Fuel materials to withstand high temperatures
Europe's energy demands continue to rise and investing in nuclear energy is imperative for a reliable, cost-effective and environmentally clean source of electricity. Over time, emphasis is placed on fuel materials as well as structural materials for cladding. Metal fuels have the advantage of much higher heat conductivity compared to oxide fuels, but cannot survive equally high temperatures because of high thermal expansion. To address this phenomenon, scientists processed a different phase structure of uranium that presents better homogeneous thermal expansion with EU funding of the project 'Uralloy' (URALLOY). The project tested uranium alloying with a range of transition metals, such as molybdenum, zirconium and tantalum. This was achieved through an entirely novel and inherently safe approach: thin-film synthesis via magnetron sputtering. Through this process, scientists prepared polycrystals or single crystals of thin-film samples of many different binary alloy combinations. Use of a capping layer served to prevent film oxidation. The purity and perfect flatness of the thin-film samples provided a simplified model surface that can subsequently be exposed to different environmental conditions during characterisation testing. URALLOY investigated the oxidation and corrosion properties of uranium alloy fuel materials especially during interim storage and under certain waste disposal conditions. In the case of the uranium-molybdenum system, the relatively low molybdenum content functioned as a stabilising factor of the single-phase uranium at a high temperature. Regarding the uranium-zirconium system, it demonstrated a mixture of low- and high-temperature uranium phases, whereas the uranium-tantalum system did not form an alloy at high temperatures. An important project outcome is that the generated thin-film uranium alloy samples have low mass and radioactivity. This is eliminating the need for designated infrastructure, making it easier for researchers to access, transport and experiment on such materials. Project findings will prove very useful for the very-high-temperature nuclear reactor that is one of the six technologies of Generation IV nuclear reactors.
