Development of oxidation resistant W based alloys for fusion reactor materials

Abstract

Pure tungsten is currently the main candidate material for the fusion reactor. Compared to other materials, tungsten has the advantage of very low sputter erosion under bombarded with high-energy deuterium, tritium or helium ions and atoms from the plasma. Also tungsten has a high melting point and good thermal conductivity at elevated temperatures. Because of the advantages mentioned above, tungsten is considered as the main material of fusion reactor. However, in the case of accident scenario, the use of pure tungsten has a potential problem. In an accident situation, the tungsten plasma-facing components will be heated up to about 1473 K after 10-30 days due to the nuclear decay heat of in-vessel component. In such a situation intense air ingress into the reactor vessel would lead to the strong exothermic formation of $WO_3$ which is radioactive and highly volatile which has the possibility of leakage to outside. To avoid the formation of hazardous tungsten oxide self-passivating tungsten alloys have to be developed. These materials have the ability to adapt their properties to the environment. During the normal plasma operation in the reactor, lighter elements will be sputtered earlier and only a nearly pure tungsten surface will be facing the plasma. However, in the case of an accident, the alloying elements in the bulk of self-passivating tungsten alloys react with oxygen and generate their own stable oxides to protect a base material. The selected alloying elements must fulfil different criteria’s, mainly the formation of a protective and stable oxide layer and low neutron activation. Chromium is appropriate as oxide former because it forms a stable and protective oxide called chromia. Among the various systems which include chromium, the tungsten-chromium-titanium (W-Cr-Ti) ternary system showed excellent performance in terms of oxidation resistance. There has been no study of the oxidation behavior with various chromium contents in that ternary system. Also previous studies carried out oxidation test only up to $1000^\circ C$. Because the internal temperature of the reactor rises up to about $1200^\circ C$ in the accident scenario, it is indispensable to conduct the oxidation test at $1200^circ C$. In this study, the bulk alloys required for the oxidation test were fabricated by high purity raw powders of tungsten, chromium and titanium via powder metallurgy method. Samples of the various compositions of tungsten as a base material and chrome as an oxide former element were also fabricated. Oxidation test was conducted using the samples which were fabricated with various compositions via powder metallurgy at $800^\circ C$, $1000^\circ C$ and $1200^\circ C$ for 1 hour each.