Thorium

Thorium is a naturally radioactive silvery metal that is about three times more abundant than uranium in the Earth's crust. It was once used to make gas mantles glow brilliantly white in camping lanterns. Today it is attracting interest as a nuclear fuel, thorium reactors could potentially generate power more safely and with less long-lived radioactive waste than uranium reactors.

• Atomic Number9090 protons, 90 electrons
• Atomic Mass232.038 u90× heavier than hydrogen
• State at Room TempSolidSolid
• Density11.72 g/cm³
• Melting / Boiling1749.8°C / 4787.9°C
• Discovered1828

What is Thorium?

Thorium is an actinide with 90 protons and primarily a +4 oxidation state. Its most stable isotope, thorium-232, has a half-life of 14 billion years, about the age of the universe, making it effectively primordial. Thorium-232 is "fertile": it absorbs a neutron and eventually converts to uranium-233, which is fissile and can sustain a chain reaction.

Named after Thor, the Norse god of thunder. Discovered by Jöns Jacob Berzelius in 1829 while analysing a mineral sent from a Norwegian priest. The mineral was later named thorite in honour of the element.

Fact India, which has enormous thorium deposits but limited uranium, has developed the world's most advanced thorium nuclear programme. India aims to run a large fraction of its future electricity from thorium in advanced reactor designs that breed uranium-233 from thorium-232.

Where you find Thorium

On Earth

Thorium is about three times more abundant than uranium in the Earth's crust at approx. 6-10 parts per million.

• Monazite sands. The primary thorium mineral, found in beach and river sands in India, Brazil, Australia, Norway and the USA.
• Thorite. A thorium silicate mineral found in Norway, where the element was discovered.

How we use Thorium

• Thorium fuel cycle.. Thorium-232 can be bred into fissile uranium-233 in nuclear reactors. Thorium reactors may produce much less long-lived radioactive waste than uranium reactors.
• Gas mantles.. Thorium oxide made incandescent gas lantern mantles glow with a brilliant white light. Now largely replaced by less radioactive alternatives.
• Tungsten electrodes.. Thoriated tungsten electrodes for TIG welding give better arc stability and easier starting.

Did you know? Thorium was proposed as a reactor fuel as early as the 1950s, and prototype molten salt thorium reactors were operated at Oak Ridge National Laboratory in the 1960s. The thorium fuel cycle produces far less plutonium and long-lived actinides than the uranium fuel cycle, which has renewed interest in thorium as a safer alternative. India, China and Norway are actively developing thorium reactor technologies.

How it was discovered

Discovered in 1829 by Jöns Jacob Berzelius, who analysed a black mineral found in Norway and sent to him by a pastor named Esmark. Berzelius recognised a new element and named it after the Norse god of thunder.

Deeper dive: thorium properties and applications

The thorium fuel cycle works differently from uranium. Thorium-232 is not fissile itself, it cannot sustain a chain reaction. But when it absorbs a neutron, it converts through two beta decays to uranium-233, which is fissile. A thorium reactor needs a small amount of fissile material (uranium-233 or uranium-235) to start the reaction; the thorium then breeds more uranium-233 to sustain it. Molten salt reactors, in which thorium and uranium are dissolved in a fluoride salt instead of solid fuel rods, are being developed in China, Canada, the USA and other countries as potentially safer designs.

Moving to 91 protons on the periodic table brings us to Protactinium.