Who discovered plutonium. What is plutonium
Story
Discovered in 1940 by Glenn Seaborg, Edwin McMillan, Kennedy and Arthur Wahl in 1940 in Berkeley (USA) during the bombardment of a uranium target with deuterons accelerated in a cyclotron.
Origin of the name
Plutonium was named after the planet Pluto, as the previous chemical element discovered was called Neptunium.
Receipt
Plutonium is produced in nuclear reactors.
The isotope 238 U, which makes up the bulk of natural uranium, is not very suitable for fission. For nuclear reactors, uranium is slightly enriched, but the share of 235 U in nuclear fuel remains small (approximately 5%). The main part in fuel rods is 238 U. During operation of a nuclear reactor, part of the 238 U nuclei captures neutrons and turns into 239 Pu, which can later be isolated.
It is quite difficult to distinguish plutonium among the products of nuclear reactions, since plutonium (like uranium, thorium, neptunium) belongs to actinides with very similar 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 extracted using the Metallothermy method (reduction of active metals from oxides and salts in a vacuum):
PuF 4 +2 Ba = 2BaF 2 + Pu
Isotopes
More than a dozen isotopes of plutonium are known, all of them 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 very small quantities (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 a crystalline structure called ?-phase. The atoms are connected by a covalent bond (instead of a metal bond), so the physical properties are 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 mole 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 substances known. The toxicity of plutonium is due not so much to its chemical properties (although plutonium is perhaps as toxic as any heavy metal), but rather to 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 the 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.
The plutonium isotope 238 Pu was first artificially obtained on February 23, 1941 by a group of American scientists led by G. Seaborg by irradiating uranium nuclei with deuterons. Only then was plutonium discovered in nature: 239 Pu is usually found in negligible quantities in uranium ores as a product of the radioactive transformation of uranium. Plutonium is the first artificial element obtained in quantities available for weighing (1942) and the first whose production began on an industrial scale.
The element's name continues the astronomical theme: it is named after Pluto, the second planet after Uranus.
Being in nature, receiving:
In uranium ores, as a result of the capture of neutrons (for example, neutrons from cosmic radiation) by uranium nuclei, neptunium (239 Np) is formed, the product b- the decay of which is natural plutonium-239. However, plutonium is formed in such microscopic quantities (0.4-15 parts Pu per 10 12 parts U) that its extraction from uranium ores is out of the question.
Plutonium is produced 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.
Plutonium-244 also accumulated in a nuclear reactor. Isotope of element No. 95 - americium, 243 Am, having captured a neutron, turned into americium-244; americium-244 transformed into curium, but in one out of 10 thousand cases a transition occurred into plutonium-244. A preparation of plutonium-244 weighing only a few millionths of a gram was isolated from a mixture of americium and curium. But they were enough to determine the half-life of this interesting isotope - 75 million years. Later it was refined and turned out to be equal to 82.8 million years. In 1971, traces of this isotope were found in the rare earth mineral bastnäsite. 244 Pu is the longest-lived of all isotopes of transuranium elements.
Physical properties:
Silvery-white metal, has 6 allotropic modifications. Melting point 637°C, boiling point - 3235°C. Density: 19.82 g/cm3.
Chemical properties:
Plutonium is capable of reacting with oxygen to form oxide(IV), which, like all the first seven actinides, has a weak basic character.
Pu + O 2 = PuO 2
Reacts with dilute sulfuric, hydrochloric, perchloric acids.
Pu + 2HCl(p) = PuCl 2 + H 2 ; Pu + 2H 2 SO 4 = Pu(SO 4) 2 + 2H 2
Does not react with nitric and concentrated sulfuric acids. 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.
The most important connections:
Plutonium(IV) oxide, PuO 2 , has a weak basic character.
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Application:
Plutonium was widely used in the production of nuclear weapons (so-called “weapons-grade plutonium”). The first plutonium-based nuclear device was detonated on July 16, 1945 at the Alamogordo test site (test codenamed Trinity).
It is used (experimentally) as nuclear fuel for nuclear reactors for civil and research purposes.
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-251 from grams of plutonium. Plutonium-242 is not fissile by thermal neutrons, and even in large quantities it can be irradiated in intense neutron fluxes. Therefore, in reactors, all elements from californium to einsteinium are “made” from this isotope and accumulated in weight quantities.
Kovalenko O.A.
HF Tyumen State University
Sources:
"Harmful chemicals: Radioactive substances" Directory L. 1990 p. 197
Rabinovich V.A., Khavin Z.Ya. "A short chemical reference book" L.: Chemistry, 1977 p. 90, 306-307.
I.N. Beckman. Plutonium. (textbook, 2009)
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.
Background and history
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 Neptune is not the last in the solar system, even further lies the orbit of Pluto - a planet about which almost nothing is still known... A similar construction We also see on the “left flank” of the periodic table: uranium - neptunium - plutonium, however, 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.
Riddles for cryptographers
The first isotope of element No. 94, plutonium-238, has found practical use today. 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 isotopes of plutonium 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, like uranium-235, to 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 construction material for some parts), then in the codes, along with “copper,” “genuine copper” appeared.
"The Tree of the Knowledge of Good and Evil"
In 1941, the most important isotope of plutonium was discovered - an isotope with mass number 239. And almost immediately the theorists' prediction was confirmed: the nuclei of plutonium-239 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.
It is widely believed 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 turned on. 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 world war broke out, nuclear weapons would be used.
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.”
How to measure, what to compare with
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...
Energy of stones
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. This isotope is useless for nuclear energy. 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.
400 thousand times less than radium
It has already been said that plutonium isotopes 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 maintained in the natural mixture of 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.
Let's see what the resulting neutrons are spent on. In any reactor, one neutron is needed to maintain a nuclear chain reaction. 0.1 neutrons are absorbed by the construction materials of the installation. The “excess” is used to accumulate plutonium-239. In one case the “excess” is 1.13, in the other it is 1.60. After the “burning” of a kilogram of plutonium in a “fast” reactor, colossal energy is released and 1.6 kg of plutonium is accumulated. And uranium in a “fast” reactor will provide the same energy and 1.1 kg of new nuclear fuel. In both cases, expanded reproduction is evident. But we must not forget about the economy.
Due to a number of technical reasons, the plutonium reproduction cycle takes several years. Let's say five years. This means that the amount of plutonium per year will increase by only 2% if η=2.23, and by 12% if η=2.7! Nuclear fuel is capital, and any capital should yield, say, 5% per annum. In the first case there are large losses, and in the second there are large profits. This primitive example illustrates the “weight” of every tenth of a number in nuclear energy.
Something else is also important. Nuclear power must keep pace with growing energy demand. Calculations show that his condition is fulfilled in the future only when η approaches three. If the development of nuclear energy sources lags behind society’s energy needs, then there will be two options left: either “slow down progress” or take energy from some other sources. They are known: thermonuclear fusion, annihilation energy of matter and antimatter, but are not yet technically accessible. And it is not known when they will become real sources of energy for humanity. And the energy of heavy nuclei has long become a reality for us, and today plutonium, as the main “supplier” of atomic energy, has no serious competitors, except, perhaps, uranium-233.
Sum of many technologies
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.
There is no need to describe in detail all the stages of the chemical separation of plutonium and uranium. Usually, their separation begins with the dissolution of uranium bars in nitric acid, after which the uranium, neptunium, plutonium and fragmentation elements contained in the solution are “separated”, using traditional radiochemical methods for this - precipitation, extraction, ion exchange and others. The final plutonium-containing products of this multi-stage technology are its dioxide PuO 2 or fluorides - PuF 3 or PuF 4. They are reduced to metal with barium, calcium or lithium vapor. However, the plutonium obtained in these processes is not suitable for the role of a structural material - fuel elements of nuclear power reactors cannot be made from it, and the charge of an atomic bomb cannot be cast. Why? The melting point of plutonium - only 640°C - is quite achievable.
No matter what “ultra-gentle” conditions are used to cast parts from pure plutonium, cracks will always appear in the castings during solidification. At 640°C, solidifying plutonium forms a cubic crystal lattice. As the temperature decreases, the density of the metal gradually increases. But then the temperature reached 480°C, and then suddenly the density of plutonium drops sharply. The reasons for this anomaly were discovered quite quickly: at this temperature, plutonium atoms are rearranged in the crystal lattice. It becomes tetragonal and very “loose”. Such plutonium can float in its own melt, like ice on water.
The temperature continues to fall, now it has reached 451°C, and the atoms again formed a cubic lattice, but located at a greater distance from each other than in the first case. With further cooling, the lattice first becomes orthorhombic, then monoclinic. In total, plutonium forms six different crystalline forms! Two of them are distinguished by a remarkable property - a negative coefficient of thermal expansion: with increasing temperature, the metal does not expand, but contracts.
When the temperature reaches 122°C and the plutonium atoms rearrange their rows for the sixth time, the density changes especially dramatically - from 17.77 to 19.82 g/cm 3 . More than 10%!
Accordingly, the volume of the ingot decreases. If the metal could still resist the stresses that arose at other transitions, then at this moment destruction is inevitable.
How then to make parts from this amazing metal? Metallurgists alloy plutonium (adding small amounts of the required elements to it) and obtain castings without a single crack. They are used to make plutonium charges for nuclear bombs. The weight of the charge (it is determined primarily by the critical mass of the isotope) is 5-6 kg. It could easily fit into a cube with an edge size of 10 cm.
Heavy isotopes of plutonium
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. In its 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 reactor cooling. 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 - with a further increase in the 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.
Half-lives of some isotopes of plutonium
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.
A 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 thermonuclear explosions produce all isotopes of plutonium, up to 257 Pu. But their lifetime is tenths of a second, and many short-lived isotopes of plutonium have not yet been studied.
Possibilities of the first plutonium isotope
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 a chemical reaction, the same energy is released when several million atoms are oxidized. An electricity source containing one kilogram of plutonium-238 develops a thermal power of 560 watts. The maximum power of a chemical current source of the same mass is 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 stimulators.
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.
- HOW TO CARRY PLUTONIUM. Radioactive and toxic plutonium requires special care during transportation. A container was designed specifically for its transportation - a container that is not destroyed even in aircraft accidents. It is made quite simply: it is a thick-walled stainless steel vessel surrounded by a mahogany shell. Obviously, plutonium is worth it, but imagine how thick the walls must be if you know that a container for transporting only two kilograms of plutonium weighs 225 kg!
- POISON AND ANTIDOTE. On October 20, 1977, Agence France-Presse reported that a chemical compound had been found that can remove plutonium from the human body. A few years later, quite a lot became known about this compound. This complex compound is a linear carboxylase catechinamide, a substance of the chelate class (from the Greek “chela” - claw). The plutonium atom, free or bound, is captured in this chemical claw. In laboratory mice, this substance was used to remove up to 70% of absorbed plutonium from the body. It is believed that in the future this compound will help extract plutonium from both production waste and nuclear fuel.
Plutonium, element number 94, was discovered by Glenn Seaborg, Edwin McMillan, Kennedy, and Arthur Wahl in 1940 at Berkeley by bombarding a uranium target with deuterons from a sixty-inch cyclotron. In May 1940, the properties of plutonium were predicted by Lewis 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.
Pu-239 is present in natural uranium in the form of traces (the amount is one part per 1015); it is formed there as a result of the capture of a neutron by the U-238 nucleus. Extremely small amounts of Pu-244 (the longest-lived isotope of plutonium, with a half-life of 80 million years) have been found in cerium ore, apparently left over from the formation of the Earth.
There are a total of 15 known isotopes of plutonium, all of which are radioactive. The most significant for the design of nuclear weapons:
Pu238 -> (86 years old, alpha decay) -> U234
Pu239 -> (24,360 years, alpha decay) -> U235
Pu240 -> (6580 years, alpha decay) -> U236
Pu241 -> (14.0 years, beta decay) -> Am241
Pu242 -> (370,000 years, alpha decay) -> U238 Physical properties of plutonium
Plutonium is a very heavy silvery metal, shiny like nickel when freshly refined. It is an extremely electronegative, chemically reactive element, much more so than uranium. It quickly fades to form an iridescent film (like an iridescent oil film), initially light yellow, eventually turning dark purple. If the oxidation is quite severe, an olive green oxide powder (PuO2) appears on its surface.
Plutonium readily oxidizes and quickly corrodes even in the presence of slight moisture. Strangely, it rusts in an atmosphere of inert gas with water vapor much faster than in dry air or pure oxygen. The reason for this is that the direct action of oxygen forms an oxide layer on the surface of plutonium, which prevents further oxidation. Exposure to moisture produces a loose mixture of oxide and hydride. A drying oven is required to prevent oxidation and corrosion.
Plutonium has four valencies, III-VI. It dissolves well only in very acidic media, such as nitric or hydrochloric acids; it also dissolves well in hydroiodic and perchloric acids. 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.
Due to its radioactivity, plutonium is warm to the touch. A large piece of plutonium in a thermally insulated shell is heated to a temperature exceeding the boiling point of water.
Basic physical properties of plutonium:
Melting point: 641 °C;
Boiling point: 3232 °C;
Density: 19.84 (in alpha phase).
Plutonium has many specific properties. It has the lowest thermal conductivity of all metals, the lowest electrical conductivity, with the exception of manganese (according to other sources, it is still the lowest of all metals). In its liquid phase it is the most viscous metal.
When temperature changes, plutonium undergoes the most severe and unnatural changes in density. Plutonium has six different phases (crystal structures) in solid form, more than any other element (actually, by more stringent terms, there are seven). Some transitions between phases are accompanied by dramatic changes in volume. In two of these phases - delta and delta prime - plutonium has the unique property of contracting as the temperature increases, and in the others it has an extremely high temperature coefficient of expansion. When melted, the plutonium contracts, allowing the unmelted plutonium to float. In its densest form, the alpha phase, plutonium is the sixth densest element (only osmium, iridium, platinum, rhenium and neptunium are heavier). In the alpha phase, pure plutonium is brittle, but flexible alloys exist.
Description of plutonium
Plutonium(Plutonium) is a silvery heavy chemical element, a radioactive metal with atomic number 94, which is represented in the periodic table 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 received its name in honor of the planet of the same name, this chemical owes its name to the planet Pluto, since the predecessors of the radioactive element in Mendeleev’s periodic table of chemical elements are and neptunium, which were also named after distant cosmic planets in our Galaxy.
Origin of plutonium
Element plutonium was first discovered in 1940 at the University of California by a group of radiologist and scientific researchers G. Seaborg, E. McMillan, Kennedy, A. Walch when bombarding a uranium target from a cyclotron with deuterons - heavy hydrogen nuclei.
In December of the same year, scientists discovered plutonium isotope– Pu-238, whose half-life is more than 90 years, and it was found that under the influence of complex nuclear chemical reactions the neptunium-238 isotope is initially produced, after which the isotope is already formed plutonium-238.
In early 1941, scientists discovered plutonium 239 with a decay period of 25,000 years. Isotopes of plutonium can have different neutron contents in the nucleus.
A pure compound of the element was only obtained at the end of 1942. Every time radiological scientists 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 time 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 has significantly greater activity than, for example, uranium and is one of the most expensive technically important and significant substances of a chemical nature.
For example, the cost of a gram of plutonium is several times more than one gram of,, or other equally valuable metals.
The production and extraction of plutonium is considered costly, and the cost of one gram of metal in our time confidently remains at around 4,000 US dollars.
How is plutonium obtained? Plutonium production
The production of the chemical element occurs in nuclear reactors, inside which uranium is split under the influence of complex chemical and 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 allow the metal to be separated from uranium.
To obtain isotopes, namely plutonium-238 and weapons-grade plutonium-239, which are intermediate decay products, neptunium-237 is irradiated with neutrons.
A tiny fraction of plutonium-244, which is the longest-lived isotope due to its long half-life, was discovered in cerium ore, which is likely preserved from 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 fairly heavy radioactive chemical element with a silvery color that only shines when purified. Nuclear mass of metal plutonium equal to 244 a. e.m.
Due to its high radioactivity, this element is warm to the touch and can heat up to a temperature that exceeds the boiling temperature of water.
Plutonium, under the influence of oxygen atoms, quickly darkens and is covered with an iridescent thin film of initially light yellow, and then a rich or brown hue.
With strong oxidation, the formation of PuO2 powder occurs on the surface of the element. This type of chemical metal is subject to strong oxidation processes and corrosion even at low levels of humidity.
To prevent corrosion and oxidation of the metal surface, a drying facility is necessary. Photo of plutonium can be viewed below.
Plutonium is a tetravalent chemical metal; it dissolves well and quickly in hydroiodic substances and acidic environments, for example, in chloric acid.
Metal salts are quickly neutralized in neutral media and alkaline solutions, 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 temperatures, unnatural changes in the density of the metal occur. In its form, plutonium has various phases and has six crystal structures.
During the transition between phases, significant changes in the volume of the element occur. The element acquires its most dense form in the sixth alpha phase (the last stage of the transition), while the only things heavier than the metal in this state are neptunium and radium.
When melted, the element undergoes strong compression, so the metal can float on 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 a closed space over a short period of time, its concentration in the air increases several times.
The main physical properties of the metal include: low degree, level of thermal conductivity of all existing and known chemical elements, 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 danger of radiation to the human body, which occurs due to active alpha radiation, therefore all work must be performed with the utmost care and only in special suits with chemical protection.
You can read more about the properties and theories of the origin of a unique metal in the book Obruchev "Plutonia"" Author V.A. Obruchev invites readers to plunge into the amazing and unique world of the fantastic country of Plutonia, which is located deep in the bowels of the Earth.
Applications of plutonium
The industrial chemical element is usually classified into weapons-grade and reactor-grade (“energy-grade”) plutonium.
Thus, for the production of nuclear weapons, of all 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 significantly complicates the production of military projectiles.
Plutonium-238 is used for the operation of small-sized radioisotope sources of electrical energy, for example, as a source of energy for space technology.
Several decades ago, plutonium was used in medicine in pacemakers (devices for maintaining heart rhythm).
The first atomic bomb created in the world had a plutonium charge. Nuclear plutonium(Pu 239) is in demand as nuclear fuel to ensure the functioning of power reactors. This isotope also serves as a source for producing transplutonium elements in reactors.
If we compare nuclear plutonium with pure metal, the isotope has higher metallic parameters and does not have transition phases, so it is widely used in the process of obtaining fuel elements.
Oxides of the Plutonium 242 isotope are also in demand as a power source for space lethal units, equipment, and fuel rods.
Weapons-grade plutonium is an element that is presented in the form of a compact metal that 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 produced in specialized industrial nuclear reactors that operate on natural or low-enriched uranium as a result of the capture of neutrons.