Thorium in the context of "Thorium series"

Uranium is a chemical element; it has symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium radioactively decays, usually by emitting an alpha particle. The half-life of this decay varies between 159,200 and 4.5 billion years for different isotopes, making them useful for dating the age of the Earth. The most common isotopes in natural uranium are uranium-238 (which has 146 neutrons and accounts for over 99% of uranium on Earth) and uranium-235 (which has 143 neutrons). Uranium has the highest atomic weight of the primordially occurring elements. Its density is about 70% higher than that of lead and slightly lower than that of gold or tungsten. It occurs naturally in low concentrations of a few parts per million in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite.

Many contemporary uses of uranium exploit its unique nuclear properties. Uranium is used in nuclear power plants and nuclear weapons because it is the only naturally occurring element with a fissile isotope – uranium-235 – present in non-trace amounts. However, because of the low abundance of uranium-235 in natural uranium (which is overwhelmingly uranium-238), uranium needs to undergo enrichment so that enough uranium-235 is present. Uranium-238 is fissionable by fast neutrons and is fertile, meaning it can be transmuted to fissile plutonium-239 in a nuclear reactor. Another fissile isotope, uranium-233, can be produced from natural thorium and is studied for future industrial use in nuclear technology. Uranium-238 has a small probability for spontaneous fission or even induced fission with fast neutrons; uranium-235, and to a lesser degree uranium-233, have a much higher fission cross-section for slow neutrons. In sufficient concentration, these isotopes maintain a sustained nuclear chain reaction. This generates the heat in nuclear power reactors and produces the fissile material for nuclear weapons. The primary civilian use for uranium harnesses the heat energy to produce electricity. Depleted uranium (U) is used in kinetic energy penetrators and armor plating.

Having a mean density of 3,346.4 kg/m, the Moon is a differentiated body, being composed of a geochemically distinct crust, mantle, and planetary core. This structure is believed to have resulted from the fractional crystallization of a magma ocean shortly after its formation about 4.5 billion years ago. The energy required to melt the outer portion of the Moon is commonly attributed to a giant impact event that is postulated to have formed the Earth-Moon system, and the subsequent reaccretion of material in Earth orbit. Crystallization of this magma ocean would have given rise to a mafic mantle and a plagioclase-rich crust.

Geochemical mapping from orbit implies that the crust of the Moon is largely anorthositic in composition, consistent with the magma ocean hypothesis. In terms of elements, the lunar crust is composed primarily of oxygen, silicon, magnesium, iron, calcium, and aluminium, but important minor and trace elements such as titanium, uranium, thorium, potassium, sulphur, manganese, chromium, and hydrogen are present as well. Based on geophysical techniques, the crust is estimated to be on average about 50 km thick.

Nuclear material refers to the elements uranium, plutonium, and thorium, in any form, according to the IAEA. This is differentiated further into "source material", consisting of natural and depleted uranium, and "special fissionable material", consisting of enriched uranium (U-235), uranium-233, and plutonium-239. Uranium ore concentrates are considered to be a "source material", although these are not subject to safeguards under the Nuclear Non-Proliferation Treaty.

According to the Nuclear Regulatory Commission (NRC), there are four different types of regulated nuclear materials: special nuclear material, source material, byproduct material and radium. Special nuclear materials have plutonium, uranium-233 or uranium with U or U that has a content found more than in nature. Source material is thorium or uranium that has a U content equal to or less than what is in nature. Byproduct material is radioactive material that is not source or special nuclear material. It can be an isotope produced by a nuclear reactor, the tailings and waste that is produced or extracted from uranium or thorium from an ore that processed mainly for its source material content. Byproduct material can also be discrete sources of radium-226 or discrete sources of accelerator-produced isotopes or naturally occurring isotopes that pose a threat greater or equal to a discrete source of radium-226. Radium is also a regulated nuclear material that is found in nature and produced by the radioactive decay of uranium. The half-life of radium is approximately 1,600 years.

Uraninite, also known as pitchblende, is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO₂ but because of oxidation typically contains variable proportions of U₃O₈. Radioactive decay of the uranium causes the mineral to contain oxides of lead and trace amounts of helium. It may also contain thorium and rare-earth elements.

In nuclear physics, a decay product (also known as a daughter product, daughter isotope, radio-daughter, or daughter nuclide) is the remaining nuclide left over from radioactive decay. Radioactive decay often proceeds via a sequence of steps (decay chain). For example, U decays to Th which decays to Pa which decays, and so on, to Pb (which is stable):

Thorium (₉₀Th) has seven naturally occurring isotopes but none are stable. One isotope, Th, is relatively stable, with a half-life of 1.40×10 years, considerably longer than the age of the Earth, and even slightly longer than the generally accepted age of the universe. This isotope makes up nearly all natural thorium, so thorium was considered to be mononuclidic. However, in 2013, IUPAC reclassified thorium as binuclidic, due to large amounts of Th in deep seawater. Thorium has a characteristic terrestrial isotopic composition and thus a standard atomic weight can be given.

Thirty-one radioisotopes have been characterized, with the most stable being Th, Th with a half-life of 75,400 years, Th with a half-life of 7,916 years, and Th with a half-life of 1.91 years. All of the remaining radioactive isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes. One isotope, Th, has a nuclear isomer (or metastable state) with a remarkably low excitation energy, recently measured to be 8.355733554021(8) eV It has been proposed to perform laser spectroscopy of the Th nucleus and use the low-energy transition for the development of a nuclear clock of extremely high accuracy.

Uranium–lead dating, abbreviated U–Pb dating, is one of the oldest and most refined of the radiometric dating schemes. It can be used to date rocks that formed and crystallised from about 1 million years to over 4.5 billion years ago with routine precisions in the 0.1–1 percent range.

The method is usually applied to zircon. This mineral incorporates uranium and thorium atoms into its crystal structure, but strongly rejects lead when forming. As a result, newly-formed zircon crystals will contain no lead, meaning that any lead found in the mineral is radiogenic. Since the exact rate at which uranium decays into lead is known, the current ratio of lead to uranium in a sample of the mineral can be used to reliably determine its age.

KREEP, an acronym built from the letters K (the atomic symbol for potassium), REE (rare-earth elements) and P (for phosphorus), is a geochemical component of some lunar impact breccia and basaltic rocks. Its most significant feature is somewhat enhanced concentration of a majority of so-called "incompatible" elements (those that are concentrated in the liquid phase during magma crystallization) and the heat-producing elements, namely radioactive uranium, thorium, and potassium (due to presence of the radioactive K).