Mechanical properties of a forged ferritic steel vessel shell containinghydrogen flakes

International audienceDuring the 2012 outage of the Doel 3 and Tihange 2 nuclear power plants (NPP), in-service inspectionsrevealed the existence of numerous quasi-laminar indications in the reactor pressure vessel(RPV). The indications were quickly identified as being hydrogen flakes formed in ghost lines at themanufacturing stage.Independently of the Doel 3 and Tihange 2 NPP utility (1), IRSN conducted a test program aiming atverifying that the presence of hydrogen flakes is not symptomatic of a weaker material. AREVA NPprovided the investigated material, namely two blocks of a steam generator vessel shell in 18MND5steel, so called VB395. This shell was rejected because of an incident which occurred during the degassingheat treatment. One block was a sound block without flake - the reference block - and thesecond block included a high density of hydrogen flakes. Those blocks were provided after quenching,tempering and simulated Post Weld heat treatment.Tensile tests and fracture toughness tests were performed in the ductile-to-brittle transition range aswell as in the ductile range of the materials. In the flaked block, different regions were identified andcharacterized material without flake nearby the flaked region, material between the hydrogen flakesand material in a large zone within the flaked region. Particular attention was paid to the sampling sothat the quenching effect related to the vessel shell thickness is the same for all the specimens. Torefine the interpretation of the test results, chemical composition behind the crack front of the specimenswas measured after fractographic examinations of the broken surfaces

Mechanical behaviour of a forged ferritic steel shell containing numerous hydrogen flakes

International audienceFollowing the flaw indications found in summer 2012 in two Belgian Reactors Pressure Vessels (RPV), WENRA recommended the nuclear safety authorities in Europe to verify the material quality and integrity of the RPV in a 2-step approach 1) a comprehensive review of the manufacturing and inspection records of the forgings of the RPV, 2) an additional UT examination of the base material of the vessels if needed.In this context, and to consolidate scientific base on this issue, IRSN, the French technical safety organization, conducted, with CEA support, a test program aiming at studying the consequences of hydrogen flakes in large forgings of primary equipment (RPV, steam generator, pressurizer). AREVA NP provided the material to be investigated, namely two blocks of a steam generator vessel shell in 18MND5 steel a block without flake - the reference block - and a block including a high density of hydrogen flakes. This shell - so called VB395 - was rejected because of an incident which occurred during the degassing heat treatment.Fracture toughness has been evaluated from 85 tests in the ductile range and the ductile-to-brittle transition range of the material. Usual fatigue precracked 1/2T-CT specimens have been compared to specimens containing a hydrogen flake replacing the fatigue precrack. At 100DC, the test results have shown that J0.2 between flakes was much higher than the minimum values specified by the RCC-M code.In the ductile-to-brittle transition range, the reference temperature T0 was determined for different regions in the VB395 shell material in the reference block, material without flake nearby the flaked region, material between the hydrogen flakes and material in a large zone within the flaked region. For the precracked specimens, the variation from T0 = 128DC to T0 = 99DC is mainly related to the variation of the carbon content measured close to the crack tip (0.19% to 0.22%).Tests on specimens with flake as precrack have been analyzed using 3D elastic-plastic X-FEM simulations allowing the modelling of the irregular flake geometry and the calculation of the stress intensity factor (KJ) evolution along the flake front. The reference temperature obtained on specimens with flake as precrack was similar to that of the material between flakes ( 102DC vs. 99DC). It results that hydrogen flakes behave like fatigue precracks.Furthermore, large scale bending specimens with multiple flakes have been tested at 100DC. These tests were interpreted thanks to 3D X-FEM simulations allowing the analysis of the hydrogen flake interaction in terms of KJ

Dynamic effects on fracture toughness for ferritic steel in the ductile-to-brittle transition

International audienceDynamic loading effects on ferritic steel toughness havebeen evaluated in the brittle-to-ductile transition, consideringloading rates representative of object drops. To verify that thebrittle-to-ductile transition curve, initially defined from statictests, tends to shift to higher temperatures due to dynamiceffects even in the case of object drops, experiments on16MND5 steel have been performed.A three-point bending set-up and a thermal chamber havebeen designed in order to perform dynamic fracture tests onlarge Single Edge-notched Bending SE(B) specimen, at verylow temperature using a drop-shock machine. In a first step,considering that the reference temperature of the material(according to the master curve concept) is -122 °C, dynamictests at -120 °C have been performed. These tests haveconfirmed that the fracture mode is still brittle at thistemperature, when an impact speed of 4.85 m/s is used.Elastic-plastic or viscoplastic dynamic simulations of thetests, compared to classical static analysis, have demonstratedthat the effects of inertia and viscosity on fracture toughness arenegligible considering the very low values obtained on thesetests at -120 °C. These results also confirm the decrease offracture toughness due to dynamic loading compared toexperimental data from static tests. A further step will be tocomplete this demonstration with dynamic tests at highertemperatures in the brittle-to-ductile transition

Size characterization of airborne SiO2 nanoparticles with on-line and off-line measurement techniques: Results of an interlaboratory comparison

Day after day, new applications using manufactured nanoparticles appear in industry. To evaluate the occupational risk associated with airborne nanoparticles, it is important to have reliable, accurate, and standardized measurement methods. It is therefore necessary to work on pre- standardization projects to develop reference methods to characterize different parameters such as the number and the size distribution of airborne nanoparticles

Size characterization of airborne SiO2 nanoparticles with on-line and off-line measurement techniques: an interlaboratory comparison study

Results of an interlaboratory comparison on size characterization of SiO2 airborne nanoparticles using on-line and off-line measurement techniques are discussed. This study was performed in the framework of Technical Working Area (TWA) 34\u2014\u2018\u2018Properties of Nanoparticle Populations\u2019\u2019 of the Versailles Project on Advanced Materials and Standards (VAMAS) in the project no. 3 \u2018\u2018Techniques for characterizing size distribution of airborne nanoparticles\u2019\u2019. Two types of nano-aerosols, consisting of (1) one population of nanoparticles with a mean diameter between 30.3 and 39.0 nm and (2) two populations of non-agglomerated nanoparticles with mean diameters between, respectively, 36.2\u201346.6 nm and 80.2\u201389.8 nm, were generated for characterization measurements. Scanning mobility particle size spectrometers (SMPS) were used for on-line measurements of size distributions of the produced nano-aerosols. Transmission electron microscopy, scanning electron microscopy, and atomic force microscopy were used as off-line measurement techniques for nanoparticles characterization. Samples were deposited on appropriate supports such as grids, filters, andmica plates by electrostatic precipitation and a filtration technique using SMPS controlled generation upstream. The results of the main size distribution parameters (mean and mode diameters), obtained from several laboratories, were compared based on metrological approaches including metrological traceability, calibration, and evaluation of the measurement uncertainty. Internationally harmonized measurement procedures for airborne SiO2 nanoparticles characterization are proposed