Nobuyuki Kimura

Experimental Study on Gas Entrainment at Free Surface in Reactor Vessel of a Compact Sodium-Cooled Fast Reactor

An innovative sodium-cooled fast reactor has been investigated as part of the fast reactor cycle technology development (FaCT) project. In the reactor, a compact reactor vessel (R/V) with increased sodium flow velocity was designedto reduce the construction cost. One of the thermal hydraulic problems associated with this design is gas entrainment at the free surface in the R/V. Horizontal plates are set below the free surface in order to prevent the gas entrainment. A water experiment was performed using a partial model built to 1/1.8 scale. The objective was to investigate the occurrence of gas entrainment under certain conditions and the mechanism of the gas entrainment. It was found that there were two types of gas entrainment phenomenon, and the conditions for their occurrence were far different from the rated condition in the reactor. One type of gas entrainment occurred at the wake region of flow around the cold leg pipe created due to the larger horizontal velocity in the R/V. The other type of gas entrainment occurred at the region between the hot leg pipe and the R/V wall when the coolant level was low and the downward velocity was large. The mechanisms of the gas entrainment at the two regions were clarified by the detailed measurement of transient flow velocity field.

Transient Behavior of Gas Entrainment Caused by Surface Vortex

A compact sodium-cooled reactor is an important candidate as a fast breeder reactor (FBR) and has been investigated in the feasibility study of FBR cycle. Due to the compact sizing of the reactor vessel, gas entrainment at the free surface of sodium coolant becomes one of the significant issues for reactor design, and it is required to clarify the criterion of gas entrainment at free surface and the tolerance. In the present study, some visualization experiments were performed in a water-air system focusing on the gas entrainment due to surface vortex and its transient phenomena. Influences of horizontal velocity were clarified by the visualization. The gas entrainment due to the surface vortex occurs intermittently. Time trends of circulation and length of gas core for the intermittent surface vortices were measured by the particle image velocimetry and visualization. It was found that the gas core length extends with time delay to the increase of circulation around the vortex.

Study on Thermal Striping at UIS of Advanced Loop Type Fast Reactor: Water Experiment Using a 1/3 Scale 60 Degree Sector Model

An advanced loop type sodium cooled fast reactor, JSFR, has been investigated in the frame work of Fast Reactor Cycle Technology Development Project (FaCT). As the temperatures difference between the control rod channels and the core fuel subassemblies is around 100 °C, temperature fluctuation due to the fluid mixing at the core outlet may cause high cycle thermal fatigue at the bottom of Upper Internal Structure (UIS). Then, a water experiment was conducted using an 1/3 scale 60 degree sector model. Temperature and its fluctuation intensity distributions around the control rod were measured and an effect of the improved structure against the thermal fatigue was examined.© 2009 ASME

Development of High Cycle Thermal Fatigue Evaluation Method Based on Time Interval of Peak-to-Peak of Fluid Temperature

Hot and cold fluids are mixed at the core outlet of sodium cooled fast reactors. The temperature fluctuation causes high cycle thermal fatigue in structural components. The temperature fluctuation at the core outlet region does not have always a sinusoidal waveform but a sharp edged waveform. The temperature shows intermittent and sudden decrease and recovery like a spike form. It is necessary to take into account the spiky waveform of temperature fluctuation for the construction of an evaluation method of the high cycle thermal fatigue. The conventional method uses the amplitude and cycle number of waves without reference to the frequency of temperature fluctuation. In this study, the time interval of each wave based on the rainflow method was applied to consider frequency characteristics against the conversion from fluid temperature to thermal stress in structure. The thermal stress obtained from the new method was compared to the results of FEM analysis. It was found that the consideration of frequency characteristics of waves could evaluate the fatigue damage in structure. Furthermore, the frequency characteristics of waves in this method were expressed as the unified curve independent of the velocity. Hereby the new evaluation method could evaluate the thermal fatigue in the reactor.Copyright

Experimental Study on Gas Entrainment Due to Non-Stationary Vortex in a Sodium Cooled Fast Reactor: Comparison of Onset Conditions Between Sodium and Water

An innovative sodium cooled fast reactor has been investigated as part of the fast reactor cycle technology development (FaCT) project. In the reactor, a compact reactor vessel (R/V) with increased sodium flow velocity was designed to reduce the construction cost. One of the thermal hydraulic problems in this design is gas entrainment at the free surface in the R/V. In most of past studies, water experiments were performed to investigate the gas entrainment in the reactor. It is necessary to evaluate an influence of fluid physical property on the gas entrainment phenomena. In this study, sodium experiments were carried out to clarify the onset criteria of the gas entrainment due to a free surface vortex. Water experiments using a test section in which geometry is the same as that in the sodium tests were also performed. The gas entrainment in water slightly tended to take place in comparison with that in sodium under low velocity conditions. Overall onset condition map on lateral and downward flow velocities in the sodium and water experiments were in good agreement.Copyright

Direct Observation and Control of Liquid Sodium Flow Dynamics Using VUV-LIF-PIV Technique Under EXB Lorentz Force

Based on a heuristic hypothesis that alkaline metals with one single electron on the outermost shell would little interact with externally applied RF field above plasma frequency, a rudimentary experiment as well as the theoretical estimate of the energy structure been performed to further explore the spectroscopic properties of liquid sodium (Na)[1–3] . Consequently, it was successfully proven that Na is reasonably transparent to the VUV (vacuum ultraviolet) laser radiation, although the liquid Na surface is highly reflective, being like a mirror to human eyes. The impact of this result is that the velocity field information inside the liquid Na can be visualized by implementing the well developed PIV (particle image velocimetry) technique[4, 5] . A large eddy simulation (LES) code has also been developed for comparison with the experimental results[6] . Furthermore, the newly developed Na loop is designed so as to enable the application of electric and magnetic field in the orthogonal direction to each other that vigorous dynamics of vortices inside the liquid Na are resolved in the phase space under the Lorentz force. The results herein obtained contribute not only to the thermal hydraulics in fast reactors[7, 8] but also space physics, such as the spiral galaxy formation[9] and solar flare activities[10] .Copyright

Experimental Study on Thermal Stratification in a Reactor Vessel of Innovative Sodium-Cooled Fast Reactor—Mitigation Approach of Temperature Gradient across Stratification Interface—

An innovative sodium-cooled fast reactor has been investigated as part of the fast reactor cycle technology development project (FaCT). Thermal stratification after a scram is one of the main thermal loads of a reactor vessel (R/V). R/V has an upper inner structure (UIS), which consists of perforated horizontal plates and control rod guide tubes, and has a slit in the radial direction for fuel handling. The UIS slit causes an asymmetric flow pattern in R/V. A water experiment using a 1/10-scale model was carried out. A steep temperature gradient across the stratification interface was observed at the neighborhood of the UIS slit in the experiment. This means that the jet through the UIS slit entrains the bottom of the stratification interface. In order to mitigate the temperature gradient across the stratification interface, the height of a plug, which was installed in the upper plenum to infill a hole at the dipped plates where a fuel handling machine was inserted during a fuel exchange operation, was changed as the parameter in the experiment. The experimental result shows that the temperature gradient near the R/V wall in the case of the lower plug position was 21% smaller than that in the case of the higher plug position. This reduction of the temperature gradient was sufficient to maintain the structural integrity of the R/V wall against the thermal stratification.

NUMERICAL STUDY ON PASSIVE CONTROL OF THERMAL STRIPING PHENOMENON USING LORENTZ FORCE IN FAST REACTOR

Liquid sodium, used as a coolant of fast reactor (FR), is an electromagnetic fluid. When a magnetic field is embedded to liquid sodium flow, the Lorentz force will be induced and flow structure such as a turbulent mixing phenomenon will be influenced. In a FR piping system, thermal fatigue is one of key issues that may occur after a piping junction of different fluid temperature. In the present paper, a numerical study of sodium flow induced by the Lorentz force on a turbulent mixing after the junction has been carried out. The large eddy simulation (LES) coupled with the electromagnetic field analysis is used for this purpose. The governing equations of the electric density field and the magnetic flux density fields are solved separately so that the solenoidal condition is satisfied noniteratively. The fractional step method is applied to the present coupling in the simulation.