Performance of structural materials for the DEMO divertor

Abstract

One of the most critical issues to the development of nuclear fusion, as a viable and sustainable source of energy, is the design of structural materials that can withstand the harsh operating conditions inside the reactor. These conditions are especially hostile for the components that line the vacuum vessel and are exposed directly to the plasma (PFM, Plasma Facing Materials) in the divertor. They should work in particularly extreme conditions (both mechanical, thermal and electrical) while subjected to high particle and neutron flux. Furthermore, the joint between these PFMs and the proposed cooling systems of the reactor is an issue that has not been solved yet. The difference between the coefficients of thermal expansion and conductivity of the materials under research: tungsten base alloys as PFM and copper or CuCrZr pipes through which the cooling fluid will circulate, make it necessary the development and characterization of novel materials. Therefore, the scope of this thesis is to study the thermo‐mechanical performance of novel tungsten‐based alloys for future fusion reactors, for use both as PFM, in heat sinks (Heat Sink Materials) or as thermal barriers. The limited amount of available material (as they are in the laboratory stage), as well as the focus on simulating the operation conditions of pressure and temperature, made it essential to develop new techniques for mechanical characterization. Thus, it has been possible to examine the fracture behaviour of these materials, obtaining relevant variables such as the R‐curve as a function of temperature. This is something entirely new due to the characteristics of the materials under study. With the aim of improving the ductility of tungsten, for its use as PFM, several alloys have been studied (W 2 wt% Ti, W‐1 wt% TiC, W 5 wt% TiC, W 5 wt% Ta and W 15 wt % Ta). These were produced by powder metallurgy and consolidated by hot isostatic pressing (HIP). The effect of the different alloying elements (Ti, TiC and Ta) and their percentage, as well as the processing parameters (grinding and milling time), were analysed through flexural and fracture tests in the temperature range between 25 °C and 1200 °C, both in air and under high vacuum atmospheres. Additionally, the micro‐mechanisms of failure were identified by scanning electron microscopy. Thus, the processing parameters to achieve the optimal densification and microstructure were identified. However, despite of this, the results revealed brittle mechanical behaviour in almost the entire temperature range for the alloys, with no improvement in the ductile to brittle transition temperature, as compared to pure tungsten. Metal matrix composites produced by melt infiltration of copper and CuCrZr in a porous preform of tungsten have also been characterized up to 800 °C temperature and under high vacuum, for their use as heat sinks. In addition, it is worth highlighting the development and implementation of an experimental setup to perform tensile tests in this environment through digital image correlation. Accordingly, the mechanical and thermal properties (coefficients of thermal expansion and conductivity) of these materials have been obtained, and they are superior to those of the current commercial products. Furthermore, thermal properties can be tailored by controlling the porosity of the initial W preform, hence the composition of the final product. In order to increase the operating temperature of the reactor, without compromising the joint between the PFM and the cooling system, it is necessary to develop materials that act as a thermal barrier. For this purpose, WC‐Cu base cermets with different compositions (25, 50 and 75 vol% Cu) have also been studied. The mechanical and microstructural analysis was performed up to 800 °C, which is higher than the service temperature. The fracture surfaces of the test specimens revealed two predominant fracture mechanisms: (i) intragranular fracture by cleavage of WC grains; and ii) ductile rupture of the copper phase, whose effect on mechanical properties is more evident as its content in the cermet increases. Moreover, it has been shown that a small proportion of Cu in the cermet can considerably reduce the high difference between the thermal expansion coefficients of W and CuCrZr, used as PFM and cooling system, respectively, without significantly increasing its thermal conductivity. In the overall, a comprehensive characterization of the mechanical behaviour as a function of temperature of three groups of materials, with complementary tasks in the future fusion reactors ITER and DEMO, has been achieved. Furthermore, the understanding of fracture behaviour of tungsten‐based materials has been broadened and improved. To achieve it, new characterization techniques and a novel methodology for obtaining the R‐curve at high temperature, have been developed.