Improvements to the Sink Strength Theory Used in Multi-Scale Rate Equation Simulations of Defects in Solids

The application of mean-field rate theory equations have proven to be a versatile method in simulating defect dynamics and temporal changes in the microstructure of materials. The reliability and usefulness of the method, however, depends critically on the defect interaction parameters used. In this study, we show that the main interaction parameter, the sink strength, intrinsically depends on the detrapping, or the dissociation process itself. We present a theory on how to determine the appropriate sink strengths. The correct sink strength required for a detrapping defect, is considerably larger than the values commonly used, and thus should not be neglected.Peer reviewe

Multiscale study on hydrogen mobility in metallic fusion divertor material

For achieving efficient fusion energy production, the plasma-facing wall materials of the fusion reactor should ensure long time operation. In the next step fusion device, ITER, the first wall region facing the highest heat and particle load, i.e. the divertor area, will mainly consist of tiles based on tungsten. During the reactor operation, the tungsten material is slowly but inevitably saturated with tritium. Tritium is the relatively short-lived hydrogen isotope used in the fusion reaction. The amount of tritium retained in the wall materials should be minimized and its recycling back to the plasma must be unrestrained, otherwise it cannot be used for fueling the plasma. A very expensive and thus economically not viable solution is to replace the first walls quite often. A better solution is to heat the walls to temperatures where tritium is released. Unfortunately, the exact mechanisms of hydrogen release in tungsten are not known. In this thesis both experimental and computational methods have been used for studying the release and retention of hydrogen in tungsten. The experimental work consists of hydrogen implantations into pure polycrystalline tungsten, the determination of the hydrogen concentrations using ion beam analyses (IBA) and monitoring the out-diffused hydrogen gas with thermodesorption spectrometry (TDS) as the tungsten samples are heated at elevated temperatures. Combining IBA methods with TDS, the retained amount of hydrogen is obtained as well as the temperatures needed for the hydrogen release. With computational methods the hydrogen-defect interactions and implantation-induced irradiation damage can be examined at the atomic level. The method of multiscale modelling combines the results obtained from computational methodologies applicable at different length and time scales. Electron density functional theory calculations were used for determining the energetics of the elementary processes of hydrogen in tungsten, such as diffusivity and trapping to vacancies and surfaces. Results from the energetics of pure tungsten defects were used in the development of an classical bond-order potential for describing the tungsten defects to be used in molecular dynamics simulations. The developed potential was utilized in determination of the defect clustering and annihilation properties. These results were further employed in binary collision and rate theory calculations to determine the evolution of large defect clusters that trap hydrogen in the course of implantation. The computational results for the defect and trapped hydrogen concentrations were successfully compared with the experimental results. With the aforedescribed multiscale analysis the experimental results within this thesis and found in the literature were explained both quantitatively and qualitatively.Kevyiden atomien fuusiossa vapautuvaa energiaa eräänä ihmiskunnan energiatuotantomuotona on tutkittu yli puoli vuosisataa. Fuusioenergian etuja ovat muun muuassa lähes rajoittamattomat polttoainereservit, se ettei se tuota kasvihuonekaasuja eikä fuusioreaktioissa synny pitkäikäistä radioaktiivista jätettä. Merkittävä edistysaskel kohti ensimmäisen fuusiovoimalan toteutumista otettiin vuonna 2005, kun suuren kansainvälisen testireaktorin, ITERin, sijoituspaikasta sovittiin. Vielä nykyään on monia avoimia kysymyksiä, jotka liittyvät ensiseinämateriaalivalintoihin fuusioreaktorin vaativiin olosuhteisiin. Materiaalin tulee kestää hiukkaspommitusta sekä korkeita lämpötiloja. Ensiseinämateriaalin ja fuusiopolttoaineen vuorovaikutusmekanismit on tunnettava. Tässä työssä tutkittiin vuorovaikutuksista aiheutuvien ensiseinävaurioiden syntymekanismeja sekä niiden merkitystä polttoaineesta irtaantuvien hiukkasten kerääntymiseen ensiseinään. Fuusioreaktorin polttoaine koostuu vedyn sekä sen isotooppien deuteriumin ja tritiumin muodostamasta plasmasta. Plasmaa ohjataan ja pyritään pitämään eristettynä ensiseinästä voimakkailla magneettikentillä. Plasmasta karanneet ionit kiertävät magneettikenttäviivoja pitkin ja lopulta iskeytyvät ensiseinään. ITERin kaltaisissa nk. tokamak-tyyppisissä fuusioreaktoreissa suurin hiukkas- ja lämpötilakuorma kohdistuu kammion pohja-alueella sijaitsevalle poikkeuttajalle. Sopivana poikkeuttajan ensiseinämateriaalina pidetään tällä hetkellä volframia, joka on hiukkaskuormitusta kestävä metalli ja jolla on hyvä lämmönjohtokyky. Vedyn ja volframin vuorovaikutuksia tutkittiin sekä kokeellisesti että laskennallisesti. Kokeellinen osuus koostui vedyn ioni-istutuksista volframiin, istutuksista aiheutuneiden kidevirheiden analysoinnista sekä kidevirheisiin loukkuuntuneiden vetyatomien lukumäärän määrittämisestä eri lämpötiloissa. Laskennallisilla menetelmillä määritettiin istutuksissa syntyneiden kidevirheiden tyyppi ja lukumäärä sekä niiden muodostumisen energetiikka. Lisäksi määriteltiin vedyn erityyppisistä loukuista vapautumiseen vaadittavat energiat sekä verrattiin niitä kokeellisesti mitattuihin vapautumislämpötiloihin. Laskennallisessa osuudessa käytettiin menetelmiä, jotka yhdessä kattavat monta aika- ja pituusmittakaavaa. Tällä tehokkaalla laskentatavalla päästiin vertaamaan mikroskooppisia, laadullisia ilmiöitä kokeellisesti saatuihin määrällisiin arvoihin. Merkittävimpinä tuloksina mainittakoon vedyn uudet diffuusionopeudet sekä eri kidevauriotyyppien laadullinen määrittäminen ja syntymekanismit. Tehty tutkimus on perustutkimusta ja se antaa uutta tarkempaa tietoa vetykaasun liikkumiseen ja loukkuuntumiseen vaikuttavista tekijöistä metalleissa yleensä, mutta sillä on myös suora teknologinen sovellus ITERin ensiseinämateriaalien suunnittelussa

Hydrogen isotope exchange experiments in high entropy alloy WMoTaNbV

Plasma–facing components in future fusion reactors must endure high temperatures as well as high fluxes and fluences of high energy particles. Currently tungsten has been chosen as the primary plasma-facing material due to its good thermal conductivity, low erosion rate and low fuel retention. Materials with even better properties are still being investigated to be used in reactor regions with demanding plasma conditions. High entropy alloys (HEA) are a new class of metallic alloys and their exploitation in fusion applications has not been widely studied. In this work, the hydrogen isotope exchange effect in an equiatomic HEA containing W, Mo, Ta, Nb, and V was studied. Deuterium was implanted into HEA samples with 30 keV/D energy and the HEA and reference samples were annealed in H2 atmosphere and in vacuum at various temperatures up to 400 °C, respectively. The near-surface D concentration profiles were measured with ERDA and the isotope exchange was observed to remove over 90 % of the trapped deuterium from the implantation region at temperatures above 200 °C. TDS was used to measure retention deeper in the bulk in which the reduction of trapped deuterium was significantly lower. High total retention of H was found in the bulk after H2 atmosphere annealing which indicates permeation and deep trapping of H in the material.Plasma-facing components in future fusion reactors must endure high temperatures as well as high fluxes and fluences of high energy particles. Currently tungsten has been chosen as the primary plasma-facing material due to its good thermal conductivity, low erosion rate and low fuel retention. Materials with even better properties are still being investigated to be used in reactor regions with demanding plasma conditions. High entropy alloys (HEA) are a new class of metallic alloys and their exploitation in fusion applications has not been widely studied. In this work, the hydrogen isotope exchange effect in an equiatomic HEA containing W, Mo, Ta, Nb, and V was studied. Deuterium was implanted into HEA samples with 30 keV/D energy and the HEA and reference samples were annealed in H2 atmosphere and in vacuum at various temperatures up to 400 °C, respectively. The near-surface D concentration profiles were measured with ERDA and the isotope exchange was observed to remove over 90 % of the trapped deuterium from the implantation region at temperatures above 200 °C. TDS was used to measure retention deeper in the bulk in which the reduction of trapped deuterium was significantly lower. High total retention of H was found in the bulk after H2 atmosphere annealing which indicates permeation and deep trapping of H in the material.Peer reviewe

Deposition of impurity metals during campaigns with the JET ITER-like Wall

Post mortem analysis shows that mid and high atomic number metallic impurities are present in deposits on JET plasma facing components with the highest amount of Ni and W, and therefore the largest sink, being found at the top of the inner divertor. Sources are defined as “continuous” or “specific”, in that “continuous” sources arise from ongoing erosion from plasma facing surfaces and “specific” are linked with specific events which decrease over time until they no longer act as a source. This contribution evaluates the sinks and estimates sources, and the balance gives an indication of the dominating processes. Charge exchange neutral erosion is found to be the main source of nickel, whereas erosion of divertor plasma facing components is the main source of tungsten. Specific sources are shown to have little influence over the global mid- and high-Z impurity concentrations in deposits.Peer reviewe

Effect of composition and surface characteristics on fuel retention in beryllium- containing co-deposited layers

We have investigated retention of deuterium in beryllium- containing, laboratory-made films whose properties resemble co- deposits observed on JET-ILW or predicted for ITER. The samples were prepared using High Power Impulse Magnetron Sputtering and Thermo-Vacuum Arc Deposition. We have observed that retention depends on the flux of D atoms on the growing film, but even more prominently on its composition, structure, and morphology. Especially, inclusion of carbon by 10-15 at.% in the layers can increase retention by a factor of 2-10. This we attribute to increasing number of defects as well as aromatic and aliphatic C-D bonds in the samples. Other impurities do not significantly alter the D inventory while more D is retained in samples with rough or highly modified surfaces. Our results show that reproducing the reported D concentrations of ~5 at.% in JET-ILW- like deposits requires keeping the sample temperature at 100- 200°C during the production phase and optimizing the uniformity of deposition fluxes. Data from Be-D samples further indicate that fuel retention in more ITER-relevant co-deposits would be around 1-2 at.%

Irradiation Damage Independent Deuterium Retention in WMoTaNbV

High entropy alloys are a promising new class of metal alloys with outstanding radiation resistance and thermal stability. The interaction with hydrogen might, however, have desired (H storage) or undesired effects, such as hydrogen-induced embrittlement or tritium retention in the fusion reactor wall. High entropy alloy WMoTaNbV and bulk W samples were used to study the quantity of irradiation-induced trapping sites and properties of D retention by employing thermal desorption spectrometry, secondary ion mass spectrometry, and elastic recoil detection analysis. The D implantation was not found to create additional hydrogen traps in WMoTaNbV as it does in W, while 90 at% of implanted D is retained in WMoTaNbV, in contrast to 35 at% in W. Implantation created damage predicted by SRIM is 0.24 dpa in WMoTaNbV, calculated with a density of 6.044×1022 atoms/cm3. The depth of the maximum damage was 90 nm. An effective trapping energy for D in WMoTaNbV was found to be about 1.7 eV, and the D emission temperature was close to 700 °C

Overview of the JET ITER-like wall divertor

The work presented draws on new analysis of components removed following the second JET ITER-like wall campaign 2013–14 concentrating on the upper inner divertor, inner and outer divertor corners, lifetime issues relating to tungsten coatings on JET carbon fibre composite divertor tiles and dust/particulate generation. The results show that the upper inner divertor remains the region of highest deposition in the JET-ILW. Variations in plasma configurations between the first and second campaign have altered material migration to the corners of the inner and outer divertor. Net deposition is shown to be beneficial in the sense that it reduces W coating erosion, covers small areas of exposed carbon surfaces and even encapsulates particles.The work presented draws on new analysis of components removed following the second JET ITER-like wall campaign 2013–14 concentrating on the upper inner divertor, inner and outer divertor corners, lifetime issues relating to tungsten coatings on JET carbon fibre composite divertor tiles and dust/particulate generation. The results show that the upper inner divertor remains the region of highest deposition in the JET-ILW. Variations in plasma configurations between the first and second campaign have altered material migration to the corners of the inner and outer divertor. Net deposition is shown to be beneficial in the sense that it reduces W coating erosion, covers small areas of exposed carbon surfaces and even encapsulates particles.Peer reviewe

Data on erosion and hydrogen fuel retention in Beryllium plasma-facing materials

ITER will use beryllium as a plasma-facing material in the main chamber, covering a total surface area of about 620 m(2). Given the importance of beryllium erosion and co-deposition for tritium retention in ITER, significant efforts have been made to understand the behaviour of beryllium under fusion-relevant conditions with high particle and heat loads. This paper provides a comprehensive report on the state of knowledge of beryllium behaviour under fusion-relevant conditions: the erosion mechanisms and their consequences, beryllium migration in JET, fuel retention and dust generation. The paper reviews basic laboratory studies, advanced computer simulations and experience from laboratory plasma experiments in linear simulators of plasma-wall interactions and in controlled fusion devices using beryllium plasma-facing components. A critical assessment of analytical methods and simulation codes used in beryllium studies is given. The overall objective is to review the existing set of data with a broad literature survey and to identify gaps and research needs to broaden the database for ITER.Peer reviewe

Thermodynamics of impurity-enhanced vacancy formation in metals

Hydrogen induced vacancy formation in metals and metal alloys has been of great interest during the past couple of decades. The main reason for this phenomenon, often referred to as the superabundant vacancy formation, is the lowering of vacancy formation energy due to the trapping of hydrogen. By means of thermodynamics, we study the equilibrium vacancy formation in fcc metals (Pd, Ni, Co, and Fe) in correlation with the H amounts. The results of this study are compared and found to be in good agreement with experiments. For the accurate description of the total energy of the metal-hydrogen system, we take into account the binding energies of each trapped impurity, the vibrational entropy of defects, and the thermodynamics of divacancy formation. We demonstrate the effect of vacancy formation energy, the hydrogen binding, and the divacancy binding energy on the total equilibrium vacancy concentration. We show that the divacancy fraction gives the major contribution to the total vacancy fraction at high H fractions and cannot be neglected when studying superabundant vacancies. Our results lead to a novel conclusion that at high hydrogen fractions, superabundant vacancy formation takes place regardless of the binding energy between vacancies and hydrogen. We also propose the reason of superabundant vacancy formation mainly in the fcc phase. The equations obtained within this work can be used for any metal-impurity system, if the impurity occupies an interstitial site in the lattice. Published by AIP Publishing.Peer reviewe