Structure design on improving injection performance for venturi scrubber working in self-priming mode

The venturi scrubber working in self-priming mode is one of the most efficient gas cleaning devices to remove the radioactive particles and gases from gaseous stream during severe accident in nuclear power plant. This paper focus on improving injection performance in a split type self-priming venturi scrubber, the static pressure distributions at gas and liquid channels in the scrubber are studied emphatically, the experimental results indicate that pressure at the center of nozzle exit is lower than at the wall, and the variation laws in radial are different with increasing gas velocity. When the average gas velocity at throat U_avg = 64.3 m/s, the static pressure difference between center and wall is 2.1 KPa; with the increase of gas velocity, pressure at centre and wall reduce gradually and the pressure difference become significantly, when the average gas velocity U_avg = 225.8 m/s, pressure at the center is lower 23.2 KPa than at the wall, relative deviation is about 45.6%. However, when U_avg ≥ 230 m/s, the static pressure is not continue to decrease but reverse to recover with increasing gas velocity, and recovery rate at the wall is greater than at the center. The condition before the transition point (U_avg < 230 m/s) is defined as velocity dominate area, in this area, pressure will always decrease with increasing gas velocity; the condition after the point (U_avg ≥ 230 m/s) is defined as resistance dominate area, while pressure in this area will reverse to recover, the venturi scrubber design should be ensured in velocity dominate area. The injection performance of self-priming venturi scrubber is closely relate to pressure distribution characteristics at nozzle exit, in condition of no injection or injecting air, pressure at liquid channel is consistent with the pressure at the wall of nozzle exit, which is higher than the average static pressure; when injecting water, an additional pressure increment will generate at liquid channel duo to the momentum exchange between gas and liquid, and lead to the effective pressure difference for injection reduce further. On this occasion, the influence of liquid channel area and resistance coefficient on injection performance become important, increase liquid channel area is effective for improving injection flow rate.     

自吸式文丘里水洗器引射特性研究

以安全壳过滤排放系统中的自吸式文丘里水洗器为研究对象,采用空气和水为工质,在不同空气流量、液位及系统压力下对文丘里水洗器的引射特性进行实验研究。结果表明,随着喉部气相折算流速(简称喉部流速)的增加,吸液口两侧压差近似呈抛物线规律增长,引射量线性增加。压力容器内的液位对引射特性的影响与其相对于文丘里水洗器出口的位置有关,当液位低于文丘里水洗器出口时,随着液位的提升,引射量获得明显的提高,并随喉部流速的变化表现出分区效应,喉部流速较低时,增加液位对于改善引射量更加有效;当液位在文丘里水洗器出口以上变化时,对引射量几乎无影响。压力是影响自吸式文丘里水洗器引射特性的重要因素,这主要归因于气体密度的变化,在0~150kPa范围内,随着压力的提升,引射量得到明显改善,且在高喉部流速区域压力变化对引射量的影响较在低喉部流速区域更加显著。     

De-agglomeration mechanisms of TiO2 aerosol agglomerates in PWR steam generator tube rupture conditions

A steam generator tube rupture (SGTR) in a pressurized water reactor (PWR) might be a major source of accidental release of radioactive aerosols into the environment during severe accident due to its potential to by-pass the reactor containment. In the ARTIST program, tests were carried out at flow conditions typical to SGTR events to determine the retention of dry aerosol particles inside a steam generator tube. The experiments with TiO₂ agglomerates showed that for high velocities in the range of 100-350 m/s, the average particle size at the outlet of the tube was significantly smaller than at the inlet due to particle de-agglomeration. Earlier, particle de-agglomeration has not been considered significant in nuclear reactor severe accidents. However, the tests in ARTIST program have shown that there is a possibility that TiO₂ aerosol particles de-agglomerate inside a tube and in the expansion zone after the tube exit under SGTR conditions.In this investigation, we measured TiO₂ aerosol de-agglomeration in the tube with very high flow velocities with two different TiO₂ aerosols. The de-agglomeration was determined by measuring the size of the agglomerates at the inlet and outlet of the test section. The test section was composed of tubes with three different lengths, 0.20, 2.0 and 4.0 m, followed by an expansion zone.The main results were: (i) the de-agglomerate process was relatively insensitive to the initial particle size distribution, (ii) the agglomerates were observed to de-agglomerate in all the tubes, and the resulting particle size distributions were similar for both TiO₂ aerosols, (iii) at high flow rates, increasing the gas mass flow rate did not produce further de-agglomeration, and (iv) the agglomerates did not de-agglomerate to primary particles. Instead, after de-agglomeration the particles had a median outer diameter D_c = 1.1 μm. Based on analysis using computational fluid dynamics (CFDs), the de-agglomeration was caused by the turbulent shear stresses due to the fluid velocity difference across the agglomerates in the viscous subrange of turbulence.It has to be noted that the particles used in this investigation were TiO₂ agglomerates, and not prototypical nuclear aerosols with significantly different characteristics. Therefore, the results of this investigation cannot be directly used to determine whether the nuclear aerosol particles may de-agglomerate in SGTR sequences. However, this investigation highlights the possibility of particle de-agglomeration under SGTR conditions, and identifies the mechanism of the de-agglomeration inside the broken tube and when the aerosol is discharged to an open space.     

上升气泡内气溶胶沉降机理研究

气溶胶池洗过滤是反应堆严重事故中去除放射性源项的重要手段。本文以严重事故条件下上升气泡中气溶胶的滞留过程为背景，设计搭建了可视化单气泡鼓泡实验装置。通过该装置研究了气溶胶在上升气泡中的沉降效率，并与MELCOR中的气溶胶沉降模型计算结果进行了对比。结果表明，气溶胶沉降效率对气泡尺寸的变化较为敏感，当气体流量大于0.1 L/min时，气泡等效直径迅速增加，相应的气溶胶沉降效率快速降低；与MELCOR模型计算结果的对比表明，两者在总体趋势上呈现出较好的一致性，但计算结果低估了液相对气溶胶的实际去除能力，导致这种偏差的主要原因是气泡在上升过程中存在无规则的晃动以及气液界面的波动。     

低流速排放下气溶胶水洗模型

在发生核电厂严重事故时,乏池水洗作为放射性气溶胶去除的手段之一,应用于先进非能动压水堆中。为评估气溶胶水洗效果,本文建立了蒸汽冷凝、惯性碰撞、重力沉降、离心沉积和布朗扩散等典型气溶胶去除机制模型,同时考虑了可溶性气溶胶颗粒增大现象,采用FORTRAN语言实现了气溶胶水洗效果分析程序。通过构建LACE Espana实验装置分析模型,模拟了3种典型低流速工况,并开展了气溶胶粒径、蒸汽份额以及淹没深度等关键因素的影响分析。结果表明：水洗净化系数（DF）计算值与实验结果的符合程度合理,模型有效性得到初步验证;DF随气溶胶粒径、蒸汽份额以及淹没深度的增大而增大,可溶性气溶胶颗粒增大将显著提高DF。     

基于FLUENT的核电用文丘里管稳流性能研究

采用Realizable k-?湍流模型和Zwart空化模型对某核电用空化型文丘里管的空化流动进行了数值模拟。模拟在特定工况条件下文丘里管内流动情况,得到流量变化曲线,预测空化区域,分析稳流原理和规律。模拟不同喉部直径文丘里管稳流性能,探究喉部直径变化对空化的影响。研究结果表明:随着入口压力的增大,文丘里管将发生空化塞流。将流量变化控制在一定范围内,达到相对稳流的作用。稳流时,管路压力每升高0.1 MPa,流量增加0.06 m3·h-1;喉部直径的尺寸直接影响水力空化初生与流量增幅;在一定范围内,文丘里管喉部直径大,空化流动发展迅速且流量增幅大,喉部直径小,管路流量增长幅值小。      

Research on Aerosol Deposition Efficiency under Bubbly Scrubbing Condition

Radioactive aerosols as one of the most important products in serious nuclear reactor accidents are generated from leakage of solid fission products and condensation of gaseous fission products. Bubbly scrubbing is an effective way to deposite radioactive aerosols. It is of great significance for post-accident source term control and accident analysis and evaluation to accurately grasp its filtration efficiency. In this paper, an in-depth basic research was carried out on the aerosol deposition characteristics in rising bubbles. With the help of advanced particle size spectrum analysis technology, the influence of parameters such as liquid submersion depth and apparent gas phase velocity on the deposition efficiency of submicron aerosols was studied to explore the deposition mechanism of aerosols in rising bubbles. The research results of this project can be used to verify the aerosol deposition efficiency model, so as to improve the uncertainty of the analysis results of source term concentration under severe accident conditions.     

池式鼓泡条件下气溶胶沉降效率研究

放射性气溶胶是核反应堆严重事故中最重要的产物之一,来源于固体裂变产物外漏和气体裂变产物的凝聚成核。池式鼓泡水洗是去除放射性气溶胶的有效途径,准确掌握其过滤效率,对于事故后源项控制和事故分析评价都具有重要意义。本文针对池式鼓泡条件下的气溶胶沉降特性展开深入的基础研究,借助先进的粒径谱分析技术,研究液相淹没深度、气相表观流速等参数对亚微米级气溶胶沉降效率的影响,探究气溶胶在上升气泡群内的沉降机理。本项目的研究成果可用于气溶胶沉降效率模型验证。     

Experiments on sodium fires and their aerosols

A number of new sodium fire and aerosol experiments were undertaken to provide data for LMFBR safety analyses: (1) Experiments on the burning of single drops of liquid sodium falling in air have been performed to aid in model development for sodium spray fire codes. (2) The leakage of sodium oxide aerosols through a straight smooth capillary tube, representative of the maximum size of a hypothetical gas leak in the wall of the secondary containment of an LMFBR, has been studied. Even in those cases in which the capillary did not plug, <11% of the entering mass was of a respirable size as it emerged from the capillary. In addition, there were a number of conditions under which the capillary plugged. (3) Experiments on the behavior of high temperature, high concentration aerosols have shown a rapid depletion of the aerosol concentration in the first 6 sec following injection of 800 g/m³ aerosols at 1000°C into a closed vessel. This depletion has been correlated with the early formation of 100 to 200 μm agglomerates which fall out promptly.     

钢制安全壳窄缝内气溶胶冷凝滞留实验研究

目前核电厂安全壳放射性评估中未考虑狭窄裂缝（简称窄缝）对气溶胶的滞留效果,但与常规尺寸相比,窄缝的高表面/体积比对气溶胶泄漏具有可观的滞留,评估结果过于保守。通过开展矩形直通道内气溶胶泄漏实验,获得缝高约100 μm钢制安全壳窄缝内气溶胶滞留效率,观察到窄缝通道入口区域为主要的粒子沉积区域。同时,通过在窄缝流动方向上建立并维持一定的温度梯度,模拟安全壳非能动冷却系统投运时安全壳窄缝内气溶胶泄漏过程。结果表明,窄缝对亚微米粒径气溶胶具有良好的滞留效果,温度梯度引入的蒸汽冷凝能显著提高气溶胶滞留效率至91%左右,且缩小了泄漏面积。      

ARTIST: introduction and first results

Aerosol Trapping In a Steam Generator (ARTIST) is a seven-phase international project (2003–2007) which investigates aerosol and droplet retention in a model steam generator under dry, wet and accident management conditions, respectively. The test section is comprised of a scaled steam generator tube bundle consisting of 270 tubes and three stages, one 1:1 separator unit, and one 1:1 dryer unit.As a prelude to the ARTIST project, four tests are conducted in the ARTIST bundle within the 5th EU FWP SGTR. These first tests address aerosol deposition phenomena on two different scales: near the tube break, where the gas velocities are sonic, and far away from the break, where the flow velocities are three orders of magnitude lower. With a dry bundle and the full flow representing the break stage conditions, there is strong evidence that the TiO₂ aerosols used (AMMD 2–4 μm, 32 nm primary particles) disintegrate into much smaller particles because of the sonic conditions at the break, hence promoting particle escape from the secondary and lowering the overall DF, which is found to be between 2.5 and 3. With a dry bundle and a small flow reproducing the far-field velocities, the overall bundle DF is of the order of 5, implying a DF of about 1.9 per stage.Extrapolating the results of the dry tests, it turns out that for steam generators with nine or more stages, it is expected that substantial DF’s could be achieved when the break is located near the tube sheet region. In addition, better decontamination is expected using more representative proxies of severe accident aerosols (sticky, multi-component particles), a topic which is yet to be investigated.When the bundle is flooded, the DF is between 45 and 5740, depending on the mass flow rate, the steam content, and the water submergence. The presence of steam in the carrier gas and subsequent condensation inside the broken tube causes aerosol deposition and blockages near the break, leading to an increase in the primary pressure. This has implications for real plant conditions, as aerosol deposits inside the broken tube will cause more flow to be diverted to the intact tubes, with a corresponding reduction in the source term to the secondary.     

Visualized measurement of extremely high-speed droplets in Venturi scrubber

Venturi scrubbers for filtered venting have been installed in nuclear power plants worldwide. Venturi scrubbers can eliminate fission products from a polluted gas by interaction through gas–liquid interfaces. Therefore, droplet diameter is important from the viewpoint of decontamination. When Venturi scrubbers are used in severe accidents, the gas flow velocity might be extremely high. In these studies, the authors did not measure droplet diameter under extremely high gas velocity conditions. In the scenarios, experimental data pertaining to droplet diameter, under the extremely high gas flow velocity, are required. Therefore, this objective is to evaluate the diameter of extremely high-speed droplets. A visualization experiment was conducted using a Venturi scrubber. The droplet diameter distribution and the Sauter mean diameter (SMD) were determined. By comparing between the experimental value of SMDs and the one evaluated using Nukiyama–Tanasawa equation, it was confirmed that the Nukiyama–Tanasawa equation can be used to evaluate SMD with good accuracy in the gas velocity range of 82–250 m/s, except the highest gas velocity conditions. Furthermore, the droplet generation mechanism in the Venturi scrubber was considered to clarify the main reason why the Nukiyama–Tanasawa equation can be used to evaluate SMD droplet diameter.     

Pressure release of containments during severe accidents in Switzerland

As required by the Swiss Federal Nuclear Safety Inspectorate (HSK) all Switzerland's five nuclear power plants have to install a containment filtered venting system. The integrity of the containment (the last barrier for radioactive releases to the environment) can be threatened by overpressure due to inadequate heat removal. Design requirements have been provided for a specific class of severe accident scenarios. In general the capacity of the system is considered sufficient if it is able to vent the steam production corresponding to a decay heat level of 1% of the thermal reactor power. The mitigation capacity for the reduction of released radioactive material is specified by a retention factor of 1000 for aerosols to prevent or limit a long term ground contamination and a factor of 100 for elementary iodine for prevention or limiting of thyroid doses and to avoid short term evacuation. Besides existing requirements for design, maintenance and operation, additional claims such as passivity and operability at any pressure conditions inside the containment have to be met. Passivity implies that the system can be initiated after a severe accident without any operator action. The system also has to allow early manual venting. Various filtered venting systems are presently available. The nuclear power plants of Beznau, Gosgen, Leibstadt and Muhleberg have already selected such systems and already implemented them or are going to install them step by step. Beznau selected the Sulzer-EWI system which is using a water pool with nozzles-baffle plates and mixing elements to achieve the required filtration of the aerosols. In both Beznau units, the systems are installed and in standby mode. Gosgen, a pressurized water reactor as well as Beznau, is going to implement a filter system developed by Siemens-KWU, known as sliding pressure venting process, combining a venturi scrubber in a water pool and a mesh filter. The boiling water reactor of Leibstadt also selected the same system as Beznau while Müheberg choose the ABB system but not in the common design. The venturi pipes are thereby integrated in the water pool of the outer torus. The system in all five nuclear power plants is fully operable and in standby mode since December 1993.     

Analysis of radionuclide retention in water pools

An analysis is presented of the removal of aerosol particles and gaseous fission products from steam-noncondensable gas mixtures rising through water pools. The pool is divided into a gas injection zone, a bubble rise zone and a pool surface zone. The formulation of the governing conservation equations is relatively general with a quasi-steady one-dimensional formulation for the gas phase, and an unsteady, well stirred, formulation for the liquid phase. An associated computer code for performing the calculations, SUPRA, is described. Results of parametric calculations are given for conditions expected in a BWR severely degraded core accident sequence. Parameters studied include aerosol particle size and distribution, mass fraction of noncondensable gas, gas mass flow rate, quencher submergence depth, and pool water temperature.     

Aerosol retention in the flooded steam generator bundle during SGTR

A steam generator tube rupture in a pressurized water reactor may cause accidental release of radioactive particles into the environment. Its specific significance is in its potential to bypass the containment thereby providing a direct pathway of the radioactivity from the primary circuit to the environment. Under certain severe accident scenarios, the steam generator bundle may be flooded with water. In addition, some severe accident management procedures are designed to minimize the release of radioactivity into the environment by flooding the defective steam generator secondary side with water when the steam generator has dried out.To extend our understanding of the particle retention phenomena in the flooded steam generator bundle, tests were conducted in the ARTIST and ARTIST II programs to determine the effect of different parameters on particle retention. The effects of particle type (spherical or agglomerate), particle size, gas mass flow rate, and the break submergence on particle retention were investigated.Results can be summarized as follows: increasing particle inertia was found to increase retention in the flooded bundle. Particle shape, i.e., agglomerate or spherical structure, did not affect retention significantly. Even with a very low submergence, 0.3 m above the tube break, significant aerosol retention took place underlining the importance of the jet-bundle interactions close to the tube break. Droplets were entrained from the water surface with high gas flow rates carrying aerosol particles with them. However, compared to particle retention in the water close to the tube break, the effect of droplet entrainment on particle transport was small.     

Evolution of the structure of a gas–liquid two-phase flow in a large vertical pipe

The evolution of the structure of a gas–liquid flow in a large vertical pipe of 195 mm inner diameter was investigated at the TOPFLOW test facility in Rossendorf. Wire-mesh sensors were used to measure sequences of two-dimensional distributions of local instantaneous gas fraction within the complete pipe cross-section. The sensors own a resolution of 3 mm at a frequency of 2500 Hz. Superficial velocities were varied in a range covering flow regimes from bubbly to churn-turbulent flow. The distance between the gas injection and the sensor position was changed using a so-called variable gas injection system. It consists of six gas injection units, each equipped with three rings of injection orifices in the pipe wall (orifice diameter: 1 and 4 mm), which are fed from ring chambers. The gas flow towards these distributor chambers is individually controlled by valves. Measured bubble-size resolved radial gas fraction profiles reveal differences in the lateral migration of bubbles of different size starting from the injection at the wall. The evolution of bubble-size distributions allows to study bubble coalescence and break-up. The influence of the physical properties of the fluid was studied by comparing cold air–water experiments with steam–water tests at 65 bar.     

基于正交试验的核主泵导叶水力性能数值优化

为提高核主泵整机水力性能,实现叶轮、导叶与环形压水室的最优匹配,以AP1000核主泵为研究对象,保持叶轮与蜗壳几何参数不变,选择导叶进口冲角、导叶包角和导叶出口角为正交试验方法的3个因素,并根据各因素的值确定取值范围。基于雷诺时均N-S方程、RNGk-ε湍流模型和SIMPLEC算法,应用CFD技术对核主泵进行了正交试验和数值优化。正交试验和因素显著性分析表明:额定工况下,优化后的模型泵较原模型泵扬程提高0.55m、效率提高0.66%;小流量工况下,优化后的杨程和效率提升更加明显;导叶包角和导叶出口角对泵水力性能的影响较为显著,导叶流道扩散程度决定了导叶流道的水力损失;导叶进口冲角、导叶出口角和导叶包角之间的相互作用对泵水力性能的影响不显著,可忽略。对导叶包角的研究表明,在小流量工况下,导叶包角与泵的效率呈正比,在大流量工况下,导叶包角与泵的效率呈反比。     

Containment analysis on the PHEBUS FPT-0, FPT-1 and FPT-2 experiments

In a severe accident, most of the fission-product species are already condensed in aerosols when they are released to the containment. The behaviour of these aerosol particles controls the fission-product transport into the containment and affects the global Source Term. The calculations presented here were performed using the CPA module (Containment Package implemented in the European integral code ASTEC) for the in-pile PHEBUS FPT-0, FPT-1 and FPT-2 experiments and are focused on the aerosol transport. A detailed thermal-hydraulic model was used in the CPA/ASTEC code to evaluate the gas circulation pattern in the closed containment volume. The comparison of ASTEC results showed that the patterns are similar to the ones predicted by the CFD-based codes. Good agreement was reached with the measured average thermo-hydraulic parameters such as containment gas pressure, temperature and the condensation rate on the condensers. The calculations with the detailed simulation of the flow in the PHEBUS containment qualitatively predicted the particle settling on the elliptic bottom and deposition on the painted wet condenser surfaces. It was shown that the influence of the gas circulation leads to a relatively quick mixing of aerosols in the containment atmosphere. In the tests investigated, the effect of the gas circulation on the airborne aerosol mass during the aerosol injection period is small because the injected mass flux is significantly higher compared to the deposition fluxes on the vessel surfaces. During the long-term aerosol deposition phase, the flow fields predicted by CPA/ASTEC have a medium impact on the evolution of the airborne mass in the PHEBUS containment.     

氡子体α能谱真空测量RaB的反冲损失率

氡子体α能谱法测量时的真空度使RaB发生反冲损失,这会对氡子体测量精度产生影响并造成测量腔室的污染。本实验选用0.8 μm孔径的Millipore AA型滤膜,在流率为6.3、12.3和15.6 L/min下,使用ELPI粒径分析仪、α测量仪和α谱仪分别对氡室气溶胶粒径分布、滤膜自吸收系数及反冲损失率进行测量。结果显示：氡室内气溶胶浓度呈单峰分布;相应流率下的自吸收系数为0.978、0.980、0.989,反冲损失率为0.362 8、0.368 4和0.362 9;所得反冲损失率并未表现出较大的差异,但滤膜样品上由RaA衰变产生的RaB原子却有较大的反冲损失。     

Description of Aerosol Release Associated with Sodium Burning

Incipient temperatures of sodium oxidation, sodium oxide aerosol release and sodium ignition were observed in relation with oxygen concentration in supplying gas by a thermobalance method. On this results, a burning rate and an aerosol release fraction were determined by burning about 0.5 g of sodium samples on a crucible in a burn tube under conditions ranging 400 to 600°C in sodium temperature, 0.1 to 1.0 Nl/min in the supplying gas flow rate and 2.5 to 20v/0 in the oxygen concentration.

Particle sizes of sodium aerosols released from the samples made a log-normal distribution, of which mass median diameter ranged from 0.6 to 2.0 μm. The burning rate increased with the increases of the sodium temperature, of the gas flow rate and of the oxygen concentration, and the aerosol release fraction also increased in proportion to the burning rate at fixed oxygen concentrations. It was found that the aerosol release rate defined as a product of the burning rate and the aerosol release fraction describes fairly well not only the present experimental data but also reported ones.