Thorium
Thorium produces a radioactive gas, radon-220, as one of its decay products. Secondary decay products of thorium include radium and actinium. In nature, virtually all thorium is found as thorium-232, which undergoes alpha decay with a half-life of about 14.05 billion years. Other isotopes of thorium are short-lived intermediates in the decay chains of higher elements, and only found in trace amounts. Thorium is estimated to be about three to four times more abundant than uranium in the Earth's crust, and is chiefly refined from monazite sands as a by-product of extracting rare earth metals.
Thorium was once commonly used as the light source in gas mantles and as an alloying material, but these applications have declined due to concerns about its radioactivity. Thorium is also used as an alloying element in nonconsumable TIG welding electrodes.
Canada, China, Germany, India, the Netherlands, the United Kingdom, and the United States have experimented with using thorium as a substitute nuclear fuel in nuclear reactors. When compared to uranium, there is a growing interest in developing a thorium fuel cycle due to its greater safety benefits, absence of non-fertile isotopes, and its higher occurrence and availability. India's three stage nuclear power programme is possibly the most well known and well funded of such efforts.
Occurrence
Thorium is found in small amounts in most rocks and soils; it is three times more abundant than tin in the Earth's crust and is about as common as lead. Soil commonly contains an average of around 6 parts per million (ppm) of thorium. Thorium occurs in several minerals including thorite (ThSiO4), thorianite (ThO2 + UO2) and monazite. Thorianite is a rare mineral and may contain up to about 12% thorium oxide. Monazite contains 2.5% thorium, allanite has 0.1 to 2% thorium and zircon can have up to 0.4% thorium. Thorium-containing minerals occur on all continents. Thorium is several times more abundant in Earth's crust than all isotopes of uranium combined and thorium-232 is several hundred times more abundant than uranium-235.
232Th decays very slowly (its half-life is comparable to the age of the universe) but other thorium isotopes occur in the thorium and uranium decay chains. Most of these are short-lived and hence much more radioactive than 232Th, though on a mass basis they are negligible.
| Symbol | Th | |
| Atomic Number | 90 | |
| Atomic Weight | 232.0381 | |
| Oxidation States | +3, +4 | |
| State at RT | Solid, Metal | |
| Melting Point, K | 1842 |
Appearance and Characteristics
Harmful effects:
Thorium is radioactive. It collects in living animal bones, including human bone, where it can remain for a long period of time.
Characteristics:
- Thorium is a radioactive, bright, soft, silvery-white metal, which tarnishes extremely slowly (over many months) to the black oxide. The most stable isotope is thorium-232, with a half-life of 14.05 billion years. Nearly 100% of thorium found on Earth is thorium-232, which is only slightly radioactive because it has such a long half-life. (Uranium-235′s half-life is 700 million years, shorter by a factor of 20.)
- Thorium is chemically reactive and is attacked by oxygen, hydrogen, the halogens and sulfur. (6a) Thorium powder is pyrophoric (ignites spontaneously in air).
- Thorium is dimorphic, changing from face centered cubic to body centered cubic above 1360 oC.
- Thorium has the largest liquid range of any element, spanning over 3000 degrees between its melting point of 2023 K (1750 oC) and its boiling point of 5063 K (4790 oC).
- Thorium dioxide (thoria) has the highest melting point of any known oxide.
- Almost all naturally occurring thorium is thorium-232 which decays slowly to the Group 2 metal radium by emission of alpha particles.
- Thorium-232 can be converted by thermal (slow) neutrons to fissionable uranium-233 via the following reaction sequence:
232Th+ n ⇒ 233Th
ß decay ß decay
233Th ⇒ 233Pa ⇒ 233U
Uses of Thorium
- An exciting possibility for the future is fueling nuclear reactors with thorium. Not only is thorium more abundant on Earth than uranium, but 1 ton of mined thorium can produce as much energy as 200 tons of mined uranium.
- The difference in the energy output of the two elements arises because most uranium mined is uranium-238, which is not fissile. (Naturally occurring uranium is over 99% uranium-238 with only about 0.7% of the fissile uranium-235.) Nearly all mined thorium, however, can easily be made into the fissile uranium isotope uranium-233 through neutron bombardment (as shown above).
- Waste from a thorium reactor is expected to lose its dangerous radioactivity after about 400-500 years, compared with many thousands of years for nuclear waste produced today.
- Thorium fuel research is continuing in several countries including the USA and India.
- Most non-nuclear uses of thorium are driven by the unique properties of its oxide.
- Thorium dioxide was used in Welsbach gas mantles in the 19th century and today these mantles may still be found in camping lanterns. (Thorium dioxide’s very high melting point ensures it stays solid, glowing with an intense, bright white light at the temperature of the lantern’s burning gas.)
- Thorium dioxide is used for heat resistant ceramics.
- Glass that contains thorium dioxide has a high refractive index and low dispersion, so thorium dioxide is added to glass for use in high quality lenses and scientific equipment.
- Thorium-magnesium alloys are used in the aerospace industry for aircraft engines. These alloys are lightweight and have excellent strength and creep resistance at high temperatures.
- Thorium is used to coat tungsten filaments in light bulbs.
- The demand for thorium in non-nuclear applications is decreasing because of environmental and health concerns due to its radioactivity.
