Marta Serrano

The Master Curve (MC) methodology is a well-known approach utilized to characterize the ductile-to-brittle transition region (DBTR) of ferritic–pearlitic steels. This methodology was initially standardized in ASTM E1921 in 1997 and has undergone continuous evolution and improvement since its origin. However, the validity criterion for the crack aspect ratio (0.45 ≤ a0/W ≤ 0.55) has remained unchanged since its inception. It is worth noting that this criterion was originally established in accordance with standard ASTM E399, which characterizes fracture conditions under linear-elastic plane strain conditions, apparently for historical precedents rather than any scientific rationale. Furthermore, ASTM E1820, which is employed to characterize the fracture behavior of metallic materials in elastic–plastic conditions, permits a maximum crack length-to-width ratio of 0.70. In this context and considering that ASTM E1921 measures KJc (elastic–plastic) values of the fracture toughness, our research seeks to empirically demonstrate that the crack length-to-width criteria established in ASTM E1921 can be increased up to 0.60, without compromising the accuracy of the reference temperature calculations, at least for the specific datasets used in this work. Such a correction would offer significant advantages, especially when dealing with mini-C(T) specimens. Their subsize dimensions may result in the discarding of numerous specimens that could otherwise be effectively employed for reference temperature calculations. More research is recommended to provide additional validation of the crack aspect ratio upper limit proposed here, and even to explore the possibility of further extensions.

The present paper provides a summary of an inter-laboratory round robin on the fracture toughness testing and Master Curve application on unirradiated materials. First, a validation exercise was performed on the analysis of a given force-displacement curve. Then, a validation exercise was performed on the application of the Master Curve procedure on a given data set of fracture toughness results. Finally, the preliminary results of the Master Curve application on mini compact tension (MC(T)) specimens by 13 different labs on six different materials (four base materials and two welds) are presented. The validation exercise on fracture toughness data evaluation showed the need for a uniform conversion from front face displacement to load line displacement. The validation exercise on application of the Master Curve approach to obtain the reference temperature proved the equivalence between the T0TEM software and in-house developed softwares. The inter-laboratory round robin showed the equivalence between T0 obtained by MC(T) and larger specimens, except for A533B JRQ and 73W. The reasons for the deviations of A533B JRQ and 73W are under investigation. As a general comment, it can be stated that the results from MC(T) specimens often indicate material inhomogeneity as compared to the result from larger specimens. This is likely the consequence of the small sampling volume of the MC(T) specimens.

This paper gives an overview of the embrittlement evaluation carried out in Spain as part of the RPV surveillance programs. Special emphasis is given to the use of embrittlement trend curves and their use for structural integrity assessment. The paper also covers other important aspects, such as international research projects with relevant contributions from Spanish organizations and the monitoring of neutron fluence facing long-term operation.

Mechanical properties of membrane-electrode assemblies (MEAs) for proton exchange membrane fuel cells (PEMFCs) have been studied with the small-punch technique. MEAs were operated according to a non-accelerated aging protocol, with changes in humidification and cell temperature. After operation, degradation of the MEA is reflected by a decrease in power density, increase in the internal resistance, and decrease in open circuit voltage. Small disks (8 mm diam.) were taken from different locations of the degraded MEA to test the mechanical properties by small-punch. The results show an increase in plastic deflection and a decrease in maximum load of degraded MEAs, that result in lower apparent Young modulus. The degradation is larger in the area close to oxygen outlet. SEM microscopy shows that degradation affects majorly to the fibers of the carbon cloth gas diffusion layer (GDL). Combined small-punch and SEM correlate the change in mechanical properties on aged MEA with carbon fibers degradation. Such GDL degradation must also be principal responsible for the increase in the internal resistance.

The H2020 FRACTESUS project is aimed at the validation of miniaturized compact tension (MCT) specimen. More specifically the usage of the MCT with the Master Curve (MC) oriented fracture toughness testing of reactor pressure vessel (RPV) materials in hot cell conditions will be examined. In the first stage of the project, a general strategy of material selection and testing processes has been established. The choice of the selected RPV materials is based on the widest possible range of mechanical properties expected for baseline materials, but also resulting from their exposure to neutron irradiation, in terms of different MC reference temperature T0, and properties determined from Charpy impact testing. Moreover, in order to validate the use of the MCT in a broad application space, it was decided to perform FT tests for both base metals and welds. The largest challenge was related to the availability of those materials. They need to be tested in numerous, planned round robin exercises. They should have already an existing extensive database of fracture toughness results obtained using large specimens. The final version of the test matrix was prepared, keeping all those requirements in mind. An irradiated round robin exercise is planned for one type of weld material, namely 73W, that will be tested by seven partners. Additionally, the FRACTESUS partners were divided into smaller groups with 3–5 participants who will test a sub-selection of unirradiated materials in six round robin exercises. This paper presents the summary of the material selection activities in the FRACTESUS project. The materials are briefly described and rationale for their usage within the project is provided.

The standard ASTM E900-15 provides an analytical expression to determine the transition temperature shift exhibited by Charpy V-notch data at 41-J for irradiated pressure vessel materials as a function of the variables copper, nickel, phosphorus, manganese, irradiation temperature, neutron fluence, and product form. The 26 free parameters included in this embrittlement correlation were fitted through maximum likelihood estimation using the PLOTTER—BASELINE database, which contains 1878 observations from commercial power reactors. The complexity of this model, derived from its high number of free parameters, invites a consideration of the possible existence of overfitting. The undeniable goal of a good predictive model is to generalize well from the training data that was used to fit its free parameters to new data from the problem domain. Overfitting takes place when a model, due to its high complexity, is able to learn not only the signal but also the noise in the training data to the extent that it negatively impacts the performance of the model on new data. This paper proposes the resampling method of Monte Carlo cross-validation to estimate the putative overfitting level of the ASTM E900-15 predictive model. This methodology is general and can be employed with any predictive model. After 5000 iterations of Monte Carlo cross-validation, large training and test datasets (7,035,000 and 2,355,000 instances, respectively) were obtained and compared to measure the amount of overfitting. A slightly lower prediction capacity was observed in the test set, both in terms of R2 (0.871 vs. 0.877 in the train set) and the RMSE (13.53 °C vs. 13.22 °C in the train set). Besides, strong statistically significant differences, which contrast with the subtle differences observed in R2 and RMSE, were obtained both between the means and the variances of the training and test sets. This result, which may seem paradoxical, can be properly interpreted from a correct understanding of the meaning of the p-value in practical terms. In conclusion, the ASTM E900-15 embrittlement trend curve possess good generalization ability and experiences a limited amount of overfitting.

The long-term operating strategy of nuclear plants must ensure the integrity of the vessel, which is subjected to neutron irradiation, causing its embrittlement over time. Embrittlement trend curves used to predict the dependence of the Charpy transition-temperature shift, ΔT41J, with neutron fluence, such as the one adopted in ASTM E900-15, are empirical or semi-empirical formulas based on parameters that characterize irradiation conditions (neutron fluence, flux and temperature), the chemical composition of the steel (copper, nickel, phosphorus and manganese), and the product type (plates, forgings, welds, or so-called standard reference materials (SRMs)). The ASTM (American Society for Testing and Materials) E900-15 trend curve was obtained as a combination of physical and phenomenological models with free parameters fitted using the available surveillance data from nuclear power plants. These data, collected to support ASTM’s E900 effort, open the way to an alternative, purely data-driven approach using machine learning algorithms. In this study, the ASTM PLOTTER database that was used to inform the ASTM E900-15 fit has been employed to train and validate a number of machine learning regression models (multilinear, k-nearest neighbors, decision trees, support vector machines, random forest, AdaBoost, gradient boosting, XGB, and multi-layer perceptron). Optimal results were obtained with gradient boosting, which provided a value of R2 = 0.91 and a root mean squared error ≈10.5 °C for the test dataset. These results outperform the prediction ability of existing trend curves, including ASTM E900-15, reducing the prediction uncertainty by ≈20%. In addition, impurity-based and permutation-based feature importance algorithms were used to identify the variables that most influence ΔT41J (copper, fluence, nickel and temperature, in this order), and individual conditional expectation and interaction plots were used to estimate the specific influence of each of the features.

The use of small specimen test techniques (SSTT) to determine the mechanical properties of irradiated materials has been studied over the past decades both in fission and fusion programs, but also to characterise and optimise new materials by nuclear and non-nuclear communities. Currently a number of activities are running that focus on the standardisation of SSTT to determine fracture toughness properties for fusion reactor materials (IAEA [1], EUROfusion [2], F4E [3]), and to support the long-term operation of light-water reactors (CRIEPI [4]). The determination of the T0 reference temperature (ASTM E1921 [5]) has been successfully achieved by testing small compact tension (C(T)) specimens (W = 8mm, B = 4mm) of non-irradiated and irradiated pressure vessel materials. However, some concerns exist regarding the use of the Master Curve (MC) on ferritic-martensitic steels, not only with SSTT but also with standard specimens. The main concern is the slope of the MC [6, 7], that seems to be steeper than the standard one. In this paper, the fracture toughness of Eurofer97 has been obtained by testing small C(T) specimens with the geometry selected in IFMIF-DONES (W = 9.2mm, B = 4.6mm) in the transition region. T0 has been determined and compared to the one obtained from 0.5T-C(T) specimens (both normalised to 1T). The scatter of the results has also been assessed to validate the scatter description of the MC.

In this work, new oxide dispersion strengthened (ODS) ferritic steels have been produced by powder metallurgy using an alternative processing route and characterized afterwards by comparing them with a base ODS steel with Y2O3 and Ti additions. Different alloying elements like boron (B), which is known as an inhibitor of grain growth obtained by pinning grain boundaries, and complex oxide compounds (Y-Ti-Zr-O) have been introduced to the 14Cr prealloyed powder by using mechanical alloying (MA) and were further consolidated by spark employing plasma sintering (SPS). Techniques such as x-ray diffraction (XRD), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) were used to study the obtained microstructures. Micro-tensile tests and microhardness measurements were carried out at room temperature to analyze the mechanical properties of the differently developed microstructures, which was considered to result in a better strength in the ODS steels containing the complex oxide Y-Ti-Zr-O. In addition, small punch (SP) tests were performed to evaluate the response of the material under high temperatures conditions, under which promising mechanical properties were attained by the materials containing Y-Ti-Zr-O (14Al-X-ODS and 14Al-X-ODS-B) in comparison with the other commercial steel, GETMAT. The differences in mechanical strength can be attributed to the precipitate’s density, nature, size, and to the density of dislocations in each ODS steel.

Two different zirconium contents (0.45 and 0.60 wt.%) have been incorporated into a Fe-14Cr-5Al-3W-0.4Ti-0.25Y2O3 oxide dispersion-strengthened (ODS) steel in order to evaluate their effect on the final microstructure and mechanical properties. The powders with the targeted compositions were obtained by mechanical alloying (MA), and subsequently processed by spark plasma sintering (SPS) at two different heating rates: 100 and 400 °C·min−1. Non-textured bimodal microstructures composed of micrometric and ultrafine grains were obtained. The increase in Zr content led to a higher percentage of Zr nano-oxides and larger regions of ultrafine grains. These ultrafine grains also seem to be promoted by higher heating rates. The effective pinning of the dislocations by the Zr dispersoids, and the refining of the microstructure, have significantly increased the strength exhibited by the ODS steels during the small punch tests, even at high temperatures (500 °C).

The study of the enhanced creep strength of conventional ferritic-martensitic (F/M) grade 91 steel by a thermomechanical treatment (TMT) to increase the precipitation of MX particles in the matrix was performed.

Creep properties were evaluated by tests at constant load at temperatures that varied from 600 °C to 700 °C with different levels of stress for both steels: T91 and T91-TMT. The creep curves and main parameters for both steels in the different conditions were analysed.

Results show a great improvement of creep strength of the T91 after the thermomechanical treatment in comparison with the conventional steel. T91-TMT presents a rupture life significantly higher than T91 and a decrease of the values of the minimum creep rate.

An increase of the density of MX precipitates in the matrix of the T91-TMT due to the thermomechanical treatment in comparison with T91 can be also observed.

A change in the fractography was also detected. T91-TMT specimens showed signs of brittle fracture instead of the ductile fracture, with the common necking effect detected in the T91.

The 4-years European project ATLAS+ project was launched in June 2017. Its main objective is to develop advanced structural assessment tools to address the remaining technology gaps for the safe and long term operation of nuclear reactor pressure coolant boundary systems.

The transferability of ductile material properties from small scale fracture mechanics specimens to large scale components is one of the topics of the project. A large programme of experimental work is to be conducted in support of the development and validation of advanced tools for structural integrity assessment within the framework of the work-package 1 (WP 1): Design and execution of simulation oriented experiments to validate models at different scales.

The experimental work is based on a full set of fracture mechanics experiments conducted on standard specimens and large scale components (several pipes and one mock-up), including a full materials characterization. Three materials are considered:

• a ferritic steel 15NiCuMoNb5 (WB 36)

• an aged austenitic stainless steel weld

• a VVER (eastern PWR) dissimilar metal weld (DMW)

The paper presents the WP 1, the experimental programme and summarizes the first results.

The major challenge in a heat-resistant steel is to generate thermally stable microstructures that allow increasing the operating temperature, which will improve the thermal efficiency of the power plant without diminishing strength or time to rupture. The strengthening mechanism in tempered martensitic 9Cr steels comes mainly from the combination of solid solution effect and of precipitation hardening by fine MX carbo-nitrides, which enhance the sub-boundary hardening. This work is focused on the effect of ausforming processing on MX nanoprecipitation, on both their distribution and number density, during the subsequent tempering heat treatment. The creep strength at 700 oC was evaluated by small punch creep tests. The creep results after ausforming were compared to those obtained after conventional heat treatment concluding, in general, that ausforming boosts the creep strength of the steel at 700 oC. Therefore, conventional ausforming thermomechanical treatment is a promising processing route to raise the operating temperature of 9Cr heat-resistant steels.

This contribution deals with the determination of the reference temperature of A533 Cl.1 steel using miniaturized specimens. The dimensions of the miniaturized specimens used were 3 × 4 × 27 mm (thickness × width × length). This specimen type allows utilizing a limited amount of test material or the broken halves of precracked Charpy or larger specimens. The test material comes from the broken halves of 0.5 T SE(B) specimens previously tested for the determination of the reference temperature at Ciemat, Madrid. The fracture toughness tests were performed in the transition region of the steel according to the recommendations of the standard ASTM E1921 and according to Wallin's recommended temperature range of miniaturized specimens. The reference temperature of the Master Curve was very similar to the ones obtained from three-point-bend specimens of sizes 0.2 T, 0.4 T, and 0.5 T. The results obtained confirm a necessity to conduct tests at low temperatures and to test a sufficient number of specimens in order to generate enough valid data for the determination of the reference temperature.

Based on the good experiences gained by using small specimens made of ferritic RPV materials, the Master Curve fracture toughness approach was applied to determine the fracture mechanical properties of oxide dispersion strengthened (ODS-) materials. A ferritic ODS-alloy (Fe-14Cr-1W-Ti-Y2O3) has been produced through the powder metallurgical production path via hot extrusion and hot isostatic pressing (HIP). Optimized oxide dispersion strengthened (ODS)-alloys have a promising potential to meet the foreseen requirements of components in future Gen IV power plants due to their high creep strength and swelling resistance under irradiation at elevated operational temperatures. The fracture toughness was characterized with mini 0.2T C(T) specimens in different material orientations (R-L / L-R) in the ductile-brittle and upper shelf region in the un-irradiated state, accounting especially for the ODS-material’s anisotropy as one key effect of manufacturing. Despite all tests were performed in orientation required by ASTM standards E 1921 and E 1820 not all validity criteria (e.g. height of yield strength, evenness of the crack, admissible K during testing or admissible stable crack growth) were met by the ODS-material: consequently, a valid T0 value and a standard-compliant Master Curve could not be determined for the ODS-material in the transition region especially in the respective R-L orientation, also due to a comparably low fracture toughness over the whole evaluated temperature range. Promising fracture toughness properties were obtained in the crack growth direction perpendicular to the prior main deformation (extrusion) direction, where a KJQ value of 196 MPa√m at T = 22°C was measured. Within the ductile regime, only a JQ = J0.2BL technical initiation toughness value could be calculated and at T = 22°C, a comparably large JQ of 137kJ/m2 is obtained for specimens with crack growth direction perpendicular to the extrusion direction, while in extrusion direction the toughness is again low.

In addition two further ODS-materials (14YWT and PM2000) were tested and compared to the alloys above. Non-conformances of ODS relating to the material requirements in ASTM standards E1921 and E1820 were finally detected and explained.

This contribution deals with determination of the reference temperature of JRQ steel using miniaturized specimens. The dimensions of used miniaturized specimens were 3 × 4 × 27 mm (thickness × width × length). This specimen type offers the utilization of limited amount of test material or broken halves of precracked Charpy and larger specimens. The test material comes from the broken halves of 0.5T SEB specimens previously tested for purposes of the reference temperature determination in Ciemat, Madrid. The fracture toughness tests of specimens were performed in the transition region of the steel according to the recommendations of standard ASTM E1921 and according Wallin’s recommended temperature range for miniaturized specimens. The determined reference temperature of the Master Curve was very similar to the determined ones from three-point-bend specimen of sizes 0.2T, 0.4T and 0.5T. The obtained results confirm a necessity of conduct of tests at low temperatures and testing sufficient number of specimens in order to generate enough valid data for determination of the reference temperature.

Powders with nominal composition Fe–14Cr–2W–0·4Ti were mechanically alloyed (MA) with Y₂O₃ in a planetary ball mill at two different rotational speeds. Consolidation of the as milled powders was performed by spark plasma sintering (SPS). As milled powders showed a highly deformed microstructure with elongated nanometre grains and, depending upon the rotational speed, different stages of the nanocluster evolution were observed to be produced. In the case of consolidated materials, grain growth occurred during the SPS process and it was possible to observe the influence of the MA parameters, with larger and more homogeneously distributed grains at the higher rotational speed. Additionally, Ti was observed to be incorporated to the nanoclusters after SPS, indicating a further step in their evolution during consolidation. The mechanical behaviour of the SPS compacts was evaluated by tensile and small punch testing also showing the influence of the MA parameters in the material behaviour.

Ferritic oxide dispersion strengthened (ODS) alloys are candidate materials to be used as cladding for long term fast reactors, due to their high strength at high temperature and good swelling and irradiation resistances. The fabrication of cladding tubes is usually made by a succession of cold deformation steps where a deformation induced anisotropic microstructure could take place, which would affect the mechanical behaviour of the tube. The characterisation of this microstructural anisotropy is one of the key issues in the development of cladding ODS tubes. In this paper, the microstructural anisotropy of a Fe–14Cr–ODS extruded bar and a Fe–12Cr–ODS plate is characterised and its effect on the mechanical properties is analysed by tensile, impact and small punch testing. In both materials, a reduction of the ductility is observed in the transverse specimens. In addition, the fracture behaviour seems to be strongly dependent on the location of the crack plane regarding the elongated grained microstructure.

Las tecnologías y medios para desarrollar plantas de biomasa con alta eficiencia en la conversión de energía son esenciales para asentar la biomasa como una fuente de energía renovable. Los sistemas de turbinas de gas de ciclo combinado (CCGT) permiten elevar la eficiencia de las plantas de biomasa del 35 % actual al 45 %. Sin embargo, para conseguir estos niveles de eficiencia en la conversión de energía, el intercambiador de calor de la caldera debe trabajar en condiciones extremas de temperatura (por encima de 1100 °C) y presión (en torno a 15-30 bar). Los aceros ODS ferríticos son la clase de material avanzado específicamente diseñado para trabajar en ambientes altamente corrosivos y a temperaturas elevadas. Pero para mejorar la resistencia a la fluencia a altas temperaturas en dirección circunferencial, la microestructura generada en el proceso de recristalización ha de ser diferente a la microestructura axi-simétrica altamente anisótropa que tiene este tipo de aceros por defecto. En este sentido, este trabajo detalla los resultados obtenidos en la aleación reforzada por dispersión de óxidos PM 2000 que ha sido fabricada por técnicas pulvimetalúrgicas, consolidada por extrusión y conformada por laminación en caliente y en frio. De los resultados obtenidos puede concluirse que una microestructura heterogénea estimula su recristalización, de forma que grandes gradientes de deformación generan microestructuras más finas e isótropas. La comparación de estos resultados con simulaciones por elementos finitos ha permitido dilucidar el papel que tienen las tensiones residuales subyacentes en la microestructura en la generación de la microestructura recristalizada.

En este documento se reflejan los primeros resultados referentes a la caracterización de dos coladas de un acero 9 Cr ferrítico/martensítico de activación reducida (RAFM) producido en España, denominadas AF1B y AF2A. Los resultados obtenidos en dicha caracterización se compararán con su homólogo europeo, el EUROFER97-2, el cual se eligió como material de referencia. Todas las actividades detalladas se realizaron en la Unidad de Materiales Estructurales del CIEMAT, dentro del proyecto TECNO-FUS CONSOLIDER INGENIO. Ambas coladas, producidas en una planta piloto de la Fundación ITMA, presentan el mismo proceso de producción y tratamiento térmico final. Asimismo presentan un comportamiento a tracción similar al del EUROFER97-2. Por el contrario, las propiedades a impacto son menores a las del homólogo europeo. En la microestructura de la colada AF1B se han detectado inclusiones bifásicas de gran tamaño que afectan a sus propiedades mecánicas, sobre todo a las de impacto. Estas inclusiones no se detectaron en la AF2A.

En este documento se reflejan los primeros resultados referentes a la caracterización de dos coladas de un acero 9 Cr ferrítico/martensítico de activación reducida (RAFM) producido en España, denominadas AF1B y AF2A. Los resultados obtenidos en dicha caracterización se compararán con su homólogo europeo, el EUROFER97-2, el cual se eligió como material de referencia. Todas las actividades detalladas se realizaron en la Unidad de Materiales Estructurales del CIEMAT, dentro del proyecto TECNO-FUS CONSOLIDER INGENIO. Ambas coladas, producidas en una planta piloto de la Fundación ITMA, presentan el mismo proceso de producción y tratamiento térmico final. Asimismo presentan un comportamiento a tracción similar al del EUROFER97-2. Por el contrario, las propiedades a impacto son menores a las del homólogo europeo. En la microestructura de la colada AF1B se han detectado inclusiones bifásicas de gran tamaño que afectan a sus propiedades mecánicas, sobre todo a las de impacto. Estas inclusiones no se detectaron en la AF2A.

There is strong interest from the nuclear industry to use the precracked Charpy single-edge notched bend, SE(B), specimen (PCVN) to enable determination of the reference temperature, T0, with reactor pressure vessel surveillance specimens. Unfortunately, for many different ferritic steels, tests with the PCVN specimen (10×10×55 mm) have resulted in T0 temperatures up to 25°C lower than T0 values obtained using data from compact, C(T), specimens. This difference in T0 reference temperature has often been designated a specimen bias effect, and the primary focus for explaining this effect is loss of constraint in the PCVN specimen. The International Atomic Energy Agency has developed a three-part coordinated research project (CRP) to evaluate various issues associated with the fracture toughness Master Curve for application to light-water reactor pressure vessels. One part of the CRP is focused on the issue of test specimen geometry effects, with emphasis on the PCVN bias. This topic area was organized in two parts, an experimental part and an analytical part with a view towards each part complementing the other. Within the analytical part, elastic plastic finite element methods are extensively used in order to access local stress and strain information that is the basic ingredient for most of the micromodels of cleavage fracture developed to date. In the framework of the international qualification and acceptance of such a tool for actual loss of constraint prediction, the validation of such tool is of prime importance. Therefore, a round robin exercise has been proposed and performed by ten laboratories from nine different countries. The round robin focuses on the modeling of realistic three-dimensional geometries containing shallow and deep crack. This round robin has been useful to qualify different finite element codes and to identify possible errors in the input file. The round robin demonstrates that errors in the input file can be easily introduced. Some remaining differences cannot be attributed to one particular finite element code or to actual errors. Those differences are attributed to the so called “user effect” which can only be reduced through in depth discussion and deep understanding of each finite element code. Independently of the used code and of relatively small user effect differences, it is found that shallow crack specimens are more sensitive to loss of constraint than deep crack specimens for a given specimen size. The difference in terms of reference temperature between the two geometries is evaluated to be about 40 °C. For a deep crack, loss of constraint is identified to appear at M values around 200. This value is larger than the one specified in current standard (M = 30). Increasing the M value to 200 will jeopardize the use of PCVN for the nuclear industry on the other hand bias introduced by M value of 30 is acceptable.

The precracked Charpy single-edge notched bend, SE(B), specimen (PCC) is the most likely specimen type to be used for determination of the reference temperature, T0, with reactor pressure vessel (RPV) surveillance specimens. Unfortunately, for many RPV steels, significant differences have been observed between the T0 temperature for the PCC specimen and that obtained from the 25-mm thick compact specimen [1TC(T)], generally considered the standard reference specimen for T0. This difference in T0 has often been designated a specimen bias effect, and the primary focus for explaining this effect is loss of constraint in the PCC specimen. The International Atomic Energy Agency (IAEA) has developed a coordinated research project (CRP) to evaluate various issues associated with the fracture toughness Master Curve for application to light-water RPVs. Topic Area 1 of the CRP is focused on the issue of test specimen geometry effects, with emphasis on determination of T0 with the PCC specimen and the bias effect. Topic Area 1 has an experimental part and an analytical part. Participating organizations for the experimental part of the CRP performed fracture toughness testing of various steels, including the reference steel JRQ (A533-B-1) often used for IAEA studies, with various types of specimens under various conditions. Additionally, many of the participants took part in a round robin exercise on finite element modeling of the PCVN specimen, discussed in a separate paper. Results from fracture toughness tests are compared with regard to effects of specimen size and type on the reference temperature T0. It is apparent from the results presented that the bias observed between the PCC specimen and larger specimens for Plate JRQ is not nearly as large as that obtained for Plate 13B (−11°C vs −37°C) and for some of the results in the literature (bias values as much as −45°C). This observation is consistent with observations in the literature that show significant variations in the bias that are dependent on the specific materials being tested. There are various methods for constraint adjustments and two methods were used that reduced the bias for Plate 13B from −37°C to −13°C in one case and to − 11°C in the second case. Unfortunately, there is not a consensus methodology available that accounts for the differences observed with different materials. Increasing the Mlim value in the ASTM E-1921 to ensure no loss of constraint for the PCC specimen is not a practicable solution because the PCC specimen is derived from CVN specimens in RPV surveillance capsules and larger specimens are normally not available. Resolution of these differences are needed for application of the master curve procedure to operating RPVs, but the research needed for such resolution is beyond the scope of this CRP.

Eurofer’97 is the structural reference material that will be tested in the ITER modules. Its metallurgical properties have been well characterized during the last years. However, more investigations related to the fracture toughness of this material are needed to assess its integrity because this property is one of the most important to design structural components. In the case of structural materials for fusion reactors, the small specimen test technology is being actively developed to investigate the fracture toughness among other mechanical properties. The use of small specimens is due to the small available irradiation volume of the International Fusion Materials Irradiation Facility (IFMIF) and also due to the high fluence expected in the fusion reactor. The aim of this paper is to determine the fracture toughness of the Eurofer’97 steel by testing small specimens of different geometry in the ductile-to-brittle transition region with the application of the Master Curve methodology. The tests and data analysis have been performed following the Master Curve approach included in the ASTM Standard E1921-05, “Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range.” Specimen size effects and comparison of the fracture toughness results with available data in the literature are also discussed.

Uno de los componentes críticos de una central nuclear es la vasija del reactor, debido a su función de contención del núcleo. Dicha vasija está sometida a irradiación neutrónica, lo que provoca cambios microestructurales en el material y pérdida de propiedades mecánicas. Debido a estos efectos, es necesario monitorizar su integridad estructural a lo largo de su vida de operación. Para ello se establecen los llamados programas de vigilancia. El objetivo final de estos ensayos es el de determinar la tenacidad de fractura del material. Actualmente, esto se consigue indirectamente mediante técnicas de predicción establecidas en diferentes normativas. El objetivo de este trabajo es el de determinar la tenacidad de fractura del material de la vasija directamente a través del ensayo Charpy V instrumentado. Para ello se ha desarrollado en el CIEMAT una metodología de ensayos y análisis de resultados.