Xinxin Wu

Flow excursion phenomenon and its mechanism in natural circulation

Abstract An experiment was performed on the test loop (HRTL-5), which simulates the geometry and system design of a 5-MW nuclear heating reactor. In a wide range of inlet subcoolings, different flow modes, such as single-phase stable flow, subcooled boiling stable flow, subcooled boiling static flow excursion, density-wave oscillation and stable two-phase flow in the natural circulation system have been described. The phenomenon and mechanism of the static flow-excursion, which has never been studied well on this field, is especially interpreted. The experimental results show that, in the process of flow excursion, the mass flow rate and the inlet temperature decreases, while the exit temperature increases smoothly. As the process of the excursion continues for about 1 h, short period dynamic flow oscillation occurs, which can only be seen in the process of this static flow excursion, and has also never been studied well. These static and dynamic flow instabilities combine together and continue for about 2 h, then a point is reached, at which the static flow excursion disappears, but the dynamic flow oscillation continues. The mechanism of the static flow excursion is interpreted through two sets of curves for flow resistance pressure drop and driven head in natural circulation, and one curve for the natural circulation operation under special thermohydraulic condition. The study of the flow excursion and its concerned dynamic flow oscillation is of great significance for the development of the nuclear heating reactor under natural circulation.

Microstructure and molten salt impregnation characteristics of a micro-fine grain graphite for use in molten salt reactors

Abstract The microstructure and molten salt impregnation characteristics of a micro-fine grain isotropic graphite ZXF-5Q from Poco Inc. was investigated. The microstructural characteristics of the pores caused by gas evolution, calcination cracks, Mrozowski cracks, and the crystal structure were characterized by optical microscopy, mercury porosimetry, helium pycnometry, transmission electron microscopy, X-ray diffraction and Raman spectroscopy. Results show that the ZXF-5Q has uniformly-distributed pores caused by gas evolution with very small entrance diameters (∼0.4 μm), and numerous lenticular Mrozowski cracks. Molten salt impregnation with a molten eutectic fluoride salt at 650 °C and 1, 3 and 5 atm, indicate that ZXF-5Q could not be infiltrated even at 5 atm due to its very small pore entrance diameter. Some scattered global salt particles found inside the ZXF-5Q are possibly formed by condensation of the fluoride salt steam during cooling.

Thermal Analysis of a Helical Tube Once Through Steam Generator of HTGR

Steam Generator (SG) is one of the most important pieces of equipment in High Temperature Gas-cooled Reactor (HTGR). It requires high reliability in a very critical working condition. The thermal analysis of HTGR SG and its uncertainty becomes very important. Large thermal non-uniformity and the resulting high temperature will damage the structure. The SG of HTGR is very different from the boilers of conventional thermal power plant. The heat transfer almost all contributes to convection (counter flow pattern) but not radiation. One dimensional (1D) and two dimensional (2D) codes were developed for the thermal analysis of the SG of HTGR. The 1D code is able to calculate the overall performance. It solves the one dimensional equations for both the shell and tube side. The 2D code is designed to analyze the temperature non-uniformity in the SG. Two dimensional Reynolds-Averaged Navier Stokes equations are solved for the shell side, one dimensional equations are solved for the tube side. The thermal mixing effect in the shell side tube bundle can be included. The thermal deviations caused by secondary side flow rate uncertainty and manufacturing tolerance of tube helical diameter are analyzed. The results show that radiation only contributes to about 0.6 percent of the total thermal power. Secondary flow rate fluctuation of 1% causes an outlet steam temperature variation of 3 °C. Heat transfer tube helical diameter tolerance of 1mm causes an outlet steam temperature deviation of 4 °C.Copyright