Uranium enrichment technologies. See what “Uranium enrichment” is in other dictionaries Weakly enriched uranium

Natural uranium contains three isotopes of uranium: 238 U (mass fraction 99.2745%), 235 U (share 0.72%) and 234 U (share 0.0055%). The isotope 238 U is a relatively stable isotope, not capable of a nuclear chain reaction on its own, unlike the rare 235 U. Currently, 235 U is the primary fissile material in the nuclear reactor and nuclear weapons technology chain. However, for many applications the proportion of the 235 U isotope in natural uranium is small and the preparation of nuclear fuel usually includes a uranium enrichment step.

Reasons for getting rich

A nuclear chain reaction implies that at least one neutron from the decay of a uranium atom will be captured by another atom and, accordingly, cause its decay. To a first approximation, this means that the neutron must “bump into” the 235 U atom before it leaves the reactor. This means that the design with uranium must be compact enough so that the probability of finding the next uranium atom for a neutron is quite high. But as the reactor operates, 235 U gradually burns out, which reduces the probability of a meeting between a neutron and a 235 U atom, which forces a certain reserve of this probability to be built into reactors. Accordingly, the low proportion of 235 U in nuclear fuel necessitates:

• a larger reactor volume so that the neutron stays in it longer;
• a larger proportion of the reactor volume should be occupied by fuel in order to increase the likelihood of a collision between a neutron and a uranium atom;
• more often it is necessary to reload fuel with fresh fuel in order to maintain a given volumetric density of 235 U in the reactor;
• high proportion of valuable 235 U in spent fuel.

In the process of improving nuclear technologies, economically and technologically optimal solutions were found that required increasing the 235 U content in the fuel, that is, uranium enrichment.

In nuclear weapons, the enrichment task is almost the same: it is required that in an extremely short time of a nuclear explosion, the maximum number of 235 U atoms find their neutron, decay and release energy. This requires the maximum possible volume density of 235 U atoms, achievable with maximum enrichment.

Uranium enrichment levels

Natural uranium with a 235 U content of 0.72%, it is used in some power reactors (for example, in Canadian CANDU), in plutonium production reactors (for example, A-1).

Uranium with a content of 235 U up to 20% is called low enriched(eng. Low enriched uranium, LEU). Uranium with 2-5% enrichment is now widely used in power reactors around the world. Uranium enriched up to 20% is used in research and experimental reactors.

Uranium with a 235 U content of more than 20% is called highly enriched(eng. Highly enriched uranium, HEU) or weapons. At the dawn of the nuclear era, several types of gun-type nuclear weapons were built based on uranium with an enrichment of about 90%. Highly enriched uranium can be used in thermonuclear weapons as tamper(compressive shell) thermonuclear charge. In addition, highly enriched uranium is used in nuclear power reactors with long fuel cycles (that is, with infrequent or no refueling), such as spacecraft reactors or ship reactors.

Remains in the waste dumps of processing plants depleted uranium with a 235 U content of 0.1-0.3%. It is widely used as cores for armor-piercing artillery shells due to the high density of uranium and the low cost of depleted uranium. In the future, it is envisaged to use depleted uranium in fast neutron reactors, where non-chain-reaction Uranium-238 can be transmuted into chain-reaction-supporting Plutonium-239. The resulting MOX fuel can be used in traditional thermal neutron power reactors.

Technologies

Many of the methods have been attempted for industrial uranium enrichment, but currently almost all enrichment facilities operate on the basis of gas centrifugation. Along with centrifugation, the gaseous diffusion method was widely used in the past. At the dawn of the nuclear era, electromagnetic, thermal diffusion, and aerodynamic methods were used. Today, centrifugation demonstrates the best economic parameters for uranium enrichment. However, research is underway into promising separation methods, such as laser isotope separation.

Production of enriched uranium in the world

Isotope separation work is calculated in special separation work units (SWP, English Separative work unit, SWU). Capacity of uranium isotope separation plants in thousands of SWU per year according to the WNA Market Report with development forecast.

A country	Company, factory	2012	2013	2015	2020
Russia

The content of the article

URANIUM INDUSTRY. Uranium is the main energy source of nuclear power, generating about 20% of the world's electricity. The uranium industry covers all stages of uranium production, including exploration, development and ore beneficiation. The processing of uranium into reactor fuel can be considered a natural branch of the uranium industry.

Resources.

The worldwide sufficiently reliably explored resources of uranium, which could be isolated from ore at a cost of no more than $100 per kilogram, are estimated at approximately 3.3 billion kg of U 3 O 8 . Approximately 20% of this (approx. 0.7 billion kg U 3 O 8, cm. Figure) falls on Australia, followed by the USA (approx. 0.45 billion kg U 3 O 8). South Africa and Canada have significant resources for uranium production.

Uranium production.

The main stages of uranium production are the extraction of ore by underground or open-pit mining, enrichment (sorting) of ore and extraction of uranium from ore by leaching. At the mine, uranium ore is extracted from the rock mass using a drilling-explosive method, the crushed ore is sorted and crushed, and then transferred into a strong acid solution (sulfuric) or an alkaline solution (sodium carbonate, which is most preferable in the case of carbonate ores). A solution containing uranium is separated from undissolved particles, concentrated and purified by sorption on ion exchange resins or extraction with organic solvents. The concentrate, usually in the form of U 3 O 8 oxide called yellowcake, is then precipitated from solution, dried and placed in steel containers with a capacity of approx. 1000 l.

In situ leaching is increasingly being used to extract uranium from porous sedimentary ores. Alkaline or acidic solution is continuously driven through wells drilled into the ore body. This solution, with the uranium transferred into it, is concentrated and purified, and then yellowcake is obtained from it by precipitation.

Processing of uranium into nuclear fuel.

Natural uranium concentrate—yellowcake—is a feedstock in the nuclear fuel cycle. To convert natural uranium into fuel that meets the requirements of a nuclear reactor, three more stages are needed: conversion to UF 6, uranium enrichment and production of fuel elements (fuel elements).

Conversion to UF6.

To convert uranium oxide U 3 O 8 to uranium hexafluoride UF 6, yellowcake is usually reduced with anhydrous ammonia to UO 2, from which UF 4 is then obtained using hydrofluoric acid. At the last stage, acting on UF 4 with pure fluorine, UF 6 is obtained - a solid product that sublimes at room temperature and normal pressure, and melts at elevated pressure. The five largest uranium producers (Canada, Russia, Niger, Kazakhstan and Uzbekistan) together can produce 65,000 tons of UF 6 per year.

Uranium enrichment.

At the next stage of the nuclear fuel cycle, the content of U-235 in UF 6 increases. Natural uranium consists of three isotopes: U-238 (99.28%), U-235 (0.71%) and U-234 (0.01%). A fission reaction in a nuclear reactor requires a higher content of the U-235 isotope. Uranium enrichment is carried out by two main methods of isotope separation: the gas diffusion method and the gas centrifugation method. (The energy expended in uranium enrichment is measured in separation work units, SWU.)

With the gas diffusion method, solid uranium hexafluoride UF 6 is converted into a gaseous state by decreasing pressure, and then pumped through porous tubes made of a special alloy, through the walls of which gas can diffuse. Because U-235 atoms have less mass than U-238 atoms, they diffuse more easily and quickly. During the process of diffusion, the gas is enriched in the U-235 isotope, and the gas passed through the tubes is depleted. The enriched gas is again passed through the tubes, and the process continues until the content of the U-235 isotope in the sample reaches the level (3–5%) required for the operation of a nuclear reactor. (Weapons-grade uranium requires enrichment to levels greater than 90% U-235.) Only 0.2–0.3% of the U-235 isotope remains in the enrichment waste. The gas diffusion method is characterized by high energy intensity. Factories based on this method are available only in the USA, France and China.

In Russia, Great Britain, Germany, the Netherlands and Japan, the centrifugation method is used, in which UF 6 gas is rotated very quickly. Due to the difference in the mass of atoms, and therefore in the centrifugal forces acting on the atoms, the gas near the axis of rotation of the flow is enriched in the light isotope U-235. The enriched gas is collected and extracted.

Manufacturing of fuel rods.

Enriched UF 6 arrives at the plant in 2.5-ton steel containers. From it, UO 2 F 2 is obtained by hydrolysis, which is then treated with ammonium hydroxide. The precipitated ammonium diuranate is filtered and fired to produce uranium dioxide UO 2 , which is pressed and sintered into small ceramic pellets. The tablets are placed in tubes made of zirconium alloy (Zircaloy) and fuel rods are obtained, the so-called. fuel elements (fuel elements), which combine approximately 200 pieces into complete fuel assemblies, ready for use at nuclear power plants.

Spent nuclear fuel is highly radioactive and requires special precautions during storage and disposal. In principle, it can be reprocessed by separating the fission products from the remaining uranium and plutonium, which can be reused as nuclear fuel. But such processing is expensive and commercial facilities are available only in a few countries, such as France and the UK.

Volume of production.

By the mid-1980s, as hopes for rapid growth in nuclear power failed, uranium production fell sharply. The construction of many new reactors was suspended, and reserves of uranium fuel began to accumulate at existing enterprises. With the collapse of the Soviet Union, the supply of uranium in the West increased further.

The content of the article

URANIUM INDUSTRY. Uranium is the main energy source of nuclear power, generating about 20% of the world's electricity. The uranium industry covers all stages of uranium production, including exploration, development and ore beneficiation. The processing of uranium into reactor fuel can be considered a natural branch of the uranium industry.

Resources.

The worldwide sufficiently reliably explored resources of uranium, which could be isolated from ore at a cost of no more than $100 per kilogram, are estimated at approximately 3.3 billion kg of U 3 O 8 . Approximately 20% of this (approx. 0.7 billion kg U 3 O 8, cm. Figure) falls on Australia, followed by the USA (approx. 0.45 billion kg U 3 O 8). South Africa and Canada have significant resources for uranium production.

Uranium production.

The main stages of uranium production are the extraction of ore by underground or open-pit mining, enrichment (sorting) of ore and extraction of uranium from ore by leaching. At the mine, uranium ore is extracted from the rock mass using a drilling-explosive method, the crushed ore is sorted and crushed, and then transferred into a strong acid solution (sulfuric) or an alkaline solution (sodium carbonate, which is most preferable in the case of carbonate ores). A solution containing uranium is separated from undissolved particles, concentrated and purified by sorption on ion exchange resins or extraction with organic solvents. The concentrate, usually in the form of U 3 O 8 oxide called yellowcake, is then precipitated from solution, dried and placed in steel containers with a capacity of approx. 1000 l.

In situ leaching is increasingly being used to extract uranium from porous sedimentary ores. Alkaline or acidic solution is continuously driven through wells drilled into the ore body. This solution, with the uranium transferred into it, is concentrated and purified, and then yellowcake is obtained from it by precipitation.

Processing of uranium into nuclear fuel.

Natural uranium concentrate—yellowcake—is a feedstock in the nuclear fuel cycle. To convert natural uranium into fuel that meets the requirements of a nuclear reactor, three more stages are needed: conversion to UF 6, uranium enrichment and production of fuel elements (fuel elements).

Conversion to UF6.

To convert uranium oxide U 3 O 8 to uranium hexafluoride UF 6, yellowcake is usually reduced with anhydrous ammonia to UO 2, from which UF 4 is then obtained using hydrofluoric acid. At the last stage, acting on UF 4 with pure fluorine, UF 6 is obtained - a solid product that sublimes at room temperature and normal pressure, and melts at elevated pressure. The five largest uranium producers (Canada, Russia, Niger, Kazakhstan and Uzbekistan) together can produce 65,000 tons of UF 6 per year.

Uranium enrichment.

At the next stage of the nuclear fuel cycle, the content of U-235 in UF 6 increases. Natural uranium consists of three isotopes: U-238 (99.28%), U-235 (0.71%) and U-234 (0.01%). A fission reaction in a nuclear reactor requires a higher content of the U-235 isotope. Uranium enrichment is carried out by two main methods of isotope separation: the gas diffusion method and the gas centrifugation method. (The energy expended in uranium enrichment is measured in separation work units, SWU.)

With the gas diffusion method, solid uranium hexafluoride UF 6 is converted into a gaseous state by decreasing pressure, and then pumped through porous tubes made of a special alloy, through the walls of which gas can diffuse. Because U-235 atoms have less mass than U-238 atoms, they diffuse more easily and quickly. During the process of diffusion, the gas is enriched in the U-235 isotope, and the gas passed through the tubes is depleted. The enriched gas is again passed through the tubes, and the process continues until the content of the U-235 isotope in the sample reaches the level (3–5%) required for the operation of a nuclear reactor. (Weapons-grade uranium requires enrichment to levels greater than 90% U-235.) Only 0.2–0.3% of the U-235 isotope remains in the enrichment waste. The gas diffusion method is characterized by high energy intensity. Factories based on this method are available only in the USA, France and China.

In Russia, Great Britain, Germany, the Netherlands and Japan, the centrifugation method is used, in which UF 6 gas is rotated very quickly. Due to the difference in the mass of atoms, and therefore in the centrifugal forces acting on the atoms, the gas near the axis of rotation of the flow is enriched in the light isotope U-235. The enriched gas is collected and extracted.

Manufacturing of fuel rods.

Enriched UF 6 arrives at the plant in 2.5-ton steel containers. From it, UO 2 F 2 is obtained by hydrolysis, which is then treated with ammonium hydroxide. The precipitated ammonium diuranate is filtered and fired to produce uranium dioxide UO 2 , which is pressed and sintered into small ceramic pellets. The tablets are placed in tubes made of zirconium alloy (Zircaloy) and fuel rods are obtained, the so-called. fuel elements (fuel elements), which combine approximately 200 pieces into complete fuel assemblies, ready for use at nuclear power plants.

Spent nuclear fuel is highly radioactive and requires special precautions during storage and disposal. In principle, it can be reprocessed by separating the fission products from the remaining uranium and plutonium, which can be reused as nuclear fuel. But such processing is expensive and commercial facilities are available only in a few countries, such as France and the UK.

Volume of production.

By the mid-1980s, as hopes for rapid growth in nuclear power failed, uranium production fell sharply. The construction of many new reactors was suspended, and reserves of uranium fuel began to accumulate at existing enterprises. With the collapse of the Soviet Union, the supply of uranium in the West increased further.

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From the editor: News reports about Iran's nuclear activities once again demonstrate the relevance of the topic of uranium enrichment. This issue of SDA magazine aims to support reasoned debate with information and analysis about the status and process of uranium enrichment.

The article describes the process and technologies of uranium enrichment, and also provides a short historical background. IN Information on the operating status of uranium enrichment plants in various countries of the world is briefly presented. You can test your knowledge in the field of uranium enrichment by answering .

Article, table and test are based on the report , published in October 2004 by IEER for the Nuclear Policy Research Institute. Links are provided in the report.

Knowledge and capabilities in the field of uranium enrichment have become quite widespread in both nuclear energy and the creation of nuclear weapons. In many ways, this process is already out of control. And this is of particular concern in light of emerging proposals that may well stimulate greater use of nuclear energy around the world in the coming decades.

For example, to fuel a thousand 1000 MW nuclear power plants (a common example in many nuclear development programs) would require a global uranium enrichment capacity that is approximately 9-10 times greater than the production currently operating in the United States. If even one percent of this capacity were used to produce highly enriched uranium (HEU), then such volumes of HEU would be produced annually that would make it possible to create from 175 to 310 nuclear weapons. Given the expanded trade in specialized materials needed to build and operate gas-generating centrifuges and other enrichment plants that could lead to increased nuclear energy production, determining the legality of trade and distribution of supposedly “peaceful” technologies will become even more difficult.

It is important to pay attention to states, such as Iran, that are currently making progress in their efforts to build support for a nuclear weapons program. However, it is equally important to remember how widespread uranium enrichment technology is and how much the threat could increase if these technologies are allowed to be applied anywhere in the world in an effort to expand the use of nuclear energy. In other words, we do well not to ignore countries that have advanced nuclear weapons and atomic energy programs, but to take into account their impressive proliferation potential and less impressive record in this area 1 . All five nuclear powers party to the Nuclear Non-Proliferation Treaty (NPT) - the US, Russia, UK, France and China - have uranium enrichment plants that were once used to produce weapons-grade HEU. All five states also have full-scale enrichment facilities that have been involved in the production of low-enriched uranium (LEU) used as fuel for industrial nuclear reactors.

Besides the five known nuclear weapons states, only three other countries have uranium enrichment facilities, which have been used to produce large quantities of fuel for industrial nuclear reactors. However, there are a number of other countries that have been involved in enrichment technologies, and some of them have been observed or suspected of using enrichment potential for military purposes. The information available today on the operating status of uranium enrichment plants in various countries of the world is briefly presented.

In Pakistan, one of those states that created nuclear weapons without being parties to the NPT, there are plants where they enriched HEU for military purposes. As is known, South Africa also produced nuclear weapons using enriched uranium obtained from its own production. On the other hand, India and Israel created atomic bombs from plutonium-239 (which is produced in nuclear reactors when non-fissionable uranium-238 absorbs a low-energy neutron). North Korea, which withdrew from the NPT in January 2003 without giving the required three months' notice, raises serious suspicions that it has produced small quantities of nuclear weapons using plutonium. The question of the possible continuation of the uranium enrichment program in North Korea also remains open.

Uranus

Only one element that occurs in nature is the raw material for making atomic bombs. This is uranium, chemical sign "U" 2. The distinctive property of uranium, which is necessary for the production of nuclear weapons and atomic energy, is its ability to fission or divide into two lighter fractions by irradiation with neutrons and to release energy in this process.

Natural uranium (that is, that which is mined from the depths of the earth) occurs as a combination of three different isotopes, that is, atoms with three different atomic masses, having essentially the same chemical, but different nuclear properties. These isotopes are uranium-234, uranium-235 and uranium-238. Uranium-234 is a highly radioactive trace element found in natural uranium. Uranium-235 is the only fissile substance found in nature in significant quantities. Uranium-238 - this isotope predominates in natural uranium (99.284% of the mass of a sample of natural uranium is uranium-238), but it cannot be fissioned. However, uranium-238 can be separated by high-energy neutrons, releasing large amounts of energy, and is therefore often used to increase the explosive power of thermonuclear or hydrogen bombs.

Some of the properties of these three isotopes found in natural uranium are summarized in Table 1. Since uranium-234 makes up a very small fraction of the total mass of natural uranium and is not used in any serious programs, in this article we will dwell in detail only on the other two isotopes - uranium-235 and uranium-238.

Table 1: Summary of uranium isotopes

Thanks to small amounts of U-235, natural uranium can support a chain reaction under certain conditions, and is thus a fuel for certain types of reactors (graphite nuclear reactors and heavy water nuclear reactors 3 - the latter commercially sold by Canada on an industrial scale). In the most common type of reactor today (light water nuclear), in which ordinary water serves as a cooling and moderating agent, to maintain the reaction the share of U-235 in the fuel must exceed 0.7% - the level of its content in natural uranium.

The set of production processes carried out to increase the percentage of U-235 in a specified amount of uranium is called "uranium enrichment". Here the term "enrichment" means increasing the percentage of the fissile isotope U-235. Light water nuclear reactors typically use 3 to 5 percent enriched uranium, meaning the fuel contains 3 to 5 percent U-235 and the rest is actually U-238. A substance with this level of U-235 is called "low enriched uranium" or LEU.

Atomic bombs cannot be created from natural or low-enriched uranium. The proportion of U-235 is too small to produce a growing "supercritical" chain reaction in a short enough time to produce an explosion. To create an atomic bomb, the U-235 content in uranium must be at least about 20%. However, a bomb made from uranium enriched to such a minimal degree would be too bulky to deliver, since it would require a huge amount of uranium and even more conventional explosives to compress it into supercritical mass.

In practice, uranium, which contains at least 90% U-235, has already been used to create nuclear weapons. A substance with this level of enrichment is called highly enriched uranium, or HEU. The atomic bomb that destroyed Hiroshima on August 6, 1945 was created from approximately 60 kilograms of HEU. Highly enriched uranium is also used in research and naval nuclear reactors - on aircraft carriers and submarines. HEU intended for nuclear research reactors may be of particular interest to those who would like to commit nuclear sabotage because it is generally less secure and is often located in cities or on university grounds. Unlike irradiated nuclear reactor fuel, unirradiated HEU does not pose a radioactive hazard.

The same process and production can be used to enrich uranium for fuel in industrial light water reactors, that is, to create LEU, as well as to obtain HEU for atomic bombs. Thus, all uranium enrichment technologies are potential sources of nuclear weapons proliferation. In addition, some other methods of uranium enrichment are much more difficult to detect, adding additional concerns about the possible existence of illegal programs.

Uranium enrichment

Since all uranium isotopes have virtually the same chemical properties, the increase in the proportion of uranium-235 in the sample depends on the difference in the atomic masses of the isotopes (which are assigned the following numbers: 234, 235 and 238). U-238 is slightly more than one percent heavier than U-235. If uranium is turned into a gas, then molecules containing lighter U-235 will, on average, move at a higher speed (at a given temperature) compared to heavier molecules containing U-238.

During a typical enrichment process, a stream of natural uranium gas containing U-235 and U-238 is split into two streams due to the slight difference in mass of the two isotopes. One stream becomes richer in uranium-235 (the "enriched" uranium stream), while the other becomes poorer in this isotope (the "depleted" uranium stream, where the term "depleted" means a lower percentage of U-235 relative to natural uranium). More detailed information about enrichment processes is presented below, in the chapter “Enrichment Technologies” 4.

The capacity of a uranium enrichment plant to increase the percentage of U-235 is expressed in units called kilograms of Separative Work Units (SWU, pronounced "swuz" in English). In production-level enterprises, plant capacities typically range from several hundred to several thousand metric tons of SWU (MTEPP) per year. (1 MTERP = 1000 SWU.) The unit of separation work is a complex unit that depends both on the proportion of U-235 that is desired in the enriched stream and on how much U-235 from the starting material remains in the depleted stream isotope. SWU can be thought of as the amount of effort required to achieve a specified enrichment rate. The less U-235 from the feedstock that must be retained in the depleted uranium, the more SWU is needed to achieve the desired degree of enrichment 5 .

The amount of SWU provided by an enrichment plant directly depends on the amount of energy consumed by this plant. The two most common enrichment technologies today, which are described in detail below, differ significantly in their energy consumption. Modern gaseous diffusion plants typically require 2,400 to 2,500 kilowatt-hour (kWh) of electricity per SWU, while gasifier centrifuge plants require only 50 to 60 kWh of electricity per SWU.

To power a typical 1,000 megawatt light water nuclear reactor using enriched uranium as fuel, approximately 100,000 to 120,000 SWU uranium enrichment services per year would be required. If such enrichment were provided by a gaseous diffusion plant (like the one currently operating in Paducah, Kentucky, USA), then the enrichment process would consume approximately 3-4% of the volume of electricity generated by this reactor 6 . On the other hand, if uranium fuel enrichment were carried out in gas generator centrifuges (which operate in many parts of the world today), then the enrichment process would consume less than 0.1% of the electricity generated by a nuclear plant per year.

In addition to the kilogram of SWU, it is worth considering another important parameter. This is the mass of natural uranium that is needed to obtain the desired mass of enriched uranium. As with the amount of SWU, the amount of feed material required will also depend on the degree of enrichment desired, as well as the amount of U-235 that remains in the depleted uranium. The amount of natural uranium required will decrease as the proportion of U-235 that must be retained in the depleted uranium decreases.

For example, when enriching LEU for a light water nuclear reactor, the enriched stream typically contains 3.6 percent U-235 (compared to 0.7 percent in natural uranium), while the lean stream contains 0.2 to 0.3 percent U -235. To produce one kilogram of such LEU, approximately 8 kilograms of natural uranium and 4.5 SWU will be required, if the permissible share of U-235 in the depleted uranium stream is 0.3%. On the other hand, if only 0.2% U-235 remained in the depletion stream, then only 6.7 kilograms of natural uranium would be required, but about 5.7 SWU for enrichment.

To obtain one kilogram of highly enriched uranium (that is, uranium containing 90% U-235), more than 193 SWU and almost 219 kilograms of natural uranium will be required, provided that 0.3% U-235 remains in the depleted uranium. If the acceptable proportion of U-235 in depleted uranium is 0.2%, almost 228 SWU and more than 176 kilograms of natural uranium will be required.

Table 2 provides a summary of the costs (for natural uranium and enrichment services) required to produce one kilogram of LEU and one kilogram of HEU with 0.2% and 0.3% U-235 in the depleted uranium stream .

Table 2: Costs of obtaining one kilogram of low-enriched uranium
and one kilogram of highly enriched uranium

LEU = uranium containing 3.6% U-235, typically used in a light water reactor.
HEU = uranium containing 90% U-235, commonly used to make nuclear weapons.
SWU = Separation Work Unit
kg = kilogram

Given that the required volume of natural uranium and SWU in the enrichment process changes in the opposite direction for a specified degree of enrichment, natural uranium is cheap, and enrichment services are expensive, owners of enrichment plants will agree to “release” a larger share of U-235 into the depleted stream (that is, it will be more profitable for them to use more natural uranium and less SWU). On the other hand, if natural uranium is more expensive than enrichment services, then plant owners will choose the opposite option.

To enrich the uranium for an atomic bomb equivalent to the one the US dropped on Hiroshima (that's about 60 kg of HEU), it would require 10.6 to 13.1 metric tons of natural uranium, as well as 11,600 to 13,700 SWU for enrichment. However, to create more complex types of nuclear weapons would require much less than half this amount. Modern types of uranium bombs typically require only 20-25 kilograms of HEU.

If, instead of natural uranium, low-enriched uranium (containing 3.6% U-235) was used as the feedstock for HEU production, then only 70-78 SWU and 26-27 kilograms of feedstock would be required to produce one kilogram of highly enriched uranium. This means that to produce the HEU equivalent to the Hiroshima bomb, only 1.6 tons of LEU would have to be enriched, less than one-tenth of the total amount of LEU needed to fuel one 1,000 MW nuclear reactor each year. . Thus, approximately two-thirds of the total uranium enrichment services required to produce weapons-grade HEU are involved in the enrichment of uranium from natural uranium (0.7% U-235) to LEU (3.6% U-235). However, only about one third of the total volume of services is involved in the enrichment of LEU with its final processing into HEU (90% U-235), as shown in the diagram.

Thus, stocks of low-enriched uranium, if maintained in a state suitable for enrichment (that is, such as uranium hexafluoride), can become the starting material for the easier and faster production of highly enriched uranium used to create nuclear weapons. This is one of the most dangerous aspects of the widespread proliferation of enrichment technologies as part of the proliferation of nuclear power.

Enrichment services required to produce highly enriched uranium from natural uranium

Enrichment technologies

Four uranium enrichment technologies have been widely used. Three of them—gas diffusion, gas centrifugation, and nozzle/aerodynamic separation—are based on the conversion of uranium to uranium hexafluoride (UF 6) gas. The fourth method, electromagnetic separation, is based on the use of ionized uranium gas obtained from solid uranium tetrachloride (UCL 4).

Gas diffusion

The gaseous diffusion process was used to enrich almost all of the low- and high-enriched uranium that was produced in the United States. This method was first developed in the 1940s as part of the Manhattan Project and was used in part to enrich uranium for the Hiroshima bomb. All five known nuclear powers party to the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) have at one time or another commissioned gaseous diffusion facilities, but to date such facilities continue to operate only in the United States and France. The diffusion process requires pumping uranium, which is in a gaseous state, through a large number of porous barriers. This is a very energy-intensive process.

In order to convert uranium into a gaseous state in which it can participate in the process of gaseous diffusion, natural uranium is converted into uranium hexafluoride (UF 6). Uranium hexafluoride molecules containing U-235 atoms, being slightly lighter, will move through each barrier with a slightly higher degree of separation than those containing U-238 atoms. To visualize this process, we can give the example of blowing sand through many sieves. The smaller grains of sand will preferentially pass through each sieve and thus, after each sieving stage, they will constitute a slightly higher percentage of the total volume of sand grains compared to the percentage they were at the previous sieving stage. A diagram of one of these screening stages in a gas diffusion installation is shown in Figure 1.

The difference in masses, and therefore speeds, of UF 6 molecules containing U-235 and U-238 is small. Thus, to enrich large industrial or military quantities of uranium, thousands of enrichment stages are needed. In a gaseous diffusion plant, the stages are built in “cascades”, which allow each stage to increase the enrichment obtained in the previous stages, as well as to use the depleted uranium flow more efficiently. To understand the scale of such production, you need to know that at the time of construction of the gaseous diffusion plant, built in the early 1940s in Oak Ridge, Tennessee, USA, it was the largest industrial facility in the world.

The most difficult task when constructing a gaseous diffusion plant is the production of permeable barriers, which are necessary for the operation of the diffusers. The material for such barriers must be highly durable and capable of maintaining the same pore diameter over several years of plant operation. This is a very difficult task when using uranium hexafluoride gas, which is highly corrosive. Typical barriers are only 5 millimeters (less than 0.2 inches) thick, and their openings are only 30 to 300 times the diameter of a single uranium atom.

In addition to requiring large amounts of electrical power to operate the plant, compressors in gaseous diffusion plants also generate a lot of heat that must be dissipated. In American installations, heat transfer occurs using ozone-depleting chlorofluorocarbons (CFCs), such as CFC-114 refrigerant (often called Freon or Freon-114). The production, import, and use of CFCs were severely limited in 1987 by the Montreal Protocol on Substances That Deplete the Ozone Layer, which the United States implemented through the 1990 Amendments to the Air Pollution Control Act ( Clean Air Act).

As a result of such measures, freon production in the United States was discontinued in 1995. From 1991 to 2002, emissions of this substance into the atmosphere from large consumers in the United States fell by almost 60%. However, emissions from the gaseous diffusion plant in Paducah, Kentucky, USA remained virtually the same during this period, decreasing by only 7% from 1989 to 2002. In 2002, the Paducah enrichment plant released more than 197.3 metric tons of Freon into the atmosphere through leaking pipes and other equipment. This one facility alone contributed over 55% of all emissions of this ozone-depleting CFC from all major US manufacturing facilities in 2002.

Due to the fact that freon has not been produced in the United States since 1995, the American Uranium Enrichment Corporation (USEC) 7 is currently seeking a coolant that does not contain CFCs. But any other refrigerants will still have heat-trapping potential, and thus, even if they do not pose a threat to the ozone layer, they will still remain potentially dangerous in terms of global warming and climate change.

A characteristic feature of gaseous diffusion installations - large heat release - makes it possible to identify those whose operation significantly exceeds 100 MTERP per year. However, such information is likely to be relevant only for identifying activities at known installations, and not at illegal sites, since there are many other industrial processes that produce large amounts of heat. Therefore, although uranium enrichment facilities such as gaseous diffusion plants are almost impossible to hide due to their size, power requirements and heat generation, it is still extremely difficult to identify any object from a distance without access to environmental samples in the surrounding area. areas (eg soil samples) that may strongly indicate the presence of enriched uranium.

Gas centrifugation

Currently, gas centrifugation is the main method of uranium enrichment in the world. This technology was discussed in the United States as part of the Manhattan Project, but methods such as gaseous diffusion and electromagnetic separation were further developed for full-scale production. Later, the centrifugation method was developed in Russia by a group of specialists led by Austrian and German scientists who were captured during the Second World War. Over time, the head of the scientific group in Russia was released. He first brought this technology to the United States and then to Europe, where he began introducing this method to enrich industrial nuclear fuel.

Centrifugation is a common method used for a variety of purposes, such as separating plasma from the heavier red blood cells. The spin cycle in a washing machine works on a similar centrifugal principle. During the enrichment process, uranium hexafluoride gas is fed into rapidly rotating cylinders. To achieve the maximum degree of enrichment at each stage, modern centrifuges are capable of rotating at speeds close to the speed of sound. It is for this reason that it is extremely difficult to control the centrifugation process, since with a high degree of rotation it is necessary for the centrifuge to be durable, almost perfectly balanced and ready to operate in this form for many years without stopping for maintenance.

Inside a spinning centrifuge, heavier molecules containing U-238 atoms preferentially move toward the outside of the cylinder, while lighter molecules containing U-235 remain closer to the central axis. The gas in this cylinder then begins to circulate from the bottom up, pushing the depleted uranium, which is closer to the outer wall, towards the top, and the gas enriched in U-235 from the center towards the bottom. The two streams, one rich and the other lean, can then be removed from the centrifuge and introduced into adjacent stages to form the cascade described above with diffusers in gaseous diffusion plants. A diagram of such a centrifuge is shown in Figure 2.

Similar to the gaseous diffusion process, uranium enrichment by gas centrifugation requires thousands to tens of thousands of steps to enrich large volumes of uranium for industrial or military purposes. In addition, like gaseous diffusion plants, centrifuge plants must use special materials to prevent corrosion caused by uranium hexafluoride, which, when reacting with moisture, can form highly corrosive hydrofluoric acid gas. One of the most important advantages of gas centrifugation over the gas diffusion process is that when the same degree of enrichment is achieved, this process requires 40-50 times less electricity. The use of centrifuges also helps to reduce the amount of heat used that is generated when UF 6 gas is compressed, and thus reduce the amount of refrigerants such as freon required.

Although the separation power at each stage is greater than that of the gaseous diffusion process, it typically requires much less uranium, which can be centrifuged through each stage in a given time. Conventional modern centrifuges are capable of achieving approximately 2 to 4 SWU annually. Therefore, between 3,000 and 7,000 centrifuges would be required to enrich enough weapons-grade HEU per year to be used to create a nuclear weapon equivalent to that dropped on Hiroshima. Such production can consume from 580,000 to 816,000 kWh of electricity, which can be provided by an installation with a capacity of less than 100 kilowatts. With the creation of modern types of weapons, these figures can be reduced to 1000-3000 centrifuges and 193,000-340,000 kWh.

The degree of enrichment at each stage in modern centrifuge models is expected to be ten times greater than that achieved by centrifuges currently in operation. This could further reduce HEU production costs. The sale of an older model of the European centrifuge to countries such as Libya, Iran and North Korea through a network led by A.Q., sources said. Khan, who formerly led Pakistan's nuclear weapons program, is particularly worrisome from a nuclear proliferation perspective because centrifuges are smaller and require less power during the enrichment process.

Electromagnetic method for separating radioactive isotopes of uranium (EMIS)

The electromagnetic method of separating radioactive isotopes is a third type of uranium enrichment that has been widely used in the past. The electromagnetic separation facility was developed as part of the Manhattan Project in Oak Ridge, Tennessee. This method was used to enrich natural uranium and then enrich uranium originally processed at the gaseous diffusion plant, which was also located at the Oak Ridge plant. The use of this installation was suspended immediately after the war due to its high cost and low productivity.

Iraq created this technology in the 1980s as part of its HEU program because of its relative simplicity. However, it produced only small volumes of medium-enriched uranium (only above 20%).

The process of electromagnetic separation is based on the fact that, moving in a magnetic field, a charged particle follows a curved path, the radius of which depends on the mass of the particle. Heavier particles will cycle more than lighter particles, provided the particles are equally charged and moving at the same speed.

In the enrichment process, uranium tetrachloride is ionized into uranium plasma, that is, the solid compound UCL 4 is heated to create a gas, which is then irradiated with electrons to produce free uranium atoms that have lost electrons and become positively charged. The uranium ions are then accelerated and passed through a strong magnetic field. After completing half the cycle, the beam of ionized uranium atoms is divided into a depleted part, located closer to the outer wall, and into a U-235-enriched part, which is located closer to the inner wall.

Due to the high energy consumption when creating a strong magnetic field, as well as the low rate of selection of the initial uranium substance, in addition to the slower and less convenient operation of such an installation, the electromagnetic separation method is unpromising for industrial-scale enrichment plants, especially in light of the current day of highly developed models of gas generator centrifuges.

Nozzle / Aerodynamic separation

The latest uranium enrichment process that has become widely used is called aerodynamic separation. The process was first developed in Germany and used by the apartheid-era South African government in a plant supposedly built to provide low-enriched uranium for South Africa's industrial nuclear power plants, as well as to produce small amounts of highly enriched uranium to fuel a nuclear research reactor. In fact, this enrichment plant also supplied approximately 400 kilograms of uranium enriched to more than 80% for military purposes. In the early 1990s, South African President Frederik de Klerk announced the cessation of all military nuclear activities and the destruction of all existing bombs. These tasks were completed a year and a half later, just after South Africa became a party to the NPT and before International Atomic Energy Agency verifications and safeguards came into effect.

Aerodynamic isotope separation (which involves a nozzle and a spiral wave) achieves enrichment in a similar way to gas centrifugation, in that the gas is forced along a curved path that moves heavier molecules containing U-238 towards the outer wall, and the lighter molecules containing U-235 remain closer to the inner one. In nozzle installations, uranium hexafluoride gas is displaced under pressure by helium or hydrogen gas to increase the gas flow rate. This compound is then passed through a number of small circular tubes that separate the inner rich stream from the outer lean stream.

Nozzle/aerodynamic separation is one of the least economical of all enrichment technologies used, especially given the technical difficulties of producing separation nozzles and the high energy consumption when compressing UF 6 and the carrier gas mixture. As in gas diffusion plants, during the operation of an aerodynamic separation plant, large volumes of heat are also generated, which in turn require a large number of coolers such as freon.

Other technologies

There are a number of other ways to enrich uranium. These are AVLIS - technology for laser isotope separation in atomic form, MLIS - molecular method for laser isotope separation, CRISLA - chemical reaction through selective isotope laser activation, as well as chemical and ion enrichment, which have also been developed, but are mainly still in the testing stage or demonstrations and were not used to enrich uranium for industrial or military purposes.

Processes such as AVLIS, CRISLA and MLIS use the slight difference in the atomic properties of U-235 and U-238 to preferentially excite or ionize one isotope over the other using high-power lasers. The AVLIS method uses uranium metal as a starting material and uses electrostatic fields to separate positively charged U-235 ions from uncharged U-238 atoms. MLIS and CRISLA technologies use uranium hexafluoride as a starting material, combined with other process gases, and use two different lasers to excite and then chemically alter uranium hexafluoride molecules containing U-235, which can then be separated from other molecules containing U-238 that were not exposed to the laser. AVLIS technology was developed by the American Uranium Enrichment Corporation for industrial use, but was abandoned in the late 1990s due to its unprofitability. At the same time, other countries have also stopped using all known production programs with AVLIS and MLIS technologies. However, little work is still going on at the proposed research sites where these technologies are used for the isotopic separation of uranium, as well as other radionuclides, including plutonium.

There is also an enrichment method that uses small differences in the chemical properties of isotopes to separate U-235 from U-238. These are the so-called chemical and ion enrichment processes, which were developed within the framework of government programs in France and Japan. Using special solutions, the uranium can be separated into an enriched part, which is contained in one solvent stream, and a depleted part, contained in another solvent stream, which does not mix with the first - just like oil and water. This enrichment technology was used in Iraq. To date, all known programs that include this method have been closed at least since the early 1990s.

All of these enrichment technologies have not been demonstrated very widely, although some, such as AVLIS, are much further along in their development, which could advance them to the level of application at production facilities. The potential use of such alternative technologies in uranium enrichment in illegal programs continues to raise concerns, especially if the cost-effectiveness of the plant is not an issue and it is intended only to produce the fairly small amount of HEU needed for one or two bombs per year. However, today the main technology for industrial uranium enrichment in the future for nuclear energy and the potential proliferation of nuclear weapons remains gas centrifugation.