Search

The aim of the present study is to extend the knowledge about the formation and thermal stability of vacancy-type defects in tungsten under neutron irradiation, thereby mimicking the temperature and neutron flux expected in the ITER divertor. Neutron irradiation of single crystal tungsten, W(100), in the temperature range 600-1200 °C is performed up to 0.12 dpa. Positron annihilation spectroscopy is employed to detect the presence of open volume defects, while hardness tests are applied to relate the irradiation-induced defects with the modification of mechanical properties. Rationalization of the experimental results is enhanced by the application of a kinetic Monte Carlo simulation tool, applied to model the microstructural evolution under the neutron irradiation process. The relation between radiation microstructure and hardness is explained via a dispersed barrier model.

Radiation-induced embrittlement of nuclear steels is one of the main limiting factors for safe long-term operation of nuclear power plants. In support of accurate and safe reactor pressure vessel (RPV) lifetime assessments, we developed a physics-based model that predicts RPV steel hardening and subsequent embrittlement as a consequence of the formation of nano-sized clusters of minor alloying elements. This model is shown to provide reliable assessments of embrittlement for a very wide range of materials, with higher accuracy than industrial correlations. The core of our model is a multiscale modelling tool that predicts the kinetics of solute clustering, given the steel chemical composition and its irradiation conditions. It is based on the observation that the formation of solute clusters ensues from atomic transport driven by radiation-induced mechanisms, differently from classical nucleation-and-growth theories. We then show that the predicted information about solute clustering can be translated into a reliable estimate for radiation-induced embrittlement, via standard hardening laws based on the dispersed barrier model. We demonstrate the validity of our approach by applying it to hundreds of nuclear reactors vessels from all over the world., Comment: 22 pages, 9 figures

The M4F project brings together the fusion and fission materials communities working on the prediction of radiation damage production and evolution and their effects on the mechanical behaviour of irradiated ferritic/martensitic (F/M) steels. It is a multidisciplinary project in which several different experimental and computational materials science tools are integrated to understand and model the complex phenomena associated with the formation and evolution of irradiation induced defects and their effects on the macroscopic behaviour of the target materials. In particular the project focuses on two specific aspects: (1) To develop physical understanding and predictive models of the origin and consequences of localised deformation under irradiation in F/M steels; (2) To develop good practices and possibly advance towards the definition of protocols for the use of ion irradiation as a tool to evaluate radiation effects on materials. Nineteen modelling codes across different scales are being used and developed and an experimental validation programme based on the examination of materials irradiated with neutrons and ions is being carried out. The project enters now its 4th year and is close to delivering high-quality results. This paper overviews the work performed so far within the project, highlighting its impact for fission and fusion materials science. Peer Reviewed L. Malerba a,*, M.J. Caturla b, E. Gaganidze c, C. Kaden d, M.J. Konstantinovi´c e, P. Olsson f, C. Robertson g, D. Rodney h, A.M. Ruiz-Moreno i, M. Serrano a, J. Aktaa c, N. Anento j, S. Austin i, A. Bakaev e, J.P. Balbuena b, F. Bergner d, F. Boioli k, M. Boleininger l, G. Bonny e, N. Castin e, J.B. J. Chapman l, P. Chekhonin d, M. Clozel m, B. Devincre k, L. Dupuy g, G. Diego a, S.L. Dudarev l, C.-C. Fu g, R. Gatti k, L. G´el´ebart g, B. G´omez-Ferrer n, D. Gonçalves g, C. Guerrero a, P.M. Gueye n, P. H¨ahner i, S.P. Hannula o, Q. Hayat p, M. Hern´andez-Mayoral a, J. Jagielski m, N. Jennett p, F. Jim´enez a, G. Kapoor d, A. Kraych h, T. Khvan e,q, L. Kurpaska m, A. Kuronen r, N. Kvashin j, O. Libera s, P.-W. Ma l, T. Manninen o, M.-C. Marinica g, S. Merino a, E. Meslin g, F. Mompiou t, F. Mota a, H. Namburi s, C.J. Ortiz a, C. Pareige n, M. Prester u, R.R. Rajakrishnan t, M. Sauzay g, A. Serra j, I. Simonovski i, F. Soisson g, P. Sp¨atig v, D. Tanguy h, D. Terentyev e, M. Trebala o, M. Trochet g, A. Ulbricht d, M.Vallet g, K. Vogel d, T. Yalcinkaya w, J. Zhao r a Centro de Investigaciones Energ´eticas, Medioambientales y Tecnol´ogicas (CIEMAT), Madrid, Spain b Universidad de Alicante, San Vicente del Raspeig, Spain c Karlsruher Institut für Technologie (KIT), Karlsruhe, Germany d Helmholtz-Zentrum Dresden-Rossendorf Ev (HZDR), Rossendorf, Germany e Studiecentrum voor Kernenergie / Centre d’´Etude de l’´Energie Nucl´eaire (SCK CEN), Mol, Belgium f KTH Royal Institute of Technology, Stockholm, Sweden g Universit´e Paris-Saclay, Commissariat `a l’´Energie Atomique et aux ´Energies Alternatives (CEA), Gif-sur-Yvette, France h Institut Lumi`ere Mati`ere (ILM), Centre National de la Recherche Scientifique, Lyon, France i Joint Research Centre (JRC)- European Commission, Petten, the Netherlands j Universitat Polit`ecnica de Catalunya, Barcelona, Spain k Laboratoire d’Etude des Microstructures (LEM), Centre National de la Recherche Scientifique, Chˆatillon, France l United Kingdom Atomic Energy Authority (UKAEA), Culham, UK m Narodowe Centrum Badan Jadrowych (NCBJ), Swierk, Poland n Normandie Univ, UNIROUEN, INSA Rouen, CNRS, Groupe de Physique des Mat´eriaux, 76000 Rouen, France o Aalto University, Espoo, Finland p Coventry University, UK q Universit´e de Li`ege, Belgium r Helsingin Yliopisto, Helsinki, Finland s Centrum Vyzkumu ˇReˇz S.R.O. (CVR), ˇReˇz, Czech Republic t Centre pour l’´Elaboration Elaboration de Mat´eriaux et pour l’´Etude des Structures (CEMES), Centre National de la Recherche Scientifique, Toulouse, France u Institut za Fiziku, Zagreb, Croatia v Paul Scherrer Institut (PSI), Villingen, Switzerland w Middle East Technical University (METU), Ankara, Turkey

The thermal stability and the structure of solute-vacancy clusters formed by neutron irradiation are studied by means of positron annihilation spectroscopy and hardness measurements of post-irradiation annealed reactor pressure vessel steels with high and low Ni contents. Two distinct recovery stages were observed and assigned to (a) the dissolution of vacancy clusters at about 650 K, and (b) the dissolution of solute-vacancy clusters at about 750 K. In steels with high Ni content, hardening mainly recovers during the second stage. Atomistic and coarse grain models suggest that during this stage, the removal of vacancies from vacancy-solute clusters leads to complete cluster dissolution, which indicates that solute clusters are radiation induced.x.

The microstructural features of intergranular irradiation-assisted stress corrosion crack tips from a redeemed neutron-irradiated flux thimble tube (60 dpa) have been investigated using focused-ion beam analysis and (scanning) transmission electron microscopy. The current work presents a close examination of the deformation field and oxide assembly associated with intergranular cracking, in addition to the analysis of radiation-induced segregation at leading grain boundaries. Evidence of stress induced martensitic transformation extending from the crack tips is presented. Intergranular crack arrest is demonstrated on the account of the external tensile stress orientation, and as a consequence of MnS inclusion particles segregating close to the fractured grain boundary. Exclusive observations of grain boundary oxidation prior to the cracking are presented, which is in full-agreement with the internal oxidation model.(c) 2022 Elsevier B.V. All rights reserved.

T91 steel samples were irradiated in both a neutron and a neutron plus proton irradiation environments, and the resulting microstructure was studied using transmission electron microscopy. Three samples were irradiated with neutrons and protons to a cumulative dose of 0.88, 2.07, and 4.35 dpa at temperatures of 254 °C, 260 °C and 320–350 °C, respectively and two samples were irradiated with neutrons to 0.6 and 1 dpa, at a temperature of 290 °C for comparison. Radiation induced dislocation loops were observed in both groups of samples. In all samples irradiated up to 1 dpa, irrespective of irradiation conditions, dislocation loops were segregated near the line dislocations, forming a clear heterogeneous distribution. In the high dose sample, on the other hand, they were uniformly distributed over the grain interior and the middle dose sample showed a mixed character. Voids of similar average size were only observed in the middle and high dose samples, and their appearance seems to be correlated with the onset of homogeneous loop distribution. The observed defect properties are rationalized on the basis of modeling results.

A probabilistic fracture model is applied to irradiation assisted stress corrosion cracking effect, assuming that the oxidized part of stainless steel sample plays an essential role in the crack initiation, propagation and the sample failure. The Weibull statistical distribution of time-to-failures, estimated through the correlation with the statistical distribution of oxide strengths, including both the surface oxide layer and oxidized grain boundaries, fully describe the experimental scatter obtained in the constant load time-to-failure tests. Large failure uncertainties in these type of tests originate from an intrinsic stochastic behavior of the oxide cracking due to subcritical crack propagation process.

The effect of simultaneous thermal and gamma irradiation ageing on the mechanical and physicochemical properties of industrial EPDM was investigated. Accelerated ageing, covering a wide range of dose rates, doses and temperatures, was preformed in stagnant air on EPDM polymer samples extracted from the cables in use in the Belgian nuclear power plants. The mechanical properties, ultimate tensile stress and elongation at break, are found to exhibit the strong dependence on the dose, ageing temperature and dose rate. The thermal decomposition of aged polymer is observed to be the dose dependent when thermogravimetry test is performed under air atmosphere. No dose dependence is observed when thermal decomposition is performed under nitrogen atmosphere. The thermal decomposition rates are found to fully mimic the reduction of elongation at break for all dose rates and ageing temperatures. This effect is argued to be the result of thermal and radiation mediated oxidation degradation process.

Vacancy-solute clustering in neutron irradiated Fe–Cr alloys with various concentrations of Cr and minor solutes (Ni, Si and P) were studied by using coincidence Doppler broadening spectroscopy and small angle neutron scattering techniques. The results from both experiments, supported by an object kinetic Monte Carlo model, show in a very consistent way the existence and formation of vacancy-CrNiSiP clusters that play detrimental role in irradiation hardening. Similar solute cluster number density of about 30–50 × 10 16 c m − 3 and an average diameter of about 1 nm were estimated for all alloys containing minor solutes, irrespectively of the chromium content. In Fe9Cr ferritic and Fe9Cr ferritic/martensitic alloys, with significantly reduced concentration of minor solute elements, the main defects are vacancy clusters, with an average cluster size of about 10 and 2 vacancies, respectively. Large concentration of α ’ -precipitates was observed in Fe14Cr(NiSiP). However, both vacancy clusters and α ’ -precipitates provide significantly less impact to hardening in comparison to vacancy-CrNiSiP clusters. The fact that vacancy clustering in Fe9Cr ferritic alloy resembles that of pure iron suggests that Cr solutes may play lesser role in irradiation hardening of ferritic alloys and steels than previously believed.

The mechanical properties of EPDM polymers, degraded as a result of extensive thermal and radiochemical aging treatment, are studied. The focus is given to dose rate effects in polymer insulation materials extracted from industrial cables in use in Belgian nuclear power plants. All studied mechanical characteristics such as the ultimate tensile stress, the Young's modulus, and the total elongation (or elongation at break) are found to be strongly affected by the irradiation dose. The ultimate tensile stress and Young's modulus are clearly exhibiting the dose rate effect, which originated from oxidation mediated interplay of polymer cross-linking and chain scission processes. The change of crossover between these two processes is found to be gradual, without critical dose rate or temperature values. On the contrary, the total elongation is observed not to be sensitive neither to irradiation temperature nor to the dose rate. Both cross-linking and chain scission seem to affect the total elongation in a similar way by reducing the average polymers chain length. This idea is confirmed by the model which shows that all total elongation data as a function of irradiation time can be reproduced by varying a single parameter, the pre-exponential factor of the irradiation rate constant.

The expected degradation of mechanical properties of structural materials under irradiation (i.e. “in operation”) in nuclear components represents a significant challenge for the design to account the impact of long-term irradiation effects. Therefore, a development of scientific and engineering expertise to understand and possibly control the severe impact of harsh neutron irradiation on materials is one of the tasks in the current material’s research agenda. As it is well known from long-standing experience with fission systems, radiation embrittlement is caused by nano-scale features that obstruct plasticity mediated by dislocations. The present contribution highlights recent research and development efforts addressed towards the assessment of the interrelation between nano-structural features and hardening induced by neutron irradiation in fusion structural high-Cr ferritic/martensitic steels. In this work, the impact of doping by three chemical elements (nickel, silicon and phosphorus) on the microstructural evolution under neutron irradiation at 300 °C and 450 °C is studied experimentally as well as by computer simulations. The evolution of microstructure is assessed by transmission electron microscopy and object kinetic Monte Carlo simulations. The latter method utilizes a new approach enabling to follow both nano-scale irradiation defects and micro-segregation zones causing the formation of solute rich clusters, detectable by atom probe tomography. The results of the present work clearly point out that Ni–Si segregation alters the spatial and size distribution of dislocations loops already at as low dose as 0.1 dpa. The physical mechanisms behind the impact of minor alloying elements are discussed.

A probabilistic fracture mechanics model is applied to irradiation assisted stress corrosion cracking effect, assuming that the oxidized part of stainless steel sample plays an essential role in the crack propagation and sample failure. The Weibull statistical distribution of time-to-failures, estimated through the correlation with the statistical distribution of oxide strengths, fully accounts for the experimental results/scatter obtained in the constant load time-to-failure tests. Large failure uncertainties in these type of tests originate from intrinsic stochastic behavior of the oxide cracking due to subcritical crack propagation process.

The mechanical properties of spallation target components exposed to combined effects of proton and neutron irradiations and in contact with liquid metal provide important information for the assessment of structural component integrity, which is crucial for the design of accelerator driven reactor concepts such as the MYRRHA reactor. In this study we perform tensile tests on T91 steel samples extracted from the MEGAPIE, and from the TWIN-ASTIR experiment. The tests are performed at different temperatures as well as with and without the contact with liquid metal. In both groups of samples we observed significant influence of liquid metal on the tensile properties, in particular reduction of total elongation. The influence of different conditions in two irradiation programs on the mechanical properties, such as irradiation temperature fluctuations, the presence of neutron/proton irradiation, with and without the contact with lead-bismuth eutectic, different flux and fluence, are also discussed.

Thermal stability and the structure of clusters formed by neutron irradiation are studied by means of positron annihilation spectroscopy of post-irradiation annealed FeCu, FeCuMnNi and FeMnNi alloys and rigid lattice calculations. While most of the vacancy–solute clusters dissolve in the temperature range between 650 K and 700 K, the presence of Ni and Mn solutes in vacancy–MnNi clusters provides an additional thermal stability and shifts the annealing stage corresponding to the dissociation of these clusters to higher temperature. Very good agreement between the measurements and calculations is obtained for vacancy–MnNi clusters of nanometric size, containing of about 25–50% of vacancies.

The mechanical properties of neutron irradiated Fe–Cr ferritic model alloys with various concentrations of Cr and minor solutes (Ni, Si and P) were investigated along with high-Cr ferritic/martensitic steels. The neutron irradiation was performed in a materials test reactor at two temperatures, 290 ∘C and 450 ∘C, reaching a dose of 0.11 dpa. Tensile tests were subsequently performed on small flat specimens. Significant hardening of about 225 MPa was observed only in model alloys containing Ni, Si and P solute atoms, irrespective of the Cr content and the irradiation temperature. These results suggest that Cr solutes play lesser role in irradiation hardening of ferritic/martensitic alloys steels than previously believed, while minor solutes have a clear effect.

The thermal stability of Frank loops, black dots, cavities and γ′ precipitates in an irradiated 316 stainless steel was studied by transmission electron microscopy. The samples were retrieved from a thimble tube irradiated at around 320 °C up to 80 dpa in a commercial nuclear power reactor, and thermally annealed, varying both annealing temperature and time. With increasing annealing temperature the density of all defects gradually decreased, resulting in the complete removal of Frank loops at 550 °C. In contrast to other defects, the density of the γ′ precipitates sharply decreased with increasing annealing time, which indicates that the dissolution of the γ′ precipitates is governed by the iron diffusion length.

In spite of general understanding of the copper precipitation in neutron irradiated Cu-rich alloys, the role of carbon in the precipitation process as well as in the formation of other defects under neutron irradiation, and their overall effect to the dislocation dynamics, are not yet fully elucidated. The mechanical tests are performed in neutron irradiated Fe, Fe–0.1%Cu and Fe–0.3%Cu alloys at three different temperatures, and the results of the yield stress are compared with the internal friction measurements. It is found that the copper precipitation, governed by the neutron irradiation, causes the appearance of carbon redistribution. From the comparison of the yield stress changes at different temperatures and dose, the carbon contribution to the hardening is estimated.

The temperature dependent internal friction spectra of cold-worked and neutron irradiated iron and iron–copper binary alloys are investigated. By increasing dose, both γ - and Snoek-Koster-relaxation peaks exhibit strong shift towards low temperatures, as a consequence of the reduction of double kink activation energy. This shift is found to be the largest in alloys with the highest copper content. Besides, new modes appear in the spectra at energies of about 410 and 540 K. The 410 K peak intensity increases at the expense of Snoek-Koster peak intensity, indicating that redistribution of carbon takes place under irradiation, most probably as a result of grain boundary segregation. The presence of copper impedes the carbon redistribution by influencing the formation of carbon-vacancy complexes, which causes the grain boundary segregation, and activation of the 410 K relaxation process at larger neutron fluence in comparison with pure iron.

The oxide film, formed on the alloy 182 due to the exposure to simulated Pressurized Water Reactor primary water conditions, is characterized using X-ray Photoelectron Spectroscopy, Scanning Electron Microscopy and Raman spectroscopy. As a consequence of the low redox potential for nickel, loosely bound oxide film consisting mainly of chromium oxide, is formed on the surface of the alloy. The oxide film is found to have a double-layer structure, with an inner layer rich in Cr2O3, outer layer rich in FeCr2O4. Thin Ni(OH)2 and Fe3O4 clusters were observed on top of the oxide film. The morphology and thickness of the layers critically depend on the exposure times and surface treatments prior to the exposure.

Valence states of uranium in three ternary uranates KUO 3 , NaUO 3 and Ba 2 U 2 O 7 were studied using X-ray photoelectron spectroscopy (XPS). The analysis of the U4f photoelectron peak positions, peak widths at half maximum and satellite positions of U4f peaks, shows that the uranium in these compounds is present in a single valence state, namely U 5 + . Since the surface of these compounds is extremely susceptible to oxidation, the XPS spectra were measured after a carefully controlled etching of the surface using an Ar + ion beam so that the surface oxidation does not affect the XPS studies.

Tensile properties and internal friction of thermally aged (at 773 K) Fe–1 wt% Cu alloys are measured as a function of copper precipitation in the temperature region between 100 and 600 K. Two regimes have been clearly identified in terms of yield stress at low temperatures below 250 K. It was found that in the over-aged samples the yield stress decreases much more abruptly with the temperature than in the specimens annealed at shorter time. The internal friction spectra show the existence of similar features in all samples, which indicates that no drastic change of the microstructure occurs during aging. These results suggest that the lack of softening at low temperatures originates principally from thermally-activated kink-pair formation and gliding processes, controlled by the size and/or density of Cu-precipitates.

The internal friction spectra in cold-worked pure iron and iron–chromium alloys are measured in the inverted torsion pendulum, operating at 1.8 Hz and in the temperature range from 100 K to 600 K. The addition of chromium, in combination with a plastic deformation, activates the relaxation process producing an internal friction peak at about 210 K. The peak intensity exhibits a strong increase by increasing the concentration of solute atoms. On the basis of the estimated activation energy, E sk ∼ 0.57 eV, and atomistic simulations, this mode is assigned to the single-kink relaxation process corresponding to the recombination of geometrical kinks formed as a result of Cr-dislocation core interaction in the 1 / 2 〈 1 1 1 〉 screw dislocations.

Getting light out of silicon is a difficult task since the bulk silicon has an indirect energy electronic band gap structure. It is expected that this problem can be circumvented by silicon nanostructuring, since the quantum confinement effect may cause the increase of the silicon band gap and shift the photoluminescence into the visible energy range. The increase in resulting structural disorder also causes the phonon confinement effect, which can be analyzed with a Raman spectroscopy. The large phonon softening and broadening, observed in silicon nanowires, are compared with calculated spectra obtained by taking into account the anharmonicity, which is incorporated through the three and four phonon decay processes into Raman scattering cross-section. This analysis clearly shows that the strong shift and broadening of the Raman peak are dominated by the anharmonic effects originating from the laser heating, while confinement plays a secondary role.

In this paper we present a study of spin dynamics in CuO, Bi2CuO4and CuGeO3single crystals using Raman spectroscopy. The measurements of polarized Raman scattering spectra are performed in the temperature region from 10 to 300 K in various frequency ranges. We found and assigned the lines in the spectra that belong to magnetically ordered phases. The origin of these excitations, based on the temperature dependence of their energies, linewidths and symmetry arguments, is given. Also, we present calculations of two-magnon intensities, based on the densities of states, and compare them with experimental data.

We present the polarized far-infrared and Raman spectra of CuGeO3 single crystals in the temperature range between 10 and 300 K. Assignations of vibrational modes were performed on the basis of factor-group and normal-coordinate analyses; The frequencies of the infrared-active modes are determined using an oscillator-fitting procedure of reflectivity data. In the Raman-scattering spectra, besides phonon modes, two broad structures at about 500 and 1600 cm-1 are observed. The temperature dependences of the frequency, full width at half maximum (FWHM) and integrated intensities of these modes suggest their magnetic origin. In the temperature range we considered, no evidence of the spin-Peierls-transition contribution to the phonon and magnon spectra is found.

Some of the properties of the low dimensional electronically correlated materials CuGeO3 and NaV2O5 are discussed. The emphasis lies on recent results obtained using Raman scattering and optical absorption spectroscopy as a function of temperature, magnetic field and hydrostatic pressure.

The polarized Raman scattering spectra of Bi2CuO4 single crystals were measured at temperatures between 10 and 300 K. In addition to the Raman-active phonon modes, two new structures at 14 and 150 cm-1 appear at temperatures below 50 K. These modes, which are assigned as one- and two-magnon modes, have Eg symmetry and Eg and B1g symmetry, respectively. Their temperature dependences agree well with previous results of antiferromagnetic resonance experiments.

We investigated the local atomic and electronic structures of two related systematic sets of ternary uranium oxides, NaUO3–KUO3–RbUO3 and BaUO3–Ba2U2O7–BaUO4, by measuring the X-ray absorption near edge structure (XANES). The results are compared with calculations based on a self-consistent real space full multiple scattering analysis. We found a very good agreement between measured and calculated spectra, which indicates that the uranium ions are in a pure U5+ oxidation state in these compounds. The low energy shoulder observed in the U L3 edge XANES is an intrinsic feature of the uranium unoccupied 6d electronic states of the U5+ ions within the studied materials. Specific double shoulder features in the higher energy range of the U L3 edge XANES can be interpreted as indicative of the pure cubic perovskite structure.

X-ray photoelectron spectra of vanadium $2p$ states in ${\ensuremath{\alpha}}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Na}{\mathrm{V}}_{2}{\mathrm{O}}_{5}$ and $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Li}{\mathrm{V}}_{2}{\mathrm{O}}_{5}$, single crystals show distinct splitting of both $j=1∕2$ and $j=3∕2$ spin-orbit lines due to the existence of ${\mathrm{V}}^{4+}$ and ${\mathrm{V}}^{5+}$ ions. In contrast to $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Li}{\mathrm{V}}_{2}{\mathrm{O}}_{5}$, the ${\ensuremath{\alpha}}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Na}{\mathrm{V}}_{2}{\mathrm{O}}_{5}$ exhibits the disproportional intensities and larger binding energy difference of $({\mathrm{V}}^{4+})2{p}^{3∕2}$ and $({\mathrm{V}}^{5+})2{p}^{3∕2}$ lines, that we attribute to a charge delocalization effect. By increasing the sodium deficiency, both effects reduce in a very good agreement with a model that takes into account the relative abundance of ${\mathrm{V}}^{4+}$ and ${\mathrm{V}}^{5+}$ ions, and the change of degree of delocalization. We estimate that sodium deficiency of about 15% increases electron localization by about 0.4%.

In the current work, ITER grade W irradiated at 1200 oC to a dose of 0.18 dpa has been investigated by Transmission Electron Microscopy (TEM), Positron Annihilation Spectroscopy (PAS), X-ray diffraction, impulse excitation and depth-sensing micro-indentation. The results are compared with those of annealed unirradiated W at 1200 oC. In the as fabricated material dislocations and small vacancy clusters of 1 to 3 vacancies are present. After annealing at 1200 oC, the small vacancy clusters disappear and only the dislocations with almost unchanged density remain. Also, hardness is reduced by 4% and creep increases by 13%. Neutron irradiation at 1200 °C results in the increase of the dislocation line density by 3.7 times and in the formation of dislocation loops and voids. The irradiation also causes a hardness increase of about 20% and a creep decrease of 15%. Both the elastic and shear moduli decrease by 4% after irradiation at 1200 °C contrary to the effect of only annealing at this temperature which causes an increase in their values by 4%. Texture changes are observed only after neutron irradiation.