Plutonium

Plutonium is a chemical element; it has symbol Pu and atomic number 94. It is a silvery-gray actinide metal that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen, silicon and hydrogen. When exposed to moist air, it forms oxides and hydrides that can expand the sample up to 70% in volume, which in turn flake off as a powder that is pyrophoric. It is radioactive and can accumulate in bones, which makes the handling of plutonium dangerous.
Plutonium was first synthesized and isolated in late 1940 and early 1941, by deuteron bombardment of uranium-238 in the cyclotron at the University of California, Berkeley. First, neptunium-238 was synthesized, which then beta-decayed to form the new element with atomic number 94 and atomic weight 238. Since uranium had been named after the planet Uranus and neptunium after the planet Neptune, element 94 was named after Pluto, which at the time was also considered a planet. Wartime secrecy prevented the University of California team from publishing its discovery until 1948.
Plutonium is the element with the highest atomic number known to occur in nature. Trace quantities arise in natural uranium deposits when uranium-238 captures neutrons emitted by decay of other uranium-238 atoms. The heavy isotope plutonium-244 has a half-life long enough that extreme trace quantities should have survived primordially to the present, but so far experiments have not yet been sensitive enough to detect it.
Both plutonium-239 and plutonium-241 are fissile, meaning they can sustain a nuclear chain reaction, leading to applications in nuclear weapons and nuclear reactors. Plutonium-240 has a high rate of spontaneous fission, raising the neutron flux of any sample containing it. The presence of plutonium-240 limits a plutonium sample's usability for weapons or its quality as reactor fuel, and the percentage of plutonium-240 determines its grade. Plutonium-238 has a half-life of 87.7 years and emits alpha particles. It is a heat source in radioisotope thermoelectric generators, which are used to power some spacecraft. Plutonium isotopes are expensive and inconvenient to separate, so particular isotopes are usually manufactured in specialized reactors.
Producing plutonium in useful quantities for the first time was a major part of the Manhattan Project during World War II that developed the first atomic bombs. The Fat Man bombs used in the Trinity nuclear test in July 1945, and in the bombing of Nagasaki in August 1945, had plutonium cores. Human radiation experiments studying plutonium were conducted without informed consent, and several criticality accidents, some lethal, occurred after the war. Disposal of plutonium waste from nuclear power plants and dismantled nuclear weapons built during the Cold War is a nuclear-proliferation and environmental concern. Other sources of plutonium in the environment are fallout from many above-ground nuclear tests, which are now banned.

Characteristics

Physical properties

Plutonium, like most metals, has a bright silvery appearance at first, much like nickel, but it oxidizes very quickly to a dull gray, though yellow and olive green are also reported. At room temperature plutonium is in its α form. This allotrope is about as hard and brittle as gray cast iron. When plutonium is alloyed with other metals, the high-temperature δ allotrope is stabilized at room temperature, making it soft and ductile. Unlike most metals, it is not a good conductor of heat or electricity. It has a low melting point and an unusually high boiling point. This gives a large range of temperatures at which plutonium is liquid, but this range is neither the greatest among all actinides nor among all metals, with neptunium theorized to have the greatest range in both instances. The low melting point as well as the reactivity of the native metal compared to the oxide leads to plutonium oxides being a preferred form for applications such as nuclear fission reactor fuel.
Alpha decay, the release of a high-energy helium nucleus, is the most common form of radioactive decay for plutonium. A 5 kg mass of Pu contains about atoms. With a half-life of 24,100 years, about of its atoms decay each second by emitting a 5.157 MeV alpha particle. This amounts to 9.68 watts of power. Heat produced by the deceleration of these alpha particles makes it warm to the touch. due to its much shorter half life heats up to much higher temperatures and glows red hot with blackbody radiation if left without external heating or cooling. This heat has been used in radioisotope thermoelectric generators.
The resistivity of plutonium at room temperature is very high for a metal, and it gets even higher with lower temperatures, which is unusual for metals. This trend continues down to 100 K, below which resistivity rapidly decreases for fresh samples. Resistivity then begins to increase with time at around 20 K due to radiation damage, with the rate dictated by the isotopic composition of the sample.
Because of self-irradiation, a sample of plutonium fatigues throughout its crystal structure, meaning the ordered arrangement of its atoms becomes disrupted by radiation with time. Self-irradiation can also lead to annealing which counteracts some of the fatigue effects as temperature increases above 100 K.
Unlike most materials, plutonium increases in density when it melts - by 2.5% - but the liquid metal exhibits a linear decrease in density with temperature. Near the melting point, the liquid plutonium has very high viscosity and surface tension compared to other metals.

Allotropes

Plutonium normally has six allotropes and forms a seventh at high temperature within a limited pressure range. These allotropes, which are different structural modifications or forms of an element, have very similar internal energies but significantly varying densities and crystal structures. This makes plutonium very sensitive to changes in temperature, pressure, or chemistry, and allows for dramatic volume changes following phase transitions from one allotropic form to another. The densities of the different allotropes vary from 16.00 g/cm to 19.86 g/cm.
The presence of these many allotropes makes machining plutonium very difficult, as it changes state very readily. For example, the α form exists at room temperature in unalloyed plutonium. It has machining characteristics similar to cast iron but changes to the plastic and malleable β form at slightly higher temperatures. The reasons for the complicated phase diagram are not entirely understood. The α form has a low-symmetry monoclinic structure, hence its brittleness, strength, compressibility, and poor thermal conductivity.
Plutonium in the δ form normally exists in the 310 °C to 452 °C range but is stable at room temperature when alloyed with a small percentage of gallium, aluminium, or cerium, enhancing workability and allowing it to be welded. The δ form has more typical metallic character, and is roughly as strong and malleable as aluminium. The ε phase, the highest temperature solid allotrope, exhibits anomalously high atomic self-diffusion compared to other elements.

Nuclear fission

Plutonium is a radioactive actinide metal whose isotope, plutonium-239, is one of the three primary fissile isotopes ; plutonium-241 is also highly fissile. To be considered fissile, an isotope's atomic nucleus must be able to break apart or fission when struck by a slow moving neutron and to release enough additional neutrons to sustain the nuclear chain reaction by splitting further nuclei.
Pure plutonium-239 may have a multiplication factor larger than one, which means that if the metal is present in sufficient quantity and with an appropriate geometry, it can form a critical mass. During fission, a fraction of the nuclear binding energy, which holds a nucleus together, is released as a large amount of electromagnetic and kinetic energy. Fission of a kilogram of plutonium-239 can produce an explosion equivalent to. It is this energy that makes plutonium-239 useful in nuclear weapons and reactors.
The presence of the isotope plutonium-240 in a sample limits its nuclear bomb potential, as Pu has a relatively high spontaneous fission rate, raising the background neutron levels and thus increasing the risk of predetonation. Plutonium is identified as either weapons-grade, fuel-grade, or reactor-grade based on the percentage of Pu that it contains. Weapons-grade plutonium contains less than 7% Pu. Fuel-grade plutonium contains 7%–19%, and power reactor-grade contains 19% or more Pu. Supergrade plutonium, with less than 4% of Pu, is used in United States Navy weapons stored near ship and submarine crews, due to its lower radioactivity. Plutonium-238 is not fissile but can undergo nuclear fission easily with fast neutrons as well as alpha decay. All plutonium isotopes can be "bred" into fissile material with one or more neutron absorptions, whether followed by beta decay or not. This makes non-fissile isotopes of plutonium a fertile material.

Isotopes and nucleosynthesis

Twenty-two radioisotopes of plutonium have been characterized, from ²²⁶Pu to ²⁴⁷Pu. The longest-lived are Pu, with a half-life of 80.8 million years; Pu, with a half-life of 373,300 years; and Pu, with a half-life of 24,110 years. All other isotopes have half-lives of less than 7,000 years. This element also has eight metastable states, though all have half-lives less than a second. Pu has been found in interstellar space and it has the longest half-life of any non-primordial radioisotope. The main decay modes of isotopes with mass numbers lower than the most stable isotope, Pu, are spontaneous fission and alpha emission, mostly forming uranium and neptunium isotopes as decay products. The main decay mode for isotopes heavier than Pu, along with Pu and Pu, is beta emission, forming americium isotopes. Plutonium-241 is the parent isotope of the neptunium series, decaying to americium-241 via beta emission.
Plutonium-238 and 239 are the most widely synthesized isotopes. Pu is synthesized via the following reaction using uranium and neutrons via beta decay with neptunium as an intermediate:
+ -> -> ->

Neutrons from the fission of uranium-235 are captured by uranium-238 nuclei to form uranium-239; a beta decay converts a neutron into a proton to form neptunium-239 and another beta decay forms plutonium-239. Egon Bretscher working on the British Tube Alloys project predicted this reaction theoretically in 1940.
Plutonium-238 is synthesized by bombarding uranium-238 with deuterons in the following reaction:
where a deuteron hitting uranium-238 produces two neutrons and neptunium-238, which decays by emitting negative beta particles to form plutonium-238. Plutonium-238 can also be produced by neutron irradiation of neptunium-237.