Americium is a synthetic chemical element; it has symbol Am and atomic number 95. It is radioactive and a transuranic member of the actinide series in the periodic table, located under the lanthanide element europium and was thus named after the Americas by analogy.
The actinide or actinoid series encompasses at least the 14 metallic chemical elements in the 5f series, with atomic numbers from 89 to 102, actinium through nobelium. The actinide series derives its name from the first element in the series, actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide.
Curium is a synthetic chemical element; it has symbol Cm and atomic number 96. This transuranic actinide element was named after eminent scientists Marie and Pierre Curie, both known for their research on radioactivity. Curium was first intentionally made by the team of Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso in 1944, using the cyclotron at Berkeley. They bombarded the newly discovered element plutonium with alpha particles. This was then sent to the Metallurgical Laboratory at University of Chicago where a tiny sample of curium was eventually separated and identified. The discovery was kept secret until after the end of World War II. The news was released to the public in November 1947. Most curium is produced by bombarding uranium or plutonium with neutrons in nuclear reactors – one tonne of spent nuclear fuel contains ~20 grams of curium.
In nuclear engineering, fissile material is material that can undergo nuclear fission when struck by a neutron of low energy. A self-sustaining thermal chain reaction can only be achieved with fissile material. The predominant neutron energy in a system may be typified by either slow neutrons or fast neutrons. Fissile material can be used to fuel thermal-neutron reactors, fast-neutron reactors and nuclear explosives.
In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". The typical radioisotope does not decay directly to a stable state, but rather it decays to another radioisotope. Thus there is usually a series of decays until the atom has become a stable isotope, meaning that the nucleus of the atom has reached a stable state.
Mixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material, usually consisting of plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alternative to the low-enriched uranium fuel used in the light-water reactors that predominate nuclear power generation.
Fertile material is a material that, although not fissile itself, can be converted into a fissile material by neutron absorption.
Uranium (92U) is a naturally occurring radioactive element that has no stable isotope. It has two primordial isotopes, uranium-238 and uranium-235, that have long half-lives and are found in appreciable quantity in the Earth's crust. The decay product uranium-234 is also found. Other isotopes such as uranium-233 have been produced in breeder reactors. In addition to isotopes found in nature or nuclear reactors, many isotopes with far shorter half-lives have been produced, ranging from 214U to 242U. The standard atomic weight of natural uranium is 238.02891(3).
Naturally occurring samarium (62Sm) is composed of five stable isotopes, 144Sm, 149Sm, 150Sm, 152Sm and 154Sm, and two extremely long-lived radioisotopes, 147Sm (half life: 1.06×1011 y) and 148Sm (6.3×1015 y), with 152Sm being the most abundant (26.75% natural abundance). 146Sm is also fairly long-lived, but is not long-lived enough to have survived in significant quantities from the formation of the Solar System on Earth, although it remains useful in radiometric dating in the Solar System as an extinct radionuclide. A 2012 paper revising the estimated half-life of 146Sm from 10.3(5)×107 y to 6.8(7)×107 y was retracted in 2023. It is the longest-lived nuclide that has not yet been confirmed to be primordial.
Neptunium (93Np) is usually considered an artificial element, although trace quantities are found in nature, so a standard atomic weight cannot be given. Like all trace or artificial elements, it has no stable isotopes. The first isotope to be synthesized and identified was 239Np in 1940, produced by bombarding 238
U
with neutrons to produce 239
U
, which then underwent beta decay to 239
Np
.
Plutonium (94Pu) is an artificial element, except for trace quantities resulting from neutron capture by uranium, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was synthesized long before being found in nature, the first isotope synthesized being 238Pu in 1940. Twenty plutonium radioisotopes have been characterized. The most stable are plutonium-244 with a half-life of 80.8 million years, plutonium-242 with a half-life of 373,300 years, and plutonium-239 with a half-life of 24,110 years. All of the remaining radioactive isotopes have half-lives that are less than 7,000 years. This element also has eight meta states; all have half-lives of less than one second.
A minor actinide is an actinide, other than uranium or plutonium, found in spent nuclear fuel. The minor actinides include neptunium, americium, curium, berkelium, californium, einsteinium, and fermium. The most important isotopes of these elements in spent nuclear fuel are neptunium-237, americium-241, americium-243, curium-242 through -248, and californium-249 through -252.
Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon and has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nuclear weapons are the most common examples.
Plutonium-241 is an isotope of plutonium formed when plutonium-240 captures a neutron. Like some other plutonium isotopes, 241Pu is fissile, with a neutron absorption cross section about one-third greater than that of 239Pu, and a similar probability of fissioning on neutron absorption, around 73%. In the non-fission case, neutron capture produces plutonium-242. In general, isotopes with an odd number of neutrons are both more likely to absorb a neutron, and more likely to undergo fission on neutron absorption, than isotopes with an even number of neutrons.
Uranium-236 (236U) is an isotope of uranium that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste. It is found in spent nuclear fuel and in the reprocessed uranium made from spent nuclear fuel.
Plutonium-242 is one of the isotopes of plutonium, the second longest-lived, with a half-life of 375,000 years. The half-life of 242Pu is about 15 times that of 239Pu; so it is one-fifteenth as radioactive, and not one of the larger contributors to nuclear waste radioactivity. 242Pu's gamma ray emissions are also weaker than those of the other isotopes.
Nuclear fission splits a heavy nucleus such as uranium or plutonium into two lighter nuclei, which are called fission products. Yield refers to the fraction of a fission product produced per fission.
Long-lived fission products (LLFPs) are radioactive materials with a long half-life produced by nuclear fission of uranium and plutonium. Because of their persistent radiotoxicity, it is necessary to isolate them from humans and the biosphere and to confine them in nuclear waste repositories for geological periods of time.
Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed.
Americium-241 is an isotope of americium. Like all isotopes of americium, it is radioactive, with a half-life of 432.2 years. 241
Am
is the most common isotope of americium as well as the most prevalent isotope of americium in nuclear waste. It is commonly found in ionization type smoke detectors and is a potential fuel for long-lifetime radioisotope thermoelectric generators (RTGs). Its common parent nuclides are β− from 241
Pu
, EC from 241
Cm
, and α from 245
Bk
. 241
Am
is fissile and the critical mass of a bare sphere is 57.6–75.6 kilograms (127.0–166.7 lb) and a sphere diameter of 19–21 centimetres (7.5–8.3 in). Americium-241 has a specific activity of 3.43 Ci/g (126.91 GBq/g). It is commonly found in the form of americium-241 dioxide. This isotope also has one meta state, 241m
Am
, with an excitation energy of 2.2 MeV (0.35 pJ) and a half-life of 1.23 μs. The presence of americium-241 in plutonium is determined by the original concentration of plutonium-241 and the sample age. Because of the low penetration of alpha radiation, americium-241 only poses a health risk when ingested or inhaled. Older samples of plutonium containing 241
Pu
contain a buildup of 241
Am
. A chemical removal of americium-241 from reworked plutonium may be required in some cases.
