Experimental and analytical studies of pipe whip tests under PWR LOCA conditions

A series of pipe rupture tests has been performed at the Japan Atomic Energy Research Institute (JAERI) to demonstrate the safety of primary coolant circuits in the event of pipe rupture in nuclear power plants. Pipe whip tests and jet discharge tests have been conducted under boiling water reactor (BWR) and pressurized water reactor (PWR) loss-of-coolant accident (LOCA) conditions. The present paper describes the experimental and analytical results of the pipe whip tests performed under PWR LOCA conditions using 4, 6 and 8-inch test pipes. The tests were carried out at an initial pressure and temperature of 15.7 MPa and 325°C, respectively. Moreover, a dynamic analysis of pipe whip tests was carried out using the general purpose finite element program ADINA.     

Reinforced concrete membrane elements subjected to reversed cyclic in-plane shear stress

A macroscopic model has become noticeable instead of a microscopic model by FEM analysis for concrete structures. Among them the model proposed by Vecchio and Collins is considered to be one of the powerful tools because of its rationalility and simplicity. The essential point of this model is a modified compression theory of cracked concrete and the stress-strain relationship for cracked concrete.This paper follows basically Collins' theory but is revised so as to be able to predict the response of reinforced concrete elements subjected to alternate reversed cyclic in-plane shear simulating earthquake forces. The overall characteristics of the new model exist in the tensile residual strains accumulated by cycles in cracked concrete, which is different from the monotonic loading.     

Experimental studies of pipe impact on rigid restraints and concrete slabs

Experiments with a three-inch-diameter pressurized pipe whipping against a rigid restraint or a concrete slab are described. Eleven experiments have pipes with various geometries impacting a stiff structure either at an elbow or on a straight segment. Pipe motion, impact force, impact time, and pipe crush are measured. Two experiments have pipe elbows impacting a six-inch-thick and a three-inch-thick reinforced concrete slab. The thicker slab experiences little impact damage, whereas the thinner slab is perforated by the elbow. Pipe displacement, rebar strains, and forces on slab supports are measured. Experimental results are reported and compared with the results of two code calculations.     

Shear transfer constitutive model for pre-cracked RC plate subjected to combined axial and shear stress

This paper proposes the constitutive model for the shear transfer through the cracks of pre-cracked reinforced concrete (RC) plate subjected to combined axial and shear stress. The plate is a scale model of a shear wall of a nuclear power plant (NPP) building. Twelve plate specimens were initially cracked and then loaded to the failure point by increasing cyclic shear under constant axial stress. Tangential shear modulus, G_cr, values are estimated from the v–γ relationships observed in the test results and formulated to the constitutive model as the correlation function of the normal strain perpendicular to the crack plane, _cr, and shear strain, γ_cr, based on the smeared crack model concept. By incorporating the proposed model to a nonlinear FEM analysis program and comparing the analysis results with the test results, it is apparent that the program could be improved in its analytical accuracy. The proposed model will be useful for the nonlinear analysis of RC shear walls when the walls are exposed to simultaneous multi-directional load.     

On the multiaxial behaviour of concrete exposed to high temperature

An attempt is made to formulate a multiaxial constitutive model for concrete in the temperature range up to 800°C. The proposed model can be characterized as isotropic, elastic-viscoplastic-plastic in the compression region. Brittle failure is assumed in the tensile region.The thermal strain increment is assumed to be a function of both temperature and the current stress tensor. This assumption implies that the thermal strain may have deviatoric components.The volumetric thermal strain is used as a scalar damage measure instead of temperature itself. The corresponding softening function is obtained from isothermal, uniaxial tests. Also the elastic properties are taken as functions of the volumetric thermal strain.The response of the model is illustrated and compared with experimental results.     

Loading capacities and failure modes of various reinforced-concrete slabs subjected to high speed loading

The objective of this study was to clarify experimentally and analytically the loading capacities, deformations and failure modes of various types of reinforced-concrete structure subjected to loads applied at various rates. Flat slabs, slabs with beams and cylindrical walls were tested under static, low speed and high speed loading. Analysis was applied to estimate the test results by the finite element method using a layered shell element. The analysis closely simulated the experimental results until punching shear failure occured.     

Experimental and analytical studies of melt jet-coolant interactions: a synthesis

Instability and fragmentation of a core melt jet in water have been actively studied during the past 10 years. Several models, and a few computer codes, have been developed. However, there are, still, large uncertainties, both, in interpreting experimental results and in predicting reactor-scale processes. Steam explosion and debris coolability, as reactor safety issues, are related to the jet fragmentation process. A better understanding of the physics of jet instability and fragmentation is crucial for assessments of fuel-coolant interactions (FCIs). This paper presents research, conducted at the Division of Nuclear Power Safety, Royal Institute of Technology (RIT/NPS), Stockholm, concerning molten jet-coolant interactions, as a precursor for premixing. First, observations were obtained from scoping experiments with simulant fluids. Second, the linear perturbation method was extended and applied to analyze the interfacial-instability characteristics. Third, two innovative approaches to computational fluid dynamics (CFD) modeling of jet fragmentation were developed and employed for analysis. The focus of the studies was placed on (a) identifying potential factors, which may affect the jet instability, (b) determining the scaling laws, and (c) predicting the jet behavior for severe accident conditions. In particular, the effects of melt physical properties, and the thermal hydraulics of the mixing zone, on jet fragmentation were investigated. Finally, with the insights gained from a synthesis of the experimental results and analysis results, a new phenomenological concept, named ‘macrointeractions concept of jet fragmentation’ is proposed.     

Experimental studies of PWR primary piping under loca conditions

The experimental program performed on AQUITAINE-II facility is directed to study the mechanical behavior of primary PWR pipes and the forces exerted on the neighbouring structures as a consequence of a breach opening. It is jointly financed by the Commissariat à l'Energie Atomique, Framatome, Electricité de France and Westinghouse. Some forty tests have been carried out with different pipe configurations (straight tube, elbow, S-or U-shaped tube) and different break types (single or double guillotine). The following aspects are investigated: (1) the dynamic behavior of the pipe and in particular the formation of a plastic hinge at the restraint; (2) impact function of a pipe on an energy-absorbing bumper; (3) lateral stability of both ends of a pipe, after a double-guillotine break.     

Dynamic behaviour of thin cylindrical shells subjected to transient inner pressures

Stress analysis is carried out for horizontal thin cylindrical shells subjected to transient inner pressures resulting from the closure of a terminal valve. The velocity of water at a valve is adjusted to become υ₀e^−bt. The relationships between the dynamic hoop stresses acting on the neutral axis of the wall of a cylinder, the hydraulic pressures, the dimensions of a cylinder and time are obtained. The results show that the maximum dynamic hoop stresses are greatly affected by the time necessary for a valve to be closed. The problem is solved by the Laplace transformation method.     

Some analytical and numerical results for cladded fuel rods subjected to thermo-irradiation induced creep

Stresses and velocities are analyzed for a hollow cylinder of fuel encased in metallic cladding and subjected to high temperature and high neutron flux fields. The material is represented by a compressible nonlinear thermo-irradiation viscoelastic model. A stress function for axisymmetric plane strain is introduced, and the problem essentially reduces to solving a nonlinear ordinary differential equation in the interfacial contact pressure. Some analytical results are obtained for the case of material properties independent of position. For the case of temperature, flux and thus material properties dependent on position, an approximate formulation is employed whereby the cylinder is divided into discrete rings with constant properties.     

Study on steel plate reinforced concrete panels subjected to cyclic in-plane shear

This paper describes the derivation of the equation for evaluating the strength of steel plate reinforced concrete structure (SC) and the experimental results of SC panels subjected to in-plane shear.Two experimental research programs were carried out. One was the experimental study in which the influence of the axial force and the partitioning web were investigated, another was that in which the influence of the opening was investigated.In the former program, nine specimens were loaded in cyclic in-plane shear. The test parameters were the thickness of the surface steel plate, the effects of the partitioning web and the axial force. The experimental results were compared with the calculated results, and good agreement between the calculated results and the experimental results was shown.In the later programs, six specimens having an opening were loaded in cyclic in-plane shear, and were compared with the results of the specimen without opening. FEM analysis was used to supplement experimental data. Finally, we proposed the equation to calculate the reduction ratio from the opening for design.     

Design considerations for concrete nuclear containment structures subjected to simultaneous pressure and seismic shear

Improvements in design code provisions for tangential shear in secondary concrete nuclear containment vessels are needed. This paper presents a brief summary of an experimental research program conducted at Cornell University on tangential shear. Six inch thick reinforced concrete panels were subjected to combined in-plane tension and shear as a behavioral model of a section of the wall of the containment under the combined loading of internal pressurization and seismic shear. Approximately 50 panels were tested. Parameters studied included: tension level and direction (biaxial or uniaxial), shear level and type (monotonic, cyclic, or a combined mode), sequence of applied loading, and reinforcing ratio and orientation.The results of the research indicate that current code provisions are overly conservative with regard to the amount of tangential shear to be carried by the orthogonally reinforced concrete. By increasing the allowable stress, the required amount of diagonal reinforcing would be reduced. This would result in savings in fabrication costs and construction time, and improved structural reliability through improved concrete placement. The research also indicates a need for a more exact consideration of containment displacements. Shear stiffnesses for the panels were extremely low, indicating that containment displacements may be larger than anticipated. The code provisions in this area are limited and unsubstantiated.     

Temperature decay in a reactor vessel subjected to thermal shock: An analytical solution

An analytical solution is derived for the time-dependent temperature profile in a reactor vessel subjected to the thermal shock of emergency cooling injection. Numerical results are given for a typical case, and it is shown how a simple correction can be used to improve the results of conventional calculational methods.     

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

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.     

Experimental studies of 4-inch pipe whip test under BWR loca conditions

Pipe whip tests or jet discharge tests have been performed at the Japan Atomic Energy Research Institute, which simulate the instantaneous circumferential guillotine break of primary coolant piping in nuclear power plants. The present paper describes the results of the pipe whip tests using test pipes of 4 inch diameter, under the BWR LOCA conditions, which were performed from 1979 to 1981. The tests were carried out at an initial pressure of about 6.8 MPa and an initial temperature of about 285°C.The test pipe was 114.3 mm (4 in) in diameter, 8.6 mm in thickness and 4500 mm in length. The four pipe whip restraints used in the tests were the U-bar type of 8 mm in diameter and fabricated from Type 304 stainless steel. The experimental parameters were the clearance (30, 50 and 100 mm) and the overhang length (250, 400, 550 and 1000 mm).The main purpose of these tests is to investigate the effects of the clearance and the overhang length on the pipe whip behavior. It has been clarified from the test results that a smaller clearance and a shorter overhang length causes the deformation of the pipe and restraints to be minimized, and the test pipe collapses near the setting point of the restraints with the overhang length of 1000 mm.     

Dynamic behaviour of thin cylindrical shells subjected to impulsive bending moments along their tips

The dynamic behaviour of thin cylindrical shells with concentrated masses at their tips is investigated when they are subjected to impulsive bending moments at their tips. The relationships between the dimensions of a cylinder, concentrated mass and maximum values of dynamic stresses are obtained. The fundamental equation of motion is solved by the Laplace transformation method. From the results of the theoretical analysis, it became evident that impulsive stresses become much larger than static ones. For the case where bending moments are applied statically, the fundamental equations are also derived, taking into account the effect of shearing force.     

Theoretical predictions of failure probabilities for PWR pressure vessels subjected to accident conditions

The Second Marshall Report (1982) presented a detailed analysis of the integrity of PWR pressure vessels. As part of that study theoretical calculations of failure probabilities were made. Since the publication of that Report modifications have been made to the theoretical model to extend the failure criterion into stable crack extension, to update the knowledge of the distribution of various parameters, to more accurately represent the stress intensities and crack shapes, and to consider a different representation of pre-service detection of defects.In this paper these modifications are summarised and the application of the model to the calculation of failure frequencies for the most severe accident conditions, the large loss-of-coolant accident and the steam break, is presented. The results indicate that the probability of vessel failure is one to two orders of magnitude lower than previously predicted.     

The prediction of failure strain for Zircaloy-4 subjected to non-stationary loading conditions

A modified Monkman-Grant relationship (MMG) is applied to predict — as far as the life time is known — the failure strain of Zry-4 subjected to tensile rupture test at load as well as temperature ramp conditions, respectively. As the analysis has shown in the first case, a simple relationship exists between the minimum creep rate and the stress rate. Thus, this quantity appearing in the MMG is phenomenologically connected with the test conditions. For failure strain predictions in temperature ramp tests the introduction of an effective temperature has shown to be advantageous. As compared to the peculiarities of the problem, the agreement between experiments and calculations is encouraging.     

DEMT experimental and analytical studies on seismic isolation

This paper reviews the experimental and theoretical studies performed at CEA/DEMT related to the overall behavior of isolated structures. The experimental work consists of the seismic shaking-table tests of a concrete cylinder isolated by neoprene sliding pads, and the vibrational tests on the reaction mass of the TAMARIS seismic facility. The analytical work consists of the development of procedures for dynamic calculation methods: for soil—structure interaction where pads are placed between an upper raft and pedestals, for time-history calculations where sliding plates are used, and for fluid—structure interaction where coupled fluid and structure motions and sloshing modes are important.Finally, this paper comments on the consequences of seismic isolation for the analysis of fast-breeder reactor (FBR) vessels. The modes can no longer be considered independent (SRSS Method leads to important errors), and the sloshing increases.