Thermal-mechanical influence aspects and evaluation of helical cruciform single rod in fluoride-salt-cooled high-temperature advanced reactor

Helical cruciform fuels are novel in nuclear reactors, potential to increase reactor's power density. However, the geometry is complicated so influence of it on the fuel performance is not identified yet. To evaluate flow and heat transfer performance of the fuel, thermal and mechanical characteristics of fuel used in Fluoride-Salt-cooled high-Temperature Advanced Reactor are analyzed. Impact of power density, cross-section parameters, and twist pitch on the fuel is discussed separately based on fluid-thermal-mechanical coupling. In general, twist pitch is vital to helical cruciform fuel while others have few effects considering thermal-mechanical features. For thermal features, increase in twist pitch leads to temperature rise owing to weaker mixing effects. Fuel center temperature at middle plane of 300 mm-pitch rod is 829.58 °C, 54.66 °C higher than that of 100 mm-pitch rod. Besides, axial temperature of cladding outer surface increases wavelike due to complex geometry. For mechanical features, not geometry sizes but temperature affects stress distribution. Maximum Von-Mises stress appears at the elbow, where maximum temperature exists, 116.8 MPa under normal conditions, lower than tensile strength of the material. After identifying the influence of these factors, five dimensionless parameters are proposed to evaluate and rate fuel performance based on Technique for Order Preference by Similarity to an Ideal Solution. As a result, an optimization comes up, scoring 0.310, twice more than the original design, owing to the thermal uniformity and mechanical safety. This study provides a reference for identifying the performance of helical structure and a new fuel design in Fluoride-Salt-cooled high-Temperature Reactor.