Dynamic testing of large AISI-316L steel specimens behaviour using LDTF

The paper presents the results of a series of dynamic experiments on AISI-316L steel specimens having a cross section of 2000 mm² and a gauge length of 350 mm subject to dynamic traction up to rupture. Results for dynamic tests at a nominal strain rate up to 50 s⁻¹ are compared to quasi-static ones, showing an important increase in both yield stress and flow stress. The experimental techniques are described, with special emphasis on full-field optical strain measurements based on Moiré interferometry. A 1-D numerical analysis of the experiments is presented.     

A study of fuel behavior in PWR design basis accident: an analysis of results from the PHEBUS and EDGAR experiments

As part of the French PWR safety study programme, fuel behavior during a design basis accident has been investigated in three parallel directions:

• - separate effect tests in the EDGAR apparatus for developing and validating cladding deformation models,

• - integral tests in PHEBUS for verifying codes,

• - development of fuel behaviour codes for plant calculation after assessment against experimental results. After describing the objectives and content of each of these programmes, the main findings are highlighted and discussed.

Fracture mechanics analysis of CT-specimens and vessels using FEM-analysis and simplified solutions

To secure the safety of a structure, a reliable capability of assessing the behaviour of an assumed initial crack is required.In the framework of a research and development program for the analysis of the fracture behaviour of vessels and pipes detailed FE-analyses as well as engineering approaches for instance such as based on the Fracture-Handbook (EPRI/GE) are performed in the Gesellschaft für Reaktorsicherheit (GRS) mbH.To investigate the stable crack growth, numerical models for calculating the J-line-integral (Rice), the J according to the virtual crack extension method (Parks) and a topological model for the crack growth (de Lorenzi) have been implemented to the nonlinear FE-program ADINA.Calculations have been performed on several experimental setups such as fracture mechanics specimens and burst vessels, among others on the experiments CT-36 and BVS 030 of the Materialprüfungsanstalt der Universität Stuttgart. Comparisons between the analytical and the xperimental results showed satisfactory agreement. The one parameter J-integral-concept, based on the comparison of the applied crack driving energy with the crack extension resistance of the material has proven to be a suitable tool to deal with a small amount of stable crack growth until the beginning of crack instability. Both techniques, the FE-analysis as well as the Fracture-Handbook evaluation, have been successfully applied.     

Three-dimensional interpretation of cleavage fracture tests of cladded specimens with local approach to cleavage fracture

Electricité de France has conducted during these last years an experimental and numerical research programme in order to evaluate fracture mechanics analyses used in nuclear reactor pressure vessels integrity assessment, regarding the risk of brittle fracture. Two cladded specimens made of ferritic steel A508 Cl3 with stainless steel cladding, and containing shallow subclad flaws, have been tested in four point bending at very low temperature to obtain cleavage failure. The crack instability was obtained in base metal by cleavage fracture, without crack arrest. The tests have been interpreted by local approach to cleavage fracture (Beremin model) using three-dimensional finite element computations. After the elastic–plastic computation of stress intensity factor K_J along the crack front, the probability of cleavage failure of each specimen is evaluated using m, σ_u Beremin model parameters identified on the same material. The failure of two specimens is conservatively predicted by both analyses. The elastic–plastic stress intensity factor K_J in base metal is always greater than base metal fracture toughness K_1c. The calculated probabilities of cleavage failure are in agreement with experimental results. The sensitivity of Beremin model to numerical aspects is finally exposed.     

The new code “RCC-MR” — Rules for design and construction of LMFBR components

In 1978, Commissariat à l'Energie Atomique, Electricité de France, and Novatome decided to undertake a common effort to gather a complete collection of rules to apply for design of LMFBR components. The first issue of this work is now being published by AFCEN as the “RCCM” code. The preparation of the design rules used largely the experience gained in Superphenix components analysis, and the results of the large R&D program performed as a support for the design of this plant or at longer term perspective, coordinated by a scientific advisary council of AFCEN (Association Française pour les règles de Conception et de Construction des matériels des Chaudières Electronucléaires).     

Accident analyses and passive measures reducing the consequences of a core-melt in CAPRA/CADRA reactor cores

As part of the Combustion Améliorée du Plutonium dans les Réacteurs Avancés/Consommation D'Actinides et de Déchets dans les Réacteurs Avancés (CAPRA/CADRA) program the feasibility of reactor systems with different neutron spectra and coolants is investigated to burn plutonium and also to destruct minor actinides and long lived fission products. In this paper, we deal with reactor cores with fast spectrum and metal cooling. The design of this type of CAPRA/CADRA cores shows significant differences compared e.g. to conventional fast reactor cores. The high Pu-enrichment and the high minor actinide load have an important influence on the core meltdown behavior and the associated recriticality risk. To cope with this risk, inherent design features and special measures/devices are investigated for their potential of early fuel discharge to reduce the criticality of the reactor core. An assessment of such measures/devices, which could provide an additional line of defense against severe accident development, is given. Within the CAPRA/CADRA program, also accelerator driven subcritical systems are investigated for performing the task of transmutation and incineration. In these fast neutron systems with a strong external neutron source, the kinetic behavior is different to a critical core and new strategies and measures for accident prevention have to be investigated.     

Transferability of post yield fracture mechanics properties from small scale specimens to large scale specimens and components

This contribution describes a method for the determination of the J-integral as a function of the load-line displacement for arbitrary specimen geometries.A correspondence could be found between the approximation method and the results determining with the Rice integral by means of a FE-calculation. Using the initiation values of the J-integral as a fracture mechanics parameter determined from the J_R-curve, correspond with failure values of double-édged notched tensile specimens and circumferentially notched round tensile specimens of which crack initiation was tantamount to instability. Consequently, it could be proved that the J-integral is a transferable parameter that may be ascertained from simple determinable deformation values. The application to real components seems to be promising, due to these good results.     

Ductile fracture parameters of RPV-materials

Ductile fracture material parameters have been determined for a reactor pressure vessel material to characterize its fracture resistance in the upper shelf toughness regime. Three different methods (the multiple specimen unloading (MSU), direct current potential drop (DCPD) and single specimen partial unloading compliance (SSPUC) methods) have been applied to test different CT-specimen geometries at temperatures between 25 and 300°C.It is shown that there are principle differences between J-R-curves measured by different experimental procedures, because of different methods for the measurement of crack lengths and crack growth. For instability analyses, using a complete J-R-curve, these differences seem to be negligible. For the determination of critical material parameters at or close to initiation of stable crack growth these differences may cause systematic errors tending to higher values for DCPD as compared to MSU-results and to lower values for SSPUC respectively.Procedures can be defined to evaluate comparable critical material parameters from the different experimental procedures, if J_i is known in a good approximation allowing to consider only the real crack extension without blunting, or if in addition the real (or realistically modelled) blunting and the effective blunting of the specific method are known. The differences in material parameters will depend quantitatively on the type of material and its toughness (slope of J-R-curve). They may be in the range of the experimental scatter observed in testing and seem to be negligible, but their systematic character should be kept in mind, e.g. when ranking different materials according to their critical parameters determined by different methods.     

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.     

A comparison between Japanese and French A16 defect assessment procedures for thermal fatigue crack growth

This paper presents the results of a benchmark on thermal fatigue crack growth evaluation for a thick-wall cylinder subjected to cyclic thermal transients. The simplified crack growth evaluation methods of both JNC in JAPAN and A16 procedures proposed by CEA in France are presented. The predictions obtained using both methods are compared with the experimental data. The JNC method, which accounts for the non-linear stress component provides predictions of crack advance in a good agreement with the experimental data. In contrast, significant differences are observed between the A16 predictions and the experimental data. The discrepancies are mainly due to the non-linear stress component which is not accounted for in the A16 method. When using the JNC stress intensity factor solution determined by finite element analysis to account for the non-linear stress component, the A16 method well predicts the thermal fatigue crack growth behavior.     

Irradiation embrittlement monitoring of WWER-440/213 TYPE RPVs

In the paper the main goals and progress of the surveillance specimen programme for the RPVs WWER-440/213 in Jaslovské Bohunice V-2 and Mochovce NPPs are presented. At Jaslovské Bohunice V-2, the standard surveillance specimen programme (SSSP) was finished and so-called ‘Extended Surveillance Specimen Programme’ (ESSP) was prepared on the base of its critical analysis. For first two units of the Mochovce NPP completely new programmes of irradiation embrittlement monitoring called ‘Modern Surveillance Specimen Programme’ is prepared. It is based on the experience with SSSP and ESSP as well as the recommendations of IAEA experts. This programme will serve for Mochovce NPP during all planned service life. The experience of ESSP application on the 3rd and 4th units in Jaslovské Bohunice V-2 NPP are presented in the paper too.     

Phébus-FP: Results and significance for plant safety in Switzerland

Phébus-FP is a multi-national collaborative programme comprising four integral and one debris bed melting and fission product release experiments. The integral experiments simulate the heat-up, degradation and fission product release and transport, thermal hydraulic response, aerosol behaviour and iodine chemistry in the containment. The total programme, including interpretation effort, runs from 1993 to ca. 2007. The experiments demonstrated new characteristics of core degradation, fission product and iodine chemistry and accident integral behaviour. PSI has participated extensively through planning support, interpretation of data from the experiments and analyses of the results to establish confidence in the models being used for plant application. A significant part of the PSI effort was performed to interpret the significance of silver iodide as an effective retention agent under typical reactor conditions. The conclusions are of an interim nature, since the data reduction and interpretation are not yet complete. The significance of the Phébus results for plant safety will be fully realised only after successful benchmarking of the computer codes against Phébus, and application to plant sequences. Such work remains in progress within the Phébus project and in various national programmes.     

Experimental investigation in the TOSQAN facility of heat and mass transfers in a spray for containment application

TOSQAN is an experimental program undertaken by the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) in order to perform thermal hydraulic containment studies. The TOSQAN facility is a large enclosure devoted to simulate typical accidental thermal hydraulic flow conditions in nuclear-pressurized water reactor (PWR) containment. The TOSQAN facility which is highly instrumented with non-intrusive optical diagnostics is particularly adapted to nuclear safety CFD code validation. The present work is devoted to studying the interaction of a water spray injection used as a mitigation means in order to reduce the gas pressure and temperature in the containment, to produce gases mixing and washout of fission products. In order to have a better understanding of heat and mass transfers between spray droplets and the gas mixture, and to analyze mixing effects due to spray activation, we perform detailed characterization of the two-phase flow.     

Analysis of startup oscillations in natural circulation boiling systems

The aim of the present work is to get an insight of the phenomena behind the low-pressure low-power transients that occur during startup of a natural circulation boiling system. A RELAP5 model developed for a test facility and its prototype is used to record additional system parameters that were not included in the data obtained from experiments. The flow oscillations observed during experimental and numerical studies are analyzed and classified. It is inferred that the low amplitude oscillations are not condensation induced geysering instabilities, but a density wave instability supported by flashing. The similarity between the nature of startup transients observed in the test facility and the prototype is also examined. The effect of flashing is more pronounced in the prototype due to the strong variation of saturation temperature as the length scale is 4 times that of the model. The time series data obtained from experimental observations and numerical simulations are analyzed to identify the structural nature of flow oscillations. The power spectral density estimated using fast Fourier transform (FFT) algorithm illustrates the chaotic nature of the signals. The nonlinear time series analysis (TISEAN) package has been used for the estimation of Lyapunov exponent and the Poincaré section. The Poincaré section and the Lyapunov exponent confirm the chaotic nature of the flow oscillations.     

A comparison between Japanese and French A16 defect assessment procedures for fatigue crack growth

This paper presents the results of a benchmark on fatigue crack growth evaluation for plates subjected to cyclic bending loads. The simplified fatigue crack growth evaluation methods of JNC in Japan and A16 procedures proposed by CEA in France are presented. The methods, based on the reference stress approach, are compared with each other. They are found to differ in estimating crack closure, in the expression used for the reference stress solution and in the formulations used to take plasticity into account. The methods are then employed to predict the fatigue crack growth behavior observed experimentally. At R=0.1, the methods provide predictions of crack growth in good agreement with the experimental data. At R=−1.0, significant differences are observed between the predictions. The discrepancies are mainly due to the crack closure effect used to calculate the effective SIF range.     

Defect assessment procedures at high temperature

Within Nuclear Electric PLC, a comprehensive assessment procedure for the high-temperature response of structures is being produced. The procedure is referred to as R5 and is written as a series of step-by-step instructions in a number of volumes. This paper considers in detail those parts of R5 which address the behaviour of defects. The defect assessment procedures may be applied to defects found in service, postulated defects, or defects formed during operation as a result of creep-fatigue loading. In the last case, a method is described for deducing from endurance data the number of cycles to initiate a crack of a specified size. Under steady loading, the creep crack tip parameter C* is used to assess crack growth. Under cyclic loading, the creep crack growth during dwell periods is still governed by C* but crack growth due to cyclic excursions must also be included. This cyclic crack growth is described by an effective stress intensity factor range. A feature of the R5 defect assessment procedures is that they are based on simplified methods and approximate reference stress methods are described which enable C* in a component to be evaluated. It is shown by comparison with theoretical calculations and experimental data that reliable estimates of C* and the associated crack growth are obtained provided realistic creep strain rate data are used in the reference stress approximation.     

Application and evaluation of different numerical methods for determining crack resistance curves

The engineering assessment of precracked components is frequently carried out with the help of crack resistance curves on the basis of the J-integral. Nevertheless, there are severe uncertainties regarding the validity of the J-integral in the case of large plastic deformations and unloading processes due to crack growth. Numerical and theoretical derivations are used to examine the influence of large plastic deformations and stable crack growth on the calculation of the J-integral. Numerical investigations were carried out on the example of a CT 25-specimen made of 20 MnMoNi 5 5 by means of the finite-element method. The following methods of calculations were selected:

• - Calculation of the specimen behaviour without consideration of the stable crack growth.

• - Calculation of the specimen behaviour using a J-Δa-curve as crack crack growth law.

• - Calculation of the specimen behaviour using a damage model (“ocal approach”) to compute the crack growth.

The results of the calculation carried out on the basis of the damage model makes it possible to carry out an assessment of the various methods for the experimental determination of the J-value.     

Combined experimental and analytical method for a realistic seismic qualification of equipment

The seismic qualification of equipment/structures are, in general, carried out either exclusively by analysis or exclusively by testing using a shake table. The analytical methods have the risk of the model not being a true reflection of the structure unless very elaborate modelling techniques are used. Even with an elaborate model there are many idealisations made which may not actually be realised in practice. The shake-table testing, avoids the modelling deviations to a large extent, but is also not without drawbacks. The important ones are the cost and the availability of a shake table of the required size and capacity. The shake-table testing is also carried out on the isolated equipment without the piping/structural connections from other components. The present paper suggests a combined experimental and analytical method on the ‘as installed’ equipment as an attractive alternative which overcomes the above drawbacks. In contrast to the existing practice of using the experimental results just to validate the analytical model, the suggested method uses the experimentally obtained dynamic characteristics of the ‘as installed’ equipment to obtain the response to the design seismic load. The paper brings out through an example of a simple storage tank which is too heavy for a shake table, the large deviations in its actual behaviour vis-à-vis an idealised analytical model.     

On the determination of fracture-mechanics data using the nearfield at the tip of dynamically loaded cracks

This paper discusses some common methods of data evaluation in dynamic fracture mechanics. This is done in regard to the restriction arising from the available testing-machines as well as due to the dynamic effects in the specimen which control the crack initiation process. It is concluded that only the process at the crack tip itself determine the dynamic crack inititation at very high K-values and consequently only data from the crack tip's nearfield will yield an correct interpretation of the events in the specimen. Based on this some new experimental methods are under development at the MPA Stuttgart, Fed. Rep. Germany which are discussed in the second part of the paper.     

Crack depth measurements in stainless steel pipes - methods and reliability

It still is a challenging development goal to improve sizing of di discontinuities by using NDT, since essential characteristics, as for instance crack depth extension, in many cases only can be evaluated within the scope of a large scale measurement: New NDT-techniques such as specially designed UT-probes to suit the test problem, and also a substantially extended theoretical background contribute to reliable statements concerning crack depth.The development of imaging UT-reconstruction methods will help to improve this situation. Practical experience with genuine testing problems are needed to judge the evidence of these methods. Experience with manual crack depth measurements is quite useful to create a practicable and reliable instrument based on reconstruction methods.