Effect of Changes in Feed Rate and Cut on Circulating Flow within Thermal Diffusion Column for Isotope Separation

A two-dimensional axisymmetric convectional flow field of an isotope separating thermal diffusion column with continuous feed and draw-offs is analyzed through the Newton iterative numerical solution of the equations of change without the Boussinesq approximation. Computations are performed for Ar gas within an inner hot radius of 0.2 mm and an outer cold radius of 5 mm, between which the temperature differs by 300 K. The rate of feed F, supplied into the middle point of the column, is varied from 1 to 5 cm³/s, while the cut θ, or the ratio of the upper drawing-off to feeding rates, from 0.1 to 0.9. Comparison of flow vectors, temperature and density profiles among various sets of (F, θ) makes it clear that the flow fields agree with one another for the same value of the total transport; that is, θF_pc (pc: feed gas density) in the upper section and -(1-θ)pc in the lower section of the column. Observations on the mass flow vectors near the feed slit lead to the finding that the multiplicative effect of separation cannot be expected in the column with the inner radius r_c of 5 mm at the feed rate F exceeding 2 cm³/s(i.e F/ΔTr_c ⁴ )>0.1 [1/(s·K·cm)]), because the feed flow interrupts the whole-length circulation of the natural convection.     

Evaluation of leak flow rate and jet impingement related to leak before break

Leak flow rate and jet impingement load are experimentally evaluated using very narrow and short artificial slits which simulate a through-wall fatigue crack. The following items are clarified in the experiments under BWR fluid conditions: (1) the leak flow rate from the rectangular slit is experimentally clarified and it agrees well with the analytical result when the estimated thermodynamic nonequilibrium parameter N in the present experiment is introduced; (2) the orifice effects are experimentally evaluated concerning the through-wall fatigue crack, in which the discharge coefficients C_D between 0.75 and 0.25 are obtained as a function of the exit to the stagnation area ratio A_e/A_o; (3) when very narrow and short slits are used in the jet impingement experiments, the maximum pressure on the target is higher than that of a large diameter pipe and it increases with the decrease of the flow path length to hydraulic diameter ratio L/D of the slit because of the thermodynamic nonequilibrium effect.     

Efficient Energy Utilization in Laser-Assisted Isotope Separation of Tritium

Optimum irradiation conditions in laser isotope separation of tritium were investigated experimentally by using trifluoromethane from the viewpoint of the energy efficiency in product formation. The absorbed energy ε_abs and the fractional conversion of CTF₃ per pulse q_T were measured as the functions of irradiation wavenumber, laser pulse duration, laser fluence, gas pressure and temperature. The results of optimization were shown in contour maps, where the ratio of the fractional conversion of CTF3 per pulse to the absorbed energy q_T /ε_abs was adopted as the objective function. The value of the objective function at the optimum condition was related to the maximum flow rate of the working substance attainable for given laser power.     

Effects of Partition Wall within Packings on Separative Performances of Water Distillation for H2O-HTO Isotope Separation

For the performance assessment of H₂O-HTO isotope separation by distillation, three-dimensional internal vapor flow and concentration profiles were obtained numerically for two types of packing of 6 mm of inner diameter and 6 mm of height: One is a simple cylindrical tube (packing I) and the other diametrically partitioned with an internal wall (packing II). The results of the concentration profiles were used to estimate separative performances of the packings. The total mass transfer rate of HTO from vapor to liquid per unit area of vapor-liquid interface per unit vapor flow rate decreased with an increase in the vapor flow rate, and the value for the packing II is smaller than that for the packing I in the range of relatively small vapor flow rate. Each separative power for the packing I and II has a maximum value of 1.4×10^–7 g/s at the Reynolds number of the vapor Re_v of 54 and 3.7×10^–7 g/s at Re_v =160, respectively.     

Influences of Gas Stream Conditions on Efficiency of Tritiated Moisture Collection with P2O5-Desiccant and Isotope Effect

A method was proposed previously for collection and measurement of tritiated moisture in gas stream using P₂O₅-desiccant. Influences of the gas humidity, the gas flow rate and the distance between gas nozzle and P₂O₅-desiccant layer surface on the moisture collection efficiency have been examined through experiments, and the isotope effect on the collection has been investigated. The collection efficiency is the ratio of collected to supplied moisture, and the moisture supplying rate is in proportion to the humidity and flow rate of feed gas. The experiments show that; the collection efficiency does not depend on the gas humidity, but is affected by the gas flow rate and by the nozzle-layer distance. The effects of the flow rate and the nozzle position are related to the mass transfer distance from the bulk of gas stream to the desiccant layer surface in the collection cell. The moisture collecting rate is promoted by the approach of the gas stream to the layer surface. An expression of effective separation factor has been derived to explain the isotope effect on the moisture collection. Experimental data distribution of the separation factor have been reasonably simulated by the analysis.     

Numerical Analysis of Pressure Effects on Heavy Water Separation Characteristics for a Pair of Dual Temperature Multistage-Type H2/H2O-Exchange Columns

In order to study how the operating pressure has influence on the heavy water separation characteristics for a pair of dual temperature multistage-type H₂/H₂O-exchange columns, the authors utilized the analytical expressions derived from the finite difference equation describing the material balance in the exchange column. The separation characteristics of the system were studied theoretically under the conditions where the temperature in a hot column t_h=200°C, the pressure in a hot column π≦50atm and the pressure in a cold column π = 5–50atm.

As the results, the optimum modified specific column volume v^*_opt decreases with the increase of π and then keeps the nearly constant value, and is smaller at ≦=50atm than at ≦=30atm in the case where the temperature in a cold column t_c is constant. Other important parameters such as the optimum operating temperature in a cold column t_copt the optimum numbers of stages in a hot column M_opt and in a cold column t_opt, the optimum hydrogen to feed water molar flow ratio γ _opt and the optimum degradation ratio δ_opt were also calculated.     

Analysis of Stratified Cocurrent Flow of Gas-Liquid Mixtures in Horizontal Pipes

A very simple model, as compared with those presented thus far, is developed to describe the stratified cocurrent flow of gas-liquid mixtures in a horizontal pipe. Assuming densities of both phases are constant in location, mass and momentum equations are solved to obtain pressure gradient and volume fractions of both phases at steady state.

Several experimental data by others are found in good agreements with the calculated results by the model. When the interface is smooth, i.e. the volumetric liquid flux J_l <0.lft/s, it is a good approximation to use the Blasius equation to estimate interfacial shear stress. For 0.1?J_l ?1.0ft/s where the interface is not smooth but ripply or wavy, the Wallis correlation is found applicable. If we assume stratified flow geometry forj_l >1.0ft/s, agreements with experiment cannot be obtained, since the transition to slug or elongated bubble flow has already taken place. The transition from stratified to annular mist flow is found to occur at gas volume fraction α≈0.9 regardless of J_l .     

Single and multiple discharge from a stratified two-phase region through small branches

Experimental data are presented for the mass flow rate and quality during single, dual and triple discharge from a stratified air–water region through small side branches (d=6.35 mm) installed on a semicircular wall. Dimensions of the semicircular wall and branches were chosen such that interaction among the branches is possible under certain flow conditions. All the branches were adjusted to have the same hydraulic resistance (R=1000 (kg m)^−1/2) and for the cases of dual and triple discharge, the same pressure drop ΔP was imposed across all active branches. Tests were conducted at two system pressures P₀=316 and 517 kPa and the pressure drop was varied within the range 40≤ΔP≤235 kPa. Data analysis is presented with emphasis on the effect of wall curvature and also the effect of additional discharges on the flow from a certain branch. The present data can serve as benchmark data for testing numerical safety codes and they should guide future research on the flow from two-phase headers.     

微流体惯性冲击器过滤性能模拟计算

用微流体惯性冲击器收集μm尺寸气溶胶颗粒是一种新型的过滤方式,弥补了现有过滤收集方式的不足,已有学者进行了可行性论证,但气流在现有的T型惯性冲击器内压力损失较大且冲击器的加工工艺较为繁琐。本文设计并研究了一种新型微流体惯性冲击器,采用CFD软件对微流体惯性冲击器的过滤性能进行了数值模拟。建立了数学模型,对气相采用层流模型,对颗粒相采用离散相模型(DPM)。通过模拟计算,分析了不同尺寸的微流体惯性冲击器对粒子收集效率的影响。结果表明,冲击器转折角和尺寸D对收集效率均有较大影响:转折角越小,收集效率越高;当D≤1/2时,效率曲线基本不变。     

Experimental study on combustion characteristics of sodium fire in a columnar flow

In the operation of the sodium-cooled fast reactor, the accident caused by the leakage and combustion of liquid sodium is common and frequent in sodium-related facilities. This paper is based on an experimental study of sodium fire in a columnar flow, which was carried out to focus on the burning characteristics by analyzing the temperature fields in the burner. The injection of 200 °C liquid sodium with the flux of 0.5 m³/h was poured into a 7.9 m³ volume stainless steel cylindrical burner to shape a sodium fire, and the data of temperature fields in the burner have been collected by dozens of thermocouples which are laid in the combustion space and sodium collection plate. These results show that the sodium fire in a columnar flow is composed of the foregoing centered columnar fire, the subsequent spray fire caused by atomization and the pool fire on the collection plate. The temperature close to the burning sodium flow maximally reaches up to 950 °C. The radial temperatures apart from the sodium flow are relatively low and generally about 200 °C, and maximally just 300 °C even when close to the sodium collection plate. The maximum temperature of the burning sodium dropping on the collection plate rises in the center of plate, about 528 °C. This study is helpful to evaluate the combustion characteristics, formation process and composing forms of the sodium fire in the sodium-related facilities.     

Sodium Vapor Deposition,on Simulated Model of LMFBR Rotating Shield Plug Annulus, (I)

Experimental study was made to investigate the controlling factors on the vapor deposition rate on reactor operational shield plug annulus, which is exposed to the vapor entrained cover gas during reactor operation. Two simulated test assemblies having annuli were made for this purpose and were installed into a small test vessel. In the experiment, the average deposition rates on the annular walls of the test assemblies were measured for various pool temperatures, and their dependents upon such parameters as pool temperature, T_s (or the saturated vapor pressure P_s at pool surface), cover gas pressure P_g , and temperature drop ΔT_a across cover gas, were studied.

The results revealed that the dominant controlling factor was the vapor pressure P_s at pool surface. Dependent of the average deposition rate φbar_d. upon the above parameters was simply expressed by: φbar_d=Bσp_sD_sΔT_G , where, σ_s is the saturated vapor concentration at pool surface, D_s , the vapor diffusion coefficient, and B the proportional constant.

To these experimental results, the previously published evaporation rate data and the theoretical evaporation rate equation based on Epstein & Rosner's theory were reviewed. Then correlation between the deposition and the evaporation rates was discussed.     

Measurement of Propagation Velocities of Pressure and Void by Cross-Correlation of Fluctuations in Nitrogen-Water Flow

A framework of boiling channel stability theory is analyzed. The fundamental equations are those of STABLE code: Three conservation laws of mass, energy and momentum applied to one-dimensional channel, together with Bankoff slip and Marinelli-Nelson's pressure drop correlation. These equations are analyzed to yield “Void Equation”, “Linearized Void Equation”, “Volume Conservation Law” and the “Flow Impedance” R(s), defined by the dynamic response of pressure drop to the inlet flow. The impedance contains all the information such a stability index, dominant frequency and damping ratio. It is shown that R is a sum of the form R _IA+N _F ^?1 R _D+N _R R _R+N _OR, where N's are non-dimensional parameters and R's characteristic impedances determined by three kinds of parameters, N_x , N_s and the power distribution parameter. Systematic edition of the characteristic impedances according to the non-dimensional parameters will reduce the need for case-by-case STABLE calculations. Hydraulic stability of BWR channels under constant system pressure, is a phenomenon with three parameters in view of complexity. Furthermore an analysis is conducted to confirm the above stability structure and three typical instabilities are identified.     

Fracture toughness of Zr-2.5 wt% Nb pressure tubes

Measurements of fracture toughness of HT Zr-2.5 wt% Nb pressure tubes have been made by studying internally pressurizing (burst) test specimens and small bending test specimens. These tests were conducted from a viewpoint of the effects of hydrogen content, hydride orientation, temperature and crack configuration on the fracture thoughness K_c. Results of the experiments showed that K_c decreased with increasing hydrogen content, but is little affecting by hydrogen content at reactor operating temperature. The value of K_c can be quantitatively evaluated by RHC defined by radial hydride content (RHC) perpendicular to the tensile stress.     

Combined Forced and Free Convective Heat Transfer Characteristics in a Narrow Vertical Rectangular Channel with 2.5 mm in Gap Heated from Both Sides

Heat transfer characteristics of mainly combined forced and free convective flow in a vertical rectangular flow channel with a gap of 2 5 mm, which was quite narrow compared with those investigated in previous experiments, were studied experimentally for water. As a result, the following heat transfer characteristics were made clear, using a non-dimensional parameter Gr _x /Re ²¹ _x /⁸ Pr ^1/2 similarly to the case for the 18 mm gap which was already reported by the authors.

(1) When the Gr _x /Re ²¹ _x /⁸ Pr ^1/2 is less than 10^?4, both upward flow and downward flow show the nature of forced convective heat transfer.

(2) When the Gr _x /Re ²¹ _x /⁸ Pr ^1/2 is between 10^?4 and 10^?2, heat transfer coefficients for both upward flow and downward flow are higher than any of those predicted by the previous correlations for turbulent forced convection along a flat plate and turbulent free convection along a vertical flat plate. This is, differently from the case of 18 mm gap, due to the effect of the acceleration of main flow induced by the development of the boundary layer along the channel.

(3) When the Gr _x /Re ²¹ _x /⁸ Pr ^1/2 is larger than 10^?2, the upward flow shows the nature of free convective flow even with the gap as narrow as 2.5 mm in the vertical rectangular flow channel. Heat transfer correlations which have been developed for the 18 mm gap channel, are also available for the described-above regions of 2.5 mm gap channel.     

Frictional Pressure Drop for NaK-N2 Two-Phase Flow in Rectangular Cross Section Channel of Large Aspect Ratio

In this paper, the frictional pressure drop in an isothermal liquid metal-gas two-phase flow through a rectangular channel with large width-to-height ratio is treated semiempirically for a NaK-N₂ two-phase flow system.

The frictional pressure drop in the two-phase flow is compared with the following two reference values :

1. The frictional pressure drop in the liquid flowing alone in single phase with the same velocity as that of the liquid in the two-phase mixture.

2. The frictional pressure drop in the liquid flowing alone in single phase with the same mass flow rate as that of the liquid in the two-phase mixture.

The comparison with the former reference value is necessary for the prediction of friction loss in a liquid metal MHD generator channel whose medium would be two-phase mixture.

The semiempirical analysis was performed assuming the two-phase mixture to be a continuous medium with its properties, e.g. viscosity and density, defined by void fraction and the velocity determined by the total mass flow rate.

In the region of low slip and density ratio ρ_g/ρ_l the frictional pressure drop in the two-phase flow appeared to be smaller than that due to the liquid flowing alone with the same velocity as that of the liquid in the two-phase flow.

The experiments have been undertaken with the NaK-N₂ two-phase mixture flowing through a rectangular channel (4 × 60 mm²).

Data were taken over the following parameter range:

NaK velocity: 5～30 m/sec, Void fraction: 0～70%

Density ratio: 0.006～0.013, Quality: 0.07～1.10%.     

Irradiation of Aqueous Solution of Thiocyanate by Radiation Emitted from 10B(n, α)7 Li Reaction

A dual temperature hydrogen isotopic exchange reaction system between water and hydrogen gas is numerically analyzed. The system has two features; high efficiency of isotope exchange reaction and operation under atmospheric pressure. To achieve them, the low temperature section of the system is composed of water mist and hydrogen gas co-current reactor units. For the high temperature section, a multistage-type reactor, in which a bubble plate, superheater and catalyst bed are alternatively arranged, is applied.

From a material balance between these reactors, enrichment and decontamination factors for the system are expressed as functions of seven parameters: unit number of the low temperature co-current reactor (X); stage number of the high temperature section (Y); flow ratio of tritium enriched water to decontaminated water (P/W), flow ratio of feed water to hydrogen gas (F/G); reaction temperatures of the low and high temperature sections (T _c , T _h ); and bubble plate temperature (T _b ). Numerical calculations show that enrichment factor depends remarkably on F/G and T _b as well as X and Y.

In order to understand the separation characteristics visually, the McCabe-Thiele diagrams for the present system are drawn and compared with the results calculated.     

Droplet entrainment correlation in vertical upward co-current annular two-phase flow

Upward annular two-phase flow in a vertical tube is characterized by the presence of liquid film on the tube wall and entrained droplet laden gas phase flowing through the tube core. Entrainment fraction in annular flow is defined as a fraction of the total liquid flow flowing in the form of droplets through the central gas core. Its prediction is important for the estimation of pressure drop and dryout in annular flow. In the following study, measurements of entrainment fraction have been obtained in vertical upward co-current air–water annular flow covering wide ranges of pressure and flow conditions. Comparison of the experimental data with the existing entrainment fraction prediction correlations revealed their inadequacies in simulating the trends observed under high flow and high pressure conditions. Furthermore, several correlations available in the literature are implicit and require iterative calculations.Analysis of the experimental data showed that the non-dimensional numbers, Weber number (We = ρ_gj_g²D/σ(Δρ/ρ_g)^1/4) and liquid phase Reynolds number (Re_f = ρ_fj_fD/μ_f), successfully collapse the data. In view of this, simple, explicit correlation was developed based on these non-dimensional numbers for the prediction of entrainment fraction. The new correlation successfully predicted the trends under the high flow and high pressure conditions observed in the current experimental data and the data available in open literature. However, in order to use the proposed correlation it is necessary to predict the maximum possible entrainment fraction (or limiting entrainment fraction). In the current analysis, an experimental data based correlation was used for this purpose. However, a better model or correlation is necessary for the maximum possible entrainment fraction. A theoretical discussion on the mechanism and modeling of the maximum possible entrainment fraction condition is presented.     

Critical Heat Flux in a Heater Rod Bundle Cooled by R-134a Fluid near the Critical Pressure

In the development of supercritical pressure water cooled reactors, it is important to understand the characteristics of a heat transfer near the thermodynamic critical point. An experimental study on the critical heat flux near the critical pressure has been performed with a 5 × 5 square array heater rod bundle cooled by R-134a fluid (P _c = 4:059MPa, T _c = 101°C). The critical power has been accurately measured up to the reduced pressure of 0.99 (4.03 MPa). The critical power decreases sharply at a pressure of about 3.8–3.9 MPa as the pressure approaches the critical pressure. For the low mass fluxes of 50 to 250kg/m², a sharp decrease in the critical power is not observed near the critical pressure. The CHF phenomenon near the critical pressure no longer leads to an inordinate increase in the heated wall temperature such as the case of DNB at normal pressure conditions. In the pressure region close to the critical pressure, there is a threshold pressure at which the CHF phenomenon disappears. When the pressure exceeds the threshold pressure, the wall temperature increases monotonously without a CHF occurrence according to the power level applied to the heater rods. The threshold pressure moves toward the lower pressure region gradually with an increasing mass flux.