[EN] The contamination of water resources with many industrial, agricultural and other chemicals is one of the key environmental problems that humanity is facing nowadays. Despite the fact that they are usually present at very low concentration, they possess a significant risk to aquatic and human life. To address this issue many national and international institutions set different regulations to monitor and control the water quality. Currently, the monitoring of compounds included in official watch lists is conducted by chromatographic and mass spectroscopic methods. These techniques are approved as "gold standards" for analytical quantitation of organic residues in water. Although they are sensitive and reproducible, cannot be used on-site. The need of sampling and centralized laboratory measurements makes not only the overall cost high but lowering the efficiency of the analysis. Therefore, there is an urgent need to develop suitable field methods to facilitate the in situ measurements at a low cost. Biosensors are therefore an alternative technology that can provide sensitive results in a fast and affordable way. This thesis has focused on the development of a biosensor based on immunoassays and compact disk technology, for the multiplex detection of priority water contaminants. As the methods based on the immunorecognition events are challenging in terms of the selectivity and sensitivity, the major part of the thesis was the selection of the immunoreagents, assay form and procedure. For the detection part, gold nanostructures were selected as sensitive tags for signal enhancement. Therefore, different nanoparticles were studied in order to select the optimal size in terms of the signal enhancement, sensitivity and antibody amount used. Also, the assays performances with signal enhancement and without any amplification were evaluated. The best immunoassay was selected for developing the multiplexed assay. Furthermore, an approach to improve the readout sensitivity of microimmunoassays based on used of gold nanoparticles as both capture and detection species was demonstrated. The method is based on the performance of the immunorecognition event in a homogeneous mode and detection part in the heterogeneous format. Finally, representative water samples were analysed to confirm the applicability of the multi-residue assay. The analytical properties have been established for each methodology and the obtained results have been validated by comparison with reference techniques. The investigations carried out in this work, have resulted in new insights in immunoassay technique that could be useful for screening applications. / [ES] La contaminación de aguas superficiales causada por plaguicidas y productos industriales es actualmente, uno de los grandes problemas medioambientales. Aunque estas sustancias están presentes a niveles muy bajos, tienen efectos perjudiciales en el medio en general y especialmente en humanos. Por este motivo, diferentes instituciones han regulado los niveles de contaminantes en áreas de controlar de la calidad de las aguas, creando listas prioritarias de sustancias peligrosas y tóxicas para el medio ambiente. Actualmente, la monitorización de los contaminantes incluidos en las listas oficiales se realiza mediante técnicas cromatográficas y espectrometría. Estos métodos analíticos están aprobados como técnicas de referencia para la determinación de residuos orgánicos presentes en aguas naturales. A pesar de ser técnicas fiables, reproducibles y sensibles, los métodos cromatográficos no están exentos de inconvenientes. Este tipo de metodologías requiere una instrumentación costosa y una laboriosa preparación de muestra, que hacen que el análisis sea en general complejo. Por ello, el desarrollo de métodos analíticos alternativos que faciliten hacer medidas in-situ a bajo coste y con gran capacidad de trabajo es de gran utilidad. En este sentido, las técnicas inmunoquímicas tienen un gran potencial analítico ya que son en general sensibles y selectivas, se pueden utilizar en el lugar de la toma de muestra y tienen capacidad multianalito. Esta tesis se ha centrado en el desarrollo de un sistema biosensor, basado en la tecnología de disco compacto, para la detección multianalito de diversos contaminantes prioritarios en aguas naturales. Las limitaciones más críticas para el desarrollo de un biosensor multianalito mediante métodos inmunoquímicos son los relacionados con su sensibilidad y selectividad. Por lo tanto, una parte importante de la tesis se ha centrado en la selección de inmunoreactivos, formato y optimización de diferentes parámetros claves del ensayo. Una estrategia utilizada para aumentar la sensibilidad de los ensayos ha consistido en marcar la inmunoreacción con nanopartículas de oro. Para ello, se ha estudiado diferentes tipos (esféricas y cilíndricas) de distinto tamaño y se han comparado sus prestaciones analíticas (relación señal ruido, sensibilidad etc.) También, se han desarrollado inmunoensayos cuantitativos sin necesidad de amplificación de la señal. Por otro lado, se ha desarrollado una aproximación que hemos denominado "inmunocaptura" basada en el uso de nanopartículas de oro como especie de captura de analitos en disolución y que actúa como marcador de la inmunointeracción que tiene lugar en la fase sólida. Finalmente, se han analizado muestras de agua naturales dopadas con distintos niveles de los analitos objeto de estudio para evaluar la utilidad de las metodologías desarrolladas como herramienta de screening masivo en el área medioambiental. Los resultados obtenidos han sido comparados con los obtenidos mediante las técnicas de referencia. Las investigaciones realizadas han permitido desarrollar nuevos formatos de ensayo y conocimientos inmunoquímicos aplicados a la tecnología de disco compacto, aportando nuevas herramientas de screening que permiten la determinación simultánea de contaminantes en aguas naturales por debajo de las concentraciones establecidas en la normativa europea de calidad de agua. / [CA] La contaminació d'aigües superficials causada principalment per plaguicides i altres productes industrials és un dels grans problemes mediambientals actuals. Malgrat que aquestes substàncies estan presents en nivells molt baixos, tenen efectes perjudicials en humans i animals. Per aquest motiu, diferents institucions estatals han regulat els nivells de contaminants en àrees de control de la qualitat de l'aigua, creant llistes prioritàries de substàncies perilloses i tòxiques per al medi ambient. Actualment, la monitorització dels contaminants inclosos en les llistes oficials es realitza mitjançant tècniques cromatogràfiques i d'espectroscòpia de masses. Aquests mètodes analítics estan aprovats com a tècniques de referència per a la determinació de residus orgànics presents en aigües naturals. Malgrat ser tècniques fiables, reproduïbles i sensibles, els mètodes cromatogràfics no estan exempts d'inconvenients. Aquest tipus de metodologies requereix una instrumentació costosa i una laboriosa preparació de mostres que fan que l'anàlisi sigui, en general, complex. Per això, el desenvolupament de mètodes analítics alternatius que facilitin la possibilitat de fer mesures in-situ a baix cost i amb gran capacitat analítica és de gran utilitat. En aquest sentit, les tècniques inmunoquímiques tenen un gran potencial analític ja que són, en general, sensibles i selectives, es poden utilitzar en el lloc de presa de la mostra i tenen capacitat multianalit. Aquesta tesi s'ha centrat en el desenvolupament d'un sistema biosensor, basat en la tecnologia de disc compacte, per a la detecció multianalit de diversos contaminants prioritaris en aigües naturals. Les limitacions més crítiques per al desenvolupament d'un biosensor multianalit mitjançant mètodes inmunoquímics són sensibilitat i selectivitat. Per tant, una part important de la tesi es va centrar en la selecció de inmunoreactius, format i optimització de diferents paràmetres clau de l'assaig. La detecció es va dur a terme mitjançant l'ús de nanopartícules d'or com a marcadors de la inmunointeracció i amplificació de la senyal analítica. S'han estudiat diferents estructures d'or (esferes i cilindres) de diferents tamanys, i s'han comparat les seves prestacions analítiques (relació senyal-soroll, sensibilitat, etc.). També s'han desenvolupat immunoassaigs quantitatius sense necessitat d'amplificació del senyal. Per altra banda, s'ha desenvolupat una aproximació que hem denominat "inmunocaptura", basada en l'ús de nanopartícules d'or com a espècie de captura d'analits en dissolució i que actua com a marcador de la inmunointeracció que té lloc en la fase sòlida. Finalment, s'han analitzat mostres d'aigües naturals dopades amb diferents nivells dels analits objecte d'estudi per avaluar la utilitat de les metodologies desenvolupades com a eina de "screening" massiu en l'àrea mediambiental. Els resultats obtinguts han sigut avaluats per comparació amb els obtinguts mitjançant tècniques de referència. Les investigacions realitzades han permès desenvolupar nous formats d'assaig i coneixements inmunoquímics aplicats a la tecnologia de disc compacte, aportant noves eines de "screening" que permetin la determinació de contaminants en aigües naturals per sota dels límits de concentració establerts per les normes internacionals de la qualitat de l'aigua. / Dobosz, PD. (2017). Screening methodologies for the determination of water contaminant residues by compact disk technology [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/79548 / TESIS

Joints between carbon steel and Alloy 600, containing Alloy 82 weld metal, were exposed to two steam-hydrogen environments considered to simulate exposure to primary water conditions in a Pressurized Water Reactor (PWR) or Canada Deuterium Uranium (CANDU) reactor. The welds were found to have elevated and variable iron contents due to dilution by carbon steel during welding. This gave the Alloy 82 weld, near the inner surface of the component, an iron content approaching that of Alloy 800. A potentially protective external iron oxide film formed on the inner surface of the weld. However, the chromium content throughout the weld is below that which would form an external chromium oxide. The results indicate that low chromium content causes internal oxidation throughout the weld and potentially below the external iron oxide which could lead to Primary Water Stress Corrosion Cracking (PWSCC).

Joints between carbon steel and Alloy 600, containing Alloy 82 weld metal, were exposed to two steam-hydrogen environments considered to simulate exposure to primary water conditions in a Pressurized Water Reactor (PWR) or Canada Deuterium Uranium (CANDU) reactor. The welds were found to have elevated and variable iron contents due to dilution by carbon steel during welding. This gave the Alloy 82 weld, near the inner surface of the component, an iron content approaching that of Alloy 800. A potentially protective external iron oxide film formed on the inner surface of the weld. However, the chromium content throughout the weld is below that which would form an external chromium oxide. The results indicate that low chromium content causes internal oxidation throughout the weld and potentially below the external iron oxide which could lead to Primary Water Stress Corrosion Cracking (PWSCC).

Um dos principais mecanismos de falha que causam riscos de fratura a reatores de água pressurizada é a corrosão sob tensão de ligas metálicas em água do circuito primário (CSTAP). É causada por uma combinação das tensões de tração, meio ambiente em temperatura e microestruturas metalúrgicas susceptíveis. Ela pode ocorrer, dentre outros locais, nos bocais do mecanismo de acionamento das barras de controle. Essa fratura pode causar acidentes que comprometem a segurança nuclear através do bloqueio das barras de controle e vazamentos de água do circuito primário reduzindo a confiabilidade e a vida útil do reator. O objetivo desta Tese de Doutorado é o estudo de modelos e uma proposta de modelagem para fraturas por corrosão sob tensão em liga 75Ni15Cr9Fe (liga 600), em água de circuito primário de reator de água pressurizada nesses bocais. São superpostos modelos eletroquímicos e de mecânica da fratura e validados com dados obtidos em experimentos e na literatura. Na parte experimental foram utilizados resultados obtidos pelo CDTN no equipamento recém-instalado de ensaio por taxa de deformação lenta. Na literatura está proposto um diagrama que exprime a condição termodinâmica de ocorrerem diversos modos de CSTAP na liga 600: partiu-se de diagramas de potencial x pH (diagramas de Pourbaix), para a liga 600 imersa em água primária à alta temperatura (3000C a 3500C). Sobre ele, determinaram-se os submodos de corrosão, a partir de dados experimentais. Em seguida acrescentou-se uma dimensão adicional ao diagrama, correlacionando uma variável a que se denominou fração de resistência à corrosão sob tensão. No entanto, é possível acrescentar-se outras variáveis que exprimem a cinética de iniciação e/ou crescimento de trinca, provenientes de outras modelagens de CSTAP. A contribuição original deste trabalho se insere nessa fase: partindo-se de uma condição de ensaio de potencial versus pH, foram iniciadas as modelagens de um modelo empírico-comparativo, um semi-empírico-probabilístico, um de tempo de iniciação e um de taxa de deformação, a partir dos ensaios experimentais e superpostas a essa condição. Esses exprimem respectivamente a susceptibilidade à CSTAP, o tempo de falha, e nos dois últimos o tempo de iniciação de falha por corrosão sob tensão. Os resultados foram comparados com os da literatura e se mostraram coerentes. Através desse trabalho, obteve-se uma metodologia de modelagem a partir de dados experimentais. / One of the main failure mechanisms that cause risks to pressurized water reactors is the primary water stress corrosion cracking (PWSCC) occurring in alloys. It can occurs, besides another places, at the control reactor displacement mechanism nozzles. It is caused by the joint effect of tensile stress, temperature, susceptible metallurgical microstructure and environmental conditions of the primary water. These cracks can cause accidents that reduce nuclear safety by blocking the rods displacement and may cause leakage of primary water, reducing the reactors life. In this work it is proposed a study of the existing models and a modeling proposal to primary water stress corrosion cracking in these nozzles in a nickelbased Alloy 600. It is been superposed electrochemical and fracture mechanics models, and validated using experimental and literature data. The experimental data were obtained at CDTN-Brazilian Nuclear Technology Development Center, in a recent installed slow strain rate testing equipment. In the literature it is found a diagram that indicates a thermodynamic condition for the occurrence of some PWSCC submodes in Alloy 600: it was used potential x pH diagrams (Pourbaix diagrams), for Alloy 600 in high temperature primary water (3000C till 3500C). Over it, were located the PWSCC submodes, using experimental data. It was added a third parameter called stress corrosion strength fraction. However, it is possible to superpose to this diagram, other parameters expressing PWSCC initiation or growth kinetics from other models. Here is the proposition of the original contribution of this work: from an original experimental condition of potencial versus pH, it was superposed, an empiric-comparative, a semi-empiric-probabilistic, an initiation time, and a strain rate damage models, to quantify respectively the PWSCC susceptibility, the failure time, and in the two lasts, the initiation time of stress corrosion cracking. It was modeling from our experimental data. The results were compared with the literature and it showed to be coherent. From this work was obtained a modeling methodology from experimental data.

Um dos principais mecanismos de falha que causam riscos de fratura a reatores de água pressurizada é a corrosão sob tensão de ligas metálicas em água do circuito primário (CSTAP). É causada por uma combinação das tensões de tração, meio ambiente em temperatura e microestruturas metalúrgicas susceptíveis. Ela pode ocorrer, dentre outros locais, nos bocais do mecanismo de acionamento das barras de controle. Essa fratura pode causar acidentes que comprometem a segurança nuclear através do bloqueio das barras de controle e vazamentos de água do circuito primário reduzindo a confiabilidade e a vida útil do reator. O objetivo desta Tese de Doutorado é o estudo de modelos e uma proposta de modelagem para fraturas por corrosão sob tensão em liga 75Ni15Cr9Fe (liga 600), em água de circuito primário de reator de água pressurizada nesses bocais. São superpostos modelos eletroquímicos e de mecânica da fratura e validados com dados obtidos em experimentos e na literatura. Na parte experimental foram utilizados resultados obtidos pelo CDTN no equipamento recém-instalado de ensaio por taxa de deformação lenta. Na literatura está proposto um diagrama que exprime a condição termodinâmica de ocorrerem diversos modos de CSTAP na liga 600: partiu-se de diagramas de potencial x pH (diagramas de Pourbaix), para a liga 600 imersa em água primária à alta temperatura (3000C a 3500C). Sobre ele, determinaram-se os submodos de corrosão, a partir de dados experimentais. Em seguida acrescentou-se uma dimensão adicional ao diagrama, correlacionando uma variável a que se denominou fração de resistência à corrosão sob tensão. No entanto, é possível acrescentar-se outras variáveis que exprimem a cinética de iniciação e/ou crescimento de trinca, provenientes de outras modelagens de CSTAP. A contribuição original deste trabalho se insere nessa fase: partindo-se de uma condição de ensaio de potencial versus pH, foram iniciadas as modelagens de um modelo empírico-comparativo, um semi-empírico-probabilístico, um de tempo de iniciação e um de taxa de deformação, a partir dos ensaios experimentais e superpostas a essa condição. Esses exprimem respectivamente a susceptibilidade à CSTAP, o tempo de falha, e nos dois últimos o tempo de iniciação de falha por corrosão sob tensão. Os resultados foram comparados com os da literatura e se mostraram coerentes. Através desse trabalho, obteve-se uma metodologia de modelagem a partir de dados experimentais. / One of the main failure mechanisms that cause risks to pressurized water reactors is the primary water stress corrosion cracking (PWSCC) occurring in alloys. It can occurs, besides another places, at the control reactor displacement mechanism nozzles. It is caused by the joint effect of tensile stress, temperature, susceptible metallurgical microstructure and environmental conditions of the primary water. These cracks can cause accidents that reduce nuclear safety by blocking the rods displacement and may cause leakage of primary water, reducing the reactors life. In this work it is proposed a study of the existing models and a modeling proposal to primary water stress corrosion cracking in these nozzles in a nickelbased Alloy 600. It is been superposed electrochemical and fracture mechanics models, and validated using experimental and literature data. The experimental data were obtained at CDTN-Brazilian Nuclear Technology Development Center, in a recent installed slow strain rate testing equipment. In the literature it is found a diagram that indicates a thermodynamic condition for the occurrence of some PWSCC submodes in Alloy 600: it was used potential x pH diagrams (Pourbaix diagrams), for Alloy 600 in high temperature primary water (3000C till 3500C). Over it, were located the PWSCC submodes, using experimental data. It was added a third parameter called stress corrosion strength fraction. However, it is possible to superpose to this diagram, other parameters expressing PWSCC initiation or growth kinetics from other models. Here is the proposition of the original contribution of this work: from an original experimental condition of potencial versus pH, it was superposed, an empiric-comparative, a semi-empiric-probabilistic, an initiation time, and a strain rate damage models, to quantify respectively the PWSCC susceptibility, the failure time, and in the two lasts, the initiation time of stress corrosion cracking. It was modeling from our experimental data. The results were compared with the literature and it showed to be coherent. From this work was obtained a modeling methodology from experimental data.

Les aciers inoxydables austénitiques de type 304L et 316L sont largement employés dans le circuit primaire des centrales nucléaires à Réacteurs à Eau sous Pression (REP). Le retour d'expérience indique la présence d'un nombre limité de fissures intergranulaires dues à la corrosion sous contrainte (CSC) sur des composants en acier inoxydable écroui. Il a été démontré qu'une pré-déformation importante associée à un chargement cyclique favorise la propagation des fissures de CSC. L'objectif de l'étude est d'améliorer la compréhension du rôle de la pré-déformation par traction ou par laminage sur les mécanismes d'amorçage de la CSC pour les aciers inoxydables austénitiques. Le comportement mécanique des matériaux écrouis a été caractérisé et des essais d'amorçage en milieu primaire simulé ont été réalisés sur des éprouvettes entaillées. L'ensemble des essais d'amorçage réalisés a confirmé un fort effet du trajet de chargement sur la sensibilité à l'amorçage des matériaux étudiés, quel que soit le niveau de pré-déformation. Un critère global a été proposé pour réunir les deux aspects de l'amorçage de la fissuration que sont la densité de fissures et leur profondeur. Ce critère est utile pour caractériser l'amorçage tandis que la profondeur maximale de fissure est le paramètre pertinent pour définir la transition entre propagation lente et propagation rapide. Des cartes de sensibilité à l'amorçage vrai ont été établies. Une profondeur critique de fissure de 10 à 20 µm a été déterminée pour les aciers 316L A et B pré-déformés par laminage. Elle est comprise entre 20 µm et 50 µm pour les matériaux pré-déformés par traction. Une ébauche de modèle d'ingénierie applicable aux aciers inoxydables austénitiques a été proposée : l'effet de la température est négligeable dans la gamme 290°C-360°C et l'impact de la contrainte sur le temps pour obtenir la transition varie comme (max/Rp0.2, T°C)11,5. L'effet du trajet de chargement ainsi que de l'écrouissage de surface dû à l'usinage sont intégrés indirectement à l'indice de contrainte, à ce stade du modèle. L'effet « matériau » observé dans cette étude tient principalement à l'effet du trajet de déformation. La puissance élevée de la dépendance à la contrainte traduit l'intégration de différents paramètres favorisant la localisation de la déformation. Pour cette raison, il sera nécessaire de définir le champ des contraintes locales pour parvenir à une modélisation plus physique. / Austenitic stainless steels are widely used in primary circuits of Pressurized Water Reactors (PWR) plants. However, a limited number of cases of Intergranular Stress Corrosion Cracking (IGSCC) has been detected in cold-worked (CW) areas of non-sensitized austenitic stainless steel components in French PWRs. A previous program launched in the early 2000's identified the required conditions for SCC of cold-worked stainless steels. It was found that a high strain hardening coupled with a cyclic loading favoured SCC. The present study aims at better understanding the role of pre-straining on crack initiation and at developing an engineering model for IGSCC initiation of 304L and 316L stainless steels in primary water. Such model will be based on SCC initiation tests on notched (not pre-cracked) specimens under “trapezoidal” cyclic loading. The effects of pre-straining (tensile versus cold rolling), cold-work level and strain path on the SCC mechanisms are investigated. Experimental results demonstrate the dominating effect of strain path on SCC susceptibility for all pre-straining levels. Initiation can be understood as crack density and crack depth. A global criterion has been proposed to integrate both aspects of initiation. Maps of SCC initiation susceptibility have been proposed. A critical crack depth between 10 and 20 µm has been demonstrated to define transition between slow propagation and fast propagation for rolled materials. For tensile pre-straining, the critical crack depth is in the range 20 - 50 µm. Experimental evidences support the notion of a KISCC threshold, whose value depends on materials, pre-straining ant load applied. The initiation time has been found to depend on the applied loading as a function of (max/YV)11,5. The effect of both strain path and surface hardening is indirectly taken into account via the yield stress. In this study, material differences rely on strain path effect on mechanical properties. As a result, a stress high exponent has been identified which includes all micro-scale mechanisms leading to strain localisation at initiation sites.

Les métaux déposés base nickel sont utilisés pour assembler des composants du circuit primaire des centrales nucléaires à Réacteurs à Eau sous Pression (REP). Un nombre croissant de cas de fissuration par Corrosion Sous Contrainte (CSC) des soudures en alliages base nickel 182 et 82 est rapporté dans le retour d'expérience international ce qui motive le développement d'un modèle permettant de prévoir la fissuration par CSC de ces matériaux. Ce mécanisme de dégradation fait intervenir des paramètres matériaux, mécaniques ou environnementaux qui peuvent interagir entre eux. L'objectif de cette étude est de mieux comprendre les mécanismes physiques locaux (aux joints de grains) impliqués dans l'amorçage de fissures de CSC. Un essai de traction sur une éprouvette en alliage 182 préalablement oxydée en milieu primaire simulé a mis en évidence une dispersion de la sensibilité à l'oxydation des différents joints de grains. L'analyse couplée entre oxydation et fissuration a permis, à l'aide de calculs de micro-mécanique sur un agrégat polycristallin synthétique, de proposer un critère de rupture des joints de grains oxydés défini par un couple profondeur d'oxydation/ contrainte locale critique. Compte tenu du rôle clé que tient l'oxydation intergranulaire dans le mécanisme de fissuration par CSC et de la dispersion observée entre les différents joints de grains, une cinétique d'oxydation intergranulaire des alliages base nickel 182 et 82 a été identifiée prenant en compte la précipitation de carbures de chrome, la température ou encore la teneur en hydrogène dissous. Ce modèle cinétique permet d'aborder statistiquement l'oxydation des joints de grains et est intégré à un modèle d'amorçage local. Dans ce dernier l'amorçage, défini par la fissuration de l'oxyde intergranulaire est suivi d'une phase de propagation lente puis rapide jusqu'à une certaine profondeur de fissure. Des hypothèses simplificatrices ont été faites lors de l'identification des lois embarquées dans le modèle de CSC. Toutefois, celles-ci s'avèreront utiles pour cibler les conditions des futurs essais à mener afin de conforter l'identification des différents paramètres. / Nickel base welds are widely used to assemble components of the primary circuit of Pressurized Water Reactors (PWR) plants. International experience shows an increasing number of Stress Corrosion Cracks (SCC) in nickel base welds 182 and 82 which motivates the development of models predicting the time to SCC initiation for these materials. SCC involves several parameters such as materials, mechanics or environment interacting together. The goal of this study is to have a better understanding of the physical mechanisms occurring at grains boundaries involved in SCC. In-situ tensile test carried out on oxidized alloy 182 evidenced dispersion in the susceptibility to corrosion of grain boundaries. Moreover, the correlation between oxidation and cracking coupled with micro-mechanical simulations on synthetic polycrystalline aggregate, allowed to propose a cracking criterion of oxidized grain boundaries which is defined by both critical oxidation depth and local stress level. Due to the key role of intergranular oxidation in SCC and since significant dispersion is observed between grain boundaries, oxidation tests were performed on alloys 182 and 82 in order to model the intergranular oxidation kinetics as a function of chromium carbides precipitation, temperature and dissolved hydrogen content. The model allows statistical analyses and is embedded in a local initiation model. In this model, SCC initiation is defined by the cracking of the intergranular oxide and is followed by slow and fast crack growth until the crack depth reaches a given value. Simplifying assumptions were necessary to identify laws used in the SCC model. However, these laws will be useful to determine experimental conditions of future investigations carried out to improve the calibration used parameters.

Les systèmes de maintien situés sur les structures assemblages-combustibles des réacteurs nucléaires à eau sous pression (REP) sont constitués d'un empilement de lames qui agissent à la fois, comme élément accommodant les incompatibilités thermiques résultant des différences de coefficients de dilatation Acier, alliages de zirconium et principalement, comme système permettant de limiter les effets hydrodynamiques induits par le passage du fluide caloporteur à travers les assemblages. Actuellement, l'alliage 718 est le matériau constitutif de ces ressorts à lames. Il présente les performances en service nécessaires et suffisantes pour répondre aux sollicitations de ces systèmes ainsi qu'aux exigences des autorités de sûreté (dans les conditions actuelles de fonctionnement des REP). Or, dans le cadre de l'augmentation des performances générales des assemblages combustibles, l'emploi d'autres matériaux, dont les propriétés de relaxation sous flux neutronique sont supérieures à celles du 718, est envisagé par AREVA. Les matériaux étudiés sont principalement des superalliages base Ni, tels que les nuances 625+ et 725 qui à l'instar de l'alliage 718 durcissent par précipitation de phases secondaires, ainsi que des nuances d'alliage 718 riche en Molybdène. Cependant, bien que ces nouveaux matériaux présentent une relaxation sous flux neutronique améliorée, ils doivent répondre également à un cahier des charges strict, propre à leur utilisation en centrale : des propriétés mécaniques équivalentes, une bonne résistance à la corrosion sous contrainte (CSC) et une bonne résistance à la fragilisation par l'hydrogène (FPH) en milieu primaire de REP. Chacune de ces propriétés a été étudiée avec attention dans le double but de comparer ces matériaux entre eux et afin de cerner les paramètres clés contrôlant leur différence de comportement aussi bien en CSC qu'en FPH / Hold-down systems used in the fuel assembly of Nuclear Pressurized Water Reactor (PWR) are constituted by stiff springs. The role of the hold-down springs is to ensure the bond between the fuel assembly and the lower plate of the intern structure of the core, thus holding down the assembly on the bottom plate of the reactor, during all the exploitation and maintenance periods. Nowadays, alloy 718 is the constitutive material of these hold-down springs. Its properties in terms of mechanical behaviour, corrosion resistance… fill in the specifications required for such application in the present service conditions. However, in order to improve the common efficiency of fuel assemblies, the upgrading of their design as well as the use of new materials are advocated by the nuclear power plant company, AREVA. Though other Ni-base superalloys known for their good behaviour under neutronic radiation can be proposed as new materials, those superalloys must fill in all the application specifications in order to substitute alloy 718. So, sufficient mechanical properties, good resistance to Stress Corrosion Cracking (SCC) and good resistance to Hydrogen Embrittlement (HE) are also required to allow the replacement. All of these properties are carefully studied with the double aim to characterize and compare different superalloys, and to determine key parameters governing the SCC and HE behaviours of such alloys in primary water of PWR