Stabilization of defects by the presence of hydrogen in tungsten : simultaneous W-ion damaging and D-atom exposure

The possible mutual influence and synergistic effect between defect production and the presence of hydrogen isotopes in the crystal lattice of tungsten is studied. For this purpose, we perform modeling of experimental data where polycrystalline tungsten samples were in one case sequentially irradiated by 10.8 MeV tungsten ions followed by low-energy deuterium exposure and in the other case simultaneously irradiated by tungsten ions, while exposed to deuterium atoms. Modeling of the measured deuterium depth profiles and thermal-desorption spectra for different irradiation temperatures is performed by the MHIMS (migration of hydrogen isotopes in materials) code. A model of trap creation due to tungsten ion irradiation during the deuterium atom exposures is implemented. In both experimental series, the deuterium desorption peaks corresponding to defects induced by tungsten irradiation are described by the same two de-trapping energies of 1.83 and 2.10 eV. The experiments give unambiguous proof that the presence of deuterium increases the overall trap density. The modeling reveals that the two trap concentrations are affected differently by the temperature and presence of deuterium: the concentration of the low-energy trap is significantly higher in the case of simultaneous exposure compared to sequential exposure, especially at high temperature (2.2 times higher at 1000 K). The concentration of the high-energy trap is only weakly affected by the presence of hydrogen.Peer reviewe

Vpliv vodikovih izotopov na obnašanje napak v kristalni mreži volframa

Tungsten (W) is one of the primary candidates for a plasma facing material in future fusion tokamak reactors, as it has good thermal properties and low intrinsic hydrogen isotope (HI) retention. Unfortunately, HI retention will be increased by many orders of magnitude due to HI trapping in lattice defects created by 14 MeV neutrons originating from the fusion reaction. To study HI retention in displacement damaged W, usually laboratory experiments are used that employ MeV W ion irradiation to create displacement damage and deuterium (D) exposures of various types to populate the created damage. The results of such experiments are modelled using macroscopic rate equations (MRE) to determine the characteristics of the interaction between the displacement damage and HI. Recently, laboratory experiments have been conducted where MeV W ion irradiation of the material was performed while the material already contained or was simultaneously exposed to D. They showed that the presence of D during W ion irradiation increases the amount of created displacement damage compared to the case when no D is present. Although efforts have been made to explain this so-called stabilization effect of D using existing MRE models, they were not able to adequately explain the observed experimental results. In this work, we have upgraded an existing displacement damage creation model by coupling a displacement damage creation model with the kinetics of D transport and trapping. The increase of the defect density due to the presence of D was parametrized by a stabilization factor. The new displacement damage creation and stabilization model’s physical meaning is based on the assumption that defects that contain at least one D have a smaller probability of annihilation as compared to D-free defects. The model was included into the existing MHIMS-R code, which was primarily designed to replicate experiments that studied HI transport and retention. We were able to reproduce the results of two laboratory experiments available in the literature where in one case the D was present during the damage creation and in the second case the D was introduced into the sample with a prior D exposure. Despite the fact that the experiments studied D-induced stabilization with different experimental procedures, the simulation reproduced the results with similar values for the stabilization factors. Using newly gained insights, we performed several follow-up experiments. These include higher D and W flux and fluence experiments and an experiment which is a combination of the previously conducted experiments. These tested the predictions made by the model, which were mostly confirmed. However, the higher flux experiment could not be adequately reproduced, as the data showed that stabilization depends on the number of trapped D in a defect which was not included in the model at the time. For this purpose, a generalized model was developed which is backwards compatible and makes the model applicable for an even wider range of W and HI fluxes and sample temperatures. We have also conducted experiments in which we studied the influence of D on defect evolution at elevated material temperatures. No significant effect of D presence was observed. The experimental results were reproduced using the MHIMS-R code with which the annealing behaviour of individual defects at temperatures between 300 and 800 K has been determined. Based on the observed behaviour, we have developed a vacancy/vacancy cluster evolution model.Volfram (W) je eden od glavnih kandidatov za uporabo kot material v stiku s plazmo v prihodnjih fuzijskih reaktorjih tipa tokamak, saj ima dobre visokotemperaturne lastnosti in nizko naravno zadrževanje vodikovih izotopov (VI). Žal se bo v fuzijskih reaktorjih zadrževanje VI povečalo za več redov velikosti, saj se bodo le-ti ujeli v mrežnih poškodbah, ki jih bo povzročilo obstreljevanje materiala s 14-MeV nevtroni, ustvarjenimi v fuzijski reakciji. Ponavadi so za namen raziskav povezanih z zadrževanjem VI, uporabljeni MeV W ioni, ki ustvarijo mrežne poškodbe, ki so nato izpostavljene devteriju (D). Rezultate takšnih eksperimentov lahko opišemo z makroskopskimi enačbami (MRE) transporta in ujemanja D v mrežnih poškodbah. To omogoči kvantifikacijo osnovnih parametrov interakcije med VI in W. Nedavno izvedeni eksperimenti, v katerih je poškodovanje materiala potekalo ob prisotnosti D, nakazujejo, da prisotnost D poveča količino nastalih mrežnih poškodb. Kljub temu, da so v literaturi na voljo MRE simulacije predstavljenih eksperimentalnih rezultatov, so bili poskusi opisa tako imenovane stabilizacije zaradi prisotnosti D le delno uspešni. Z namenom opisa eksperimentov, ki preučujejo stabilizacijo poškodb ob prisotnosti D, smo nadgradili obstoječ model nastajanja mrežnih poškodb. Nadgradnja združuje dobro poznane procese nastanka poškodb ter kinematiko transporta in ujemanja VI v materialu. Nadgrajen model nastanjanja in stabilizacije poškodb smo vključili v obstoječo MRE kodo MHIMS-R. Porast količine nastalih mrežnih poškodb model opisuje s tako imenovanim stabilizacijskim faktorjem. Nov model temelji na predpostavki, da imajo mrežne poškodbe, ki vsebujejo vsaj en VI, manjšo verjetnost za anihilacijo kot poškodbe brez ujetih VI. Model smo najprej uspešno uporabili za opis dveh laboratorijskih eksperimentov iz literature. Pri prvem izmed eksperimentov je bil D v material uveden s predhodno izpostavitvijo, pri drugem pa hkrati z obstreljevanjem z W ioni. Kljub temu, da so bili v eksperimentih uporabljeni različni načini W-ionskega obstreljevanja in izpostavitve D, smo eksperimentalne rezultate uspešno opisali s podobnimi vrednostmi stabilizacijskega faktorja. Pridobljeno znanje smo uporabili za izvedbo dodatnih eksperimentov, s katerimi smo preizkusili podane napovedi modela. Ti vključujejo višje tokove W in D ter kombinacijo predhodne in hkratne izpostavitve materiala D v kombinaciji z obstreljevanjem z W ioni. Eksperimentalni rezultati so se večinoma dobro skladali z napovedmi modela. Vendarle pa opis eksperimenta z višjim tokom D ni bil popolnoma uspešen, saj podatki kažejo, da je stabilizacija poškodb odvisna od količine v njej ujetega D, kar v modelu še ni bilo vključeno. To dognanje je spodbudilo ustrezno posplošitev modela. Posplošitev je skladna z že izvedenimi simulacijami, prav tako pa omogoča uporabo modela za širši nabor tokov delcev W in D ter na širšem temperaturnem področju. Izvedli smo tudi eksperiment, ki je preučeval vpliv prisotnosti D na evolucijo mrežnih poškodb ob visokih temperaturah materiala. Eksperimentalni rezultati so pokazali na kvečjemu šibek vpliv D. Za opis rezultatov je bila znova uporabljena MHIMS-R koda, ki je s simulacijo eksperimentalnih rezultatov omogočila določitev obnašanja posameznih tipov mrežnih poškodb pri posameznih temperaturah materiala (300-800 K). S pomočjo rezultatov simulacije smo razvili model evolucije poškodb tipa vrzeli in skupki vrzeli

Arabidopsis halleri shows hyperbioindicator behaviour for Pb and leaf Pb accumulation spatially separated from Zn

Lead (Pb) ranks among the most problematic environmental pollutants. Background contamination of soils is nearly ubiquitous, yet plant Pb accumulation is barely understood. In a survey covering 165 European populations of the metallophyte Arabidopsis halleri, several field samples had indicated Pb hyperaccumulation, offering a chance to dissect plant Pb accumulation. Accumulation of Pb was analysed in A. halleri individuals from contrasting habitats under controlled conditions to rule out aerial deposition as a source of apparent Pb accumulation. Several elemental imaging techniques were employed to study the spatial distribution and ligand environment of Pb. Regardless of genetic background, A. halleri individuals showed higher shoot Pb accumulation than A. thaliana. However, dose–response curves revealed indicator rather than hyperaccumulator behaviour. Xylem sap data and elemental imaging unequivocally demonstrated the in planta mobility of Pb. Highest Pb concentrations were found in epidermal and vascular tissues. Distribution of Pb was distinct from that of the hyperaccumulated metal zinc. Most Pb was bound by oxygen ligands in bidentate coordination. A. halleri accumulates Pb whenever soil conditions render Pb phytoavailable. Considerable Pb accumulation under such circumstances, even in leaves of A. thaliana, strongly suggests that Pb can enter food webs and may pose a food safety risk