Thermal and mechanical characteristics of an instrumented capsule for a material irradiation test

In this study, the thermal and mechanical characteristics are analyzed for the structural integrity evaluation of the instrumented capsule used for the irradiation test of reactor vessel materials in the research reactor, hi-flux advanced neutron application reactor (HANARO). The temperature of test specimens inserted in the capsule mainbody by γ-flux is calculated using a heat transfer code, HEATING 7.2f. The maximum temperature is 556.75 K at the center of the capsule mainbody, thus the temperature satisfies the user's requirement. To estimate the mechanical characteristics of the capsule due to the pressure and thermal loading, stress analysis is carried out with a finite element analysis program, ANSYS. The strength of the capsule's external tube is also evaluated by considering the buckling stress of the capsule mainbody under coolant pressure loading. The results of the analysis show that the temperature distributions are significantly affected by the gap size between the holder and the specimen. The calculated stresses of the capsule structure are well within the allowable stress values of the ASME code. It is expected that the results presented in this paper will be useful in the design and safety evaluation of instrumented capsules for material irradiation tests.     

承压热冲击下DVI接管动态应力特性研究

压力容器直接注入（DVI）接管在热冲击下的动态应力特性对于反应堆压力容器（RPV）结构完整性评估具有重要意义。建立了含DVI接管的RPV压力壳热流固耦合数值计算模型,并进行了验证分析;然后研究了蓄压安注箱（ACC）和堆芯补水箱（CMT）安注时RPV筒体和DVI接管热工水力特性;最后分析了热冲击下RPV筒体和DVI接管连接高应力区的温度分布、等效应力和等效塑性应变分布特性。研究结果表明,ACC安注阶段RPV筒体和DVI接管连接区存在较大的温度梯度和等效应力,且发生了局部塑性变形。若发生承压热冲击事件,应控制好DVI接管连接区温差,确保反应堆压力容器的结构完整性。本文开发的热冲击下热流固耦合数值计算模型和计算方法可用于核岛内DVI接管与RPV筒体的安全性评价,也可用于类似承压结构在热冲击下的动态应力特性分析。      

Effect of specimen sizes, specimen shapes, and placement directions on compressive strength of concrete

The compressive strength of concrete is used as the most basic and important material property when reinforced concrete structures are designed. It has become a problem to use this value, however, because the control specimen sizes and shapes may be different from country to country.In this study, the effect of specimen sizes, specimen shapes, and placement directions on compressive strength of concrete specimens was experimentally investigated based on fracture mechanics. Experiments for the Mode I failure were carried out by using cylinder, cube, and prism specimens. The test results are curve-fitted using least square method (LSM) to obtain the new parameters for the modified size effect law (MSEL). The analysis results show that the effect of specimen sizes, specimen shapes, and placement directions on ultimate strength is present. In addition, correlations between compressive strengths with size, shape, and placement direction of the specimen are investigated.     

Instrumented capsule for in-reactor fuel creep studies at high fission rates

An instrumented capsule for in-reactor creep studies of ceramic nuclear fuels at high fission rates has been designed, fabricated and succesfully operated. The capsule contains a device for loading the creep specimen in compression from 0 to 13,000 psi. The temperature is adjusted by means of both internal electrical heaters and a gas gap. The deformation of the specimen during creep is continuously measured by means of a constant gas flow, variable orifice device.Creep studies have been conducted in the Battelle Research Reactor on UO₂-22 w/o PuO₂ in the temperature range of 350 to 600°C, the stress range of 4000 to 8000 psi and the fission rate of 3.8 × 10¹³ fissions/cm³ sec. The creep rate under these conditions was found to be substantially increased as a result of the fission process.     

CARR铱源试验靶件设计和试验验证

为配合中国先进研究堆(CARR)铱源辐照生产项目,设计制造了铱源试验靶件,对试验靶件的设计参数、结构尺寸进行了介绍。在堆外使用专门的传热装置模拟铱源靶件的外部和内部传热工况,测量了用于模拟辐照罐壁面温度和样品温度的传热装置的壁面温度和内部温度,结果验证了热工分析方法是合适的。入堆试验靶件由含有铱片样品的辐照罐和等量发热的模拟罐组成。堆内试验获得的数据综合验证了试验靶件物理热工的分析结果,这个结果可对CARR铱源辐照生产安全评审提供依据且偏于安全。     

First spinning cylinder test analysis using a local approach to fracture

In recent years, several experimental programmes on large-scale specimens have been organized to evaluate the capabilities of the fracture mechanics concepts employed in structural integrity assessment of pressurized water reactor pressure vessels. During the first spinning cylinder test, a geometry effect was revealed experimentally showing the difficulties of transferring toughness data from small-scale to large-scale specimens. An original analysis of this test, by means of a local approach to fracture, is presented in this paper. Both compact tension specimen and spinning cylinder fracture behaviour were computed using a continuum damage mechanics model developed at EDF. We confirmed by numerical analysis that the cylinder's resistance to ductile tearing was considerably larger than in small-scale fracture mechanics specimen tests, about 50%. The final crack growth predicted by the model was close to the experimental value. Discrepancies in J R curves seemed to be due to an effect of stress triaxiality and plastic zone evolution. The geometry effect inducing differences in resistance to ductile tearing of the material involved in the specimens can be investigated and explained using a local approach to fracture methodology.     

A Method of Temperature Control in Capsule Irradiation Vacuum Control Method, (1)

A method is presented for controlling the temperature of specimens during capsule irradiation. A narrow gas gap layer is provided in the capsule between the outer and inner cans to serve as thermal resistance barrier. The value of thermal resistance is regulated by varying the vacuum pressure in the gas gap from a control device placed outside the reactor.

The present new method has been based on data obtained from preliminary experiments to determine the relation between pressure and thermal resistance in the gas gap, and an original arrangement for controlling the gap pressure from outside the reactor through a long thin tube. Automatic temperature control has been realized through feed-back regulation governing the gap pressure.

A discussion is given of theoretical analyses made on the characteristics, the effectiveness and the limits of this temperature control mechanism, backed by experimental data from mock-up and in-pile experiments, demonstrating the practical utility of the method.

This new method of temperature control can be expected to find wide application also in other fields.     

Fracture mechanics investigations on cylindrical large scale specimens under thermal shock loading

To investigate the crack growth and crack arrest behaviour of primary circuit materials large scale experiments were conducted on component-like specimens under pressurized thermal shock loading at MPA Stuttgart. The material characteristics varied from high tough material to low tough material with higher nil ductility transition temperature to simulate EOL or beyond EOL-state. All tests started from in-service conditions and were cooled down to room temperature. The specimens showed both stable and unstable crack growth and partly crack arrest. The crack growth behaviour was verified by post test calculations and could be explained with the help of the multiaxiality of the stress state.     

中国实验快堆结构材料辐照装置设计

为在中国实验快堆(CEFR)上开展国产快堆包壳材料的辐照试验,进行了CEFR首个结构材料辐照装置的设计。材料辐照装置的创新设计基于CEFR的辐照条件和堆芯组件的基本结构,通过在辐照装置内部设置不同气隙尺寸的辐照罐,实现了在快堆不同功率稳态运行条件下(40%和100%额定功率)对材料样品不同辐照温度(450~600℃)的要求。辐照装置具有样品辐照温度与中子注量率的非在线监测功能,其结构具有通用性,能满足材料辐照标准试样最大装载的需要。通过对辐照装置进行热工分析和堆外的传热验证试验、流阻特性和结构稳定性验证试验,保证了辐照装置的设计能满足材料辐照任务的要求。     

Thermo-hydraulic and structural analysis for finger-based concept of ITER blanket first wall

The blanket first wall is one of the main plasma facing components in ITER tokamak. The finger-typed first wall was proposed through the current design progress by ITER organization. In this concept, each first wall module is composed of a beam and twenty fingers. The main function of the first wall is to remove efficiently the high heat flux loading from the fusion plasma during its operation. Therefore, the thermal and structural performance should be investigated for the proposed finger-based design concept of first wall. The various case studies were performed for a unit finger model considering different loading conditions. The finite element model was made for a half of a module using symmetric boundary conditions to reduce the computational effort. The thermo-hydraulic analysis was performed to obtain the pressure drop and temperature profiles. Then the structural analysis was carried out using the maximum temperature distribution obtained in thermo-hydraulic analysis. Finally, the transient thermo-hydraulic analysis was performed for the generic first wall module to obtain the temperature evolution history considering cyclic heat flux loading with nuclear heating. After that, the thermo-mechanical analysis was performed at the time step when the maximum temperature gradient was occurred. Also, the stress analysis was performed for the component with a finger and a beam to check the residual stress of the component after thermal shrinkage assembly.     

Iodine Stress Corrosion Cracking of Unirradiated Zircaloy-2 Tubing by Means of Ring Tension Test

Ring tension testing methods were applied for studying the iodine stress corrosion cracking (SCC) behavior of unirradiated cold-worked (CW) and recrystallized (RX) Zircaloy-2 tubing at 300°C. Two kinds of tests—constant load and constant cross-head speed—were conducted on the grooved ring tension specimen simulating a plane-strain stress state under uniaxial loading, as well as on the simple ring tension specimen. The results showed that SCC failure threshold stresses were approximately 450 and 300 MPa, respectively, for the grooved CW and RX specimens. The ratio between the threshold stress of the grooved specimen and the yield stress derived from the simple specimen is equal to 1.4 for both CW and RX materials. The threshold value of the grooved RX specimen is 1.3 times larger than found on the same material of simple shape, which corresponds to the yield stress ratio between the grooved and simple specimens. Inside surface observations of specimens stressed in iodine environment suggested that corrosion lines, presumably corresponding to corrosion pits, and chemical attacks on grain boundaries would be incipient phases of SCC for CW and RX materials, respectively. They may appear at stresses lower than those for SCC failure.     

NESC spinning cylinder experiment: pre-test structural analysis evaluation

This spinning cylinder experiment, organised under the auspices of the Network for the Evaluation of Steel Components (NESC), is designed to address the cleavage initiation behaviour of axial subclad and surface breaking flaws in end of life RPV material under pressurised thermal shock (PTS) transient conditions. Pre-test structural integrity assessments have been performed to assist in the detailed design of the experiment and in particular the specification of the initial defect sizes. The results presented offer an insight into the influences of experimental variables on the probability of realising a cleavage event during the transient.     

A three-dimensional method for integrated transient analysis of reactor-piping systems

A three-dimensional method for integrated hydrodynamic, structural, and thermal analyses of reactor-piping systems is presented. The hydrodynamics are analyzed in a reference frame fixed to the piping and are treated with a two-dimensional Eulerian finite-difference technique. The structural responses are calculated with a three-dimensional co-rotational finite-element methodology. Interaction between fluid and structure is accounted for by iteratively enforcing the interface boundary conditions.A thermal transient capability has been developed. A system energy equation is used to compute the coolant temperatures due to convection. A radial heat-conduction equation is employed to establish the temperature profile throughout the pipe-wall thickness. The constitutive equation used for the thermal-mechanical stress calculation is suited for a large number of materials under various loading conditions, such as those having thermal, plastic, and viscous effects. The flow surface, which defined the purely elastic regime, can be arbitrarily small; an associated flow rule is utilized for regimes of material plasticity.Three sample problems are presented to illustrate this method. The first one calculates the piping response under the seismic excitation. The second one validates the heat-conduction model. The third problem deals with a coupled hydrodynamic-structural-thermal analysis of a piping system. Results are discussed in detail.     

Effect of biaxial loading on the fracture behaviour of a ferritic steel component

The effect of biaxial loading on the ductile behaviour of a through-wall crack in a ferritic steel structure under contained yield is of particular interest to the structural integrity argument for reactor pressure vessels. This results from the fact that there are many instances in practice (for example a crack in a circumferential weld), where a significant applied stress is present in the direction parallel to the crack as well as in the perpendicular direction. Two large plate ductile tearing tests have been performed on centre through-crack specimens (75 mm by 2 m by 2 m) manufactured from a ferritic steel. The first test specimen was loaded in uniaxial tension and the second test specimen was loaded biaxially. This paper presents experimental details and results of the two plate tests and describes the analysis work undertaken to interpret the experiments satisfactorily.     

Fracture assessment of shallow-flaw cruciform beams tested under uniaxial and biaxial loading conditions

A technology to determine shallow-flaw fracture toughness of reactor pressure vessel (RPV) steels is being developed for application to the safety assessment of RPVs containing postulated shallow surface flaws. Matrices of cruciform beam tests were developed to investigate and quantify the effects of temperature, biaxial loading, and specimen size on fracture initiation toughness of two-dimensional (constant depth), shallow, surface flaws. The cruciform beam specimens were developed at Oak Ridge National Laboratory (ORNL) to introduce a far-field, out-of-plane biaxial stress component in the test section that approximates the nonlinear stresses resulting from pressurized-thermal-shock or pressure–temperature loading of an RPV. Tests were conducted under biaxial load ratios ranging from uniaxial to equibiaxial. These tests demonstrated that biaxial loading can have a pronounced effect on shallow-flaw fracture toughness in the lower transition temperature region for an RPV material. The cruciform fracture toughness data were used to evaluate fracture methodologies for predicting the observed effects of biaxial loading on shallow-flaw fracture toughness. Initial emphasis was placed on assessment of stress-based methodologies, namely, the J–Q formulation, the Dodds–Anderson toughness scaling model, and the Weibull approach. Applications of these methodologies based on the hydrostatic stress fracture criterion indicated an effect of loading-biaxiality on fracture toughness; the conventional maximum principal stress criterion indicated no effect. A three-parameter Weibull model based on the hydrostatic stress criterion is shown to correlate with the experimentally observed biaxial effect on cleavage fracture toughness by providing a scaling mechanism between uniaxial and biaxial loading states.     

Evaluation of The Thermal Performance of Multi—Element Doped Graphite under Steady—State High Heat Flux

Multi-element doped graphite,GBST1308 has been developed as a plasma facing material(PFM) for high heat flux components of the HT-7U device.The thermal performance of the material under steady-state(SS) high heat flux was evaluated under actively cooling conditions,the specimens were mechanically joined to copper heat sink with supercarbon sheet as a compliant layer between the interfaces.The experiments have been performed in a facility of ACT (actively cooling test stand) with a 100kW electron gun in order to test the suitability and the loading limit of such materials.The surface temperature and bulk temperature distribtuion of the specimens were investigated.The experimental results are very encouraging that when heat flux is not more than 6 MW/m^2,the surface temperature of GBST1308 is less than 1000℃,which is the lowest,compared with IG-430U and even with CX-2002U(CFC),The primary results indicate that the mechanically-joined material system by such a proper design as thin tile.Super compliant layer,GBST as PFM and copper-alloy heat sink,can be used as divertor plater for HT-7U in the first phase.     

Double cracking in brittle cylindrical specimens subjected to quenching

A series of quench tests on hollow cylindrical specimens made of the machinable ceramic, Pyrophyllite, has been performed. The specimens consistently fractured with two diametrically opposite longitudinal cracks. Finite element analysis indicates that the primary crack occurred at the stage when the strain energy of the cylinder reached its maximum during the thermal transient process, while the secondary crack was caused by stress waves generated following the primary crack. This is supported by experimental observation of the time of fracture and the crack morphology. The work reported here helps to shed light on the fracture criterion for brittle cylindrical components under transient thermal and dynamic loadings.     

Advanced flaw production method for in-service inspection qualification mock-ups

One of the key issues in in-service inspection qualification is the representativeness of the defects used in qualification specimens. The best representativeness is achieved with realistic defects. However, present specimen production techniques have some significant weaknesses, such as unrealistic defects or additional alterations induced in the surrounding material. Specimens manufactured, for example, by weld implantation or with weld solidification defects always result in one or more extra weld interfaces. These interfaces can be detected by NDT. To overcome problems with the current specimens, a new defect manufacturing technique was developed. The new technique produces natural, representative defects without introducing additional weld metal or other unwanted alterations to the specimen.The new method enables artificial production of single, separate fatigue cracks by thermal loading. The method is based on a natural thermal fatigue damage mechanism and enables production of real cracks directly into the samples. Cracks are produced without welding or machining and without any preliminary surface treatment or artificial initiator such as a notch or a precrack. Single crack or a network of cracks can be induced into the base material, welded areas, HAZ, weld claddings, threaded areas, T-joints, etc. The location, orientation and size of produced cracks can be accurately controlled. Produced cracks can be used to simulate different types of service-induced cracks such as thermal fatigue, mechanical fatigue and stress corrosion cracks. It is shown that artificially produced thermal fatigue cracks correspond well with the real, service-induced cracks and overcome the problems of traditional qualification specimen manufacturing techniques.     

Temperature evaluation of an instrumented capsule after material irradiation tests in HANARO

An instrumented capsule has been used for an irradiation test of various nuclear materials in the research reactor, HANARO. The capsule is designed to have a standard 4-hole structure for the economical test of an RPV material at 290 ± 10 °C. The temperature of the specimens for the reactor powers, 0-24 MW, is measured by 12 thermocouples, and finite element (FE) analyses are also performed to compare and verify the irradiation test results. As a result of the tests and analyses, the maximum temperature at the reactor power of 24 MW is 256 °C for an irradiation test and 202.6 °C for an FE analysis at Stage 3 of the capsule. Also, for each stage of the capsule, the temperature difference of the specimen in the axial direction is very small to within 10 °C. It is expected that the results presented in this paper will be useful when designing the instrumented capsules for an irradiation test.