Advanced fission and future fusion energy will require new high-performance structural alloys with outstanding properties that are sustained under long-term service in ultrasevere environments, including neutron damage producing up to 200 atomic displacements per atom and, for fusion, 2000 appm of He. Following a brief description of irradiation damage and damage resistance, we focus on an emerging class of nanostructured ferritic alloys (NFAs) that show promise for meeting these challenges. NFAs contain an ultrahigh density of Y-Ti-O-enriched dispersion-strengthening nanofeatures (NFs) that, along with fine grains and high dislocation densities, provide remarkably high tensile, creep, and fatigue strength. The NFs are stable under irradiation up to 800°C and trap He in fine-scale bubbles, suppressing void swelling and fast fracture embrittlement at lower temperatures and creep rupture embrittlement at high temperatures. The current state of the development and understanding of NFAs is described, along wi...

Recent Developments in Irradiation-Resistant Steels

Ferritic martensitic steels show limited swelling and susceptibility to helium effects and can be made with low activation chemical compositions. These properties make them the reference steel for the development of breeding blankets in fusion power plants. EUROFER97 is the European implementation of such a steel, where experience gained from an IEA co-operation with Japan and the US is also implemented. Results obtained so far show that EUROFER steel has attractive mechanical properties even after long ageing times. Compatibility tests in water and PbLi17 are in progress. Oxidised aluminium is the most effective protective layer in PbLi17. The displacement damage and helium formation strongly influence the hydrogen transport in the steel. Present experiments should be backed by tests in a more fusion relevant environment, e.g. IFMIF. The 2.5 dpa neutron irradiations at low temperatures result in a higher DBTT. High dose irradiations, up to 80 dpa, are underway. The early results of ODS grades with EUROFER steel composition show potential of these grades for increasing the operating temperature with 100–150 K.

The development of EUROFER reduced activation steel

https://iopscience.iop.org/article/10.1088/1741-4326/57/9/092005/pdf

Development of next generation tempered and ODS reduced activation ferritic/martensitic steels for fusion energy applications

https://pure.mpg.de/pubman/item/item_2052989_2/component/file_2052987/Roth_Eurofer.pdf

EUROFER as wall material: reduced sputtering yields due to W surface enrichment

The development of first wall, blanket and divertor materials that are capable of withstanding for many years high neutron and heat fluxes, is a critical path to fusion power. Therefore, the timely availability of a sound materials database has become an indispensable element in international fusion road maps. In order to provide materials design data for the short-term needs of ITER test blanket modules (TBMs) and for a DEMOnstration fusion reactor, a wealth of R&D results on the European reduced activation ferritic–martensitic steel EUROFER, and on oxide dispersion strengthened variants are being characterized, mainly in the temperature window 250–650°C. The characterization includes irradiations up to 15 dpa in the mixed spectrum reactor HFR and up to 75 dpa in the fast breeder reactor BOR60. Industrial EUROFER-batches of 3.5 and 7.5 tons have been produced with a variety of semi-finished, quality-assured product forms. To increase the thermal efficiency of blankets, high temperature resistant SiCf/SiC channel inserts for liquid metal coolant tubes are also developed. Regarding radiation damage resistance, broad based reactor irradiation programmes deal with several ranges from ≤ 5 dpa (ITER TBMs) up to 75 dpa (DEMO). For the European divertor designers, a materials database is currently being set up for pure W and its alloys, and related reactor irradiations are foreseen with temperatures from 650°C to 1000°C.

https://iopscience.iop.org/article/10.1088/0029-5515/45/7/013/pdf

Towards reduced activation structural materials data for fusion DEMO reactors

EUROFER 97. Tensile, charpy, creep and structural tests

Le materiau de reference europeen EUROFER pour la premiere paroi d'un reacteur a fusion de demonstration a ete fabrique comme charge de 3,5 t a differentes dimensions de semi-produit. Apres un apercu historique sur la filiere de developpement qui va de l'acier classique martensitique a 12% de Cr jusqu'a MANET et OPTIFER et EUROFER d'activation reduite (RAFM-steel), on decrit les resultats des essais faits sur 2 dimensions de semi-produits (O100 mm et tole de 14 mm) pour obtenir une caracterisation de base. On compare les caracteristiques physiques et mecaniques avec celles d'OPTIFER-1W et de l'acier a 2% de W, F82H-mod. Pour etudier le comportement de transformation, on a etabli un diagramme de transformation temps-temperature continu et determine le coefficient de dilatation pour la temperature ambiante de 1000°C. Des essais de trempe dans le domaine de temperature de 850-1120°C donnent le domaine de trempabilite maximale et l'evolution de la grosseur des grains. Des essais de revenu et des essais de recuit supplementaires dans le domaine de T de 300-875°C donnent des eclaircissements sur le comportement de revenu, et les essais de recuit supplementaire permettent de conclure a la resistance au revenu. On decrit le comportement de resilience pour differents etats de traitement et des eprouvettes de 2 semi-produits differents pour le domaine de temperature d'essai de +60 a 100°C, et l'on determine les criteres de ductilite FATT-DBTT-68 J. On met l'accent specialement sur l'influence de la grosseur des grains et de la teneur en O 2 . Les caracteristiques de resistance a la traction pour differents etats de traitement sont determinees dans le domaine de temperature d'essai entre l'ambiante et 700°C. On a etudie le comportement de vieillissement du a l'effet de temperature de longue duree sur les caracteristiques de resistance a la traction, en effectuant des recuits de stabilisation a des couples T/t differents. En ce qui concerne les caracteristiques de resistance a la traction, EUROFER ne presente pratiquement aucune influence des traitements de recuits, et pour ce qui est des caracteristiques de resilience, il ne presente qu'une augmentation moderee des temperatures pour les criteres de resilience, ce qui signifie qu'EUROFER n'est pas susceptible de veillir. Le comportement de resistance a la rupture et au fluage est etudie dans le domaine de T de 450-650°C, et jusqu'a present, une duree d'essai allant jusqu'a 15 000 h environ est couverte experimentalement. L'etat d'avancement du programme d'essai permet de definir les valeurs minimales des limites d'allongement-temps 1% et de la resistance a la rupture pour une duree de service de 20 000 h. Le comportement de fluage avec l'exposant de tension qui en resulte peut etre indique sans equivoque pour le domaine de tension couvert par l'experience. Mais pour le domaine de tension important pour la conception ≤ 100 MPa jusqu'a 550°C, on ne dispose pas de donnees de fluage. Pour repondre a cette question, on a commence en 2001 a effectuer des essais de fluage speciaux a 500° et 550°C destines a determiner experimentalement les limites d'allongement importantes pour la conception de composants ( 0.01-1%), notamment la variation de la vitesse de fluage minimale en fonction de la tension dans le domaine des basses tensions. On souligne les problemes poses par une simple extrapolation.

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Ergebnisse von Charakterisierungsuntersuchungen zu physikalischen und mechanischen Eigenschaften des martensitischen 9% Cr-Stahles EUROFER'97

EUROFER 97 - the European reference material for the first wall of a DEMO fusion reactor - was produced as 3.5 t batch of rods and plates. Following the history of the development activities from conventional martensitic 12 % Cr steel, MANET and OPTIFER up to the low or reduced activation (RAFM) EUROFER steel, results obtained from experiments on specimens from rods (O 100 mm) and plates (14 mm) are presented for a basic characterization. Physical and mechanical properties are compared with those of OPTIFER-1W and the F82H-mod 2% W steel. The transition behaviour was determined by plotting a continuous TTT (time temperature transition) diagram. In addition, extension coefficients were determined from room temperature up to 1000 °C. Hardening tests at temperatures from 850 °C to 1120 °C illustrated the range of maximum hardness as well as grain size development. Tempering tests and additional annealing experiments from 300 °C to 875 °C allowed characterizing tempering behaviour and stability. Charpy properties were examined for various heat treatments and specimen types between 60 °C and -100 °C. Further, ductility criteria like FATT, DBTT and 68 J were determined. Particular attention was paid to the influence of grain size and O 2 content. Tensile strength was measured for several heat treatments between room temperature and 700 °C. Long-term ageing was investigated by means of stabilization annealing experiments. These were carried out with various temperature/time combinations including tensile tests. In EUROFER tensile strength was hardly affected by the different heat treatments while the ductility criteria showed only a moderate increase in temperature. Therefore, it can be concluded that EUROFER is not susceptible to ageing. Creep and creep rupture properties were investigated in the temperature range of 450 °C to 650 °C. So far, creep times of up to 15000 h have been covered by the experiments. The status of the test program allows for an extrapolation of the 1 % yield limits and times to rupture to a period of 20000 h. Creep behaviour and stress exponents can be determined reliably for the experimental stress range. However, for the design relevant low stress range (= 100 MPa) up to 550 °C creep data are still lacking. Therefore, special creep tests at 500 °C and 550 °C were started in 2001. They are aimed at the experimental determination of design relevant yield limits (0.01-1%) and minimum creep rate behaviour.

S. Heger

Papers

EUROFER 97. Tensile, charpy, creep and structural tests

Ergebnisse von Charakterisierungsuntersuchungen zu physikalischen und mechanischen Eigenschaften des martensitischen 9% Cr-Stahles EUROFER'97

Tensile, charpy, creep and structural tests