Deuterium and Helium Retention in ITER Specification Tungsten before and after Plastic Deformation

Nuclear fusion, where light elements react to form heavier elements, is an alternative to nuclear fission and fossil fuels. Among its advantages are high fuel power density, no highly active long-lived nuclear waste, no emission of harmful gases like carbon dioxide. It is also important to mention, that nuclear fusion is potentially an inexhaustible source of energy, due to the availability of deuterium in sea water (about 30 mg per liter), while the technology for tritium breeding is under development. One of the most ambitious fusion projects nowadays is ITER – the International Thermonuclear Experimental Reactor under construction in Cadarache, France. ITER is not designed to convert fusion energy into electricity. Its main goal is to demonstrate the possibility to perform controlled nuclear fusion (with a plasma discharge of a given duration and density) and use it for energy generation. ITER’s experience will be very important for the following step in fusion energy generation – the construction of the demonstration fusion power plant DEMO. The majority of the interaction of plasma with the fusion chamber wall will happen in the divertor – the special component situated at the bottom of the chamber. It serves to extract heat and ash produced by the fusion reaction, and to minimize plasma contamination. Due to a number of advantages (the main of them is the high melting point – 3695 K), tungsten is chosen as a plasma-facing material for the ITER divertor. Among other attractive characteristics of tungsten, the following should be mentioned: high thermal conductivity, low erosion rate and low neutron irradiation swelling. During the operation in fusion environment, the surface of divertor components will be subjected to cyclic heat loads, and as a result a certain plastic deformation will accumulate in the material. This thesis is focused on the study of the impact of plastic deformation, given variable exposure conditions i.e. fluence and temperature, on deuterium and helium trapping and retention in tungsten. Polycrystalline tungsten provided by the Austrian company Plansee AG was exposed to plasma in linear plasma generator Pilot-PSI, Nieuwegein, the Netherlands. In particular, the material was studied in two conditions: reference (according to ITER specification) and plastically deformed (mimicking the effect of long term operation under cyclic load). Pilot-PSI allows one to perform exposures to a high-density low-temperature plasma mimicking the 'subdisplacement threshold' plasma-wall interaction conditions expected in the ITER divertor. The following plasma compositions were studied within this project: pure deuterium, pure helium and mixed beam (helium-deuterium) plasmas. The exposure temperature was varied in the range of 460-1000 K, the particle flux – (1-3) ×1024 ions /m2/s and the total fluence - 5×1025 – 1027 ions/m2. Thermal Desorption Spectroscopy (TDS) was used to measure the retention of plasma particles, the temperature ramp was kept at 0.5 K/s and the maximum (technically achievable) temperature was 1273 K. A few complementary techniques were used to support the results and discussion of the results of TDS, namely scanning electron microscopy (SEM), nanoindentation (NI), nuclear reaction analysis (NRA) and transmission electron microscopy (TEM). This thesis consists of four chapters. The first one provides a general introduction to the fundamental principles of fusion and plasma-wall interaction. It is also discussed why tungsten was chosen as plasma-facing material. Chapter 2 describes the materials studied in this project, the procedures of sample preparation and experimental facilities used for plasma exposure and subsequent analysis. Chapter 3 reports the experimental results and contains their discussion. It is divided into three sections which report (i) the details of the microstructure of the materials before the plasma exposure, (ii) results of TDS measurements, and (iii) complementary experimental analysis performed on the plasma exposed samples. Pure deuterium plasma exposure revealed that plastic deformation leads to the complex interplay in the intensity of three main deuterium release peaks. The total deuterium retention was progressively increasing with the fluence, and it is about 50% higher in plastically deformed samples compared to the reference material. Comparison of the retention in the samples exposed at similar temperatures revealed, that in the case of low temperature exposure (600 K and below) plastic deformation enhanced the total retention, while at high temperatures (800 K and above) it reduced it. Addition of helium in the plasma beam increased the integral retention of He significantly as the fraction of He raised from 80% to 100%. He seeding into the plasma also enhanced the integral retention of D in both reference recrystallized and plastically deformed samples. It was concluded that He seeding played a more important role in the D trapping than plastic deformation under mixed beam exposure. Comparison of pure deuterium and pure helium plasma exposures revealed a significant influence of the plastic deformation on the total retention of helium (the pre-straining suppressed He retention by a factor of three). However, a certain amount of helium may still remain in the samples, since TDS measurement was performed up to 1300 K, thus high temperature TDS is needed to confirm the conclusion. Complementary techniques were used to support the discussion of TDS results. Nanoindentation demonstrated that plasma exposure significantly increased the resistance to plastic penetration of the indenter in the sub-surface region affected by the exposure. Spatially resolved nanoindentation coupled with EBSD analysis was also applied to investigate the sensitivity of grains with different crystallographic orientations. It was demonstrated that oriented grains are more prone to the plasma exposure than others. SEM analysis revealed the presence of raptured blisters only on the surface of plastically deformed samples and not on the reference samples. The latter was attributed to the influence of the dislocation networks which led to a shallower nucleation of bubbles compared to the reference material. TEM was used to investigate the microstructure on the surface and in the sub-surface region after pure and mixed plasma exposures. A strong increase of the dislocation density (at least one order of magnitude higher) was revealed irrespective of the composition of the plasma. He exposure mainly induces sub-surface damage (within 5 μm), while for D exposure the damage extends to 15-20 μm in depth. The last chapter concludes and summarizes the main experimental observations reported in Chapter 3 and provides the outlook for further study.