The atomic number of plutonium. Weapon-grade plutonium: application, production, disposal. Design features of industrial reactors
Plutonium was discovered in late 1940 at the University of California. It was synthesized by McMillan, Kennedy and Wahl by bombarding uranium oxide (U 3 O 8) with deuterium nuclei (deuterons) strongly accelerated in a cyclotron. Later it was found that this nuclear reaction first produces the short-lived isotope neptunium-238, and from it already plutonium-238 with a half-life of about 50 years. A year later, Kennedy, Seaborg, Segre and Wahl synthesized a more important isotope, plutonium-239, by irradiating uranium with highly accelerated neutrons in a cyclotron. Plutonium-239 is formed by the decay of neptunium-239; it emits alpha rays and has a half-life of 24,000 years. The pure plutonium compound was first obtained in 1942. Then it became known that there is natural plutonium found in uranium ores, in particular in ores, deposits in the Congo.
The name of the element was proposed in 1948: McMillan named the first transuranic element neptunium due to the fact that the planet Neptune is the first behind Uranus. By analogy, it was decided to call element 94 plutonium, since the planet Pluto is the second after Uranus. Pluto, discovered in 1930, got its name from the god Pluto - the ruler of the underworld in Greek mythology. At the beginning of the XIX century. Clark proposed to name the element barium plutonium, deriving this name directly from the god Pluto, but his proposal was not accepted.
Plutonium (Latin Plutonium, denoted by the symbol Pu) is a radioactive chemical element with atomic number 94 and atomic weight 244.064. Plutonium is an element of the III group of the periodic system of Dmitry Ivanovich Mendeleev, belongs to the actinide family. Plutonium is a heavy (density under normal conditions 19.84 g / cm³) brittle radioactive metal of silvery-white color.
Plutonium has no stable isotopes. Of a hundred possible isotopes of plutonium, twenty-five have been synthesized. Fifteen of them studied nuclear properties (mass numbers 232-246). Four have found practical applications. The longest-lived isotopes - 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) is a β-emitter. Plutonium is naturally found in trace amounts in uranium ores (239Pu); it is formed from uranium under the action of neutrons, the sources of which are the reactions occurring during the interaction of α-particles with light elements (which are part of the ores), the 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 at Berkeley (at the University of California) during the bombardment of a uranium oxide target ( 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, a year later, 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 (isotope 239Pu) is used in nuclear weapons and serves as a nuclear fuel for power reactors operating on thermal and especially fast neutrons. The critical mass for 239Pu as a metal is 5.6 kg. Among other things, the 239Pu isotope is the starting material for the production of transplutonium elements in nuclear reactors. The 238Pu isotope is used in small-sized nuclear power sources used in space research, as well as in stimulators of human cardiac activity.
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 harmful to the body. It is worth considering that different plutonium isotopes 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.
Biological properties
Plutonium is concentrated by marine organisms: the accumulation coefficient of this radioactive metal (the ratio of concentrations in the body and in the external environment) for algae is 1000-9000, for plankton - about 2300, for starfish - about 1000, for mollusks - up to 380, for muscles, bones , liver and stomach of fish - 5, 570, 200 and 1060, respectively. Terrestrial plants assimilate plutonium mainly through the root system and accumulate it up 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 hazard (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 power: for 239Pu, the range of α-particles in air is 3.7 cm, and in soft biological tissue, 43 microns. Therefore, α-particles pose a serious danger if their source is in the body of an infected person. At the same time, they damage the body tissues surrounding the element.
At the same time, gamma 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 harm 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 the element is taken along with water and food, plutonium is less toxic than substances such as caffeine, some vitamins, pseudoephedrine, and many plants and fungi. This is due to the fact that this element is poorly absorbed by the gastrointestinal tract, even when supplied in the form of a soluble salt, this very salt is bound by the contents of the stomach and intestines. However, the absorption of 0.5 gram of plutonium in a finely divided or dissolved state can lead to death from acute irradiation of the digestive system in a few days or weeks (for cyanide this value is 0.1 gram).
From the point of view of inhalation, plutonium is an ordinary toxin (roughly corresponds to mercury vapor). When inhaled, plutonium is carcinogenic and can cause lung cancer. So when inhaling one hundred milligrams of plutonium in the form of particles of the optimal size for holding 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 penetration of plutonium into the body is increased due to the fact that plutonium is prone to the formation of aerosols.
Despite being a metal, it is quite volatile. A short stay of metal in a room significantly increases its concentration in the air. Once in the lungs, plutonium partially settles on the surface of the lungs, partially passes into the blood, and then into the lymph and bone marrow substance. Most (about 60%) goes to bone tissue, 30% to the liver and only 10% is excreted naturally. The amount of plutonium ingested depends on the size of the aerosol particles and the 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 pull plutonium through the lymph nodes. Once in the body, plutonium is removed from it for a very long time - over the course of 50 years, only 80% will be excreted 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 the ninety-fourth element in the bones is constant.
Throughout and after World War II, scientists at the Manhattan Project, as well as scientists from the Third Reich and other research organizations, experimented with 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 was that chronically ill patients were usually injected intramuscularly with 5 μg of plutonium. As a result, it was found that the lethal dose for a patient is equal to one microgram of plutonium, and that plutonium is more dangerous than radium and is prone to accumulation in bones.
As you know, plutonium is an element practically absent in nature. However, about five tons of it was released into the atmosphere as a result of nuclear tests in the period 1945-1963. The total amount of plutonium released into the atmosphere due to nuclear tests prior to the 1980s is estimated at 10 tons. According to some estimates, the soil in the United States of America contains an average of 2 millicurie (28 mg) plutonium per km2 of fallout, and the presence of plutonium in the Pacific Ocean is increased compared to the general proliferation of nuclear materials on earth.
The latter phenomenon is associated with the conduct of US nuclear tests on the territory of the Marshall Islands in the Pacific Range in the mid-1950s. The residence time of plutonium in the surface waters of the ocean is from 6 to 21 years, however, even after this period, plutonium falls to the bottom together with biogenic particles, from which it is reduced to soluble forms as a result of microbial decomposition.
World pollution by the ninety-fourth element is associated not only with nuclear tests, but also with accidents in industries and equipment interacting with this element. So in January 1968, the US Air Force B-52, carrying four nuclear charges, crashed in Greenland. As a result of the explosion, the charges were destroyed and plutonium spilled into the ocean.
Another case of radioactive contamination of the environment as a result of the accident occurred with the Soviet spacecraft "Kosmos-954" on January 24, 1978. As a result of an uncontrolled de-orbit, a satellite with a nuclear power source on board fell into the territory of Canada. As a result of the accident, more than a kilogram of plutonium-238 was released into the environment, spreading over an area of \u200b\u200babout 124,000 m².
The most terrible example of an accidental 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 \u200b\u200babout 2200 km².
The release of plutonium into the environment is associated not only with man-made accidents. There are known cases of plutonium leakage from both laboratory and factory conditions. There are more than twenty known leakage accidents from the 235U and 239Pu laboratories. During 1953-1978. accidents led to the loss from 0.81 (Mayak, March 15, 1953) to 10.1 kg (Tomsk, December 13, 1978) 239Pu. Accidents at industrial plants resulted in a total of two deaths in the city of Los Alamos (August 21, 1945 and May 21, 1946) due to two accidents and losses of 6.2 kg of plutonium. In the city of Sarov in 1953 and 1963. about 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 by neutrons, an energy is released in the amount of 200 MeV, which is 50 million times more than during the most famous exothermic reaction: C + O2 → CO2. "Burning" in a nuclear reactor, one gram of plutonium gives 2,107 kcal - this is the energy contained in 4 tons of coal. A thimble of plutonium fuel in the energy equivalent can be equated to forty wagons of good firewood!
It is believed that the "natural isotope" of plutonium (244Pu) is 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.
Story
In 1934, a group of scientists led by Enrico Fermi made a statement that in the course of scientific work at the University of Rome they had discovered a chemical element with atomic number 94. At Fermi's insistence, the element was named the Hesperium, the scientist was convinced that he had discovered a new element, which is now called plutonium, thus making the assumption about the existence of transuranium elements and becoming their theoretical discoverer. Fermi defended this hypothesis in his Nobel lecture in 1938. Only after the discovery of nuclear fission by German scientists Otto Frisch and Fritz Strassmann, Fermi was forced to make a note in the printed version, published in Stockholm in 1939, indicating the need to revise "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 transuranic elements, as he previously believed.
The new ninety-fourth element was opened at the end of 1940. It happened at Berkeley at the University of California. When uranium oxide (U3O8) was bombarded with heavy hydrogen nuclei (deuterons), a group of American radiochemists led by Glenn T. Seaborg discovered a previously unknown emitter of alpha particles with a half-life of 90 years. This emitter turned out to be an isotope of element 94 with a mass number of 238. Thus, on December 14, 1940, the first microgram quantities of plutonium were obtained together with an admixture of other elements and their compounds.
In the course of an experiment carried out in 1940, it was found that during a nuclear reaction carried out, a short-lived isotope neptunium-238 (half-life of 2.117 days) is first obtained, and from it already plutonium-238:
23392U (d, 2n) → 23893Np → (β−) 23894Pu
Long and laborious chemical experiments to separate a 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. K. Wall thanks to the study of its first chemical properties - the ability to have 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 Val synthesized a more important isotope, plutonium-239, by irradiating uranium with neutrons strongly accelerated in a cyclotron. This isotope is produced 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 94 element was proposed in 1948 by Macmillan, who a few months before the discovery of plutonium, together with F. Abelson, received the first element heavier than uranium - element 93, which was named neptunium in honor of the planet Neptune - the first behind Uranus. By analogy, it was decided to call element 94 plutonium, since the planet Pluto is the second after Uranus. In turn, Seaborg proposed to name the new element "plutium", but then realized that the name does not sound very much 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 the periodic table. Eventually, the element was named "plutonium" after the discovery of the last planet in the solar system.
Being in nature
The longest-lived plutonium isotope has a half-life of 75 million years. The figure is quite 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 to survive to this day. And yet, this does not mean that there is no plutonium at all in the Earth. It is constantly formed in uranium ores. Capturing neutrons from cosmic radiation and neutrons formed during the spontaneous (spontaneous) fission of 238U nuclei, some - very few - atoms of this isotope are converted into 239U atoms. The nuclei of this element are very unstable, they emit electrons and thereby increase their charge, 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 quite long at 24,000 years. On average, the 239Pu content is about 400,000 times less than that of radium. Therefore, it is extremely difficult not only to extract - even to detect "terrestrial" plutonium. Small amounts of 239Pu - a trillionth - and fission products can be found in uranium ores, for example, in a natural nuclear reactor in Oklo, Gabon, West Africa. The so-called "natural nuclear reactor" is considered 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 neptunium (239Np) is formed in uranium ores as a result of the capture of neutrons by uranium nuclei, the product of β-decay of which is natural plutonium-239. Thanks to special instruments - mass spectrometers, the presence of plutonium-244 (244Pu), which has the longest half-life - about 80 million years, was detected in Precambrian bastnesite (in cerium ore). In nature, 244Pu is found mainly 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 plutonium-244 decay chain, its decay takes place, but this happens very rarely (1 case in 10,000). Very small amounts of plutonium-238 (238Pu) are attributed to the very rare double beta decay of the parent isotope, uranium-238, which has been found in uranium ores.
Traces of isotopes 247Pu and 255Pu were found in the dust collected after the explosions of thermonuclear bombs.
The minimum amount of plutonium can hypothetically be in the human body, given that a huge number of nuclear tests have been carried out 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, the ninety-fourth element is accumulated by marine organisms; terrestrial plants assimilate plutonium mainly through the root system.
It turns out that artificially synthesized plutonium still exists in nature, so why is it not mined, but produced 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 labor", and radium in nature is 400,000 times more than plutonium! For this reason, it is extremely difficult not only to extract - even to detect - "terrestrial" plutonium. This was done only after the physical and chemical properties of plutonium obtained in atomic reactors were studied.
Application
The isotope 239Pu (along with U) is used as nuclear fuel for 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 about 370 GW of electricity (or 15% of the total electricity production in the world). 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 work of the heart (pacemakers). 238Pu is used in small-sized nuclear power sources used in space research. Thus, plutonium-238 is a power source for New Horizons, Galileo and Cassini probes, the Curiosity rover and other spacecraft.
Plutonium-239 is used in nuclear weapons, since this isotope is the only suitable nuclide for use in a nuclear bomb. In addition, the more frequent use of 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, one can gain in the explosive power of the bomb due to this property.
The scheme according to which a nuclear explosion with the participation of plutonium occurs is in the construction 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 structure and due to the explosive substance surrounding this sphere. After the impact, the nucleus expands in volume and density in the shortest time - ten microseconds, the assembly jumps through the critical state on thermal neutrons and goes into a supercritical state on fast neutrons - a nuclear chain reaction begins with the participation of neutrons and element nuclei. In the final explosion of a nuclear bomb, a temperature of the order of tens of millions of degrees is released.
Plutonium isotopes have found their application in the synthesis of transplutonium (after plutonium) elements. For example, at the Oak Ridge National Laboratory, 24496Cm, 24296Cm, 24997Bk, 25298Cf, 25399Es and 257100Fm are obtained under long-term neutron irradiation of 239Pu. In the same way in 1944 americium 24195Am was first obtained. In 2010, plutonium-242 oxide bombarded with calcium-48 ions served as a source for the production of ununquadium.
δ-Stabilized plutonium alloys are used in the manufacture of fuel rods, because they have significantly better metallurgical properties in comparison with pure plutonium, which undergoes phase transitions when heated and is a very fragile and unreliable material. Alloys of plutonium with other elements (intermetallic compounds) are usually obtained by direct interaction of elements in the required ratios, 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 intermediate compounds 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) upon rapid cooling. Gallium, aluminum, americium, scandium and cerium can stabilize δ-plutonium at room temperature.
Large amounts of holmium, hafnium and thallium make it possible to store some δ-plutonium at room temperature. Neptunium is the only element that can stabilize α-plutonium at high temperatures. Titanium, hafnium and zirconium stabilize the β-plutonium structure at room temperature with sharp cooling. The use of such alloys is quite diverse. For example, a plutonium-gallium alloy is used to stabilize the δ-phase of plutonium, which avoids the α-δ phase transition. The plutonium-gallium-cobalt ternary alloy (PuGaCo5) is a superconducting alloy at a temperature of 18.5 K. There are a number of alloys (plutonium-zirconium, plutonium-cerium and plutonium-cerium-cobalt) that are used as nuclear fuel.
Production
Commercial plutonium is produced in two ways. This is either irradiation of 238U nuclei contained in nuclear reactors, or radiochemical separation (coprecipitation, extraction, ion exchange, etc.) of plutonium from uranium, transuranium elements and fission products contained in spent fuel.
In the first case, the most practical isotope 239Pu (in a mixture 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. Next, neptunium-238 fissions spontaneously, emitting beta minus particles that form plutonium-238.
Usually the content of 239Pu in a mixture is 90-95%, 240Pu-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 - fractions of a percent of the content of 242Pu. Small amounts of isotopically pure plutonium-238 are produced by neutron irradiation of neptunium-237. Light isotopes of plutonium with mass numbers 232-237 are usually obtained at a cyclotron by irradiating uranium isotopes with α-particles.
The second method for industrial production of 239Pu uses a 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, plutonium is subjected to further purification and concentration. The scheme of such a process is based on the difference in the properties of tetra- and hexavalent compounds of the separated elements.
Initially, the spent fuel rods are dismantled and the cladding containing the spent plutonium and uranium is removed by physical and chemical methods. Next, the recovered nuclear fuel is dissolved in nitric acid. After all, it is a strong oxidizing agent when dissolved, and uranium and plutonium, and impurities are oxidized. Plutonium atoms with zero valence are converted into Pu + 6, dissolution of both plutonium and uranium occurs. 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 precipitate contains neptunium and rare earth elements, but the bulk (uranium) remains in solution. Then plutonium is again oxidized to Pu + 6, and lanthanum fluoride is added again. Now the rare earth elements are deposited, and the plutonium remains in solution. Further, neptunium is oxidized to a tetravalent state with potassium bromate, since this reagent does not act on plutonium, then during the 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 (by reduction with barium, calcium or lithium vapor) metal plutonium is obtained.
The production of purer plutonium can be achieved by electrolytic refining of a pyrochemically produced metal, which is produced in electrolysis cells at a temperature of 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 obtain large quantities of plutonium, breeder reactors are built, the so-called "breeders" (from the English verb to breed - to multiply). These reactors got their name due to their ability to obtain fissile material in an amount exceeding the cost of this material to obtain. 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 them to react as much as possible with 238U.
After the reaction, 239U atoms are formed, which later 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-238 dioxide (238UO2), in which 239Pu is formed. The combined use of 238U and 235U allows "breeders" to produce energy from natural uranium 50-60 times more than other reactors. However, these reactors have a big drawback - the fuel rods must be cooled by an environment 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 to build them only in the 1950s.
Physical properties
Plutonium is a very heavy (standard density 19.84 g / cm³) silvery metal, in a purified state very similar to nickel, however, in air, plutonium oxidizes rapidly, dims, forming an iridescent film, first light yellow, then turning into dark purple. With strong oxidation, an olive-green oxide powder (PuO2) appears on the metal surface.
Plutonium is a highly electronegative and reactive metal, many times greater than even uranium. It has seven allotropic modifications (α, β, γ, δ, δ ", ε and ζ), which vary in a certain temperature segment 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 the same hard as gray cast iron, if it is not alloyed with other metals that will give the alloy plasticity and softness. In addition, in this most dense form, plutonium is the sixth densest element (only osmium, iridium, platinum, rhenium and neptunium are heavier than it). 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 (delta "And" delta-prime ") When it melts (transition from the epsilon phase to the liquid phase), plutonium also compresses, allowing the 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 specific resistance 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. Such an "anomaly" can be traced up to a temperature of 100 K - below this mark, the electrical resistance will decrease. However, from a mark of 20 K, the resistance starts to increase again due to the radiation activity of the metal.
Plutonium has the highest electrical resistivity of all the studied actinides (at the moment), which is 150 μΩ cm (at 22 ° C). This metal has a low melting point (640 ° C) and an unusually high boiling point (3,227 ° C). Closer to its 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 thermo-jacket 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 an increase in temperature above 100 K.
The presence of a large number of allotropic modifications in plutonium makes it a difficult metal to handle 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, but it tends to change and turn into a plastic material, and form 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. Alloyed with these metals, plutonium can be used in welding. In general, the delta shape has more pronounced characteristics of the metal - in terms of strength and forging ability it is close to aluminum.
Chemical properties
The chemical properties of the ninety-fourth element are largely similar to those of its predecessors in the 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 oxidation states: 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 in solution simultaneously in equilibrium, 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 the other valences (oxidation states). Plutonium ions of all oxidation states are prone to hydrolysis and complexation. The ability to form such compounds increases in the series Pu5 +
Compact plutonium slowly oxidizes in air, becoming covered with a rainbow oily oxide film. The following plutonium oxides are known: PuO, Pu2O3, PuO2 and the phase of variable composition Pu2O3 - Pu4O7 (berthollides). In the presence of a small amount of moisture, the rate of oxidation and corrosion increases significantly. If a metal is exposed to small amounts of humid air for a long enough time, plutonium dioxide (PuO2) is formed on its surface. With a lack of oxygen, its dihydride (PuH2) can also form. Surprisingly, plutonium rusts in an inert gas (such as argon) with water vapor much faster than in dry air or pure oxygen. In fact, this fact is easy to explain - the direct action of oxygen forms an oxide layer on the plutonium surface, which prevents further oxidation, the presence of moisture produces a loose mixture of oxide and hydride. By the way, thanks to just such a coating, the metal becomes pyrophoric, that is, it is capable of spontaneous combustion, for this reason, metallic plutonium, as a rule, is 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 readily 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 concentrated boiling acid. Plutonium does not noticeably dissolve in alkali solutions.
When alkalis are exposed to solutions containing tetravalent plutonium, a precipitate of plutonium hydroxide Pu (OH) 4 xH2O, which has basic properties, precipitates. Under the action of alkalis on solutions of salts containing PuO2 +, amphoteric hydroxide PuO2OH precipitates. Salts correspond to it - plutonites, for example, Na2Pu2O6.
Plutonium salts readily hydrolyze upon contact with neutral or alkaline solutions, creating insoluble plutonium hydroxide. Concentrated plutonium solutions are unstable due to radiolytic decomposition leading to precipitation.
Dose-forming radionuclides. Part 5
The date: 03/08/2011
Topic: Health
The main characteristics of dose-forming radionuclides are presented. The main focus is on the presentation of the potential hazard of radionuclides. For the purpose of safety of use, the radiotoxic and radiobiological effects of exposure to radioisotopes on the body and the environment are considered. The foregoing makes it possible to more consciously relate to the radiation hazard of dose-forming radionuclides.
11. Cesium-137
Cesium (lat. cesium - Cs, chemical element of group I of Mendeleev's Periodic Table, atomic number 55, atomic mass 132.9054. Named from Latin caesius - blue (open by bright blue spectral lines). Silvery-white metal from the alkali group; fusible, soft like wax; density is 1.904 g / cm 3 and has beats. weight 1.88 (at 15 ° C), T pl - 28.4 ° C. It ignites in air, reacts with an explosion with water. The main mineral is pollucite.
There are 34 known cesium isotopes with mass numbers of 114-148, of which only one (133 Cs) is stable, the rest are radioactive. The isotopic abundance of cesium-133 in nature is approximately 100%. 133 Cs refers to scattered elements. In small quantities, it is contained in almost all objects of the external environment. The clarke (average) content of the nuclide in the earth's crust is 3.7 ∙ 10 -4%, in the soil - 5 ∙ 10 -5%. Cesium is a constant trace element of plant and animal organisms: in living phytomass it is contained in an amount of 6 ∙ 10 -6%, in the human body - about 4 g. With a uniform distribution of cesium-137 in the human body with a specific activity of 1 Bq / kg, the absorbed dose rate, according to various authors, varies from 2.14 to 3.16 μGy / year.
In nature, this silvery-white alkali metal occurs as the stable isotope Cs-133. It is a rare element with an average content of 3.7 ∙ 10 -4% in the earth's crust. Ordinary, natural cesium and its compounds not radioactive... Only the artificially produced isotope 137 Cs is radioactive. The long-lived radioactive isotope of cesium 137 Cs is formed by fission of 235 U and 239 Pu nuclei with a yield of about 7%. During radioactive decay, 137 Cs emits electrons with a maximum energy of 1173 keV and turns into a short-lived γ-emitting nuclide 137m Ba (Table 18). It has the highest chemical activity among alkali metals; it can be stored only in sealed evacuated ampoules.
Table 18
Main characteristics of cesium-137
| Isotope | Main view radiation | Half-life, T 1/2 | HC value water , Bq / dm 3 | Natural variations in OA in waters (min-max), Bq / dm 3 |
137 Cs | β (E β max \u003d 1173 keV); | 11.0 (NRB-99) | n ∙ 10 -3 - n ∙ 10 -2 |
Metallic cesium is used in photocells and photomultipliers in the manufacture of photocathodes and as a getter in luminescent tubes. Cesium vapors are the working fluid in MHD generators and gas lasers. Cesium compounds are used in optics and night vision devices.
The products of the nuclear fission reaction contain significant amounts of decomposed cesium radionuclides, among which the most dangerous is 137 Cs. Radiochemical plants can also be a source of pollution. The release of cesium-137 into the environment occurs mainly as a result of nuclear tests and accidents at nuclear power plants. By the beginning of 1981, the total activity of 137 Cs released into the environment reached 960 PBq. The density of pollution in the Northern and Southern Hemispheres and on the average on the globe was 3.42, respectively; 0.86 and 3.14 kBq / m 2, and on the territory of the former USSR, on average - 3.4 kBq / m 2.
During the accident in the South Urals in 1957, a thermal explosion of the radioactive waste storage occurred, and radionuclides with a total activity of 74 PBq, including 0.2 PBq of 137 Cs, entered the atmosphere. During a fire at the RCZ in Windscale, Great Britain in 1957, 12 PBq of radionuclides were released, of which 46 TBq 137 Cs. Technological discharge of radioactive waste from the Mayak enterprise in the South Urals in the The flow in 1950 was 102 PBq, including 137 Cs 12.4 PBq. Wind removal of radionuclides from the floodplain of Lake Karachay in the South Urals in 1967 amounted to 30 TBq. The share of 137 Cs was 0.4 TBq.
The accident at the Chernobyl nuclear power plant (ChNPP) became a real disaster in 1986: 1850 PBq of radionuclides were ejected from the destroyed reactor, while the share of radioactive cesium was 270 PBq. The spread of radionuclides has taken on a planetary scale. In Ukraine, Belarus and the Central Region of the Russian Federation, more than half of the total amount of radionuclides settled in the CIS fell. There are known cases of environmental pollution as a result of careless storage of sources of radioactive cesium for medical and technological purposes.
Cesium-137 is used in gamma-ray flaw detection, measuring technology, for radiation sterilization of foodstuffs, medicines and drugs, in radiotherapy for the treatment of malignant tumors. Also, cesium-137 is used in the production of radioisotope current sources, where it is used in the form of cesium chloride (density 3.9 g / cm 3 , energy release about 1.27 W / cm 3 ).
Cesium-137 is used in sensors for limit levels of bulk solids in opaque bins. Cesium-137 has certain advantages over radioactive cobalt-60: a longer half-life and less harsh gamma radiation. Therefore, 137 Cs-based devices are more durable, and radiation protection is less cumbersome. However, these advantages become real only in the absence of 137 Cs impurities with a shorter half-life and harder gamma radiation.
It is widely used as a source of γ-radiation. In medicine, cesium sources, along with radium ones, are used in therapeutic γ-devices and devices for interstitial and cavity gamma therapy. Since 1967, the phenomenon of a transition between two hyperfine levels of the ground state of the cesium-137 atom has been used to determine one of the main units of time measurement, the second.
Radiocesium 137 Cs is an exclusively technogenic radionuclide, its presence in the studied environment is associated with nuclear weapons tests or with the use of nuclear technologies. 137 Cs - β-γ-emitting radioisotope of cesium, one of the main components of technogenic radioactive contamination of the biosphere. Formed as a result of nuclear fission reactions. Contained in radioactive fallout, discharges, waste of radiochemical plants. OA 137 Cs in drinking water is limited to levels of 11Bq / dm 3 or 8 Bq / dm 3.
The geochemical feature of 137 Cs is its ability to be very strongly retained by natural sorbents. As a result, upon entering the OPS, its activity rapidly decreases with distance from the source of pollution. Natural waters are relatively quickly self-purifying due to the absorption of 137 Cs by suspensions and bottom sediments.
Cesium can accumulate in significant quantities in agricultural plants, and in particular in seeds. It most intensively comes from the aquatic environment and moves at a high speed through the plant. The introduction of potash fertilizers into the soil and liming significantly reduce the absorption of cesium by plants, and the stronger, the higher the proportion of potassium.
The accumulation coefficient is especially high in freshwater algae and arctic terrestrial plants (especially lichens), from the animal world - in reindeer through reindeer, which they feed on. Inside living organisms, cesium-137 mainly penetrates through the respiratory and digestive organs. This nuclide is supplied mainly with food in an amount of 10 μg / day. It is excreted from the body mainly with urine (on average, 9 μg / day). Cesium is a permanent chemical microcomponent of plants and animals. The main store of cesium in mammals is muscles, heart, and liver. About 80% of the cesium that has entered the body is accumulated in the muscles, 8% in the skeleton, the remaining 12% is distributed evenly over other tissues.
Cesium-137 is excreted mainly through the kidneys and intestines. The biological half-life of accumulated cesium-137 for humans is considered to be equal to 70 days (according to the data of the International Commission on Radiological Protection). During the elimination process, significant amounts of cesium are reabsorbed into the bloodstream in the lower intestines. An effective means for reducing the absorption of cesium in the intestine is the ferrocyanide sorbent, which binds the nuclide into an indigestible form. In addition, to accelerate the excretion of the nuclide, natural excretory processes are stimulated, various complexing agents are used.
The development of radiation injuries in humans can be expected when a dose of about 2 Gy or more is absorbed. Doses of 148, 170 and 740 MBq correspond to mild, moderate and severe degrees of damage, however, the radiation reaction is noted already at units of MBq.
137 Cs belongs to the group of radioactive substances that are evenly distributed over organs and tissues, for this reason it belongs to the nuclides of moderately hazardous radioactive toxicity. It has a good ability to enter the body along with potassium through the food webs.
The main source of cesium entering the human body is foodstuffs of animal origin contaminated with a nuclide. The content of radioactive cesium in a liter of cow's milk reaches 0.8-1.1% of the daily intake of the nuclide, goat and sheep - 10-20%. However, it mainly accumulates in the muscle tissue of animals: 1 kg of meat of cows, sheep, pigs and chickens contains 4.8, 20 and 26% (respectively) of the daily intake of cesium. The protein of chicken eggs gets less - 1.8-2.1%. Even in large quantities, cesium accumulates in the muscle tissues of aquatic organisms: the activity of 1 kg of freshwater fish can exceed the activity of 1 liter of water by more than 1000 times (in marine fish it is lower).
The main source of cesium for the population of Russia is dairy and grain products (after the Chernobyl accident - dairy and meat products), in Europe and the United States, cesium comes mainly with dairy and meat products, and less - with grain and vegetables. The constant internal irradiation created in this way causes significantly greater harm than external irradiation with this isotope.
The published methods for measuring the activity of 137 Cs by its β-radiation suggest the radiochemical preparation of the sample and the isolation of cesium with a high degree of purity to exclude the interfering influence of other β-emitters. Modern methods for the determination of 137 Cs are based, as a rule, on the registration of gamma radiation with an energy of 661.6 keV. They are subdivided into instrumental ones, the lower limit of determination (NPL) of which is 1-10 Bq / kg (or Bq / dm 3), and methods with preliminary chemical enrichment (NPD up to 10 -2 Bq / kg). To concentrate 137 Cs from dilute solutions, it is most often used by coprecipitation with ferrocyanides of nickel, copper, zinc, iron, cobalt, calcium, magnesium or collector sorbents based on them.
12. Plutonium
Plutonium (plutonium) Pu is an artificial radioactive chemical element of the III group of Mendeleev's Periodic Table of Elements, atomic number 94, transuranic element, refers to actinides. The first nuclide 238 Pu was discovered in 1940 by G. Th.Seaborg, E.M. McMillan, J.E. Kennedy and A.C. Val ( A.Ch.Wahl). In the spring of 1941, Seaborg and his co-workers discovered and for the first time isolated a quarter of a microgram of 239 Pu after the decay of 239 Np, formed by irradiation of 238 U with heavy hydrogen nuclei (deuterons). Following uranium and neptunium, the new element was named after the planet Pluto, discovered in 1930. Since August 24, 2006, according to the decision of the International Astronomical Union, Pluto is no longer a planet of the solar system. In Greek mythology, Pluto (aka Hades) is the god of the kingdom of the dead.
Plutonium Pu is the most dangerous heavy metal. It has 15 radioactive isotopes with mass numbers from 232 to 246, mainly α-emitters. On Earth there are only traces of this element and only in uranium ores. The T½ values \u200b\u200bof all plutonium isotopes are much less than the age of the Earth, and therefore all primary plutonium (which existed on our planet during its formation) has completely decayed. However, negligible amounts of 239 Pu are constantly formed during the β-decay of 239 Np, which, in turn, occurs during the nuclear reaction of uranium with neutrons (for example, neutrons from cosmic radiation).
Therefore, traces of plutonium have been found in uranium ores in such microscopic quantities (0.4-15 parts of Pu per 10 12 parts of U) that it is out of the question to extract it from uranium ores. About 5000 kg of it was released into the atmosphere as a result of nuclear tests. According to some estimates, the soil in the United States contains an average of 2 millicurie (28 mg) plutonium per km 2 of fallout. It is a typical human handiwork; it is obtained in nuclear reactors from uranium-238, which is successively converted into uranium-239, neptunium-239 and plutonium-239.
The even isotopes of plutonium-238, -240, -242 are not fissile materials, but can be fissioned by high-energy neutrons (they are fissile). They are unable to sustain a chain reaction (with the exception of plutonium-240). Isotopes 232 Pu - 246 Pu were obtained; 247 Pu and 255 Pu were also found among the explosion products of thermonuclear bombs. The most stable is the inaccessible 244 Pu (α-decay and spontaneous fission, T 1/2\u003d 8.2 10 7 years, atomic mass 244.0642). A brittle silvery-white metal in free form. Traces of isotopes 247 Pu and 255 Pu were found in the dust collected after the explosions of thermonuclear bombs.
Enormous forces and resources were thrown into nuclear research and the creation of the atomic industry in the USA, as later in the USSR. The nuclear and physicochemical properties of plutonium were studied in a short time (Table 19). The first plutonium-based nuclear charge was detonated on July 16, 1945 at the Alamogordo test site (test codenamed "Trinity"). In the USSR, the first experiments to produce 239 Pu were started in 1943-1944. under the leadership of academicians I.V. Kurchatov and V.G. Khlopin. For the first time, plutonium was isolated in the USSR from uranium irradiated with neutrons. In 1945 and 1949, the first radiochemical separation plant began operating in the USSR.
Table 19
Nuclear properties of the most important isotopes of plutonium
| Nuclear properties | Plutonium-238 | Plutonium-239 | Plutonium-240 | Plutonium-241 | Plutonium-242 |
Half-life, years | |||||
Activity, Ci / g | |||||
Type of radioactive decay | alpha decay | alpha decay | alpha decay | beta decay | alpha decay |
Energy of radioactive decay, MeV |
Note. All plutonium isotopes are weak gamma emitters. Plutonium-241 converts to americium-241 (powerful gamma emitter)
Only two isotopes of plutonium are of practical use for industrial and military purposes. Plutonium-238, obtained in nuclear reactors from neptunium-237, is used to manufacture compact thermoelectric generators. Six million electron volts are released from the decay of one atomic nucleus of plutonium-238. In a chemical reaction, the same energy is released during the oxidation of several million atoms. A source of electricity containing one kilogram of plutonium-238 develops a thermal power of 560 MW. The maximum power of a chemical current source of the same mass is 5 W.
There are many emitters with similar energy characteristics, but one feature of plutonium-238 makes this isotope irreplaceable. Typically, alpha decay is accompanied by strong gamma radiation penetrating large layers of matter. 238 Pu is an exception. The energy of the gamma quanta accompanying the decay of its nuclei is small, and it is not difficult to defend against it: the radiation is absorbed by a thin-walled container. The probability of spontaneous fission of the nuclei of this isotope is also small. Therefore, he found application not only in current sources, but also in medicine. Batteries with plutonium-238 serve as a source of energy in special stimulators of cardiac activity, the service life of which reaches 5 years or more.
The plutonium-beryllium alloy works as a laboratory neutron source. The Pu-238 isotope is found in a number of nuclear thermoelectric power generators on board space research vehicles. Due to its long lifetime and high thermal power, this isotope is used almost exclusively in space RTGs, for example, on all spacecraft that flew beyond the orbit of Mars.
Of all the isotopes, Pu-239 seems to be the most interesting, with a half-life of 24110 years. As a fissile material, 239 Pu is widely used as a nuclear fuel in nuclear reactors (the energy released during the fission of 1 r 239 Pu, equivalent to the heat released during the combustion of 4000 kg of coal), in the production of nuclear weapons (the so-called "weapons-grade plutonium") and in atomic and thermonuclear bombs, as well as for nuclear reactors on fast neutrons and nuclear reactors for civil and research purposes ... As a source of α-radiation, plutonium, along with 210 Po, has found wide application in industry, in particular, in devices for the elimination of electrostatic charges. This isotope is also used in control and measuring equipment.
Plutonium has many specific properties. It has the lowest thermal conductivity of all metals, the lowest electrical conductivity, with the exception of manganese. In its liquid phase, it is the most viscous metal. Melting point -641 ° C; boiling point -3232 ° C; density - 19.84 (in alpha phase). It is a highly electronegative, reactive element, much more so than uranium. It quickly fades, forming an iridescent film (like an iridescent oil film), at first light yellow, eventually turning into a dark purple. If the oxidation is large enough, an olive-green oxide powder (PuO 2) appears on its surface. Plutonium readily oxidizes and corrodes rapidly even in the presence of low humidity.
When temperature changes, plutonium undergoes the strongest and most unnatural changes in density. Plutonium has six distinct phases (crystalline structures) in solid form, more than any other element.
Compounds of plutonium with oxygen, carbon and fluorine are used in the nuclear industry (either directly or as intermediate materials). Metallic plutonium does not dissolve in nitric acid, but plutonium dioxide dissolves in hot concentrated nitric acid. However, in a solid mixture with uranium dioxide (for example, in spent fuel from nuclear reactors), the solubility of plutonium dioxide in nitric acid increases as uranium dioxide dissolves in it. This feature is used in the reprocessing of nuclear fuel (Table 20).
Table 20
Plutonium compounds and their applications
| Plutonium compounds | Application |
Plutonium dioxide PuO 2 | Mixed with uranium dioxide (UO 2) it is used as fuel for nuclear reactors |
Plutonium carbide (PuC) | Potentially can be used as fuel for breeder reactors |
Plutonium Trifluoride (PuF 3) | They are intermediate compounds in the production of plutonium metal |
| Plutonium nitrates - Pu (NO 3) 4 and Pu (NO 3) 3 | Not used. Are products of reprocessing (when extracting plutonium from spent nuclear fuel) |
The most important plutonium compounds are: PuF 6 (a low-boiling liquid; thermally much less stable than UF 6), solid oxide PuO 2, carbide PuC and nitride PuN, which, in mixtures with the corresponding uranium compounds, can be used as nuclear fuel.
The most widely used radioisotope devices are ionization fire alarms or radioisotope smoke detectors. Plutonium easily forms aerosols when machined.
In nature, it is formed during β-decay of Np-239, which, in turn, occurs during the nuclear reaction of uranium-238 with neutrons (for example, neutrons from cosmic radiation). Industrial production of Pu-239 is also based on this reaction and takes place in nuclear reactors. Plutonium-239 is the first to be formed in a nuclear reactor when uranium-238 is irradiated; the longer this process is, the more heavier plutonium isotopes appear. Plutonium-239 must be chemically separated from fission products and uranium remaining in spent nuclear fuel. This process is called reprocessing. Since all isotopes have the same number of protons and different - neutrons, their chemical properties (chemical properties depend on the number of protons in the nucleus) are the same, so it is very difficult to separate isotopes using chemical methods.
Subsequent separation of Pu-239 from uranium, neptunium and highly radioactive fission products is carried out at radiochemical plants by radiochemical methods (coprecipitation, extraction, ion exchange, etc.). Plutonium metal is usually obtained by reduction of PuF 3, PuF 4 or PuO 2 with barium, calcium or lithium vapor.
Then they use its ability to fission under the action of neutrons in nuclear reactors, and its ability to self-sustaining fission chain reaction in the presence of a critical mass (7 kg) - in atomic and thermonuclear bombs, where it is the main component. The critical mass of its α-modification is 5.6 kg (a sphere with a diameter of 4.1 cm). 238 Pu is used in long-life "atomic" electric batteries. Plutonium isotopes are used as raw materials for the synthesis of transplutonium elements (Am, etc.).
By irradiating Pu-239 with neutrons, it is possible to obtain a mixture of isotopes, of which the Pu-241 isotope, as well as Pu-239, is fissile and could be used to generate energy. However, its half-life is 14.4 years, which does not allow it to be preserved for a long time, moreover, when decaying, it forms non-fissionable Am-241 (α-, γ-radioactive) with a half-life of 432.8 years. It turns out that approximately every 14 years, the amount of Am-241 in the environment doubles. It is difficult to detect it, like other transuranic elements, with conventional γ-spectrometric equipment, and very specific and expensive detection methods are required. The isotope Pu-242 is most similar in nuclear properties to uranium-238, Am-241, produced by the decay of the isotope Pu-241, was used in smoke detectors.
Americium-241, as well as other transuranic elements (neptunium, californium, and others), is an environmentally hazardous radionuclide, being predominantly an α-emitting element that causes internal irradiation of the body.
The plutonium accumulated on Earth is more than enough. Its production is absolutely not required for both defense and energy. Nevertheless, out of 13 reactors that existed in the USSR that produced weapons-grade plutonium, 3 continue to operate: two of them are in Seversk. The last such reactor in the United States was shut down in 1988.
The quality of plutonium is determined by the percentage of isotopes in it (except for plutonium-239) (Table 21).
As of September 1998, plutonium prices set by the Isotope Division of Oak Ridge National Laboratory (ORNL) were: $ 8.25 / mg for plutonium-238 (97% pure); $ 4.65 / mg for plutonium-239 (\u003e 99.99%); $ 5.45 / mg for plutonium-240 (\u003e 95%); $ 14.70 / mg for plutonium-241 (\u003e 93%) and $ 19.75 / mg for plutonium-242.
Table 21Plutonium quality
This plutonium quality classification, developed by the US Department of Energy, is rather arbitrary. For example, fuel and reactor plutonium, which are less suitable for military purposes than weapons-grade plutonium, can also be used to make a nuclear bomb. Plutonium of any quality can be used to create a radiological weapon (when radioactive substances are sprayed without a nuclear explosion).
Just 60 years ago, green plants and animals did not contain plutonium, now up to 10 tons of it is sprayed into the atmosphere. About 650 tons have been generated by nuclear power and over 300 tons by military production. A significant part of all plutonium production is located in Russia.
Once in the biosphere, plutonium migrates across the earth's surface, being incorporated into biochemical cycles. Plutonium is concentrated by marine organisms: its accumulation coefficient (i.e. the ratio of concentrations in the body and in the external environment) for algae is 1000-9000, for plankton (mixed) - about 2300, for mollusks - up to 380, for starfish - about 1000 , for muscles, bones, liver and stomach of fish - 5,570, 200 and 1060, respectively. Terrestrial plants assimilate plutonium mainly through the root system and accumulate it up to 0.01% of their mass. Since the 70s. In the 20th century, the share of plutonium in the radioactive contamination of the biosphere is increasing (the irradiation of marine invertebrates due to plutonium becomes greater than due to 90 Sr and 137 Cs). The maximum permissible concentration for 239 Pu in open water bodies and in the air of working rooms is 81.4 and 3.3 ּ 10 -5 Bq / l, respectively.
The behavior of plutonium in the air determines the conditions for its safe storage and handling during production (Table 22). Oxidation of plutonium poses a health risk because plutonium dioxide, being a stable compound, easily enters the lungs when inhaled. Its specific activity is 200 thousand times higher than that of uranium; moreover, the release of the body from plutonium that has got into it practically does not occur throughout a person's life.
The biological half-life of plutonium is 80-100 years when it is in the bone tissue, its concentration there is practically constant. The half-life from the liver is 40 years. Chelating additives can accelerate the removal of plutonium.
Changes in the properties of plutonium in air
| Form and environmental conditions | Plutonium reaction |
Metal ingots | Relatively inert |
Metal powder | Reacts quickly to form |
Fine powder: | Spontaneously ignites: |
At elevated temperatures and humidity | Reacts to form |
Plutonium is called "nuclear poison", its permissible content in the human body is estimated in nanograms. The International Commission on Radiological Protection (ICRP) has set an annual absorption rate of 280 nanograms. This means that for professional exposure, the concentration of plutonium in the air should not exceed 7 picoCurie / m 3. The maximum allowable concentration of Pu-239 (for professional personnel) is 40 nanoCuri (0.56 microgram) and 16 nanoCuri (0.23 microgram) for lung tissue.
The absorption of 500 mg of plutonium as a finely divided or dissolved material can lead to death from acute irradiation of the digestive system in a few days or weeks. Inhalation of 100 mg of plutonium in the form of particles of 1-3 microns optimal for retention in the lungs leads to death from pulmonary edema in 1-10 days. Inhalation of a dose of 20 mg leads to death from fibrosis in about a month. For doses much lower than these values, a chronic carcinogenic effect appears.
Throughout life, the risk of developing lung cancer for an adult depends on the amount of plutonium that has entered the body. Ingestion of 1 microgram of plutonium poses a 1% risk of developing cancer (normal cancer rate is 20%). Accordingly, 10 micrograms increases the risk of cancer from 20% to 30%. Ingestion of 100 micrograms or more guarantees the development of lung cancer (usually over several decades), although evidence of lung damage may appear within a few months. If it enters the circulatory system, it will most likely begin to concentrate in tissues containing iron: bone marrow, liver, spleen. If 1.4 micrograms are accommodated in the bones of an adult, immunity will deteriorate as a result and cancer may develop after a few years.
The fact is that Pu-239 is an α-emitter, and each of its α-particles in biological tissue generates 150 thousand pairs of ions along its short path, damaging cells, producing various chemical transformations. 239 Pu belongs to substances with a mixed type of distribution, since it accumulates not only in the bone skeleton, but also in the liver. It is very well retained in bones and is practically not removed from the body due to the slowdown of metabolic processes in bone tissue. For this reason, this nuclide belongs to the category of the most toxic.
Being in the body, plutonium becomes a constant source of α-radiation for humans, causing bone tumors, liver cancer and leukemia, hematopoietic disorders, osteosarcomas, lung cancer, thus being one of the most dangerous carcinogens (Table 23).
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Description of plutonium
Plutonium (Plutonium) is a heavy, silvery chemical element, a radioactive metal with an atomic number of 94, which is designated in the periodic one by the symbol Pu.
This electronegative active chemical element belongs to the group of actinides with an atomic mass of 244.0642, and, like neptunium, which got its name after the planet of the same name, this chemical owes its name to the planet Pluto, since the precursors of the radioactive element in the periodic table of chemical elements of Mendeleev are and neptunium, which were also named after the distant cosmic planets of our Galaxy.
Origin of plutonium
Element plutonium was first discovered in 1940 at the University of California by a group of radiologists and scientific researchers G. Seaborg, E. Macmillan, Kennedy, A. Walhom while bombarding a uranium target from a cyclotron with deuterons - heavy hydrogen nuclei.
In December of the same year, scientists discovered plutonium isotope - Pu-238, the half-life of which is more than 90 years, while it was found that under the influence of the most complex nuclear chemical reactions, the isotope neptunium-238 is initially obtained, after which the isotope is already formed plutonium-238.
In early 1941, scientists discovered plutonium 239 with a decay period of 25,000 years. Plutonium isotopes can have different content of neutrons in the nucleus.
A pure compound of the element could only be obtained at the end of 1942. Every time radiologists discovered a new isotope, they always measured the half-lives of the isotopes.
At the moment, plutonium isotopes, of which there are 15 in total, differ in duration. half-life... It is with this element that great hopes and prospects are associated, but at the same time, serious fears of humanity.
Plutonium is much more active than, for example, uranium and belongs to the most expensive technically important and significant chemical substances.
For example, the cost of a gram of plutonium is several times more than one gram, or other equally valuable metals.
Production and mining of plutonium is considered costly, and the cost of one gram of metal in our time is confidently kept at around 4000 US dollars.
How is plutonium obtained? Plutonium production
The production of a chemical element takes place in nuclear reactors, inside of which uranium is split under the influence of complex chemical-technological interrelated processes.
Uranium and plutonium are the main, main components in the production of atomic (nuclear) fuel.
If it is necessary to obtain a large amount of a radioactive element, the method of irradiation of transuranic elements, which can be obtained from spent nuclear fuel and irradiation of uranium, is used. Complex chemical reactions can separate the metal from uranium.
To obtain isotopes, namely plutonium-238 and weapon-grade plutonium-239, which are intermediate decay products, neutron irradiation of neptunium-237 is used.
A negligible fraction of plutonium-244, which is the most "long-lived" variant of the isotope, due to its long half-life, was found in studies in cerium ore, which, most likely, has survived since the formation of our planet Earth. This radioactive element does not occur naturally in nature.
Basic physical properties and characteristics of plutonium
Plutonium is a rather heavy radioactive chemical element of silvery color, which shines only in purified form. Atomic plutonium metal mass is equal to 244 amu. eat.
Due to its high radioactivity, this element is warm to the touch, it can heat up to a temperature that exceeds the temperature indicator when water boils.
Plutonium, under the influence of oxygen atoms, quickly darkens and becomes covered with an iridescent thin film of initially light yellow, and then saturated - or brown hue.
With strong oxidation, the formation of PuO2 powder on the element surface occurs. This type of chemical metal is susceptible to strong oxidation and corrosion processes even at low humidity levels.
To prevent corrosion and oxidation of the metal surface, drying is required. Plutonium photo can be seen below.
Plutonium belongs to tetravalent chemical metals, dissolves well and quickly in hydrogen iodide substances, acidic media, for example, in chlorine,.
Metal salts are quickly neutralized in media with a neutral reaction, alkaline solutions, while forming insoluble plutonium hydroxide.
The temperature at which plutonium melts is 641 degrees Celsius, the boiling point is 3230 degrees.
Under the influence of high temperature conditions, unnatural changes in the density of the metal occur. In the form of plutonium, it has different phases and six crystal structures.
During the transition between phases, significant changes in the volume of the element occur. The element acquires the most dense form in the sixth alpha phase (the last stage of the transition), while only, neptunium, radium is heavier than metal in this state.
During melting, the element is strongly compressed, so the metal can stick to the surface of water and other non-aggressive liquid media.
Despite the fact that this radioactive element belongs to the group of chemical metals, the element is quite volatile, and when it is in an enclosed space for a short period of time, its concentration in the air increases and increases several times.
The main physical properties of the metal include: a low degree, the level of thermal conductivity of all existing and known chemical elements, a low level of electrical conductivity, in the liquid state plutonium is one of the most viscous metals.
It is worth noting that any plutonium compounds are toxic, poisonous and pose a serious risk of radiation to the human body, which occurs due to active alpha radiation, therefore, all work must be performed extremely carefully and only in special suits with chemical protection.
More about the properties, theories of the origin of the unique metal can be found in the book Obruchev "Plutonium". Author V.A. Obruchev invites readers to plunge into the amazing and unique world of the fantastic country Plutonium, which is located deep in the bowels of the Earth.
Plutonium use
It is customary to classify an industrial chemical element into weapons-grade and reactor ("power") plutonium.
So, for the production of nuclear weapons, of all the existing isotopes, it is permissible to use only plutonium 239, which should not contain more than 4.5% plutonium 240, since it is subject to spontaneous fission, which greatly complicates the manufacture of military shells.
Plutonium-238 finds application for the functioning of small-sized radioisotope sources of electrical energy, for example, as an energy source for space technology.
Several decades ago, plutonium was used in medicine in pacemakers (devices for maintaining a heart rate).
The world's first atomic bomb had a plutonium charge. Nuclear plutonium (Pu 239) is in demand as a nuclear fuel for the operation of power reactors. Also, this isotope serves as a source for the production of transplutonium elements in reactors.
If we compare nuclear plutonium with pure metal, the isotope has higher metallic parameters, does not have transition phases, therefore it is widely used in the process of obtaining fuel elements.
Plutonium 242 isotope oxides are also in demand as a power source for space lethal units, equipment, and fuel rods.
Weapon-grade plutonium Is an element that is presented in the form of a compact metal, which contains at least 93% of the Pu239 isotope.
This type of radioactive metal is used in the production of various types of nuclear weapons.
Weapons-grade plutonium is obtained in specialized industrial nuclear reactors that operate on natural or low-enriched uranium, as a result of the capture of neutrons by it.
This metal is called precious, but not for its beauty, but for its irreplaceability. In Mendeleev's periodic table, 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 mankind.
Plutonium: description
It is a silvery white metal in appearance. It is radioactive and can be represented as 15 isotopes with different half-lives, for example:
- Pu-238 - about 90 years old
- Pu-239 - about 24 thousand years old
- Pu-240 - 6580 years old
- Pu-241 - 14 years old
- Pu-242 - 370 thousand years old
- Pu-244 - about 80 million years old
This metal cannot be extracted from the ore, since it is a product of the radioactive conversion of uranium.
How is plutonium obtained?
Plutonium production requires uranium fission, which can only be done in nuclear reactors. If we talk about the presence of the element Pu in the 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 isotope 239-Pu) in an amount of several kilograms, it is necessary to carry out a complex technological process in a nuclear reactor.
Plutonium properties
The radioactive metal plutonium has the following physical properties:
- density 19.8 g / cm 3
- melting point - 641 ° C
- boiling point - 3232 ° C
- thermal conductivity (at 300 K) - 6.74 W / (m K)
Plutonium is radioactive and therefore warm to the touch. Moreover, this metal is characterized by the lowest thermal and electrical conductivity. Liquid plutonium is the most viscous of all metals in existence.
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 of the minimum mass of a fissionable substance at which the course of fission (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 fuel. To obtain plutonium in large quantities, two technologies are used:
- irradiation of uranium
- irradiation of transuranic elements derived from spent fuel
Both methods are separation of plutonium and uranium as a result of a chemical reaction.
To obtain pure plutonium-238, neutron irradiation of neptunium-237 is used. The same isotope is involved in the creation of weapons-grade plutonium-239, in particular, it is an intermediate decay product. $ 1 million is the price for 1 kg of plutonium-238.