Thorium is a naturally radioactive metal found chiefly as a companion mineral in monazite. Chemically close to uranium, it has long been regarded as a candidate resource for future thorium-based nuclear fuel cycles. In the nuclear field, thorium-232 captures neutrons and is converted into fissile uranium-233, forming the thorium fuel cycle, which—compared with conventional uranium-fueled reactors—offers potential advantages such as lower waste radiotoxicity and a more abundant fuel resource; several countries operate research reactors and experimental programs exploring this route. In industry, thorium dioxide was once the core material of incandescent gas lamp mantles: the thorium in the mantle emits a bright white light at high temperature, which was thorium's largest commercial use in the early twentieth century and has now been replaced by rare-earth oxides and LEDs. In welding, tungsten electrodes containing about 2% thorium (thoriated tungsten electrodes) have long been used as the non-consumable electrodes in gas tungsten arc welding (TIG), because thorium improves electron emission and arc stability, although radiation concerns are gradually leading to replacement with thorium-free cerium-tungsten and lanthanum-tungsten electrodes. Small amounts of thorium have also been used in heat-resistant crucibles and as additives in specialty optical glass.
Thorium dioxide (ThO2), with its extremely high melting point and high-temperature chemical stability, was used to make gas lamp mantles and refractory crucibles and is also the primary form of fuel pellet in the thorium fuel cycle. Thorium nitrate (Th(NO3)4) was historically the key compound for making lamp mantles: fabric was dipped in a thorium nitrate solution and then ignited, leaving a luminous coating of thorium and rare-earth oxides; today it is used only in small amounts in front-end nuclear-fuel processing. Thorium tetrafluoride (ThF4) is an important intermediate in thorium-fuel reprocessing and molten-salt reactor research, used to purify thorium from ore and to prepare thorium metal; at present it exists only in small batches in laboratories and demonstration reactors.
