Toshiharu Muramatsu

Experimental and analytical studies of the thermal stratification phenomenon in the outlet plenum of fast breeder reactors

Abstract Water and sodium experiments were conducted with scaled models to study the in-vessel thermal stratification. The models simulate the outlet plenum in a loop-type LMFBR with an inner barrel and a closed-type UIS (upper instrumentation structure). An examination of the threshold for thermal stratification, the rising rate of the stratification interface, and the flow regime above the interface has been conducted and is reported in the present paper. In parallel, the phenomenon in the experiments has been evaluated with the multi-dimensional analysis.

Development of thermohydraulics computer programs for thermal striping phenomena

Thermal striping phenomena characterized by stationary random temperature fluctuations are observed in the region immediately above the core exit of liquid-metal-cooled fast reactors (LMFRs) due to the interactions of cold sodium flowing out of a control rod (C/R) assembly and hot sodium flowing out of adjacent fuel assemblies (F/As). Two thermohydraulics computer programs AQUA and DINUS-3, which are represented by both time- and volume-averaged transport analysis and direct numerical simulation of turbulence, respectively, were developed and validated for the evaluation of thermal striping phenomena. These codes were incorporated with higher order difference schemes to approximate the convection terms in conservation equations and adaptive time-step size control systems based on the fuzzy theory to eliminate numerical instabilities. From validation analyses with fundamental experiments in water and sodium, it was concluded that (a) thermal striping conditions such as spatial distributions of the intensity and the frequency of the fluid temperature fluctuations can be estimated efficiently by a combined approach incorporating the AQUA code and the DINUS-3 code, and (b) the thermal striping phenomena for the in-vessel components of actual liquid-metal-cooled fast reactors can be evaluated by the numerical method without conventional approaches such as large scale model experiments using sodium.

Application of an eddy-current type flowmeter to void detection at the LMFBR core exit

Abstract To apply an eddy-current type flowmeter to void (bubble) detection, out-of-pile test has been carried out in a sodium loop, and the void detection technique is applied to the subassembly outlet flowmeters of JOYO. The experimental apparatus used 7 simulated fuel subassemblies under 500°C sodium conditions with a volumetric void fraction of up to 2%. It was found that the phase of the void signal (vector) is different from that of the flow signal (vector). Therefore, the signal-to-noise ratio of void detection was highly improved by choosing the optimum phase angle for synchronous detection. At the optimum phase, the flow-induced fluctuation signal (background) is minimum and the void signal nearly maximum. In addition to the improved void-detection technique, basic information about amplitude probability density functions and power spectral densities of void signals are presented. Using the flowmeter installed in the core of JOYO, void-detecting characteristics have been studied.

Investigation of turbulence modelling in thermal stratification analysis

Abstract In-vessel thermal stratification analysis was carried out using a multidimensional thermohydraulic analysis code, in which a higher-order finite difference scheme was applied to the convection terms. Discussions centred on the buoyancy modelling in the vicinity of the stratification interface through comparisons between experiment and calculation. Computational results were obtained from the following three turbulence models: (i) the k−ϵ model with a constant turbulent Prandtl number Prt, (ii) the k−ϵ model with the turbulent Prandtl number being dependent on the local Richardson number Ri, and (iii) the algebraic stress model. Numerical analysis of the stratification phenomena using the higher-order scheme showed that, in general, the modelling of the buoyancy terms appearing in the turbulence transport equations was the most important key to successful results. When the k−ϵ model was used, it was pointed out that a dependence on the local Richardson number must be carefully included in the turbulent Prandtl number. In this case, however, the range of applicability was limited to the phenomena observed in the water system in general because the model was constructed and calibrated for water experiments. Overall it was found that the calculated stratification interface rise agreed well with experimental results in water and sodium insofar as the algebraic stress model was utilized. As a conclusion, in predicting the behaviour of the thermal stratification phenomena in liquid metal cooled reactors, the coupled use of the higher-order difference scheme and the algebraic stress model was most appropriate and recommended.

Influence of Secondary Flow Generated in a 90-deg Bend on the Thermal-Hydraulic Characteristics in a Mixing Tee

Abstract This study aims at clarifying a relationship between nonisothermal fluid mixing in a T-junction area with a 90-deg bend upstream and temperature fluctuations induced by the unstable mixing, by visualizing the flow fields with particle image velocimetry and measuring fluid-temperature fluctuation in the vicinity of a wall. From the visualization, it is clarified that a high-temperature jet flowing out from a branch pipe swings and sways near the wall, though the mixing patterns are basically classified into the same ones without the 90-deg bend upstream. Furthermore, there are cautionary conditions in which the temperature fluctuation is maximized in a transition regime between a stratified flow and a turn-jet flow. It seems that the principal cause is repetitional generation and disappearance of a circulating flow formed behind the jet due to an interaction between unsteady behavior of a secondary flow in a decay process after the bend and the wakes formed behind the jet, which leads to the vigorous oscillation of the jet near the wall.

In-vessel thermohydraulics evaluation of an unprotected transient overpower accident and delayed neutron precursor concentration transport analysis using a multidimensional code

This paper reports on a three-dimensional in-vessel thermohydraulics analysis that is carried out for the early phase of an unprotected transient overpower (UTOP) accident and delayed neutron precursor concentration transport in a typical loop-type fast breeder reactor plant. In the UTOP calculations, the time at which the sodium temperature reaches the reactor trip level is evaluated based on calculated upper plenum flow and temperature distributions. For fission product release from the core assemblies, the delayed neutron precursor concentration in the sodium that reaches the detectors depends on the location of the faulted assembly. Three-dimensional flow patterns, and hence, the residence time in the upper plenum. Delayed neutron precursors that bypassed the recirculation flow to appear in the plenum primarily contribute to the peak concentration.

Flows in T-Junction Piping System (2nd Report, Numerical Analysis of Vortex Street Formed by Branch Pipe Flow)

Thermohydraulic analyses for a fundamental water experiment simulating thermal striping phenomena at T-junction piping systems were carried out using a quasi-direct numerical simulation code DINUS-3. Calculated results were compared with the experimental results on the flow patterns in the downstream region of the systems, the arched vortex structures under the deflection jet condition, the generation frequency of the arched vortex in the various conditions ; i.e., diameter ratio α, flow velocity ratio β and Reynolds number. From the comparisons, it was confirmed that (1) the DINUS-3 code is applicable to the flow pattern classifications in the downstream region of the T-junction piping systems, (2) the arched vortex characteristics with lower frequency components can be estimated numerically by the DINUS-3 code, and (3) special attentions should be paid to the arched vortex generations with lower frequency components of fluid temperature fluctuations, which might induced high-cycle thermal fatigue for the design of T-junction systems.

Development of Analytical Method for Thermal Striping Phenomena.

A thermal striping phenomenon characterized by a random temperature fluctuation occurs in the region immediately above the fast breeder reactor core due to the temperature difference of the core outlet coolant between subassemblies. Then the random temperature fluctuation applies a thermal stress to in-vessel structures. In this paper, we have investigated the intensity of the temperature fluctuation in water by implementing the algebraic stress turbulence model (ASM) including the second-order momentum of turbulence and the fuzzy controller to determine an optimum time step size in a general-purpose thermohydraulic analysis code. From the analysis, it is concluded that the characteristics of the temperature fluctuation intensity can be estimated efficiently by the ASM.