Post irradiation examination of RAF/M steels after fast reactor irradiation up to 71 dpa and < 340°C (ARBOR 2) : RAFM steels: metallurgical and mechanical characterisation. (KIT Scientific Reports ; 7596)

The objective of the ARBOR 2 irradiation programme was to study the effects of low-temperature (330-340 °C) high dose (up to 70 dpa) neutron irradiation on the mechanical properties of the European reference structural material for the Demonstration Reactor (DEMO), EUROFER97, its Oxide Dispersion Strengthened (ODS) variants, selected technological specimens and other international RAFM steels. The impact, tensile and LCF properties have been studied by implying small specimen testing technology

Review and critical assessment of dislocation loop analyses on EUROFER97

The understanding of microstructural defects behavior after neutron irradiation is crucial for assessing the applicability of reduced activation ferritic/martensitic (RAFM) steel EUROFER 97 in future fusion reactors. Formation and evolution of dislocation loops is believed to play the major role in material's hardening under neutron irradiation. In this work, transmission electron microscopy (TEM) data on dislocation loop size distribution is provided after different irradiation campaigns to determine the role of neutron dose on the dislocation loop evolution. A comparison of investigations on dislocation loop behavior and appearance yield considerable differences. For a conclusive interpretation, this work reviews available data, and possible reasons for the observed differences are discussed. Recommendation for future TEM investigation are given. Keywords: TEM, Microstructural defects, Neutron irradiation, Fusion, Reduced activation ferritic/martensitic RAFM steel

Effect of neutron irradiation to 0.7 and 1.4 dpa on the tensile mechanical properties and microstructure of EUROFER97 steel

Several grades of reduced-activation ferritic-martensitic (RAFM) steels have been proposed for fusion applications (e.g., blanket first wall) since the 1990s all over the world. Four batches of the European reference RAFM steel EUROFER97 have been produced since 1998. The RCC-MRx design code, developed, among others, for fusion reactors, currently contains a provisional section dedicated to EUROFER97, encompassing properties of the first two batches, whereas minimum three batches are required for a full qualification and final inclusion of a material into RCC-MRx. The EUROfusion project coordinates efforts to broaden the knowledge of EUROFER97 properties relevant for fusion reactors ITER and DEMO, preparing them for closing the database gaps in RCC-MRx and aggregating them in the DEMO material property handbook (MPH). Its purpose is to provide average and minimum curves of required properties according to the DEMO engineering design and manufacturing needs. The present work reports mechanical properties and fractographic analysis of batch 4 of neutron-irradiated EUROFER97 for the first time. The measured strength and ductility are in line with the data already aggregated in the MPH. SEM investigation confirms that the dimple fracture is retained in the material after neutron irradiation up to 1.4 dpa in the temperature range 25…550 °C

Embrittlement of WCLL blanket and its fracture mechanical assessment

In the European fusion programme, the Water Cooled Lithium Lead breeding blanket (WCLL BB) uses EUROFER as a structural material cooled with water at temperatures between 295 °C–328 °C and a pressure of 155 bar. The WCLL BB will be significantly irradiated (>2 dpa), while some parts will not receive significant heat loads, e.g. the sidewalls or the back-supporting structures. The irradiation, together with the irradiation temperature of EUROFER below 350 °C, produces a shift of the ductile-to-brittle-transition temperature (DBTT) to levels above room temperature at neutron doses, causing material damage as low as 2–3 dpa. Even though the DBTT does not reach the operating temperature level, brittle/non-ductile fracture is a concern during in-vessel maintenance when the BB temperature is below the DBTT. Two loading scenarios were identified as severe in this respect: (i) re-pressurization of the WCLL BB cooling loop after in-vessel maintenance, and (ii) dead weight loads during lifting of the BB segment. The embrittlement of the WCLL BB was investigated by quantifying the local DBTT shift in its parts based on current knowledge of the embrittlement behaviour of EUROFER under neutron irradiation. Therefore, a suitable, not overly conservative procedure was derived considering dpa damage and transmuted helium effects. The results demonstrate the ability to identify the 3D spread of the severely embrittled zones in the structure whose impact on the structural integrity was assessed considering the risk of brittle/non-ductile fracture. Thereby, the fracture mechanics approach established in nuclear codes was applied assuming its applicability to EUROFER. The embrittled zones in the first wall (FW) and its sidewalls pass the criteria when assessing the relatively low stresses resulting from the coolant pressure. The assessment was then continued considering stresses appearing in the FW during maintenance, in particular, when lifting the BB segment and transporting it out of the vacuum vessel. In this context, the maximum tolerable flaw sizes were determined in a parameter study considering designs of the FW with different cooling channel wall thicknesses

Comparative small angle neutron scattering (SANS) study of Eurofer97 steel neutron irradiated in mixed (HFR) and fast spectra (BOR60) reactors

AbstractThis contribution presents a comparative microstructural investigation, carried out by Small-Angle Neutron Scattering (SANS), of ferritic/martensitic steel Eurofer97 (0.12 C, 9 Cr, 0.2V, 1.08Wwt%) neutron irradiated at two different neutron sources, the HFR-Petten (SPICE experiment) and the BOR60 reactor (ARBOR experiment). The investigated “SPICE” sample had been irradiated to 16dpa at 250°C, the investigated “ARBOR” one had been irradiated to 32dpa at 330°C. The SANS measurements were carried under a 1 T magnetic field to separate nuclear and magnetic SANS components; a reference, un-irradiated Eurofer sample was also measured to evaluate as accurately as possible the genuine effect of the irradiation on the microstructure. The detected increase in the respective SANS cross-sections of these two samples under irradiation is attributed primarily to the presence of micro-voids, for neutron contrast reasons; it is quite similar in the two samples, despite the higher irradiation dose and temperature of the “ARBOR” sample with respect to the “SPICE” one. This is tentatively correlated with the higher helium content produced under HFR irradiation, playing an important role to stabilize the micro-voids under irradiation. In fact, the size distributions obtained by transformation of the SANS data yield a micro-void volume fraction of 1.3% for the “SPICE” sample and of 0.6% for the “ARBOR” one

Microstructural defects in EUROFER 97 after different neutron irradiation conditions

Characterization of irradiation induced microstructural evolution is essential for assessing the applicability of structural steels like the Reduced Activation Ferritic/Martensitic steel EUROFER 97 in upcoming fusion reactors. In this work Transmission Electron Microscopy (TEM) is used to determine the defect microstructure after different neutron irradiation conditions. In particular dislocation loops, voids and precipitates are analyzed concerning defect nature, density and size distribution after irradiation to 15 dpa at 300 °C in the mixed spectrum High Flux Reactor (HFR). New results are combined with previously obtained data from irradiation in the fast spectrum BOR-60 reactor (15 and 32 dpa, 330 °C), which allows for assessment of dose and dose rate effects on the aforementioned irradiation induced defects and microstructural characteristics