Experiments on natural circulation of lead-bismuth in the TALL test facility

Full text of publication follows: Lead-bismuth eutectic (LBE) is a potential candidate coolant for next generation liquid metal reactors due to its favorable properties such as being chemical inert and low melting point, in comparison with sodium and lead considered as coolants in FBRs. Having a high atomic number of LBE allows it be well suited as a spallation target for accelerator-driven systems (ADS) which have been proposed for the transmutation of nuclear waste. Due to its strong buoyancy, the LBE-cooled system should also have significant natural circulation, which is desirable for so-called Generation IV nuclear reactors, which like to employ passive safety and reliability. But so far, very little experimental data have been published on the natural circulation thermal-hydraulics of LBE-cooled systems. Motivated by the increasing interest in LBE-cooled fast reactors and ADS, a test facility called Thermal-hydraulic ADS Lead-bismuth Loop (TALL) was designed and constructed at KTH to investigate the thermalhydraulic characteristics of liquid LBE. The facility consists of a primary loop (LBE loop) and a secondary loop (oil loop). The LBE loop consists of sump tank, core tank, expansion tank, heat exchanger, EM pump, EM flowmeter, electric heaters and instrumentation. The heating of LBE in the core tank and its cooling in the heat exchanger allows natural convection flows as should occur in the prototypic vessel. Recently, our experimental study on natural circulation was performed on the TALL test facility. This paper will present the experimental results and analysis. The facility is of 6.8 m height which is comparable to the full height of the LBE heat exchange circuit in the ANSALDO ADS reactor vessel design, and has been scaled for prototypic (power/volume) ratio to represent the main components. Their LBE volume, flow velocity and heating rates correspond to one tube of the heat exchanger design chosen. During the experiments, the main adjustable parameters are the input power in core tank and the flowrate of the secondary loop. Under the low flowrate conditions which are beyond the range of the EM flowmeter, the flowrate was obtained by differential pressure transducer and heat balance, respectively. The experiments include 1)the start-up of natural circulation from different initial conditions; 2)the natural performance and its influencing parameters; 3)computer code analysis. Significant natural convection flow was observed in the experiment. It was also found that the natural circulation is easily established in the TALL test facility. It took minutes to have a stable natural circulation prevailing from cold conditions. The natural circulation flowrate mainly depends on the loop resistance, and the temperature difference between the hot leg and cold leg, which is determined by the power level and heat sink capacity. The ongoing experiments show that the maximum flowrate of natural circulation is {approx}0.4 kg/s (corresponding to {approx}0.4 m/s in the heat exchanger), resulting in total heat removal of {approx}15 kW from the core tank, which is comparable to the capacity of {approx}100 W/cm of the electric heater elements. The experimental data are in reasonable agreement with the RELAP5 analysis. (authors)