The influence of hydrogen isotopes on the behaviour of crystal lattice defects in tungsten

Pečovnik, Matic (author), Markelj, Sabina (mentor)

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.

• Language: English
• Keywords: Hydrogen isotopes, tungsten, hydrogen isotope retention, lattice defects, defect stabilization, macroscopic rate-equations
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2021
• PID: 20.500.12556/RUL-125631
• COBISS.SI-ID: 59557123
• Publication date in RUL: 28.03.2021

Secondary language

• Language: Slovenian
• Title: Vpliv vodikovih izotopov na obnašanje napak v kristalni mreži volframa

Abstract

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.

• Keywords: Vodikovi izotopi, volfram, zadrževanje vodikovih izotopov, mrežne poškodbe, stabilizacija poškodb, enodimenzionalne zasedbene enačbe