Weapons-grade plutonium is plutonium in the form of a compact metal containing at least 93.5% of the 239Pu isotope. Intended for the creation of nuclear weapons.
1.Name and features
They call it “weapon-grade” to distinguish it from “reactor-grade”. Plutonium is formed in any nuclear reactor operating on natural or low-enriched uranium, containing mainly the 238U isotope, when it captures excess neutrons. But as the reactor operates, the weapons-grade isotope of plutonium quickly burns up, and as a result, a large number of isotopes 240Pu, 241Pu and 242Pu accumulate in the reactor, formed by the successive capture of several neutrons - since the burnup depth is usually determined by economic factors. The lower the burnup depth, the fewer isotopes 240Pu, 241Pu and 242Pu will contain plutonium separated from irradiated nuclear fuel, but the less plutonium is formed in the fuel.
Special production of plutonium for weapons containing almost exclusively 239Pu is required mainly because isotopes with mass numbers 240 and 242 create a high neutron background, making it difficult to design effective nuclear weapons, in addition, 240Pu and 241Pu have a significantly shorter half-life than 239Pu, due to which the plutonium parts heat up, and it is necessary to additionally introduce heat removal elements into the design of the nuclear weapon. Even pure 239Pu is warmer than the human body. Additionally, the decay products of heavy isotopes spoil the crystal lattice of the metal, which can lead to a change in the shape of plutonium parts, which can lead to the failure of a nuclear explosive device.
In principle, all these difficulties can be overcome, and nuclear explosive devices made from “reactor” plutonium have been successfully tested, however, in ammunition, where compactness, light weight, reliability and durability play an important role, exclusively specially produced weapons-grade plutonium is used. The critical mass of metallic 240Pu and 242Pu is very large, 241Pu is slightly larger than that of 239Pu.
2.Production
In the USSR, the production of weapons-grade plutonium was carried out first at the Mayak plant in Ozersk (formerly Chelyabinsk-40, Chelyabinsk-65), then at the Siberian Chemical Plant in Seversk (formerly Tomsk-7), and later the Krasnoyarsk Mining Plant was put into operation -chemical plant in Zheleznogorsk (also known as Sotsgorod and Krasnoyarsk-26). Production of weapons-grade plutonium in Russia ceased in 1994. In 1999, the reactors in Ozyorsk and Seversk were shut down, and in 2010 the last reactor in Zheleznogorsk was shut down.
In the United States, weapons-grade plutonium was produced in several places, such as the Hanford complex in Washington state. Production was closed in 1988.
3.Synthesis of new elements
The transformation of some atoms into others occurs through the interaction of atomic or subatomic particles. Of these, only neutrons are available in large quantities. A gigawatt nuclear reactor produces about 3.75 kg (or 4 * 1030) neutrons over the course of a year.
4.Plutonium production
Plutonium atoms are formed as a result of a chain of atomic reactions beginning with the capture of a neutron by a uranium-238 atom:
U238 + n -> U239 -> Np239 -> Pu239
or, more precisely:
0n1 + 92U238 -> 92U239 -> -1e0 + 93Np239 -> -1e0 + 94Pu239
With continued irradiation, some atoms of plutonium-239 are able, in turn, to capture a neutron and turn into the heavier isotope plutonium-240:
Pu239 + n -> Pu240
To obtain plutonium in sufficient quantities, strong neutron fluxes are needed. These are exactly what are created in nuclear reactors. In principle, any reactor is a source of neutrons, but for the industrial production of plutonium it is natural to use one specially designed for this purpose.
The world's very first commercial plutonium production reactor was the B-reactor at Hanford. Worked on September 26, 1944, power - 250 MW, productivity - 6 kg of plutonium per month. It contained about 200 tons of uranium metal, 1200 tons of graphite and was cooled with water at a rate of 5 cubic meters/min.
Loading panel of the Hanford reactor with uranium cassettes:
Scheme of its work. In a reactor for irradiating uranium-238, neutrons are created as a result of a stationary chain reaction of fission of uranium-235 nuclei. On average, 2.5 neutrons are produced per fission of U-235. To maintain the reaction and simultaneously produce plutonium, it is necessary that on average one or two neutrons be absorbed by U-238, and one would cause the fission of the next U-235 atom.
Neutrons produced during the fission of uranium have very high speeds. Uranium atoms are arranged in such a way that the capture of fast neutrons by the nuclei of both U-238 and U-235 is unlikely. Therefore, fast neutrons, having experienced several collisions with surrounding atoms, gradually slow down. In this case, U-238 nuclei absorb such neutrons (intermediate velocities) so strongly that nothing is left to fission U-235 and maintain the chain reaction (U-235 is divided from slow, thermal neutrons).
This is counteracted by a moderator, some light substance surrounding the uranium blocks. In it, neutrons are decelerated without absorption, experiencing elastic collisions, in each of which a small part of the energy is lost. Good moderators are water and carbon. Thus, neutrons slowed down to thermal speeds travel through the reactor until they cause fission of U-235 (U-238 absorbs them very weakly). With a certain configuration of the moderator and uranium rods, conditions will be created for the absorption of neutrons by both U-238 and U-235.
The isotopic composition of the resulting plutonium depends on the length of time the uranium rods are in the reactor. A significant accumulation of Pu-240 occurs as a result of prolonged irradiation of a cassette with uranium. With a short residence time of uranium in the reactor, Pu-239 is obtained with an insignificant content of Pu-240.
Pu-240 is harmful to weapons production for the following reasons:
1. It is less fissile than Pu-239, so slightly more plutonium is required to make weapons.
2. Second, much more important reason. The level of spontaneous fission in Pu-240 is much higher, which creates a strong neutron background.
In the very early years of atomic weapon development, neutron emission (high neutron background) was a problem in achieving a reliable and effective charge due to premature detonation. Strong neutron fluxes made it difficult or impossible to compress a bomb core containing several kilograms of plutonium into a supercritical state - before this it was destroyed by the strongest, but still not the maximum possible energy output. The advent of mixed nuclei - containing highly enriched U-235 and plutonium (in the late 1940s) - overcame this difficulty when it became possible to use relatively small amounts of plutonium in mostly uranium nuclei. The next generation of charges, fusion amplified devices (in the mid-1950s), completely eliminated this difficulty, guaranteeing high energy release even with low-power initial fission charges.
Plutonium produced in special reactors contains a relatively small percentage of Pu-240 (<7%), плутоний "оружейного качества"; в реакторах АЭС отработанное ядерное топливо имеет концентрацию Pu-240 более 20%, плутоний "реакторного качества".
In special-purpose reactors, uranium is present for a relatively short period of time, during which not all U-235 burns out and not all U-238 turns into plutonium, but a smaller amount of Pu-240 is formed.
There are two reasons for producing plutonium with low Pu-240 content:
Economic: the only reason for the existence of plutonium special reactors. Decaying plutonium by fission or converting it into less fissile Pu-240 reduces returns and increases production costs (to the point where its price balances with the cost of processing irradiated fuel with low plutonium concentrations).
Handling Difficulty: While neutron emission is not a major concern for weapon designers, it can create manufacturing and handling challenges for such a charge. Neutrons create an additional contribution to occupational exposure to those who assemble or maintain weapons (neutrons themselves do not ionize, but they create protons that can). In fact, charges that involve direct contact with people, such as the Davy Crocket, may require ultra-pure, low-neutron-emitting plutonium for this reason.
The actual casting and processing of plutonium is done by hand in sealed chambers with operator gloves. Like these:
This implies very little protection for humans from neutron-emitting plutonium. Therefore, plutonium with a high content of Pu-240 is processed only by manipulators, or the time each worker works with it is strictly limited.
For all these reasons (radioactivity, worse properties of Pu-240) it is explained why reactor-quality plutonium is not used for the manufacture of weapons - it is cheaper to produce weapons-grade plutonium in special. reactors. Although, apparently, it is also possible to make a nuclear explosive device from a reactor one.
Plutonium ring
This ring is made of electrolytically purified plutonium metal (over 99.96% pure). Typical of the rings prepared at Los Alamos and sent to Rocky Flats for weapon making until production was recently suspended. The mass of the ring is 5.3 kg, sufficient for the manufacture of a modern strategic charge, the diameter is approximately 11 cm. The ring shape is important for ensuring critical safety.
Casting of plutonium-gallium alloy recovered from a weapons core:
Plutonium during the Manhattan Project
Historically, the first 520 milligrams of plutonium metal produced by Ted Magel and Nick Dallas at Los Alamos on March 23, 1944:
Press for hot pressing of plutonium-gallium alloy in the form of hemispheres. This press was used at Los Alamos to make plutonium cores for the charges detonated at Nagasaki and Operation Trinity.
Products cast on it:
Additional by-product isotopes of plutonium
Neutron capture, not accompanied by fission, creates new isotopes of plutonium: Pu-240, Pu-241 and Pu-242. The last two accumulate in small quantities.
Pu239 + n -> Pu240
Pu240 + n -> Pu241
Pu241 + n -> Pu242
A side chain of reactions is also possible:
U238 + n -> U237 + 2n
U237 -> (6.75 days, beta decay) -> Np237
Np237 + n -> Np238
Np238 -> (2.1 days, beta decay) -> Pu238
The overall measure of irradiation (waste) of a fuel cell can be expressed in megawatt days/ton (MW-day/t). Weapons grade plutonium quality is obtained from elements with a small amount of MW-day/t, it produces fewer by-product isotopes. Fuel cells in modern pressurized water reactors reach levels of 33,000 MW-day/t. Typical exposure in a weapons breeder (with expanded breeding of nuclear fuel) reactor is 1000 MW-day/t. Plutonium in the Hanford graphite-moderated reactors is irradiated up to 600 MW-day/t, in Savannah the heavy water reactor produces plutonium of the same quality at 1000 MW-day/t (possibly due to the fact that some of the neutrons are spent on the formation of tritium) . During the Manhattan Project, natural uranium fuel received only 100 MW-day/t, thus producing very high quality plutonium-239 (only 0.9-1% Pu-240, other isotopes in even smaller quantities).
Related information.
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) highly accelerated in a cyclotron. It was later found that this nuclear reaction first produces the short-lived isotope neptunium-238, and from it 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 from the decay of neptunium-239; it emits alpha rays and has a half-life of 24,000 years. Pure plutonium compound was first obtained in 1942. Then it became known that there was natural plutonium found in uranium ores, in particular in ores deposited 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 beyond Uranus. By analogy, they decided to call element 94 plutonium, since the planet Pluto is second after Uranus. Pluto, discovered in 1930, received its name from the name of the god Pluto, the ruler of the underworld in Greek mythology. At the beginning of the 19th century. Clark proposed calling the element barium plutonium, deriving this name directly from the name of the god Pluto, but his proposal was not accepted.
This metal is called precious, but not for its beauty, but for its irreplaceability. In the periodic table of Mendeleev, this element occupies cell number 94. It is with it that scientists pin their greatest hopes, and it is plutonium that they call the most dangerous metal for humanity.
Plutonium: description
In appearance it is a silvery-white metal. It is radioactive and can be represented in the form of 15 isotopes with different half-lives, for example:
- Pu-238 – about 90 years
- Pu-239 – about 24 thousand years
- Pu-240 – 6580 years
- Pu-241 – 14 years
- Pu-242 – 370 thousand years
- Pu-244 – about 80 million years
This metal cannot be extracted from ore, since it is a product of the radioactive transformation of uranium.
How is plutonium obtained?
The production of plutonium requires the fission of uranium, which can only be done in nuclear reactors. If we talk about the presence of the element Pu in 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.
Properties of plutonium
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, which is why it is warm to the touch. Moreover, this metal is characterized by the lowest thermal and electrical conductivity. Liquid plutonium is the most viscous of all existing metals.
The slightest change in the temperature of plutonium leads to an instant change in the density of the substance. In general, the mass of plutonium is constantly changing, since the nuclei of this metal are in a state of constant fission into smaller nuclei and neutrons. The critical mass of plutonium is the name given to the minimum mass of a fissile substance at which fission (a nuclear chain reaction) remains possible. For example, the critical mass of weapons-grade plutonium is 11 kg (for comparison, the critical mass of highly enriched uranium is 52 kg).
Uranium and plutonium are the main nuclear fuels. To obtain plutonium in large quantities, two technologies are used:
- uranium irradiation
- irradiation of transuranium elements obtained from spent fuel
Both methods involve the 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.
Humanity has always been in search of new sources of energy that can solve many problems. However, they are not always safe. So, in particular, those widely used today, although they are capable of generating simply colossal amounts of electrical energy that everyone needs, still carry a mortal danger. But, in addition to peaceful purposes, some countries on our planet have learned to use it for military purposes, especially to create nuclear warheads. This article will discuss the basis of such destructive weapons, the name of which is weapons-grade plutonium.
Brief information
This compact form of the metal contains a minimum of 93.5% of the 239Pu isotope. Weapons-grade plutonium was named so so that it could be distinguished from its “reactor counterpart.” In principle, plutonium is always formed in absolutely any nuclear reactor, which, in turn, operates on low-enriched or natural uranium, containing, for the most part, the 238U isotope.
Application in the military industry
Weapons-grade plutonium 239Pu is the basis of nuclear weapons. At the same time, the use of isotopes with mass numbers 240 and 242 is irrelevant, since they create a very high neutron background, which ultimately complicates the creation and design of highly effective nuclear ammunition. In addition, the plutonium isotopes 240Pu and 241Pu have a significantly shorter half-life compared to 239Pu, so plutonium parts become very hot. It is in this regard that engineers are forced to additionally add elements to remove excess heat into nuclear weapons. By the way, 239Pu in its pure form is warmer than the human body. It is also impossible not to take into account the fact that the products of the decay process of heavy isotopes subject the crystal lattice of the metal to harmful changes, and this quite naturally changes the configuration of plutonium parts, which, in the end, can cause a complete failure of a nuclear explosive device.
By and large, all of the above difficulties can be overcome. And in practice, tests have already been carried out more than once on the basis of “reactor” plutonium. But it should be understood that in nuclear weapons their compactness, low dead weight, durability and reliability are by no means the least important. In this regard, they use exclusively weapons-grade plutonium.
Design features of production reactors
Almost all plutonium in Russia was produced in reactors equipped with a graphite moderator. Each of the reactors is built around cylindrically assembled blocks of graphite.
When assembled, the graphite blocks have special slots between them to ensure continuous circulation of the coolant, which uses nitrogen. The assembled structure also has vertically located channels created for the passage of water cooling and fuel through them. The assembly itself is rigidly supported by a structure with openings under the channels used to discharge already irradiated fuel. Moreover, each of the channels is located in a thin-walled tube cast from a lightweight and extremely strong aluminum alloy. Most of the described channels have 70 fuel rods. Cooling water flows directly around the fuel rods, removing excess heat from them.
Increasing the power of production reactors
Initially, the first Mayak reactor operated with a thermal power of 100 MW. However, the main leader of the Soviet nuclear weapons program made a proposal that the reactor should operate at a power of 170-190 MW in winter, and 140-150 MW in summer. This approach allowed the reactor to produce almost 140 grams of precious plutonium per day.
In 1952, full-fledged research work was carried out in order to increase the production capacity of operating reactors using the following methods:
- By increasing the flow of water used for cooling and flowing through the cores of a nuclear plant.
- By increasing resistance to the phenomenon of corrosion that occurs near the channel liner.
- Reducing the rate of graphite oxidation.
- Increasing temperature inside fuel cells.
As a result, the throughput of circulating water increased significantly after the gap between the fuel and the channel walls was increased. We also managed to get rid of corrosion. For this, the most suitable aluminum alloys were selected and sodium bichromate began to be actively added, which ultimately increased the softness of the cooling water (pH became about 6.0-6.2). The oxidation of graphite ceased to be a pressing problem after nitrogen was used to cool it (previously only air was used).
In the late 1950s, the innovations were fully realized in practice, reducing the highly unnecessary inflation of uranium caused by radiation, significantly reducing the heat hardening of uranium rods, improving cladding resistance, and increasing production quality control.
Production at Mayak
"Chelyabinsk-65" is one of those very secret plants where weapons-grade plutonium was created. The enterprise had several reactors, and we will take a closer look at each of them.
Reactor A
The installation was designed and created under the leadership of the legendary N. A. Dollezhal. It operated with a power of 100 MW. The reactor had 1149 vertically arranged control and fuel channels in a graphite block. The total weight of the structure was about 1050 tons. Almost all channels (except 25) were loaded with uranium, the total mass of which was 120-130 tons. 17 channels were used for control rods, and 8 for experiments. The maximum design heat release of the fuel cell was 3.45 kW. At first, the reactor produced about 100 grams of plutonium per day. The first metallic plutonium was produced on April 16, 1949.
Technological disadvantages
Almost immediately, quite serious problems were identified, which consisted of corrosion of aluminum liners and coating of fuel cells. The uranium rods also swelled and became damaged, causing cooling water to leak directly into the reactor core. After each leak, the reactor had to be stopped for up to 10 hours in order to dry the graphite with air. In January 1949, the channel liners were replaced. After this, the installation was launched on March 26, 1949.
Weapons-grade plutonium, the production of which at reactor A was accompanied by all sorts of difficulties, was produced in the period 1950-1954 with an average unit power of 180 MW. Subsequent operation of the reactor began to be accompanied by more intensive use, which quite naturally led to more frequent shutdowns (up to 165 times a month). As a result, the reactor was shut down in October 1963 and resumed operation only in the spring of 1964. It completely completed its campaign in 1987 and over the entire period of many years of operation it produced 4.6 tons of plutonium.
AB reactors
It was decided to build three AB reactors at the Chelyabinsk-65 enterprise in the fall of 1948. Their production capacity was 200-250 grams of plutonium per day. The chief designer of the project was A. Savin. Each reactor consisted of 1996 channels, 65 of which were control channels. The installations used a technical innovation - each channel was equipped with a special coolant leak detector. This move made it possible to change the liners without stopping the operation of the reactor itself.
The first year of operation of the reactors showed that they produced about 260 grams of plutonium per day. However, already from the second year of operation, the capacity was gradually increased, and already in 1963 its figure was 600 MW. After the second overhaul, the problem with the liners was completely resolved, and the power was already 1200 MW with an annual production of plutonium of 270 kilograms. These indicators remained until the reactors were completely closed.
AI-IR reactor
The Chelyabinsk enterprise used this installation from December 22, 1951 to May 25, 1987. In addition to uranium, the reactor also produced cobalt-60 and polonium-210. Initially, the facility produced tritium, but later began to produce plutonium.
Also, the plant for processing weapons-grade plutonium had in operation reactors operating on heavy water and a single light water reactor (its name was “Ruslan”).
Siberian giant
"Tomsk-7" was the name of the plant, which housed five reactors for the creation of plutonium. Each of the units used graphite to slow down the neutrons and ordinary water to ensure proper cooling.
The I-1 reactor operated with a cooling system in which water passed through once. However, the remaining four installations were equipped with closed primary circuits equipped with heat exchangers. This design made it possible to additionally generate steam, which in turn helped in the production of electricity and heating of various living spaces.
Tomsk-7 also had a reactor called EI-2, which, in turn, had a dual purpose: it produced plutonium and, due to the steam generated, generated 100 MW of electricity, as well as 200 MW of thermal energy.
Important information
According to scientists, the half-life of weapons-grade plutonium is about 24,360 years. Huge number! In this regard, the question becomes especially acute: “How to properly deal with the waste from the production of this element?” The best option is considered to be the construction of special enterprises for the subsequent processing of weapons-grade plutonium. This is explained by the fact that in this case the element can no longer be used for military purposes and will be under human control. This is exactly how weapons-grade plutonium is disposed of in Russia, but the United States of America has taken a different route, thereby violating its international obligations.
Thus, the American government proposes to destroy highly enriched material not by industrial means, but by diluting plutonium and storing it in special containers at a depth of 500 meters. It goes without saying that in this case the material can easily be removed from the ground at any time and used again for military purposes. According to Russian President Vladimir Putin, initially the countries agreed to destroy plutonium not by this method, but to carry out disposal at industrial facilities.
The cost of weapons-grade plutonium deserves special attention. According to experts, tens of tons of this element may well cost several billion US dollars. And some experts have even estimated 500 tons of weapons-grade plutonium at as much as 8 trillion dollars. The amount is really impressive. To make it clearer how much money this is, let’s say that in the last ten years of the 20th century, Russia’s average annual GDP was $400 billion. That is, in fact, the real price of weapons-grade plutonium was equal to twenty annual GDP of the Russian Federation.
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 75 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 No. 93. This element was called neptunium, and the 94th 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 application these days. But in the early 40s they didn’t even think about it. It is possible to obtain plutonium-238 in quantities of practical interest only by relying on the powerful nuclear industry. At that time it was just in its infancy. But it was already clear that by releasing the energy contained in the nuclei of heavy radioactive elements, it was possible to obtain weapons of unprecedented power. The Manhattan Project appeared, which had nothing more than a name in common with the famous New York area. This was the general name for all work related to the creation of the first atomic bombs in the United States. It was not a scientist, but a military man, General Groves, who was appointed head of the Manhattan Project, who “affectionately” called his highly educated charges “broken pots.”
The leaders of the “project” were not interested in plutonium-238. Its nuclei, like the nuclei of all 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.
* We call low-energy neutrons neutrons whose energy does not exceed 10 keV. Neutrons with energy measured in fractions of an electronvolt are called thermal, and the slowest neutrons, with energy less than 0.005 eV, are called cold. If the neutron energy is more than 100 keV, then such a neutron is considered fast.
How can we explain this? Physicists understood: the synthesis of plutonium isotopes with odd mass numbers was a matter of time, and not too long. Odd isotopes were expected to, like uranium-235, be able to support a nuclear chain reaction. Some people saw them as potential nuclear explosives, which had not yet been received. And plutonium, unfortunately, justified these hopes.
In encryption of that time, element No. 94 was called nothing more than... copper. And when the need arose for copper itself (as a structural material for some parts), then in the codes, along with “copper,” “genuine copper” appeared.
"The Tree of the Knowledge of Good and Evil"
In 1941, the most important isotope of plutonium was discovered - an isotope with a mass number of 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.
There is a widespread belief that humanity was clearly in a hurry with the discovery of the nuclear chain reaction (the inevitable consequence of which was the creation of a nuclear bomb). You can think differently or pretend to think differently - it’s more pleasant to be an optimist. But even optimists inevitably face the question of the responsibility of scientists. We remember the triumphant June day of 1954, the day when the first nuclear power plant in Obninsk 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 unbridled 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 prospect of 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 2·10 7 kcal. In order not to break traditions (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 3.5·10 5 kcal of energy. It's a lot, but...
Processing granite and extracting uranium from it requires spending an even larger amount of energy - about 10 6 ...10 7 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 already be from 8·10 7 to 5·10 8 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 isotopes of plutonium have not been preserved since the synthesis of elements during the formation of our planet. But this does not mean that there is no plutonium in the Earth.
It is formed all the time in uranium ores. By capturing neutrons from cosmic radiation and neutrons produced by the spontaneous fission of uranium-238 nuclei, some - very few - atoms of this isotope turn into atoms of uranium-239. These nuclei are very unstable; they emit electrons and thereby increase their charge. Neptunium, the first transuranium element, is formed. Neptunium-239 is also highly unstable, and its nuclei emit electrons. In just 56 hours, half of the neptunium-239 turns into plutonium-239, the half-life of which is already quite long - 24 thousand years.
Why isn't plutonium extracted from uranium ores? Low, too low concentration. “A gram of production is a year of work” - this is about radium, and the ores contain 400 thousand times less plutonium 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.
When 2.70 >> 2.23
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 electron volt), 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 η. 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, η 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 structural materials of the installation. The “excess” is used to accumulate plutonium-239. In one case, the “excess” is 1.13, in the other – 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 give 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 the number η in nuclear power.
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 - coprecipitation with carriers, 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 dropped 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 drop, 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
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 - this is ballast in plutonium. From 241, americium is obtained - element No. 95. In their pure form, without admixture of other isotopes, dlutonium-240 and plutonium-241 can be obtained by electromagnetic separation of plutonium accumulated in a 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-251 from grams of plutonium.
It is possible to reduce the accumulation time of higher isotopes by increasing the intensity of the neutron flux in the reactor. This is what they do, but then you cannot irradiate large amounts of plutonium-239. After all, this isotope is divided by neutrons, and too much energy is released in intense flows. Additional difficulties arise with cooling the container and reactor. To avoid these difficulties, it would be necessary to reduce the amount of plutonium irradiated. Consequently, the yield of californium would again become scanty. Vicious circle!
Plutonium-242 is not fissile by thermal neutrons, it can be irradiated in large quantities in intense neutron fluxes... Therefore, in reactors, all elements from californium to einsteinium are “made” from this isotope and accumulated in weight quantities.
Not the heaviest, but the longest lived
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.)
Rice. 8.
Look at the graph showing the dependence of the half-life of even isotopes of plutonium on the mass number. As the mass increases, the “lifetime” of the isotope also increases. A few 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? Exactly 1 , which corresponds to a lifetime of 30 million years, or to the point 2 , which has been answering for 300 million years? The answer to this question was very important for geosciences. In the first case, if 5 billion years ago the Earth consisted entirely of 244 Pu, now only one atom of plutonium-244 would remain in the entire mass of the Earth. If the second assumption is true, then plutonium-244 may be in the Earth in concentrations that could already be detected. If we were lucky enough to find this isotope in the Earth, science would receive the most valuable information about the processes that took place during the formation of our planet.
A few years ago, scientists were faced with the question: is it worth trying to find heavy plutonium in the Earth? To answer it, it was necessary first of all to determine the half-life of plutonium-244. Theorists could not calculate this value with the required accuracy. All hope was only for experiment.
Plutonium-244 accumulated in a nuclear reactor. Element No. 95, americium (isotope 243 Am), was irradiated. Having captured a neutron, this isotope turned into americium-244; americium-244 in one out of 10 thousand cases turned into plutonium-244.
The preparation of plutonium-244 was isolated from a mixture of americium and curium. The sample weighed only a few millionths of a gram. But they were enough to determine the half-life of this interesting isotope. It turned out to be equal to 75 million years. Later, other researchers clarified the half-life of plutonium-244, but not by much - 82.8 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 14 million years. It was not possible to break the record of plutonium-244 - it is the longest-lived of all isotopes of transuranium elements.
Even heavier isotopes of plutonium undergo beta decay, and their lifetimes range from a few days to a few tenths of a second. We know for sure that all isotopes of plutonium are formed in thermonuclear explosions, up to 257 Pu. But their lifetime is tenths of a second, and many short-lived isotopes of plutonium have not yet been studied.
Possibilities of the first 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, i.e. 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 stimulants.
But 238 Pu is not the lightest known isotope of element No. 94; isotopes of plutonium have been obtained with mass numbers from 232 to 237. The half-life of the lightest isotope is 36 minutes.
Plutonium is a big topic. The most important things are told here. After all, it has already become a standard phrase that the chemistry of plutonium has been studied much better than the chemistry of such “old” elements as iron. Whole books have been written about the nuclear properties of plutonium. The metallurgy of plutonium is another amazing section of human knowledge... Therefore, you should not think that after reading this story, you truly learned plutonium - the most important metal of the 20th century.
