Modelling deuterium release during thermal desorption of D-irradiated tungsten

Thermal desorption profiles were modelled based on SIMS measurements of implantation profiles and using the multi-trap diffusion code TMAP7 [G.R. Longhurst, TMAP7: Tritium Migration Analysis Program, User Manual, Idaho National Laboratory, INEEL/EXT-04-02352 (2004)]. The thermal desorption profiles were the result of 500 eV/D⁺ irradiations on single crystal tungsten at 300 and 500 K to fluences of 10²²-10²⁴ D⁺/m². SIMS depth profiling was performed after irradiation to obtain the distribution of trapped D within the top 60 nm of the surface. Thermal desorption spectroscopy (TDS) was performed subsequently to obtain desorption profiles and to extract the total trapped D inventory. The SIMS profiles were calibrated to give D concentrations. To account for the total trapped D inventory measured by TDS, SIMS depth distributions were used in the near-surface (surface to 30 nm), NRA measurements [V.Kh. Alimov, J. Roth, M. Mayer, J. Nucl. Mater. 337-339 (2005) 619] were used in the range 1-7 μm, and a linear drop in the D distribution was assumed in the intermediate sub-surface region (∼30 nm to 1 μm). Traps were assumed to be saturated so that the D distribution also represented the trap distribution. Three trap energies, 1.07 ± 0.03, 1.34 ± 0.03 and 2.1 ± 0.05 eV were required to model the 520, 640 and 900 K desorption peaks, respectively. The 1.34 and 1.07 eV traps correspond to trapping of a first and second D atom at a vacancy, respectively, while the 2.1 eV trap corresponds to atomic D trapping at a void. A fourth trap energy of 0.65 eV was used to fit the 400 K desorption peak observed by Quastel et al. [A.D. Quastel, J.W. Davis, A.A. Haasz, R.G. Macaulay-Newcombe, J. Nucl. Mater. 359 (2006) 8].     

Mechanical properties of Eurofer 97 in Pb-16Li and irradiation effect

To be used in a fusion reactor, structural materials, and in particular steels, has to be selected and optimised in their composition to achieve a reduction in the long-term radioactive waste. A reduction in the long-term radioactive inventory could be reached substituting elements like molybdenum, niobium and nickel with other ones like tantalum and tungsten which have the same functions as alloying elements and, if irradiated, do not produce long lived radioisotopes. The martensitic steel belonging to the family of 8-9% Cr Eurofer 97 is considered the reference structural steel for fusion application. However, only few information are available about its mechanical properties in the liquid eutectic alloy Pb-16%Li. Particularly, the problem of liquid metal embrittlement (LME) has not been studied in detail and the effect of neutron irradiation on LME has not been investigated at all so far. This work presents the results obtained irradiating tensile specimens of Eurofer 97 up to 5.9 dpa in lead lithium. Tensile tests of samples have been performed out of pile in the same alloy at the same temperature at which irradiation was carried out.     

Epitaxial growth and surface morphology of aluminum films deposited on mica studied by transmission electron microscopy and atomic force microscopy

The bulk structure and epitaxial growth of aluminum films deposited on mica substrates by thermal evaporation in a wide temperature range (16-550 °C) in high vacuum were investigated by transmission electron microscopy and transmission electron diffraction. The surface morphology of the films was observed and analyzed by atomic force microscopy. The films prepared at room temperature consist of single crystals having a diameter of 90 ± 40 nm with (111) planes. The surface of the films comprises spherical grains with morphology that is caused by self-shadowing during the deposition. The surface of the films becomes smoother as the temperature increases, and atomically-smooth surfaces with a root-mean-square roughness of about 0.45 nm over an area of 1 μm² are obtained at 250-350 °C. The crystals are oriented randomly along the [111] direction perpendicular to the substrate. The surface of the films consists of larger (> 300 nm) grains with terraces, and the surface becomes rough above 400 °C. Films with well-oriented single crystals along the [111] direction perpendicular to the substrate are obtained above 520 °C. The films grown epitaxially at 520-550 °C are characterized by the isolated grains with a diameter of 1220 ± 450 nm.     

Monitoring the embrittlement of reactor pressure vessel steels by using the Seebeck coefficient

The degree of embrittlement of the reactor pressure vessel (RPV) limits the lifetime of nuclear power plants. Therefore, neutron irradiation-induced embrittlement of RPV steels demands accurate monitoring. Current federal legislation requires a surveillance program in which specimens are placed inside the RPV for several years before their fracture toughness is determined by destructive Charpy impact testing. Measuring the changes in the thermoelectric properties of the material due to irradiation, is an alternative and non-destructive method for the diagnostics of material embrittlement. In this paper, the measurement of the Seebeck coefficient () of several Charpy specimens, made from two different grades of 22 NiMoCr 37 low-alloy steels, irradiated by neutrons with energies greater than 1 MeV, and fluencies ranging from 0 up to 4.5 × 10¹⁹ neutrons per cm², are presented. Within this range, it was observed that increased by ≈500 nV/°C and a linear dependency was noted between and the temperature shift ΔT_41 J of the Charpy energy vs. temperature curve, which is a measure for the embrittlement. We conclude that the change of the Seebeck coefficient has the potential for non-destructive monitoring of the neutron embrittlement of RPV steels if very precise measurements of the Seebeck coefficient are possible.     

Radiation damage effects in the uranium-bearing δ-phase oxide Y6U1O12

Ion irradiation damage effects in delta (δ) Y₆U₁O₁₂ were characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Experimental results revealed no amorphization transformation occurs in Kr-ion irradiated Y₆U₁O₁₂ to a maximum displacement damage dose of ∼50 displacements per atom at cryogenic temperature. Density functional theory calculations indicate that δ-Y₆U₁O₁₂ possesses a relatively low cation antisite formation energy, which may help to explain the observed resistance of δ-Y₆U₁O₁₂ to irradiation-induced amorphization of δ-Y₆U₁O₁₂.     

Confinement of motion of interstitial clusters and dislocation loops in BCC Fe-Cr alloys

This work is devoted to the study of the effect of Cr solutes on the mobility of self interstitial atom (SIA) clusters and small interstitial dislocation loops (of size up to a few nanometers) in concentrated Fe-Cr alloys. Atomistic simulations have been performed to characterize the variation of the free energy of interstitial loops in the Fe-15Cr alloy using the experimentally determined profile of Cr distribution along the path of a loop. It is shown that the presence of randomly distributed Cr in Fe leads to the creation of local trapping configurations for small SIA clusters. The strength (trapping energy) and density of these configurations depend on the Cr content. On the contrary, large SIA clusters (which can be described as 1/2〈1 1 1〉 dislocation loops) are strongly affected by the presence Cr-Cr pairs and larger Cr clusters, which act as barriers to their motion.     

Secondary electron yield of Au and Al2O3 surfaces from swift heavy ion impact in the 2.5-7.9 MeV/amu energy range

We report on the secondary electron yields of Au and oxidized aluminum (Al₂O₃) by impact of heavy ions with energies ranging from 7.92 MeV/amu (¹²C₆) to 2.54 MeV/amu (¹⁰⁷Ag₄₇). The obtained results, the first in this energy range using medium-heavy ions, extend the validity of proposed scaling laws obtained with lighter ions. Measurements have been performed using the SIRAD irradiation facility at the 15 MV Tandem of the INFN Laboratory of Legnaro (Italy), to evaluate the performance of ion electron emission microscopy at SIRAD.     

Dosimetric calibration of solid state detectors with low energy β sources

A PTW Optidos plastic scintillation and a PTW natural diamond detectors were calibrated in terms of absorbed dose to water with β fields produced by ⁹⁰Sr + ⁹⁰Y and ⁸⁵Kr reference sources. Each source was characterized at the Italian National Metrological Institute - the Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti of ENEA (ENEA-INMRI) - for two different series, 1 and 2, of ISO reference β-particle radiation fields. Beam flattening filters were used for the series 1 β fields to give uniform absorbed dose rates over a large area at a source-to-reference plane distance of 30 cm. The series 2 β fields were produced at source-to-reference plane distance of 10 cm, without the beam flattening filters, in order to obtain higher absorbed dose rates.The reference absorbed dose rate values were directly determined by the Italian national standard for β-particle dosimetry (a PTW extrapolation ionization chamber) for the series 1 β fields and by a calibrated transfer standard chamber, (a Capintec thin fixed-volume parallel plate ionization chamber) for the series 2 β fields. Finally the two solid state detectors were calibrated in terms of absorbed dose to water with the series 2 β field.The expanded uncertainties of the calibration coefficients obtained for the plastic scintillation dosimeter were 10% and 12% (2SD) for the ⁹⁰Sr + ⁹⁰Y and the ⁸⁵Kr sources, respectively. The expanded uncertainties obtained for the diamond dosimeter were 10% (2SD) and 16% (2SD) for the ⁹⁰Sr + ⁹⁰Y and the ⁸⁵Kr sources, respectively.The good results obtained with the ⁹⁰Sr + ⁹⁰Y and the ⁸⁵Kr β sources encourage to implement this procedure to calibrate this type of detectors at shorter distances and with other β sources of interest in brachytherapy, for example the ¹⁰⁶Ru source.     

Measurement of specimen thickness by phase change determination in TEM

A non-destructive method for measuring the thickness of thin amorphous films composed of light elements has been developed. The method employs the statistics of the phase of the electron exit wave function. The accuracy of this method has been checked numerically by the multislice method and compared with that based on the mean inner potential.     

Bright-field TEM imaging of single molecules: Dream or near future?

We examine the suitability of spherical aberration (C(S))-corrected (CS) and uncorrected (UC) transmission electron microscopes (TEM) for conventional bright-field imaging of radiation-sensitive materials. We have chosen an individual molecule suspended in vacuum as a hypothetical example of a well-defined radiation-sensitive sample. We find that for this particular sample, CS instruments provide about 30% improvement over an UC instrument in terms of signal/noise ratio per unit electron dose at 300kV. The lowest imaging doses can be achieved in CS instruments equipped with high-brightness electron source operated at low incident electron energies. Our calculations suggest that it may be possible to image individual, iodine- or bromine-substituted organic molecules in bright-field mode, at doses lower than the accepted values for radiation damage of aromatic molecules.