Cleavage fracture micromechanisms related to WPS effect in RPV steel

International audienceThe main objective of this paper is to characterise the changes caused by warm pre-stress (WPS) on the cleavage fracture micromechanisms of a 18MND5 (A533B) reactor pressure vessel (RPV) steel. In this purpose, different WPS fracture test results obtained on compact tensile (CT) and notched tensile (NT) geometries are presented and compared with isothermal test data. While confirming some well-established features of WPS, these experimental results, analysed through extensive fractographic investigations and finite element (FE) calculations, demonstrate a strong material aspect to WPS. One unveiled characteristic of the WPS effect is the deactivation of particles at high temperature through plastic straining under low stress, which enables breaking, or detaching from the matrix, the most critical particles without causing unstable cleavage propagation. In 18MND5 steel, these particles are TiN and carbide particles: according to the pre-stress level and fracture geometry, there is a progressive shift from TiN- to carbide-induced cleavage, up to the replacement of both by ductile type nucleation mechanisms

PVP2008-61232 ILLUSTRATION OF THE WPS BENEFIT THROUGH BATMAN TEST SERIES: TESTS ON LARGE SPECIMENS UNDER WPS LOADING CONFIGURATIONS

ABSTRACT A study combining modelling and a series of experiments on large specimens submitted to a thermal shock or isothermal cooling has been performed in CEA-Saclay in order to show the WPS benefit on large scale specimen. The test series, named BATMAN, was made on 18MND5 ferritic steel bars, containing a short or large fatigue pre-crack. For the two performed tests (fast thermal shock creating a gradient across the thickness of the bar or for the gradual uniform cooling), the effect of "Warm Pre-Stressing" was confirmed. In both cases, no propagation was observed during the thermal transient. Fracture occurred under low temperature conditions, at the end of the test when the tensile load was increased. The failure loads then recorded were substantially higher than during prestressing. To illustrate the benefit of the WPS effect, numerical interpretations were performed using either global approach or local approach criteria. The capability of models to predict the WPS effect was clearly shown. INTRODUCTION A large amount of work was performed on ferritic steels to demonstrate the benefit of the Warm Pre-Stressing (WPS) effect. Mainly on small specimens and in the frame of experimental and theoretical studies, the objectives were to show the enhancement of the cleavage fracture toughness at low temperature following the application of a preloading at higher temperature (which exceeds the fracture toughness of the virgin material at low temperature)

Evaluation of Cladding Residual Stresses in Clad Blocks by Measurements and Numerical Simulations

Reactor pressure vessels are internally clad with austenitic stainless steel. This welding operation generates residual stresses which can have an important role in integrity assessments. In order to evaluate these stresses, an experimental and numerical programme has been conducted. The experiments includes cladding operations, macrographic analyses, temperature and residual stresses measurements with different methods. According to these measurements, transversal stresses (perpendicular to the welding direction) and longitudinal stresses (parallel to the welding direction) are highly tensile in stainless steel and they are compressive in the HAZ. Finite element calculations were used to simulate both welding operations and post weld heat treatment. These calculations coupled the thermal, metallurgical and mechanical aspects in a 2D representation. Different models were studied including effect of generalised plane strain, transformation plasticity, creep and tempering. The transversal stresses calculated are similar to the measured ones, but the longitudinal stresses showed to be very sensitive to the model used. As expected because of the two-dimension model, the longitudinal stresses can't be well estimated. More work is needed to improve measurements of stresses in depth (important differences appeared between the different methods). A predictive model would be also very useful to determine the thermal loading which is at present dependant on measurements. A 3D calculation appears to be necessary to evaluate longitudinal stresses

Cleavage fracture of RPV steel following warm pre-stressing : micromechanical analysis and interpretation through a new model

International audienceIn this paper, the warm pre-stress (WPS) effect on the cleavage fracture of an 18MND5 (A533B) RPV steel is investigated. This effect, which describes the effective enhancement of the cleavage fracture toughness at low temperature following a prior loading at high temperature, has received great interest in light of its significance in the integrity assessment of structures, such as nuclear pressure vessels, subjected to thermal transients. Several loading cycles between room temperature (RT) and −150 °C are considered: Load-Unload-Cool-Fracture (LUCF), Load-Cool-Fracture (LCF) and Load-Cool with Increasing K-Fracture (LCIKF). All experiments complied with the conservative principle, which states that no fracture will occur if the applied stress intensity factor (SIF) decreases (or is held constant) while the temperature at the crack-tip decreases, even if the fracture toughness of the virgin material is exceeded. The experimental results indicate that an effective WPS effect is present even at small pre-load (Kwps = 40 MPa√m), and that a minimum critical slope (−ΔK/ΔT) in the LCIKF cycle has to be exceeded to induce cleavage fracture between RT and −150 °C. Numerical modelling was performed using mixed isotropic and kinematic hardening laws identified on notched tensile (NT) specimens, tested in tension to large strains (up to 40%), followed by large compressive strains. Detailed microstructural investigations on compact tensile (CT) and NT fracture test specimens were performed so as to determine the nature of the cleavage initiation sites, as well as the local mechanical conditions at fracture. Based on this local information, a new cleavage model was calibrated and applied to predict the probability of cleavage fracture after WPS: it is shown that the predictions are in good agreement with the experimental results

The influence of water flow on the impact/sliding wear and oxidation of PWR control rods specimens

International audienceSome mechanical components of PWR nuclear power plants can be sensitive to wear damage resulting from contacts with their supports or guides due to flow-induced vibrations. It is particularly the case of the control rods (rod cluster control assemblies (RCCAs)) and RCC guide tubes inside the reactor pressure vessel.In order to improve our knowledge about the wear behavior of such components, an extensive research program has been conducted during these past years by CEA and EDF including numerous wear tests performed on specific wear simulators. Within the framework of this program, the influence of the water flow on the wear process and damage has been studied on such components. Several wear tests have been conducted on specimens by EDF and CEA using the wear simulators ERABLE 1&2 (EDF) and SYRACUSE 1&2 (CEA) developed by Atomic Energy of Canada Limited (AECL). These simulators are operating in experimental conditions close to the PWR primary system (temperature, pressure, oxygen content, etc.). Both types of simulators are very similar except the water flow which is present only in the EDF machines. Several water flow rates have been taken into account in this program. Complementary to the tests, some extensive non destructive and destructive examinations of the specimens have been performed including the determination of the wear volumes using weights, three-dimensional (3D) profilometry and metallurgical examinations of the surfaces.The results obtained by CEA and EDF are compared and analyzed. The influence of the water flow on the behavior of the specimens is clearly shown, particularly on the dynamic behavior of the system and on the oxidation of the specimens. The effect on the wear pattern seems to be less significant even though the wear volumes are different

Effet de préchargement à chaud sur la ténacité de l’acier de cuve de réacteur à eau pressurisée 16MND5 fortement irradié

Dans une perspective d’exploitation à long terme, l’effet de l’histoire de chargement sur la résistance à la propagation de fissures de l’acier de cuve peut être amené à être pris en compte pour des transitoires thermomécaniques de type perte de réfrigérant primaire (APRP). Dans un programme commun CEA, EDF et AREVA, un acier de cuve de réacteur a été irradié à un niveau correspondant à plus de 60 ans d’exploitation. Des essais mécaniques ont permis d’obtenir la description complète des propriétés mécaniques et de leurs évolutions avec l’irradiation. Des essais spécifiques ont confirmé l’effet d’histoire de chargement sur la ténacité de l’acier de cuve irradié, pour ce niveau de fluence et des trajets de chargements de type APRP

Warm PreStress effect on highly irradiated reactor pressure vessel steel

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