Plutonium (Latin Plutonium, symbol Pu) - radioactive chemical element with atomic number 94 and atomic weight 244.064. Plutonium is an element of Group III of Dmitry Ivanovich Mendeleev’s periodic table and belongs to the actinide family. Plutonium is a heavy (density under normal conditions 19.84 g/cm³) brittle radioactive metal of a silvery-white color.
Plutonium has no stable isotopes. Of the hundred possible isotopes of plutonium, twenty-five have been synthesized. The nuclear properties of fifteen of them were studied (mass numbers 232-246). Four found practical use. The longest-lived isotopes are 244Pu (half-life 8.26-107 years), 242Pu (half-life 3.76-105 years), 239Pu (half-life 2.41-104 years), 238Pu (half-life 87.74 years) - α-emitters and 241Pu (half-life 14 years) - β-emitter. In nature, plutonium occurs in negligible quantities in uranium ores (239Pu); it is formed from uranium under the influence of neutrons, the sources of which are reactions occurring during the interaction of α-particles with light elements (included in ores), spontaneous fission of uranium nuclei and cosmic radiation.
The ninety-fourth element was discovered by a group of American scientists - Glenn Seaborg, Kennedy, Edwin McMillan and Arthur Wahl in 1940 in Berkeley (at the University of California) when bombing a target of uranium oxide ( U3O8) by highly accelerated deuterium nuclei (deuterons) from a sixty-inch cyclotron. In May 1940, the properties of plutonium were predicted by Louis Turner.
In December 1940, the plutonium isotope Pu-238 was discovered, with a half-life of ~90 years, followed a year later by the more important Pu-239 with a half-life of ~24,000 years.
Edwin MacMillan in 1948 proposed to name the chemical element plutonium in honor of the discovery of the new planet Pluto and by analogy with neptunium, which was named after the discovery of Neptune.
Metallic plutonium (239Pu isotope) is used in nuclear weapons and serves as nuclear fuel in power reactors operating on thermal and especially fast neutrons. The critical mass for 239Pu as metal is 5.6 kg. Among other things, the 239Pu isotope is the starting material for obtaining nuclear reactors transplutonium elements. The 238Pu isotope is used in small-sized nuclear power sources used in space research, as well as in human cardiac stimulants.
Plutonium-242 is important as a “raw material” for the relatively rapid accumulation of higher transuranium elements in nuclear reactors. δ-stabilized plutonium alloys are used in the manufacture of fuel cells, since they have better metallurgical properties compared to pure plutonium, which undergoes phase transitions when heated. Plutonium oxides are used as an energy source for space technology and find their application in fuel rods.
All plutonium compounds are poisonous, which is a consequence of α-radiation. Alpha particles pose a serious danger if their source is in the body of an infected person; they damage the surrounding tissue of the body. Gamma radiation from plutonium is not dangerous to the body. It is worth considering that different isotopes of plutonium have different toxicities, for example, typical reactor plutonium is 8-10 times more toxic than pure 239Pu, since it is dominated by 240Pu nuclides, which is a powerful source of alpha radiation. Plutonium is the most radiotoxic element of all actinides, however, it is considered far from the most dangerous element, since radium is almost a thousand times more dangerous than the most poisonous isotope of plutonium - 239Pu.
Plutonium is concentrated by marine organisms: the accumulation coefficient of this radioactive metal (the ratio of concentrations in the body and in external environment) for algae is 1000-9000, for plankton - approximately 2300, for starfish- about 1000, for mollusks - up to 380, for muscles, bones, liver and stomach of fish - 5, 570, 200 and 1060, respectively. Land plants absorb plutonium mainly through root system and accumulate it to 0.01% of their mass. In the human body, the ninety-fourth element is retained mainly in the skeleton and liver, from where it is almost not excreted (especially from the bones).
Plutonium is highly toxic, and its chemical danger (like any other heavy metal) is much weaker (from a chemical point of view, it is also poisonous like lead.) in comparison with its radioactive toxicity, which is a consequence of alpha radiation. Moreover, α-particles have a relatively low penetrating ability: for 239Pu, the range of α-particles in air is 3.7 cm, and in soft biological tissue 43 μm. Therefore, alpha particles pose a serious danger if their source is in the body of an infected person. At the same time, they damage the tissues of the body surrounding the element.
At the same time, γ-rays and neutrons, which plutonium also emits and which are able to penetrate the body from the outside, are not very dangerous, because their level is too low to cause harm to health. Plutonium belongs to a group of elements with particularly high radiotoxicity. At the same time, different isotopes of plutonium have different toxicity, for example, typical reactor plutonium is 8-10 times more toxic than pure 239Pu, since it is dominated by 240Pu nuclides, which is a powerful source of alpha radiation.
When ingested through water and food, plutonium is less toxic than substances such as caffeine, some vitamins, pseudoephedrine, and many plants and fungi. This is explained by the fact that this element is poorly absorbed by the gastrointestinal tract, even when supplied in the form of a soluble salt, this same salt is bound by the contents of the stomach and intestines. However, ingestion of 0.5 grams of plutonium in a finely divided or dissolved state can lead to death from acute radiation exposure digestive system over several days or weeks (for cyanide this value is 0.1 grams).
From an inhalation point of view, plutonium is an ordinary toxin (roughly equivalent to mercury vapor). When inhaled, plutonium is carcinogenic and can cause lung cancer. So, when inhaled, one hundred milligrams of plutonium in the form of particles of an optimal size for retention in the lungs (1-3 microns) leads to death from pulmonary edema in 1-10 days. A dose of twenty milligrams leads to death from fibrosis in about a month. Smaller doses lead to chronic carcinogenic poisoning. The danger of inhalation of plutonium into the body increases due to the fact that plutonium is prone to the formation of aerosols.
Even though it is a metal, it is quite volatile. A short stay of metal in a room significantly increases its concentration in the air. Plutonium that enters the lungs partially settles on the surface of the lungs, partially passes into the blood, and then into the lymph and bone marrow. Most (approximately 60%) ends up in bone tissue, 30% in the liver and only 10% is excreted naturally. The amount of plutonium that enters the body depends on the size of aerosol particles and solubility in the blood.
Plutonium entering the human body in one way or another is similar in properties to ferric iron, therefore, penetrating into the circulatory system, plutonium begins to concentrate in tissues containing iron: bone marrow, liver, spleen. The body perceives plutonium as iron, therefore, the transferrin protein takes plutonium instead of iron, as a result of which the transfer of oxygen in the body stops. Microphages carry plutonium to the lymph nodes. Plutonium that enters the body takes a very long time to be removed from the body - within 50 years, only 80% will be removed from the body. The half-life from the liver is 40 years. For bone tissue, the half-life of plutonium is 80-100 years; in fact, the concentration of element ninety-four in bones is constant.
Throughout World War II and after its end, scientists working in the Manhattan Project, as well as scientists of the Third Reich and other research organizations, conducted experiments using plutonium on animals and humans. Animal studies have shown that a few milligrams of plutonium per kilogram of tissue is a lethal dose. The use of plutonium in humans consisted of usually 5 mcg of plutonium being injected intramuscularly into chronically ill patients. It was eventually determined that the lethal dose to a patient was one microgram of plutonium, and that plutonium was more dangerous than radium and tended to accumulate in bones.
As is known, plutonium is an element practically absent in nature. However, about five tons of it were released into the atmosphere as a result nuclear tests in the period 1945-1963. The total amount of plutonium released into the atmosphere due to nuclear tests before the 1980s is estimated at 10 tons. By some estimates, soil in the United States contains an average of 2 millicuries (28 mg) of plutonium per km2 from fallout, and the occurrence of plutonium in Pacific Ocean increased compared to the overall proliferation of nuclear materials on earth.
The latest phenomenon is associated with US nuclear testing in the Marshall Islands at the Pacific Test Site in the mid-1950s. The residence time of plutonium in surface ocean waters ranges from 6 to 21 years, however, even after this period, plutonium falls to the bottom along with biogenic particles, from which it is reduced to soluble forms as a result of microbial decomposition.
Global pollution with the ninety-fourth element is associated not only with nuclear tests, but also with accidents in production and equipment interacting with this element. So in January 1968, a US Air Force B-52 carrying four nuclear warheads crashed in Greenland. As a result of the explosion, the charges were destroyed and plutonium leaked into the ocean.
Another case of radioactive contamination of the environment as a result of an accident occurred with the Soviet spacecraft Kosmos-954 on January 24, 1978. As a result of an uncontrolled deorbit, a satellite with a nuclear power source on board fell into Canadian territory. As a result of the accident in environment more than a kilogram of plutonium-238 fell, spreading over an area of about 124,000 m².
The most terrible example of an emergency leak of radioactive substances into the environment is the accident at the Chernobyl nuclear power plant, which occurred on April 26, 1986. As a result of the destruction of the fourth power unit, 190 tons of radioactive substances (including plutonium isotopes) were released into the environment over an area of about 2200 km².
The release of plutonium into the environment is not only associated with man-made incidents. There are known cases of plutonium leakage, both from laboratory and factory conditions. More than twenty accidental leaks from the 235U and 239Pu laboratories are known. During 1953-1978. accidents led to a loss of 0.81 (Mayak, March 15, 1953) to 10.1 kg (Tomsk, December 13, 1978) 239Pu. Industrial incidents resulted in a total of two deaths at Los Alamos (August 21, 1945 and May 21, 1946) due to two accidents and the loss of 6.2 kg of plutonium. In the city of Sarov in 1953 and 1963. approximately 8 and 17.35 kg fell outside the nuclear reactor. One of them led to the destruction of a nuclear reactor in 1953.
When a 238Pu nucleus fissions with neutrons, 200 MeV of energy is released, which is 50 million times more than the most famous exothermic reaction: C + O2 → CO2. “Burning” in a nuclear reactor, one gram of plutonium produces 2,107 kcal - this is the energy contained in 4 tons of coal. A thimble of plutonium fuel in energy equivalent can be equivalent to forty wagons of good firewood!
The “natural isotope” of plutonium (244Pu) is believed to be the longest-lived isotope of all transuranium elements. Its half-life is 8.26∙107 years. Scientists have been trying for a long time to obtain an isotope of a transuranium element that would exist longer than 244Pu - great hopes in this regard were pinned on 247Cm. However, after its synthesis it turned out that the half-life of this element is only 14 million years.
In 1934, a group of scientists led by Enrico Fermi made a statement that during scientific works At the University of Rome, they discovered a chemical element with serial number 94. The element was named hesperium at Fermi's insistence; the scientist was convinced that he had discovered a new element, which is now called plutonium, thus making the assumption of the existence of transuranium elements and becoming their theoretical discoverer. Fermi defended this hypothesis in his Nobel lecture in 1938. It was only after the discovery of nuclear fission by the German scientists Otto Frisch and Fritz Strassmann that Fermi was forced to make a note in the printed version published in Stockholm in 1939 indicating the need to reconsider “the whole problem of transuranium elements.” The fact is that the work of Frisch and Strassmann showed that the activity discovered by Fermi in his experiments was due precisely to fission, and not to the discovery of transuranium elements, as he had previously believed.
A new element, the ninety-fourth, was discovered at the end of 1940. It happened in Berkeley at the University of California. By bombarding uranium oxide (U3O8) with heavy hydrogen nuclei (deuterons), a group of American radiochemists led by Glenn T. Seaborg discovered a previously unknown alpha particle emitter with a half-life of 90 years. This emitter turned out to be the isotope of element No. 94 with a mass number of 238. Thus, on December 14, 1940, the first microgram quantities of plutonium were obtained along with an admixture of other elements and their compounds.
During an experiment conducted in 1940, it was found that during a nuclear reaction, the short-lived isotope neptunium-238 is first produced (half-life 2.117 days), and from it plutonium-238:
23392U (d,2n) → 23893Np → (β−) 23894Pu
Long and laborious chemical experiments to separate the new element from impurities lasted two months. The existence of a new chemical element was confirmed on the night of February 23–24, 1941 by G. T. Seaborg, E. M. Macmillan, J. W. Kennedy and A. C. Wall through the study of its first chemical properties - the ability to possess at least at least two oxidation states. A little later than the end of the experiments, it was established that this isotope is non-fissile, and, therefore, uninteresting for further study. Soon (March 1941), Kennedy, Seaborg, Segre and Wahl synthesized a more important isotope, plutonium-239, by irradiating uranium with highly accelerated neutrons in a cyclotron. This isotope is formed by the decay of neptunium-239, emits alpha rays and has a half-life of 24,000 years. The first pure compound of the element was obtained in 1942, and the first weight quantities of metallic plutonium were obtained in 1943.
The name of the new element 94 was proposed in 1948 by MacMillan, who, a few months before the discovery of plutonium, together with F. Eibelson, obtained the first element heavier than uranium - element No. 93, which was named neptunium in honor of the planet Neptune - the first beyond Uranus. By analogy, they decided to call element No. 94 plutonium, since the planet Pluto is second after Uranus. In turn, Seaborg proposed calling the new element “plutium,” but then realized that the name did not sound very good compared to “plutonium.” In addition, he put forward other names for the new element: ultimium, extermium, due to the erroneous judgment at that time that plutonium would become the last chemical element in periodic table. As a result, the element was named “plutonium” in honor of the discovery of the last planet in the solar system.
The half-life of the longest-lived isotope of plutonium is 75 million years. The figure is very impressive, however, the age of the Galaxy is measured in billions of years. It follows from this that the primary isotopes of the ninety-fourth element, formed during the great synthesis of the elements of the Universe, had no chance of surviving to this day. And yet, this does not mean that there is no plutonium in the Earth at all. It is constantly formed in uranium ores. Capturing neutrons cosmic radiation and neutrons produced during the spontaneous fission of 238U nuclei, some - very few - atoms of this isotope turn into 239U atoms. The nuclei of this element are very unstable, they emit electrons and thereby increase their charge, and the formation of neptunium, the first transuranium element, occurs. 239Np is also unstable, its nuclei also emit electrons, so in just 56 hours half of 239Np turns into 239Pu.
The half-life of this isotope is already very long and amounts to 24,000 years. On average, the content of 239Pu is about 400,000 times less than that of radium. Therefore, it is extremely difficult not only to mine, but even to detect “terrestrial” plutonium. Small quantities of 239Pu - parts per trillion - and decay products can be found in uranium ores, for example in the natural nuclear reactor at Oklo, Gabon (West Africa). The so-called “natural nuclear reactor” is considered to be the only one in the world in which actinides and their fission products are currently being formed in the geosphere. According to modern estimates, a self-sustaining reaction with the release of heat took place in this region several million years ago, which lasted more than half a million years.
So, we already know that in uranium ores, as a result of the capture of neutrons by uranium nuclei, neptunium (239Np) is formed, the β-decay product of which is natural plutonium-239. Thanks to special devices- mass spectrometers have detected the presence of plutonium-244 (244Pu), which has the longest half-life of approximately 80 million years, in Precambrian bastnaesite (cerium ore). In nature, 244Pu is found predominantly in the form of dioxide (PuO2), which is even less soluble in water than sand (quartz). Since the relatively long-lived isotope plutonium-240 (240Pu) is in the decay chain of plutonium-244, its decay does occur, but this occurs very rarely (1 case in 10,000). Very small amounts of plutonium-238 (238Pu) are due to the very rare double beta decay of the parent isotope, uranium-238, which was found in uranium ores.
Traces of 247Pu and 255Pu isotopes were found in dust collected after thermal explosions nuclear bombs.
Minimal amounts of plutonium could hypothetically be present in the human body, given that a huge number of nuclear tests have been conducted in one way or another related to plutonium. Plutonium accumulates mainly in the skeleton and liver, from where it is practically not excreted. In addition, element ninety-four is accumulated by marine organisms; Land plants absorb plutonium mainly through the root system.
It turns out that artificially synthesized plutonium still exists in nature, so why is it not mined, but obtained artificially? The fact is that the concentration of this element is too low. About another radioactive metal - radium they say: “a gram of production - a year of work,” and radium in nature is 400,000 times more abundant than plutonium! For this reason, it is extremely difficult not only to mine, but even to detect “terrestrial” plutonium. This was done only after the physical and chemical properties of plutonium obtained in nuclear reactors.
The 239Pu isotope (along with U) is used as nuclear fuel in power reactors operating on thermal and fast neutrons (mainly), as well as in the manufacture of nuclear weapons.
About half a thousand nuclear power plants around the world generate approximately 370 GW of electricity (or 15% of the world's total electricity production). Plutonium-236 is used in the manufacture of atomic electric batteries, the service life of which reaches five years or more, they are used in current generators that stimulate the heart (pacemakers). 238Pu is used in small-sized nuclear power sources used in space research. Thus, plutonium-238 is the power source for the New Horizons, Galileo and Cassini probes, the Curiosity rover and other spacecraft.
Nuclear weapons use plutonium-239 because this isotope is the only suitable nuclide for use in a nuclear bomb. In addition, more frequent use plutonium-239 in nuclear bombs is due to the fact that plutonium occupies a smaller volume in the sphere (where the bomb core is located), therefore, it is possible to gain in the explosive power of the bomb due to this property.
The scheme by which a nuclear explosion involving plutonium occurs lies in the design of the bomb itself, the core of which consists of a sphere filled with 239Pu. At the moment of collision with the ground, the sphere is compressed to a million atmospheres due to the design and thanks to the explosive surrounding this sphere. After the impact, the core expands in volume and density for shortest time- tens of microseconds, the assembly passes the critical state on thermal neutrons and goes into the supercritical state on fast neutrons - a nuclear chain reaction begins with the participation of neutrons and nuclei of the element. The final explosion of a nuclear bomb releases temperatures of the order of tens of millions of degrees.
Plutonium isotopes have found their use in the synthesis of transplutonium (next to plutonium) elements. For example, at the Oak Ridge National Laboratory, with long-term neutron irradiation of 239Pu, 24496Cm, 24296Cm, 24997Bk, 25298Cf, 25399Es and 257100Fm are obtained. In the same way, americium 24195Am was first obtained in 1944. In 2010, plutonium-242 oxide bombarded with calcium-48 ions served as a source for ununquadium.
δ-Stabilized plutonium alloys are used in the manufacture of fuel rods, because they have significantly better metallurgical properties compared to pure plutonium, which undergoes phase transitions when heated and is a very brittle and unreliable material. Alloys of plutonium with other elements (intermetallic compounds) are usually obtained by direct interaction of elements in the required proportions, while arc melting is mainly used; sometimes unstable alloys are obtained by spray deposition or cooling of melts.
The main industrial alloying elements for plutonium are gallium, aluminum and iron, although plutonium is capable of forming alloys and intermediates with most metals with rare exceptions (potassium, sodium, lithium, rubidium, magnesium, calcium, strontium, barium, europium and ytterbium). Refractory metals: molybdenum, niobium, chromium, tantalum and tungsten are soluble in liquid plutonium, but almost insoluble or slightly soluble in solid plutonium. Indium, silicon, zinc and zirconium are capable of forming metastable δ-plutonium (δ"-phase) when rapidly cooled. Gallium, aluminum, americium, scandium and cerium can stabilize δ-plutonium at room temperature.
Large quantities of holmium, hafnium and thallium allow some δ-plutonium to be stored at room temperature. Neptunium is the only element that can stabilize α-plutonium at high temperatures. Titanium, hafnium and zirconium stabilize the structure of β-plutonium at room temperature when rapidly cooled. The applications of such alloys are quite diverse. For example, a plutonium-gallium alloy is used to stabilize the δ phase of plutonium, which avoids the α-δ phase transition. Plutonium-gallium-cobalt ternary alloy (PuGaCo5) is a superconducting alloy at 18.5 K. There are a number of alloys (plutonium-zirconium, plutonium-cerium and plutonium-cerium-cobalt) that are used as nuclear fuel.
Industrial plutonium is produced in two ways. This is either irradiation of 238U nuclei contained in nuclear reactors, or separation by radiochemical methods (co-precipitation, extraction, ion exchange, etc.) of plutonium from uranium, transuranic elements and fission products contained in spent fuel.
In the first case, the most practical isotope 239Pu (mixed with a small admixture of 240Pu) is produced in nuclear reactors with the participation of uranium nuclei and neutrons using β-decay and with the participation of neptunium isotopes as an intermediate fission product:
23892U + 21D → 23893Np + 210n;
23893Np → 23894Pu
β-decay
In this process, a deuteron enters uranium-238, resulting in the formation of neptunium-238 and two neutrons. Neptunium-238 then spontaneously fissions, emitting beta-minus particles that form plutonium-238.
Typically, the content of 239Pu in the mixture is 90-95%, 240Pu is 1-7%, the content of other isotopes does not exceed tenths of a percent. Isotopes with long half-lives - 242Pu and 244Pu are obtained by prolonged irradiation with 239Pu neutrons. Moreover, the yield of 242Pu is several tens of percent, and 244Pu is a fraction of a percent of the 242Pu content. Small amounts of isotopically pure plutonium-238 are formed when neptunium-237 is irradiated with neutrons. Light isotopes of plutonium with mass numbers 232-237 are usually obtained in a cyclotron by irradiating uranium isotopes with α-particles.
With the second method industrial production 239Pu uses the Purex process, based on extraction with tributyl phosphate in a light diluent. In the first cycle, Pu and U are jointly purified from fission products and then separated. In the second and third cycles, the plutonium is further purified and concentrated. The scheme of such a process is based on the difference in the properties of tetra- and hexavalent compounds of the elements being separated.
Initially, spent fuel rods are dismantled and the cladding containing spent plutonium and uranium is removed by physical and chemical means. Next, the extracted nuclear fuel is dissolved in nitric acid. After all, it is a strong oxidizing agent when dissolved, and uranium, plutonium, and impurities are oxidized. Plutonium atoms with zero valence are converted into Pu+6, and both plutonium and uranium are dissolved. From such a solution, the ninety-fourth element is reduced to the trivalent state with sulfur dioxide and then precipitated with lanthanum fluoride (LaF3).
However, in addition to plutonium, the sediment contains neptunium and rare earth elements, but the bulk (uranium) remains in solution. Next, the plutonium is again oxidized to Pu+6 and lanthanum fluoride is added again. Now the rare earth elements precipitate, and the plutonium remains in solution. Next, neptunium is oxidized to a tetravalent state with potassium bromate, since this reagent has no effect on plutonium, then during secondary precipitation with the same lanthanum fluoride, trivalent plutonium passes into a precipitate, and neptunium remains in solution. The end products of such operations are plutonium-containing compounds - PuO2 dioxide or fluorides (PuF3 or PuF4), from which metallic plutonium is obtained (by reduction with barium, calcium or lithium vapor).
Purer plutonium can be achieved by electrolytic refining of the pyrochemically produced metal, which is done in electrolysis cells at 700° C with an electrolyte of potassium, sodium and plutonium chloride using a tungsten or tantalum cathode. The plutonium obtained in this way has a purity of 99.99%.
To produce large quantities of plutonium, breeder reactors, so-called “breeders” (from English verb to breed - to multiply). These reactors got their name due to their ability to produce fissile material in quantities exceeding the cost of obtaining this material. The difference between reactors of this type and others is that the neutrons in them are not slowed down (there is no moderator, for example, graphite) in order for as many of them as possible to react with 238U.
After the reaction, 239U atoms are formed, which subsequently form 239Pu. The core of such a reactor, containing PuO2 in depleted uranium dioxide (UO2), is surrounded by a shell of even more depleted uranium dioxide-238 (238UO2), in which 239Pu is formed. The combined use of 238U and 235U allows “breeders” to produce 50-60 times more energy from natural uranium than other reactors. However, these reactors have a big drawback - fuel rods must be cooled by a medium other than water, which reduces their energy. Therefore, it was decided to use liquid sodium as a coolant.
The construction of such reactors in the United States of America began after the end of World War II; the USSR and Great Britain began their construction only in the 1950s.
Plutonium is a very heavy (density at normal level 19.84 g/cm³) silvery metal, in a purified state very similar to nickel, but in air plutonium quickly oxidizes, fades, forming an iridescent film, first light yellow, then turning into dark purple. When severe oxidation occurs, an olive green oxide powder (PuO2) appears on the metal surface.
Plutonium is a highly electronegative and reactive metal, many times more so even than uranium. Has seven allotropic modifications(α, β, γ, δ, δ", ε and ζ), which change in a certain temperature range and at a certain pressure range. At room temperature, plutonium is in the α form - this is the most common allotropic modification for plutonium. In the alpha phase pure plutonium is brittle and very hard - this structure is about as hard as gray cast iron, if it is not alloyed with other metals, which will give the alloy ductility and softness. Additionally, in this densest form, plutonium is the sixth densest element (only osmium, iridium, platinum, rhenium and neptunium are heavier). Further allotropic transformations of plutonium are accompanied by abrupt changes in density. For example, when heated from 310 to 480 °C, it does not expand like other metals, but contracts (phases “delta” and “delta-prime”). When melted (transition from the epsilon phase to the liquid phase), the plutonium also contracts, allowing unmelted plutonium to float.
Plutonium has a large number of unusual properties: it has the lowest thermal conductivity of all metals - at 300 K it is 6.7 W/(m K); plutonium has the lowest electrical conductivity; In its liquid phase, plutonium is the most viscous metal. The resistivity of the ninety-fourth element at room temperature is very high for a metal, and this feature will increase with decreasing temperature, which is not typical for metals. This “anomaly” can be traced up to a temperature of 100 K - below this mark the electrical resistance will decrease. However, from 20 K the resistance begins to increase again due to the radiation activity of the metal.
Plutonium has the highest electrical resistivity of all the actinides studied (so far), which is 150 μΩ cm (at 22 °C). This metal has a low melting point (640 °C) and is unusual high temperature boiling (3,227 °C). Closer to the melting point, liquid plutonium has a very high viscosity and surface tension compared to other metals.
Due to its radioactivity, plutonium is warm to the touch. A large piece of plutonium in a thermal shell is heated to a temperature exceeding the boiling point of water! In addition, due to its radioactivity, plutonium undergoes changes in its crystal lattice over time - a kind of annealing occurs due to self-irradiation due to temperature increases above 100 K.
The presence of a large number of allotropic modifications in plutonium makes it a difficult metal to process and roll out due to phase transitions. We already know that in the alpha form the ninety-fourth element is similar in properties to cast iron, however, it tends to change and turn into a ductile material, and form a malleable β-form at higher temperature ranges. Plutonium in the δ form is usually stable at temperatures between 310 °C and 452 °C, but can exist at room temperature if doped with low percentages of aluminum, cerium or gallium. When alloyed with these metals, plutonium can be used in welding. In general, the delta form has more pronounced characteristics of a metal - it is close to aluminum in strength and forgeability.
The chemical properties of the ninety-fourth element are in many ways similar to the properties of its predecessors in periodic table- uranium and neptunium. Plutonium is a fairly active metal; it forms compounds with oxidation states from +2 to +7. In aqueous solutions the element exhibits the following degrees oxidation: Pu (III), as Pu3+ (exists in acidic aqueous solutions, has a light purple color); Pu (IV), as Pu4+ (chocolate shade); Pu (V), as PuO2+ (light solution); Pu (VI), as PuO22+ (light orange solution) and Pu(VII), as PuO53- (green solution).
Moreover, these ions (except for PuO53-) can be simultaneously in equilibrium in the solution, which is explained by the presence of 5f electrons, which are located in the localized and delocalized zone of the electron orbital. At pH 5-8, Pu(IV) dominates, which is the most stable among other valences (oxidation states). Plutonium ions of all oxidation states are prone to hydrolysis and complex formation. The ability to form such compounds increases in the Pu5+ series
Compact plutonium slowly oxidizes in air, becoming covered with an iridescent, oily film of oxide. The following plutonium oxides are known: PuO, Pu2O3, PuO2 and a phase of variable composition Pu2O3 - Pu4O7 (Berthollides). In the presence of small amounts of moisture, the rate of oxidation and corrosion increases significantly. If a metal is exposed to small amounts of moist air for long enough, plutonium dioxide (PuO2) forms on its surface. With a lack of oxygen, its dihydride (PuH2) can also form. Surprisingly, plutonium rusts much faster in an atmosphere of an inert gas (such as argon) with water vapor than in dry air or pure oxygen. In fact, this fact is easy to explain - the direct action of oxygen forms a layer of oxide on the surface of plutonium, which prevents further oxidation; the presence of moisture produces a loose mixture of oxide and hydride. By the way, thanks to this coating, the metal becomes pyrophoric, that is, it is capable of spontaneous combustion; for this reason, metallic plutonium is usually processed in an inert atmosphere of argon or nitrogen. At the same time, oxygen is a protective substance and prevents moisture from affecting the metal.
The ninety-fourth element reacts with acids, oxygen and their vapors, but not with alkalis. Plutonium is highly soluble only in very acidic media (for example, hydrochloric acid HCl), and also dissolves in hydrogen chloride, hydrogen iodide, hydrogen bromide, 72% perchloric acid, 85% phosphoric acid H3PO4, concentrated CCl3COOH, sulfamic acid and boiling concentrated nitric acid. Plutonium does not dissolve noticeably in alkali solutions.
When solutions containing tetravalent plutonium are exposed to alkalis, a precipitate of plutonium hydroxide Pu(OH)4 xH2O, which has basic properties, precipitates. When solutions of salts containing PuO2+ are exposed to alkalis, amphoteric hydroxide PuO2OH precipitates. It is answered by salts - plutonites, for example, Na2Pu2O6.
Plutonium salts readily hydrolyze upon contact with neutral or alkaline solutions, creating insoluble plutonium hydroxide. Concentrated solutions of plutonium are unstable due to radiolytic decomposition leading to precipitation.
Weapons-grade plutonium is plutonium in the form of a compact metal containing at least 93.5% of the 239Pu isotope. Intended for the creation of nuclear weapons.
1.Name and features
They call it “weapon-grade” to distinguish it from “reactor-grade”. Plutonium is formed in any nuclear reactor operating on natural or low-enriched uranium, containing mainly the 238U isotope, when it captures excess neutrons. But as the reactor operates, the weapons-grade isotope of plutonium quickly burns up, and as a result, a large number of isotopes 240Pu, 241Pu and 242Pu accumulate in the reactor, formed by the successive capture of several neutrons - since the burnup depth is usually determined by economic factors. The lower the burnup depth, the fewer isotopes 240Pu, 241Pu and 242Pu will contain plutonium separated from irradiated nuclear fuel, but the less plutonium is formed in the fuel.
Special production of plutonium for weapons containing almost exclusively 239Pu is required mainly because isotopes with mass numbers 240 and 242 create a high neutron background, making it difficult to design effective nuclear weapons, in addition, 240Pu and 241Pu have a significantly shorter half-life than 239Pu, due to which the plutonium parts heat up, and it is necessary to additionally introduce heat removal elements into the design of the nuclear weapon. Even pure 239Pu is warmer human body. Additionally, the decay products of heavy isotopes spoil the crystal lattice of the metal, which can lead to a change in the shape of plutonium parts, which can lead to the failure of a nuclear explosive device.
In principle, all these difficulties can be overcome, and nuclear explosive devices made from “reactor” plutonium have been successfully tested, however, in ammunition, where compactness, light weight, reliability and durability play an important role, exclusively specially produced weapons-grade plutonium is used. The critical mass of metallic 240Pu and 242Pu is very large, 241Pu is slightly larger than that of 239Pu.
2.Production
In the USSR, the production of weapons-grade plutonium was carried out first at the Mayak plant in Ozersk (formerly Chelyabinsk-40, Chelyabinsk-65), then at the Siberian Chemical Plant in Seversk (formerly Tomsk-7), and later the Krasnoyarsk Mining Plant was put into operation -chemical plant in Zheleznogorsk (also known as Sotsgorod and Krasnoyarsk-26). Production of weapons-grade plutonium in Russia ceased in 1994. In 1999, the reactors in Ozyorsk and Seversk were shut down, and in 2010 the last reactor in Zheleznogorsk was shut down.
In the United States, weapons-grade plutonium was produced in several places, such as the Hanford complex in Washington state. Production was closed in 1988.
3.Synthesis of new elements
The transformation of some atoms into others occurs through the interaction of atomic or subatomic particles. Of these, only neutrons are available in large quantities. A gigawatt nuclear reactor produces about 3.75 kg (or 4 * 1030) neutrons over the course of a year.
4.Plutonium production
Plutonium atoms are formed as a result of a chain of atomic reactions beginning with the capture of a neutron by a uranium-238 atom:
U238 + n -> U239 -> Np239 -> Pu239
or, more precisely:
0n1 + 92U238 -> 92U239 -> -1e0 + 93Np239 -> -1e0 + 94Pu239
With continued irradiation, some atoms of plutonium-239 are able, in turn, to capture a neutron and turn into the heavier isotope plutonium-240:
Pu239 + n -> Pu240
To obtain plutonium in sufficient quantities, strong neutron fluxes are needed. These are exactly what are created in nuclear reactors. In principle, any reactor is a source of neutrons, but for the industrial production of plutonium it is natural to use one specially designed for this purpose.
The world's very first commercial plutonium production reactor was the B-reactor at Hanford. Worked on September 26, 1944, power - 250 MW, productivity - 6 kg of plutonium per month. It contained about 200 tons of uranium metal, 1200 tons of graphite and was cooled with water at a rate of 5 cubic meters/min.
Loading panel of the Hanford reactor with uranium cassettes:
Scheme of its work. In a reactor for irradiating uranium-238, neutrons are created as a result of a stationary chain reaction of fission of uranium-235 nuclei. On average, 2.5 neutrons are produced per fission of U-235. To maintain the reaction and simultaneously produce plutonium, it is necessary that on average one or two neutrons be absorbed by U-238, and one would cause the fission of the next U-235 atom.
Neutrons produced during the fission of uranium have very high speeds. Uranium atoms are arranged in such a way that the capture of fast neutrons by the nuclei of both U-238 and U-235 is unlikely. Therefore, fast neutrons, having experienced several collisions with surrounding atoms, gradually slow down. In this case, U-238 nuclei absorb such neutrons (intermediate velocities) so strongly that nothing is left to fission U-235 and maintain the chain reaction (U-235 is divided from slow, thermal neutrons).
This is counteracted by a moderator, some light substance surrounding the uranium blocks. In it, neutrons are decelerated without absorption, experiencing elastic collisions, in each of which a small part of the energy is lost. Good moderators are water and carbon. Thus, neutrons slowed down to thermal speeds travel through the reactor until they cause fission of U-235 (U-238 absorbs them very weakly). With a certain configuration of the moderator and uranium rods, conditions will be created for the absorption of neutrons by both U-238 and U-235.
The isotopic composition of the resulting plutonium depends on the length of time the uranium rods are in the reactor. A significant accumulation of Pu-240 occurs as a result of prolonged irradiation of a cassette with uranium. With a short residence time of uranium in the reactor, Pu-239 is obtained with an insignificant content of Pu-240.
Pu-240 is harmful to weapons production for the following reasons:
1. It is less fissile than Pu-239, so slightly more plutonium is required to make weapons.
2. Second, much more important reason. The level of spontaneous fission in Pu-240 is much higher, which creates a strong neutron background.
In the very first years of development atomic weapons neutron emission (high neutron background) was a problem on the way to a reliable and efficient charge due to its premature detonation. Strong neutron fluxes made it difficult or impossible to compress a bomb core containing several kilograms of plutonium into a supercritical state - before this it was destroyed by the strongest, but still not the maximum possible energy output. The advent of mixed nuclei - containing highly enriched U-235 and plutonium (in the late 1940s) - overcame this difficulty when it became possible to use relatively small amounts of plutonium in mostly uranium nuclei. The next generation of charges, fusion amplified devices (in the mid-1950s), completely eliminated this difficulty, guaranteeing high energy release even with low-power initial fission charges.
Plutonium produced in special reactors contains a relatively small percentage of Pu-240 (<7%), плутоний "оружейного качества"; в реакторах АЭС отработанное ядерное топливо имеет концентрацию Pu-240 более 20%, плутоний "реакторного качества".
In special-purpose reactors, uranium is present for a relatively short period of time, during which not all U-235 burns out and not all U-238 turns into plutonium, but a smaller amount of Pu-240 is formed.
There are two reasons for producing plutonium with low Pu-240 content:
Economic: the only reason for the existence of plutonium special reactors. Decaying plutonium by fission or converting it into less fissile Pu-240 reduces returns and increases production costs (to the point where its price balances with the cost of processing irradiated fuel with low plutonium concentrations).
Handling Difficulty: While neutron emission is not a major concern for weapon designers, it can create manufacturing and handling challenges for such a charge. Neutrons create an additional contribution to occupational exposure to those who assemble or maintain weapons (neutrons themselves do not ionize, but they create protons that can). In fact, charges that involve direct contact with people, such as the Davy Crocket, may require ultra-pure, low-neutron-emitting plutonium for this reason.
The actual casting and processing of plutonium is done by hand in sealed chambers with operator gloves. Like these:
This implies very little protection for humans from neutron-emitting plutonium. Therefore, plutonium with a high content of Pu-240 is processed only by manipulators, or the time each worker works with it is strictly limited.
For all these reasons (radioactivity, worse properties of Pu-240) it is explained why reactor-quality plutonium is not used for the manufacture of weapons - it is cheaper to produce weapons-grade plutonium in special. reactors. Although, apparently, it is also possible to make a nuclear explosive device from a reactor one.
Plutonium ring
This ring is made of electrolytically purified plutonium metal (over 99.96% pure). Typical of the rings prepared at Los Alamos and sent to Rocky Flats for weapon making until production was recently suspended. The mass of the ring is 5.3 kg, sufficient for the manufacture of a modern strategic charge, the diameter is approximately 11 cm. The ring shape is important for ensuring critical safety.
Casting of plutonium-gallium alloy recovered from a weapons core:
Plutonium during the Manhattan Project
Historically, the first 520 milligrams of plutonium metal produced by Ted Magel and Nick Dallas at Los Alamos on March 23, 1944:
Press for hot pressing of plutonium-gallium alloy in the form of hemispheres. This press was used at Los Alamos to make plutonium cores for the charges detonated at Nagasaki and Operation Trinity.
Products cast on it:
Additional by-product isotopes of plutonium
Neutron capture, not accompanied by fission, creates new isotopes of plutonium: Pu-240, Pu-241 and Pu-242. The last two accumulate in small quantities.
Pu239 + n -> Pu240
Pu240 + n -> Pu241
Pu241 + n -> Pu242
A side chain of reactions is also possible:
U238 + n -> U237 + 2n
U237 -> (6.75 days, beta decay) -> Np237
Np237 + n -> Np238
Np238 -> (2.1 days, beta decay) -> Pu238
The overall measure of irradiation (waste) of a fuel cell can be expressed in megawatt days/ton (MW-day/t). Weapons grade plutonium quality is obtained from elements with a small amount of MW-day/t, it produces fewer by-product isotopes. Fuel cells in modern pressurized water reactors reach levels of 33,000 MW-day/t. Typical exposure in a weapons breeder (with expanded breeding of nuclear fuel) reactor is 1000 MW-day/t. Plutonium in the Hanford graphite-moderated reactors is irradiated up to 600 MW-day/t, in Savannah the heavy water reactor produces plutonium of the same quality at 1000 MW-day/t (possibly due to the fact that some of the neutrons are spent on the formation of tritium) . During the Manhattan Project, natural uranium fuel received only 100 MW-day/t, thus producing very high quality plutonium-239 (only 0.9-1% Pu-240, other isotopes in even smaller quantities).
Related information.
Chemistry
Plutonium Pu - element No. 94 is associated with very great hopes and very great fears of humanity. These days it is one of the most important, strategically important elements. It is the most expensive of the technically important metals - it is much more expensive than silver, gold and platinum. He is truly precious.
In the beginning there were protons - galactic hydrogen. As a result of its compression and subsequent nuclear reactions, the most incredible “ingots” of nucleons were formed. Among them, these “ingots,” there were apparently those containing 94 protons. Theorists' estimates suggest that about 100 nucleon formations, which include 94 protons and from 107 to 206 neutrons, are so stable that they can be considered the nuclei of isotopes of element No. 94.
But all these isotopes - hypothetical and real - are not so stable as to survive to this day since the formation of the elements of the solar system. The half-life of the longest-lived isotope of element No. 94 is 81 million years. The age of the Galaxy is measured in billions of years. Consequently, the “primordial” plutonium had no chance of surviving to this day. If it was formed during the great synthesis of the elements of the Universe, then those ancient atoms of it “extinct” long ago, just as dinosaurs and mammoths became extinct.
In the 20th century new era, AD, this element was recreated. Of the 100 possible isotopes of plutonium, 25 have been synthesized. The nuclear properties of 15 of them have been studied. Four have found practical application. And it was opened quite recently. In December 1940, when uranium was irradiated with heavy hydrogen nuclei, a group of American radiochemists led by Glenn T. Seaborg discovered a previously unknown alpha particle emitter with a half-life of 90 years. This emitter turned out to be the isotope of element No. 94 with a mass number of 238. In the same year, but a few months earlier, E.M. McMillan and F. Abelson obtained the first element heavier than uranium, element number 93. This element was called neptunium, and element 94 was called plutonium. The historian will definitely say that these names originate in Roman mythology, but in essence the origin of these names is rather not mythological, but astronomical.
Elements No. 92 and 93 are named after the distant planets of the solar system - Uranus and Neptune, but also Neptune in solar system- not the last, even further lies the orbit of Pluto - a planet about which almost nothing is still known... We see a similar construction on the “left flank” of the periodic table: uranium - neptunium - plutonium, but humanity knows much more about plutonium than about Pluto. By the way, astronomers discovered Pluto just ten years before the synthesis of plutonium - almost the same period of time separated the discoveries of Uranus - the planet and uranium - the element.
The first isotope of element No. 94, plutonium-238, has found practical application these days. But in the early 40s they didn’t even think about it. It is possible to obtain plutonium-238 in quantities of practical interest only by relying on the powerful nuclear industry. At that time it was just in its infancy. But it was already clear that by releasing the energy contained in the nuclei of heavy radioactive elements, it was possible to obtain weapons of unprecedented power. The Manhattan Project appeared, which had nothing more than a name in common with the famous New York area. This was the general name for all work related to the creation of the first atomic bombs in the United States. It was not a scientist, but a military man, General Groves, who was appointed head of the Manhattan Project, who “affectionately” called his highly educated charges “broken pots.”
The leaders of the “project” were not interested in plutonium-238. Its nuclei, like the nuclei of all plutonium isotopes with even mass numbers, are not fissile by low-energy neutrons, so it could not serve as a nuclear explosive. Nevertheless, the first not very clear reports about elements No. 93 and 94 appeared in print only in the spring of 1942.
How can we explain this? Physicists understood: the synthesis of plutonium isotopes with odd mass numbers was a matter of time, and not too long. Odd isotopes were expected to, like uranium-235, be able to support a nuclear chain reaction. Some people saw them as potential nuclear explosives, which had not yet been received. And these hopes plutonium, unfortunately, he justified it.
In encryption of that time, element No. 94 was called nothing more than... copper. And when the need arose for copper itself (as a structural material for some parts), then in the codes, along with “copper,” “genuine copper” appeared.
In 1941, the most important isotope of plutonium was discovered - an isotope with mass number 239. And almost immediately the theorists' prediction was confirmed: plutonium-239 nuclei were fissioned by thermal neutrons. Moreover, during their fission, no less number of neutrons were produced than during the fission of uranium-235. Ways to obtain this isotope in large quantities were immediately outlined...
Years have passed. Now it’s no secret to anyone that the nuclear bombs stored in arsenals are filled with plutonium-239 and that these bombs are enough to cause irreparable damage to all life on Earth.
There is a widespread belief that humanity was clearly in a hurry with the discovery of the nuclear chain reaction (the inevitable consequence of which was the creation of a nuclear bomb). You can think differently or pretend to think differently - it’s more pleasant to be an optimist. But even optimists inevitably face the question of the responsibility of scientists. We remember the triumphant June day of 1954, the day when the first nuclear power plant in Obninsk. But we cannot forget the morning of August 1945 - “the morning of Hiroshima”, “the black day of Albert Einstein”... We remember the first post-war years and the rampant atomic blackmail - the basis of American policy in those years. But hasn’t humanity experienced a lot of troubles in subsequent years? Moreover, these anxieties were intensified many times over by the consciousness that if a new outbreak broke out World War, nuclear weapons will be launched.
Here you can try to prove that the discovery of plutonium did not add fear to humanity, that, on the contrary, it was only useful.
Let's say it happened that for some reason or, as they would say in the old days, by the will of God, plutonium was inaccessible to scientists. Would our fears and concerns then be reduced? Nothing happened. Nuclear bombs would be made from uranium-235 (and in no less quantity than from plutonium), and these bombs would “eat up” even larger parts of the budgets than now.
But without plutonium there would be no prospects for the peaceful use of nuclear energy on a large scale. There simply would not be enough uranium-235 for a “peaceful atom”. The evil inflicted on humanity by the discovery of nuclear energy would not be balanced, even partially, by the achievements of the “good atom.”
When a plutonium-239 nucleus is split by neutrons into two fragments of approximately equal mass, about 200 MeV of energy is released. This is 50 million times more energy released in the most famous exothermic reaction C + O 2 = CO 2. “Burning” in a nuclear reactor, a gram of plutonium gives 2,107 kcal. In order not to break tradition (and in popular articles, the energy of nuclear fuel is usually measured in non-systemic units - tons of coal, gasoline, trinitrotoluene, etc.), we also note: this is the energy contained in 4 tons of coal. And an ordinary thimble contains an amount of plutonium energetically equivalent to forty carloads of good birch firewood.
The same energy is released during the fission of uranium-235 nuclei by neutrons. But the bulk of natural uranium (99.3%!) is the isotope 238 U, which can only be used by turning uranium into plutonium...
Let us evaluate the energy resources contained in natural uranium reserves.
Uranium is a trace element and is found almost everywhere. Anyone who has visited, for example, Karelia, will probably remember granite boulders and coastal cliffs. But few people know that a ton of granite contains up to 25 g of uranium. Granites make up almost 20% of the weight of the earth's crust. If we count only uranium-235, then a ton of granite contains 3.5-105 kcal of energy. It's a lot, but...
Processing granite and extracting uranium from it requires spending an even larger amount of energy - about 106-107 kcal/t. Now, if it were possible to use not only uranium-235, but also uranium-238 as an energy source, then granite could be considered at least as a potential energy raw material. Then the energy obtained from a ton of stone would be from 8-107 to 5-108 kcal. This is equivalent to 16-100 tons of coal. And in this case, granite could provide people with almost a million times more energy than all the chemical fuel reserves on Earth.
But uranium-238 nuclei do not fission by neutrons. For nuclear energy this isotope is useless. More precisely, it would be useless if it could not be converted into plutonium-239. And what is especially important: practically no energy needs to be spent on this nuclear transformation - on the contrary, energy is produced in this process!
Let's try to figure out how this happens, but first a few words about natural plutonium.
It has already been said that isotopes of plutonium have not been preserved since the synthesis of elements during the formation of our planet. But this does not mean that there is no plutonium in the Earth.
It is formed all the time in uranium ores. By capturing neutrons from cosmic radiation and neutrons produced by the spontaneous fission of uranium-238 nuclei, some - very few - atoms of this isotope turn into atoms of uranium-239. These nuclei are very unstable; they emit electrons and thereby increase their charge. Neptunium, the first transuranium element, is formed. Neptunium-239 is also highly unstable, and its nuclei emit electrons. In just 56 hours, half of the neptunium-239 turns into plutonium-239, the half-life of which is already quite long - 24 thousand years.
Why is plutonium not extracted from uranium ores?? Low, too low concentration. “Production per gram - labor per year” - this is about radium, and plutonium in ores is 400 thousand times less than radium. Therefore, it is extremely difficult not only to mine, but even to detect “terrestrial” plutonium. This was done only after the physical and chemical properties of plutonium produced in nuclear reactors were studied.
Plutonium is accumulated in nuclear reactors. In powerful neutron streams, the same reaction occurs as in uranium ores, but the rate of formation and accumulation of plutonium in the reactor is much higher - a billion billion times. For the reaction of converting ballast uranium-238 into energy-grade plutonium-239, optimal (within acceptable) conditions are created.
If the reactor operates on thermal neutrons (recall that their speed is about 2000 m per second, and their energy is a fraction of an electronvolt), then from a natural mixture of uranium isotopes an amount of plutonium is obtained that is slightly less than the amount of “burnt out” uranium-235. A little, but less, plus the inevitable losses of plutonium during its chemical separation from irradiated uranium. In addition, the nuclear chain reaction is accelerated in natural mixture uranium isotopes only until a small fraction of uranium-235 is consumed. Hence the logical conclusion: a “thermal” reactor using natural uranium - the main type of currently operating reactors - cannot ensure the expanded reproduction of nuclear fuel. But what is promising then? To answer this question, let’s compare the course of the nuclear chain reaction in uranium-235 and plutonium-239 and introduce another physical concept into our discussions.
The most important characteristic of any nuclear fuel is the average number of neutrons emitted after the nucleus has captured one neutron. Physicists call it the eta number and denote it by the Greek letter q. In “thermal” reactors on uranium, the following pattern is observed: each neutron generates an average of 2.08 neutrons (η = 2.08). Plutonium placed in such a reactor under the influence of thermal neutrons gives η = 2.03. But there are also reactors that operate on fast neutrons. It is useless to load a natural mixture of uranium isotopes into such a reactor: a chain reaction will not occur. But if the “raw material” is enriched with uranium-235, it can be developed in a “fast” reactor. In this case, c will already be equal to 2.23. And plutonium, exposed to fast neutron fire, will give η equal to 2.70. We will have “extra half a neutron” at our disposal. And this is not at all little.
When, as a result of nuclear reactions, the required amount of plutonium has accumulated in uranium, it must be separated not only from the uranium itself, but also from fission fragments - both uranium and plutonium, burned up in the nuclear chain reaction. In addition, the uranium-plutonium mass also contains a certain amount of neptunium. The most difficult things to separate are plutonium from neptunium and rare earth elements (lanthanides). Plutonium, as a chemical element, has been unlucky to some extent. From a chemist's point of view, the main element of nuclear energy is just one of fourteen actinides. Like rare earth elements, all elements of the actinium series are very similar to each other in chemical properties; the structure of the outer electron shells of the atoms of all elements from actinium to 103 is the same. What’s even more unpleasant is that the chemical properties of actinides are similar to the properties of rare earth elements, and among the fission fragments of uranium and plutonium there are more than enough lanthanides. But then element 94 can be in five valence states, and this “sweets the pill” - it helps to separate plutonium from both uranium and fission fragments.
The valency of plutonium varies from three to seven. Chemically, the most stable (and therefore the most common and most studied) compounds are tetravalent plutonium.
The separation of actinides with similar chemical properties - uranium, neptunium and plutonium - can be based on the difference in the properties of their tetra- and hexavalent compounds.
Plutonium-239 also contains in small quantities higher isotopes of this element - with mass numbers 240 and 241. The 240 Pu isotope is practically useless - it is ballast in plutonium. From 241, americium is obtained - element No. 95. B pure form, without admixture of other isotopes, plutonium-240 and plutonium-241 can be obtained by electromagnetic separation of plutonium accumulated in the reactor. Before this, plutonium is additionally irradiated with neutron fluxes with strictly defined characteristics. Of course, all this is very complicated, especially since plutonium is not only radioactive, but also very toxic. Working with it requires extreme caution.
One of the most interesting isotopes of plutonium, 242 Pu, can be obtained by irradiating 239 Pu for a long time in neutron fluxes. 242 Pu very rarely captures neutrons and therefore “burns out” in the reactor more slowly than other isotopes; it persists even after the remaining isotopes of plutonium have almost completely turned into fragments or turned into plutonium-242.
Plutonium-242 is important as a “raw material” for the relatively rapid accumulation of higher transuranium elements in nuclear reactors. If plutonium-239 is irradiated in a conventional reactor, then it will take about 20 years to accumulate microgram amounts of, for example, California-252 from grams of plutonium.
It is possible to reduce the accumulation time of higher isotopes by increasing the intensity of the neutron flux in the reactor. This is what they do, but then you cannot irradiate large amounts of plutonium-239. After all, this isotope is divided by neutrons, and too much energy is released in intense flows. Additional difficulties arise with cooling the reactor. To avoid these difficulties, it would be necessary to reduce the amount of plutonium irradiated. Consequently, the yield of californium would again become scanty. Vicious circle!
Plutonium-242 is not fissile by thermal neutrons, it can be irradiated in large quantities in intense neutron fluxes... Therefore, in reactors, all elements from americium to fermium are “made” from this isotope and accumulated in weight quantities.
Every time scientists managed to obtain a new isotope of plutonium, the half-life of its nuclei was measured. The half-lives of isotopes of heavy radioactive nuclei with even mass numbers change regularly. (This cannot be said for odd isotopes.)
As the mass increases, the “lifetime” of the isotope also increases. Several years ago, the high point of this graph was plutonium-242. And then how will this curve go? further growth mass number? To point 1, which corresponds to a lifetime of 30 million years, or to point 2, which corresponds to 300 million years? The answer to this question was very important for geosciences. In the first case, if 5 billion years ago the Earth consisted entirely of 244 Pu, now only one atom of plutonium-244 would remain in the entire mass of the Earth. If the second assumption is true, then plutonium-244 may be in the Earth in concentrations that could already be detected. If we were lucky enough to find this isotope in the Earth, science would receive the most valuable information about the processes that took place during the formation of our planet.
A few years ago, scientists were faced with the question: is it worth trying to find heavy plutonium in the Earth? To answer it, it was necessary first of all to determine the half-life of plutonium-244. Theorists could not calculate this value with the required accuracy. All hope was only for experiment.
Plutonium-244 accumulated in a nuclear reactor. Element No. 95 - americium (isotope 243 Am) was irradiated. Having captured a neutron, this isotope turned into americium-244; americium-244 in one out of 10 thousand cases turned into plutonium-244.
The preparation of plutonium-244 was isolated from a mixture of americium and curium. The sample weighed only a few millionths of a gram. But they were enough to determine the half-life of this interesting isotope. It turned out to be equal to 75 million years. Later, other researchers clarified the half-life of plutonium-244, but not by much - 81 million years. In 1971, traces of this isotope were found in the rare earth mineral bastnäsite.
Many attempts have been made by scientists to find an isotope of the transuranium element that lives longer than 244 Pu. But all attempts remained in vain. At one time, hopes were placed on curium-247, but after this isotope was accumulated in the reactor, it turned out that its half-life is only 16 million years. It was not possible to break the record of plutonium-244 - it is the longest-lived of all isotopes of transuranium elements.
Even heavier isotopes of plutonium undergo beta decay, and their lifetimes range from a few days to a few tenths of a second. We know for sure that all isotopes of plutonium are formed in thermonuclear explosions, up to 257 Pu. But their lifetime is tenths of a second, and many short-lived isotopes of plutonium have not yet been studied.
And finally - about plutonium-238 - the very first of the “man-made” isotopes of plutonium, an isotope that at first seemed unpromising. It is actually a very interesting isotope. It is subject to alpha decay, that is, its nuclei spontaneously emit alpha particles - helium nuclei. Alpha particles generated by plutonium-238 nuclei carry high energy; dissipated in matter, this energy turns into heat. How big is this energy? Six million electron volts are released from the decay of one atomic nucleus of plutonium-238. IN chemical reaction the same energy is released during the oxidation of several million atoms. In a source of electricity containing one kilogram of plutonium-238, thermal power 560 watts. Maximum power a chemical current source of the same mass - 5 watts.
There are many emitters with similar energy characteristics, but one feature of plutonium-238 makes this isotope indispensable. Alpha decay is usually accompanied by strong gamma radiation, penetrating through large layers of matter. 238 Pu is an exception. The energy of gamma rays accompanying the decay of its nuclei is low, and it is not difficult to protect against it: the radiation is absorbed by a thin-walled container. The probability of spontaneous fission of nuclei of this isotope is also low. Therefore, it has found application not only in current sources, but also in medicine. Batteries containing plutonium-238 serve as a source of energy in special cardiac stimulants.
But 238 Pu is not the lightest known isotope of element No. 94; isotopes of plutonium have been obtained with mass numbers from 232 to 237. The half-life of the lightest isotope is 36 minutes.
Plutonium is a big topic. The most important things are told here. After all, it has already become a standard phrase that the chemistry of plutonium has been studied much better than the chemistry of such “old” elements as iron. Whole books have been written about the nuclear properties of plutonium. The metallurgy of plutonium is another amazing section of human knowledge... Therefore, you should not think that after reading this story, you truly learned plutonium - the most important metal of the 20th century.
This metal is called precious, but not for its beauty, but for its irreplaceability. In the periodic table of Mendeleev, this element occupies cell number 94. It is with it that scientists pin their greatest hopes, and it is plutonium that they call the most dangerous metal for humanity.
In appearance it is a silvery-white metal. It is radioactive and can be represented in the form of 15 isotopes with different half-lives, for example:
This metal cannot be extracted from ore, since it is a product of the radioactive transformation of uranium.
The production of plutonium requires the fission of uranium, which can only be done in nuclear reactors. If we talk about the presence of the element Pu in earth's crust, then for 4 million tons of uranium ore there will be only 1 gram of pure plutonium. And this gram is formed by the natural capture of neutrons by uranium nuclei. Thus, in order to obtain this nuclear fuel (usually the 239-Pu isotope) in an amount of several kilograms, a complex technological process in a nuclear reactor.
The radioactive metal plutonium has the following physical properties:
Plutonium is radioactive, which is why it is warm to the touch. Moreover, this metal is characterized by the lowest thermal and electrical conductivity. Liquid plutonium is the most viscous of all existing metals.
The slightest change in the temperature of plutonium leads to an instant change in the density of the substance. In general, the mass of plutonium is constantly changing, since the nuclei of this metal are in a state of constant fission into smaller nuclei and neutrons. The critical mass of plutonium is the name given to the minimum mass of a fissile substance at which fission (a nuclear chain reaction) remains possible. For example, the critical mass of weapons-grade plutonium is 11 kg (for comparison, the critical mass of highly enriched uranium is 52 kg).
Uranium and plutonium are the main nuclear fuels. To obtain plutonium in large quantities, two technologies are used:
Both methods involve the separation of plutonium and uranium as a result of a chemical reaction.
It is quite difficult to isolate plutonium among the products of nuclear reactions, since plutonium (like uranium, thorium, neptunium) belongs to actinides that are very similar in chemical properties. The task is complicated by the fact that among the decay products contained rare earth elements, the chemical properties of which are also similar to plutonium. Traditional radiochemical methods are used - precipitation, extraction, ion exchange, etc. The final product of this multi-stage technology is plutonium oxides PuO 2 or fluorides (PuF 3, PuF 4).
Plutonium is mined using the metallothermy method (reduction active metals from oxides and salts in vacuum):
PuF 4 +2 Ba = 2BaF 2 + Pu
Isotopes
More than a dozen isotopes of plutonium are known, all of them are radioactive.
The most important isotope 239 Pu, capable of nuclear fission and nuclear chain reactions. It is the only isotope suitable for use in nuclear weapons. It has better neutron absorption and scattering characteristics than uranium-235, the number of neutrons per fission (about 3 versus 2.3) and, accordingly, a lower critical mass. Its half-life is about 24 thousand years. Other isotopes of plutonium are considered primarily from the point of view of their harmfulness for primary (weapon) use.
Isotope 238 Pu has powerful alpha radioactivity and, as a consequence, significant heat generation (567 W / kg). This is problematic for use in nuclear weapons, but has applications in nuclear batteries. Almost all spacecraft that have flown beyond the orbit of Mars have radioisotope reactors using 238 Pu. In reactor plutonium, the proportion of this isotope is very small.
Isotope 240 Pu is the main contaminant of weapons-grade plutonium. It has a high rate of spontaneous decay and creates a high neutron background, which significantly complicates the detonation of nuclear charges. It is believed that its share in weapons should not exceed 7%.
241 Pu has a low neutron background and moderate thermal emission. Its share is slightly less than 1% and does not affect the properties of weapons-grade plutonium. However, with its half-life, 1914 turns into americium-241, which generates a lot of heat, which can create a problem with overheating of charges.
242 Pu has a very small cross section for the neutron capture reaction and accumulates in nuclear reactors, although in a very small quantity(less than 0.1%). It does not affect the properties of weapons-grade plutonium. It is used mainly for further nuclear reactions in the synthesis of transplutonium elements: thermal neutrons do not cause nuclear fission, so any quantities of this isotope can be irradiated with powerful neutron fluxes.
Other isotopes of plutonium are extremely rare and have no effect on the manufacture of nuclear weapons. Heavy isotopes are formed in very small quantities, have a short lifetime (less than a few days or hours) and, through beta decay, are converted into the corresponding isotopes of americium. Among them stands out 244 Pu– its half-life is about 82 million years. It is the most isotope of all transuranium elements.
Application
At the end of 1995, the world had produced about 1,270 tons of plutonium, of which 257 tons were for military use, for which only the 239 Pu isotope is suitable. It is possible to use 239 Pu as fuel in nuclear reactors, but it is inferior to uranium in economic terms. The cost of reprocessing nuclear fuel to extract plutonium is much greater than the cost of low-enriched (~5% 235 U) uranium. Only Japan has a program for the energy use of plutonium.
Allotropic modifications
In solid form, plutonium has seven allotropic modifications (however, phases ? and ? 1 are sometimes combined and considered one phase). At room temperature, plutonium is crystal structure, which is called ?-phase. The atoms are connected by a covalent bond (instead of a metal bond), so physical properties closer to minerals than to metals. It is a hard, brittle material that breaks in certain directions. It has low thermal conductivity among all metals, low electrical conductivity, with the exception of manganese. The ?-phase cannot be processed using conventional metal technologies.
When temperature changes, plutonium undergoes a restructuring and experiences extremely strong changes. Some transitions between phases are accompanied by simply striking changes in volume. In two of these phases (? and?1) plutonium has a unique property - a negative temperature coefficient of expansion, i.e. it contracts with increasing temperature.
In the gamma and delta phases, plutonium exhibits the usual properties of metals, in particular malleability. However, in the delta phase, plutonium exhibits instability. Under slight pressure, it tries to settle into a dense (25%) alpha phase. This property is used in implosion devices of nuclear weapons.
In pure plutonium at pressures above 1 kilobar, the delta phase does not exist at all. At pressures above 30 kilobars, only alpha and beta phases exist.
Plutonium metallurgy
Plutonium can be stabilized in the delta phase at normal pressure and room temperature by forming an alloy with trivalent metals such as gallium, aluminum, cerium, indium in a concentration of several molar percent. It is in this form that plutonium is used in nuclear weapons.
Weaponized plutonium
To produce nuclear weapons, it is necessary to achieve a purity of the desired isotope (235 U or 239 Pu) of more than 90%. Creating charges from uranium requires many enrichment steps (because the proportion of 235 U in natural uranium is less than 1%), while the proportion of 239 Pu in reactor plutonium is usually from 50% to 80% (i.e. almost 100 times more). And in some reactor operating modes it is possible to obtain plutonium containing more than 90% 239 Pu - such plutonium does not require enrichment and can be used directly for the manufacture of nuclear weapons.
Biological role
Plutonium is one of the most toxic known substances. The toxicity of plutonium is due not so much chemical properties(although plutonium is perhaps as toxic as any heavy metal), how much is its alpha radioactivity. Alpha particles are retained even by thin layers of materials or fabrics. Let's say a few millimeters of skin will completely absorb their flow, protecting internal organs. But alpha particles are extremely damaging to the tissues with which they come into contact. So, plutonium poses a serious danger if it enters the body. It is very poorly absorbed in the gastrointestinal tract, even if it gets there in soluble form. But ingesting half a gram of plutonium can lead to death within weeks due to acute exposure to the digestive tract.
Inhalation of a tenth of a gram of plutonium dust results in death from pulmonary edema within ten days. Inhalation of a dose of 20 mg leads to death from fibrosis within a month. Smaller doses cause a carcinogenic effect. Ingestion of 1 mcg of plutonium increases the likelihood of lung cancer by 1%. Therefore, 100 micrograms of plutonium in the body almost guarantees the development of cancer (within ten years, although tissue damage may occur earlier).
In biological systems, plutonium is usually in the +4 oxidation state and shows similarities to iron. Once in the blood, it will most likely concentrate in tissues containing iron: bone marrow, liver, spleen. If even 1-2 micrograms of plutonium settles in the bone marrow, immunity will deteriorate significantly. The period of removal of plutonium from bone tissue is 80-100 years, i.e. he will remain there practically throughout his life.
The International Commission on Radiological Protection has set the maximum annual plutonium uptake at 280 nanograms.
