Thorium-232

Thorium-232 undergoes alpha decay to radium-228, with a half-life of 14.05 billion years, which is longer than the age of the universe and comparable to the age of the Earth. This decay chain, known as the thorium series, eventually ends at stable lead-208. The decay chain includes notable isotopes such as radium-224 and radon-220, the latter being a radioactive gas that can pose a health hazard when accumulated in enclosed spaces. The long half-life means that thorium-232 is only weakly radioactive, and its specific activity is about 4.07 MBq/kg. The alpha particles emitted have an energy of about 4.08 MeV, which is relatively low, but they can be harmful if thorium is ingested or inhaled. The decay chain also produces significant gamma radiation from daughter products, which must be considered in handling and storage.

Thorium-232 is a fertile isotope, meaning it can be converted into a fissile material by neutron capture. When it absorbs a neutron, it becomes thorium-233, which quickly beta-decays to protactinium-233 and then to uranium-233, a fissile isotope that can sustain a nuclear chain reaction. This process is the basis of the thorium fuel cycle, which has been proposed as an alternative to the uranium-plutonium cycle. Thorium is about three to four times more abundant in the Earth's crust than uranium, and it is often cited as a more sustainable and proliferation-resistant fuel source. However, the thorium fuel cycle requires a neutron source to initiate the conversion, and the intermediate protactinium-233 has a relatively long half-life (27 days), which can absorb neutrons and reduce efficiency. Research and development have been conducted in several countries, including India, which has large thorium reserves, and China, which has pursued molten salt reactor designs.

Historically, thorium-232 was used in gas mantles for incandescent lamps, where its oxide glows brightly when heated. It was also used in early nuclear reactors, such as the Shippingport Atomic Power Station in the United States, which operated a thorium fuel cycle in the 1970s. Thorium-232 has been used in the production of thorium dioxide, which has high melting point and is used in high-temperature ceramics and as a catalyst in chemical processes. In the medical field, thorium-232 was once used in X-ray contrast agents, but this practice was discontinued due to its radioactivity and associated health risks. The isotope is also used in the dating of marine sediments and in the study of oceanic processes, as its decay products can be used to trace sedimentation rates. Despite its potential, the use of thorium-232 in nuclear power has been limited by technical challenges and the availability of uranium.

Thorium-232 is the only naturally occurring isotope of thorium, but trace amounts of other isotopes, such as thorium-230 and thorium-228, are present in nature as decay products of uranium and thorium. The isotope's long half-life makes it a useful tool in geochronology, particularly for dating carbonate materials and deep-sea sediments. One lesser-known fact is that thorium-232 was used in the first nuclear weapon test, the Trinity test, as a tamper material, though its role was overshadowed by the plutonium core. In the 1960s, the United States and the Soviet Union explored the use of thorium-232 in nuclear rockets, but these projects were abandoned. Thorium-232 has also been proposed as a fuel for accelerator-driven systems, which could transmute nuclear waste. The isotope's high melting point and low thermal neutron absorption cross-section make it attractive for certain reactor designs, but its use requires careful handling of its decay products, which include the highly radiotoxic radium-228.

Fertile material

A nuclide that can be converted into a fissile isotope by neutron capture and subsequent beta decay.

Fissile material

A nuclide capable of sustaining a nuclear chain reaction with thermal neutrons.

Thorium fuel cycle

A nuclear fuel cycle that uses thorium-232 as the fertile isotope to produce uranium-233.

Molten salt reactor

A type of nuclear reactor that uses a liquid salt as the coolant and fuel carrier.