Characterisation of mechanical and thermal properties of tungsten for high power spallation target applications

Sammanfattning: The European Spallation Source (ESS), which is currently under construction in Lund, will use pure tungsten as the spallation neutron target. The tungsten will be irradiated by a 2 GeV proton beam, pulsed at a repetition rate of 14 Hz. Each pulse will deposit 357 kJ in the tungsten causing an instantaneous temperature rise of approximately 100 °C, during the 2.86 ms pulse duration. The target is wheel-shaped, 2.6 m in diameter, and will be rotating at 0.39 Hz to distribute the proton beam loads. The tungsten target consists of nearly 7000 bricks with dimensions 80×30×10 mm3. The bricks are separated by 2 mm wide cooling channels in which helium gas flows at a rate of 2.85 kg/s. The proton beam will be more powerful than the beam used at any other existing neutron spallation facility. Thanks to the high beam power and innovative moderator design, ESS will become the brightest neutron source in the world. However, designing the target is challenging, as its structure should withstand loads from the high beam power. The high intensity proton beam will not only cause cyclic thermo-mechanical loading of the tungsten, but also irradiation damage in the form of displacements of atoms in the microstructure, and production of a wide range of radioactive isotopes such as gaseous transmutation elements, and a significant fraction of solid transmutation elements, which will alter the thermal and mechanical properties of tungsten. Using such a powerful beam requires an optimal design of the target and a good understanding of the complex physical, mechanical, and thermal changes occurring in irradiated tungsten. It is also important to identify and mitigate potential issues and accidental scenarios during operation of the ESS target. The present work aims to characterise thermal and mechanical properties of high-energy proton and spallation neutron irradiated pure tungsten, for the use as a spallation target material. The work includes studies on fatigue properties of unirradiated tungsten from various processing routes, to determine which type of tungsten is the most durable under cyclic loads. The fatigue study served as a basis for setting the maximum acceptable stress of 100 MPa in the tungsten bricks during operation, as well as choosing rolling as the most suitable manufacturing method. The tungsten volume will be contained in a stainless-steel target vessel, which confines the helium gas target-coolant. Tungsten is known to be readily oxidised even at moderate temperatures, which makes even impurity levels of oxygen and water vapour in the helium a potential issue. Therefore, oxidation behaviour of tungsten in mildly oxidising atmospheres and temperatures relevant to ESS operation, was characterised. The results were used to set the upper temperature limit in the tungsten target during normal operation, as well as the maximum allowable temperature during off-normal incidents. However, both temperature and thermo-mechanical stress in the tungsten will alter as the properties of the material change due irradiation. The available data on proton irradiated tungsten is very limited. Therefore, in the present work, large efforts have been made for studying tungsten irradiated under similar conditions as the future ESS target material. Specifically, irradiation induced changes in thermal diffusivity, hardness, ductility, and ultimate tensile strength of tungsten irradiated by a high power proton beam at Paul Scherrer Institute (PSI), were studied. The results point towards a severe embrittlement of irradiated tungsten. Virtually zero plasticity was found in specimens tested up to 500 °C, and the hardness increased by more than 50%. Thermal diffusivity decreased by 28-51%, depending on the test temperature (25-500 °C). All tested irradiated specimens had lower irradiation damages than the ESS tungsten is expected to accumulate during the 5-year life time of the target. Finally, a concluding study is presented in which new calculations of temperature and the maximum stress in the bricks were made, based on the obtained experimental data on the thermal and mechanical properties of irradiated tungsten. Keywords: ESS, Tungsten, Spallation target, Mechanical properties, Fatigue, Oxidation, Irradiation effects, Thermal diffusivity.