and accelerator trip are chosen in the simulations. In addition, the operational transients, the startup and the shut-down of the system are also simulated, respectively. Two configurations of the facility are considered: Configuration-A with a core tank, and Configuration-B with a fuel rod simulator. The separate-effect study is conducted to investigate the effects of coolant inventory, LBE flow resistance and mass flowrate in the secondary loop on natural circulation in the primary loop. Generally speaking, for all the cases analyzed in the present study, the calculation results have a good agreement with the experimental data for the primary side (LBE) parameters (e.g., variations in temperature and mass flowrate). Specifically, the simulation for the transient loss of heat sink indicates the same tendency as in the experiment in term of temperature: it is rising at the inlet and outlet of the core simulator, as well as at those of the intermediate heat exchanger. The temperature keeps going up till the resumption of the heat sink as a protective measure, and then it decreases sharply at the very beginning and gradually returns to steady-state conditions. For the transient loss of primary pump, the temperature level is elevated and a significant natural circulation in the primary loop obtained. The simulation well reproduces the establishing process of natural circulation and final flowrate, but it overestimates the peak temperature. Such simulation outcome II applies to the transient ―loss of both primary and secondary pumps‖, except for a further elevated temperature. The calculation results of the transients startup and overpower are perfectly matching the experimental data, especially when approaching the final steady state, while the transients of shutdown and heater trip are underestimated in their final temperatures but the trends are well captured in both the transients. In general, the transient time to a steady state or the maximum temperature level of Configuration-B is much shorter than that of Configuration-A, mainly because of the larger inventory of LBE in Configuration-A. The separate-effect study also shows that the LBE inventory in the primary loop plays a mitigative role in the transients. For the loss of primary pump transient, decrease in primary flow resistance and increase in secondary flowrate all contributes to passive safety. Keywords: Heavy metal liquid, transient thermal-hydraulics, safety analysis, TRACE