Researches on Special Thermal Hydraulic Phenomena of Annular Fuel Assembly by Sub-Channel Analysis Code-NACAF

Dual cooled annular fuel is a novel fuel design, which has the potential to improve the reactor power density while maintaining or improving its safety margin. The effects of tight-lattice geometry, fuel burnup, fuel expansion, coolant channel blockage on the thermal hydraulic performance of annular fuel is studied to illustrate its special features in this paper. A sub-channel analysis code named NACAF, which includes empirical constitutive models in consideration of tight-lattice effects on prediction of pressure drop, critical heat flux and turbulent mixing, channel blockage model, heat conduction model for dual surface cooling condition, coolant flowrate distribution between inner and outer channel, is developed for annular fuel assembly or core analysis based on homogenous fluid model. Validation work is carried out with comparing NACAF results with analytical solutions, as well as numerical results of existing sub-channel code for annular fuel, such as VIPRE-01 and TAFIX. Comparison results demonstrates NACAF’s prediction error is acceptable and it has the ability to simulate thermal hydraulic performance of annular fuels or annular fuel bundles. Based on the developed and verified NACAF, the special thermal hydraulic phenomena of annular fuel are studied to clarify the features of annular fuel.

An Experimental Study of Two-Phase Flow in a Tight Lattice Using Wire-Mesh Sensor

Tight lattice bundle can improve the conversion ratio and the heat transfer coefficient between the fuel bundle and the coolant, which is widely used in the innovative reactor fuel bundle design. The P/D ratio of a tight lattice bundle is usually less than 1.1, which is smaller than that of a conventional rod bundle. In the small-break loss-of-coolant accident (LOCA), the steam-water two-phase flow will occur in the reactor. The investigation of gas-liquid two-phase flow in the tight lattice is very important to the reactor safety analysis. A dual sub-channels tight lattice was designed in this study. The original reference of the channel is the annular fuel bundle, with the fuel diameter of 15.52mm, pitch of 16.51mm, P/D = 1.06. The original reference of working condition is the stream-water two-phase flow under the pressure of 15.5MPa. The experimental condition is the air-water two-phase flow at the normal temperature and pressure. According to the ratio of a critical bubble diameter in the reactor (steam-water) to that in atmospheric conditions (air-water), the channel is zoomed in 2.7 times. The diameter of the rod in the dual sub-channels tight lattice is 42mm and the pitch is 44.6mm. The total length of the dual sub-channels tight lattice is 3m. A self-developed 16 × 32 Wire-mesh sensor (WMS) was used to measure the void fraction distribution of air-water two-phase flow in the dual sub-channels tight lattice channel. The spatial resolution of the WMS is 2.79mm and the temporal resolution is 5000fps. The WMS was installed at a distance of 2.5m from the channel inlet and 0.5m from the outlet, which can avoid the influence of outlet on bubbles. The experimental range of flow condition is 0.921–1.84m/s for the superficial liquid velocity and 0.0884–1.07m/s for the superficial gas velocity. The instantaneous and time-averaged void fraction distributions in the channel was measured. With the increase of superficial gas velocity, the distribution of void fraction distribution changed from the wall peak to the core peak. The characteristics of bubbles in the sub-channel were also discussed in this study.

Design and Construction of Magnetic Coils for Quantum Magnetism Experiments

We report on the design and construction of a spin-flip Zeeman slower, a quadrupole magnetic trap and a Feshbach field for a new machine for ultra-cold Li-7. The small mass of the Li-7 atom, and the tight lattice spacing, will enable to achieve a 100-fold increase in tunneling rates over comparable Rb-87 optical lattice emulator experiments. These improvements should enable to access new regimes in quantum magnetic phase transitions and spin dynamics.

Design and Construction of Magnetic Coils for Quantum Magnetism Experiments

We report on the design and construction of a spin-flip Zeeman slower, quadrupole magnetic trap, and Feshbach fields for a new machine for ultra-cold Li-7. The small mass of the Li-7 atom, and the tight lattice spacing, will enable us to achieve a 100-fold increase in tuneling rates over comparable Rb-87 optical lattice emulator experiments. These improvements should enable to access new regimes in quantum magnetic phase transitions and spin dynamics.