Example.
D-T synthesis starts with a deuterium and tritium atom and ends with a helium-4 atom and a neutron. Initial mass 2.013553 + 3.015500 = 5.029053. Final mass 4.001506 + 1.008665 = 5.010171. Subtracting the second from the first, we find that the mass defect is equal to 0.018882. Multiplying by 931.494028 we find the resulting energy equal to 17.58847 MeV.
Note that nuclear fusion produces energy as larger and larger atoms fuse together until they grow to the point where they become iron atoms. After this, the fusion of heavy atoms begins to consume more energy than it produces.
Particles
This table gives symbols for various particles that can be used as fusion fuel. The particle masses are given in case you want to calculate the mass defect for the reactions below and be surprised by the amount of energy obtained.
Tritium has a half-life of only 12.32 years, which makes it a little difficult to use in space, since after twelve years it will half decay into helium-3. This is why there are no natural deposits of tritium. Most reactor designs using tritium rely on tritium generators. They are usually tanks of liquid lithium surrounding the reactor. Lithium absorbs neutrons and transmutes into fresh tritium and helium-4.
The famous helium-3, which is often cited as the economic motive for space exploration, unfortunately, is not as good as one might expect. First, it is not found on Earth, making it difficult to obtain. Some enthusiasts want to mine it on the Moon, without specifying, its concentration there is very low. To obtain just a ton of helium-3, it is necessary to process 100 million tons of lunar regolith. Alternatively, it can be produced in factories, but this requires a large number of neutrons. IN general outline, you need to get tritium and wait for it to decay. Huge quantities of helium-3 are available in the atmospheres of Saturn and Uranus, but the appropriate infrastructure is needed to extract it from there. The concentration of helium-3 in their atmospheres can reach ten parts per million, which is much better than on the Moon. Jupiter also contains helium-3 in its atmosphere, but due to its enormous gravity, its extraction can be very difficult.
Introduction
This article describes, at first glance, another method of using thermonuclear energy for fast manned space flights. Previous efforts on this path were unsuccessful, largely due to the following two reasons. First, they were based on the design of fusion reactors. Straightforward application of the approaches used in reactors leads to systems with enormous mass and problems with energy dissipation. In a detailed analysis, for the most compact TOKMAK concept, a spherical torus, the mass of the ship was around 4000 tons. The maximum mass for launching into a low reference orbit using chemical rockets should not exceed 200 tons.The second reason is that, in fact, all previous propulsion systems required complex reactions producing, for the most part, charged particles. This was necessary to reduce energy losses through neutrons. The most promising were D- 3 He and P- 11 B. But these reactions require much higher plasma temperatures and were orders of magnitude more difficult to achieve than D-T fusion, which is much more accessible and is considered the only candidate for application on Earth. While less profitable, they nevertheless require enormous amounts of energy to maintain combustion, making them little better than alternative fission reactions.
Past ideas about how to use fusion energy in space propulsion systems need to be rethought. Let's look at what gives chemical rocket engines such advantages. The main reason is that the energy obtained from the chemical reaction of combustion can be as large or as small as desired. From 13 GW for a heavy Atlas launch vehicle, to 130 kW for a car. It is worth noting that at lower energy, combustion is more efficient, since the temperature can be increased without worrying about the need for intensive heat removal and thermal damage that can occur with long-term continuous operation.
As tests of atomic and hydrogen bombs have shown, the combustion of nuclear fuel can produce energy many orders of magnitude greater than the same Atlas. The problem is how to control the release of nuclear energy to obtain the characteristics necessary for space flights: a multi-megawatt plume, low specific gravity α (~ 1 kg/kW) with high specific impulse Isp (> 20000 m/s). It turns out that, at least for nuclear fission, there is no way to scale down to the required energy scale, since a certain critical mass (critical configuration) is required for the reaction to start self-sustaining. As a result, projects using nuclear fission reactions, such as Orion, typically produced millions of tons of thrust, which is only suitable for spacecraft with a mass of 10 7 kg and above.
Fortunately, the scale of fusion reactions can be much smaller and techniques such as Magneto Inertial Fusion (MIF) can produce large amounts of energy from nuclear material in systems that can accommodate space propulsion systems in their size. weight, power and cost.
Engine physics
The engine is based on the principle of three-dimensional implosion (compression by a blast wave) of metal foil around an FRC plasmoid (Field-reversed configuration) using magnetic field. This is necessary to achieve the conditions necessary to begin synthesis, such as high temperature and pressure. This approach to starting a reaction is a type of inertial fusion. To roughly understand how it works, you can take a look at Inertial Confinement Fusion (ICF). ICF synthesis is achieved using three-dimensional implosion of a spherical capsule with millimeter-sized cryogenic fuel. Implosion occurs due to the explosive evaporation of the capsule body, after it is heated using laser beams, electrons, or ions. The heated outer layer of the capsule explodes in outward direction, which produces a counterforce that accelerates the remainder of the capsule material inward, compressing it. Also, shock waves appear moving into the target. A sufficiently powerful set of shock waves can compress and heat the fuel in the center so much that a thermonuclear reaction begins. This method assumes that the inertia of a small capsule is sufficient to hold the plasma long enough for all the fuel to react and produce a useful output of G ~ 200 or more (G = fusion energy/plasma energy). The ICF approach has been pursued by the National Nuclear Security Administration (NNSA) for decades because it represents something like a miniature thermonuclear bomb. Due to its small size and weight, the capsule must be heated to the synthesis temperature within nanoseconds. It turned out that the most promising solution to this problem is an array of high-power pulsed lasers focused on a capsule with D-T fuel.I would like to note that when it comes to space flights, the main indicator is Δv - velocity increment (m/s or km/s). It is a measure of the amount of “effort” that is required to move from one trajectory to another when performing an orbital maneuver. For a spacecraft there are no such concepts as fuel reserve, maximum distance, or maximum speed, there is only Δv. The maximum Δv of a ship can be represented as the increase in speed that it will receive after using up all its fuel. It is important to know that a "mission" can be characterized according to what Δv is required to complete it. For example, ascent from Earth, Homan trajectory to Mars and landing on it requires a Δv budget of 18 km/s. If the ship has a reserve Δv greater than or equal to the mission Δv, then it can complete this mission.
In order to find out the Δv of the ship, you can use the Tsiolkovsky formula. 
Where:
V is the final (after all the fuel has been used up) speed of the aircraft (m/s);
I is the specific impulse of the rocket engine (the ratio of the engine thrust to the second fuel mass consumption, the speed of the working fluid flowing out of the nozzle, m/s);
M 1 - initial mass of the aircraft (payload + vehicle design + fuel, kg);
M 2 - final mass of the aircraft (payload + structure, kg).
A very important conclusion follows from this, which may not be very obvious at first glance. If the mission's Δv is less than or equal to the specific impulse, then the relative mass of the ship is large and it becomes possible to transport a larger payload. However, if the mission's Δv is greater than the specific impulse, the relative mass begins to decrease exponentially, making the ship a huge fuel tank with a tiny payload. Actually, it is precisely because of this that interplanetary flights using conventional chemical engines are very difficult.
Plan for a 210-day flight to Mars and back. 
90-day mission to Mars (ΔV = 13.5 km/s)
Target: best attitude payload to total weight.Advantages:
- No need for additional transport missions
- Simplified mission architecture
- Ability to bring all supplies during one mission
- Low mission cost
- Possibility to start a mission after a single launch from Earth
30-day mission to Mars (ΔV = 40.9 km/s)
Goal: fastest mission.Advantages:
- Low risk
- Minimal radiation exposure
- Apollo mission architecture
- The Key to Regularly Visiting Mars
- Developing the technologies needed to conquer deep space
NASA is currently developing the Space Launch System (SLS), a super-heavy launch vehicle capable of launching 70 to 130 tons of payload into a low reference orbit. This makes it possible to begin a 90-day mission to Mars after just one launch of such a launch vehicle.
Both missions have the ability to immediately cancel and return to Earth.
Key mission parameters
| Fuel Assumptions | |
|---|---|
| Costs for ionization of liner material | 75 MJ/kg |
| Efficiency of energy transfer to the liner (remaining energy is returned back to the capacitors) | 50% |
| Conversion efficiency to thrust η t | 90% |
| Liner weight (corresponds to gain from 50 to 500) | from 0.28 to 0.41 kg |
| Ignition factor | 5 |
| Safety margin (G F =G F(calc.) /2) | 2 |
| Mission Assumptions | |
| Mass of the Mars module (according to Design Reference Architecture 5.0) | 61 t |
| Habitable zone | 31 t |
| Return capsule | 16 t |
| Release system | 14 t |
| Relative weight of capacitors (this also includes the necessary wiring) | 1 J/g |
| Relative mass of solar panels | 200 W/kg |
| Structural factor (tanks, structure, radiators, etc.) | 10% |
| Full fuel braking, no aerobraking used | |
| Ship design | |
| Structure (fairings, power structures, communication channels, automated control systems, batteries) | 6.6 t |
| Lithium retention system | 0.1 t |
| Plasma creation and injection system | 0.2 t |
| Fuel supply mechanism | 1.2 t |
| Capacitor banks | 1.8 t |
| Liner Compression Coils | 0.3 t |
| Wiring and power electronics | 1.8 t |
| Solar panels (180 kW at 200 W/kg) | 1.5 t |
| Thermal control system | 1.3 t |
| Magnetic nozzle | 0.2 t |
| Ship mass | 15 t |
| Mass of the Mars module | 61 t |
| Lithium working fluid | 57 t |
| total weight | 133 t |
The pulse repetition rate, judging by the research plan, will be higher than 0.1 Hz. If we take into account that the specific impulse is 51400 m/s, and the mass of the working fluid is 0.37 kg per impulse, then we can calculate the impulse p = mv = 19018 kg m/s. According to the law of conservation of momentum, the speed of the ship will increase by p/M = 19018/133000 = 0.14 m/s. If we take the nozzle radius to be 1 m, then the expanding gases will press on it in the area t = r/v =1/51400 =0.00002 s. Therefore, the acceleration will be in the area a = dv/dt = 0.14/0.00002 = 7000 m/s 2 . It is obvious that either shock absorbers will be used, as in the Daedalus project, or some other technical solutions to smooth out the impulse.
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Uranium is the main element of nuclear energy, used as nuclear fuel, raw material for the production of plutonium and in nuclear weapons. The uranium content in the earth's crust is 2.5-10 -4%, and the total amount in a 20 km thick layer of the lithosphere reaches 1.3-10 14 tons. Uranium minerals are found almost everywhere. However, uranium is a trace element. This means that its concentration in rocks is often insufficient for commercially viable production. The uranium content in ore is one of the key parameters that determines the cost of production. Uranium ores containing 0.03-0.10% uranium are considered poor, ordinary - 0.10-0.25%, average - 0.25-0.5%, rich - over 0.50% 1.
Uranium has 14 isotopes, but only three of them occur in nature (Table 1.6).
Table 1.6
According to the latest data, the explored volume of uranium reserves, the cost of production of which does not exceed $130/kg U, is 5,327,200 tons. For the category with a production cost of less than $260/kg U - 7,096,600 tons. In addition, the amount of uranium in the so-called predicted and estimated reserves reach 10,429,100 tons.
Table 1.7
Countries with the largest proven reserves of uranium with a value not exceeding $130/kg U
IN last years The pattern of distribution of uranium deposits by country has changed somewhat due to the fact that during the study of a number of uranium deposits, additional resources were discovered in African countries (Botswana, Zambia, the Islamic Republic of Mauritania, Malawi, Mali, Namibia, the United Republic of Tanzania). Also, new reserves were discovered in Guyana, Colombia, Paraguay, Peru and Sweden.
The main minerals containing uranium are uraninite (a mixture of uranium and thorium oxides with the general formula (U, Th)0 2x), pitchblende (uranium oxides: U0 2, U0 3, also known as uranium pitch), carnotite - K, (U0 2)2 (V0 4) 2 -3H 2 0, uranophane - Ca (U0 2)Si0 3 (0H) 2 -5H 2 0 and others 110].
The extraction of uranium from rocks is carried out in the following ways:
- Quarry mining(open method) is used to extract ore that is located on the surface of the earth's crust or lies shallow. The method involves creating pits called quarries or cuts. To date, deposits that can be mined by open-pit mining have been practically exhausted. Production is 23%;
- Mine mining(closed method) is used for the extraction of minerals located at significant depths, and involves the construction of a complex of underground mine workings. Production - 32%;
- In-situ leaching involves pumping into the formation under pressure an aqueous solution of a chemical reagent, which, passing through the ore, selectively dissolves natural uranium compounds. The leach solution, containing uranium and associated metals, is then brought to the surface of the earth through extraction wells. Production - 39%.
- Joint mining with ores of other metals(uranium in this case is a by-product) - is 6%.
The production of dioxide fuel from uranium ore is a complex and expensive process, including the extraction of uranium from the ore, its concentration, purification (refining), conversion (production of uranium hexafluoride, enrichment, deconversion (UF translation) 6 b U0 2), production of fuel elements (fuel rods).
At the first stage of processing uranium ore mined by quarry and mine methods, it is crushed and sorted by radioactivity. After sorting, the ore pieces are further crushed and sent for leaching to convert the uranium into a soluble form. The choice of chemical solution for opening the ore depends on the type of mineral that includes uranium. In some cases, microbiological methods are used to open the ore.
As a result of leaching, a productive solution containing uranium is formed. During further processing of the productive solution by methods of ion exchange, extraction or precipitation, uranium is concentrated and undesirable impurities (Na, K, Ca, Mg, Fe, Mn, Ni, etc.) are separated. The resulting product is filtered, dried and heated to a high temperature, at which uranium oxide - yellow cake (U 3 0 8) is formed. To deeply purify uranium from impurities, refining is carried out, the traditional scheme of which is to dissolve U 3 0 8 in nitric acid and purify it by extraction (less commonly, precipitation). In this case, the final product of the refining technology is U 3 0 8 or uranium trioxide U0 3. The resulting oxide product is converted into a gaseous state - UF 6, which is most convenient for enrichment. This process is called conversion.
Crushed uranium ore (see Fig. 1.10) is supplied to a processing plant. The ore concentrate (natural uranium) is sent to the plant to produce uranium hexafluoride (UF 6).
Rice. 1.10.
Uranium from a radiochemical fuel regeneration plant is added to the cycle. Uranium hexafluoride is sent to a plant for the enrichment of natural and regenerated uranium to increase the content of the 235 U isotope. To separate uranium isotopes, special methods are required (gas diffusion and gas centrifuge), since the isotopes being separated are 23:> and 238 and represent one chemical element(i.e. cannot be divided chemical methods) and differ only in mass number (235 and 238 amu). These methods are extremely complex and require significant amounts of energy, time and special equipment. The gas diffusion method is based on the difference in the penetration rates of uranium-238 and uranium-235 hexafluorides through porous partitions (membranes). When gaseous uranium is passed through one membrane, the concentrations change by only 0.43%, i.e., the initial concentration is 2b and increases from 0.710 to 0.712%. To significantly enrich the mixture with 235 U, the separation process must be repeated many times. Thus, to obtain a mixture from natural uranium enriched to 2.4% no 235 U, and a concentration of 235 U in depleted uranium (waste) of 0.3%, about 840 steps are required. The cascade for producing highly enriched uranium (90% and above) must have 3000 stages.
The gas centrifuge method is more effective, in which hexafluorides of the isotopes of uranium-235 and 238 are introduced into a gas centrifuge, which rotates at a speed of 1500 revolutions per second. In this case, a significant centrifugal force arises, pushing uranium-238 towards the wall, and uranium-235 is concentrated in the area of the rotation axis. To achieve the required degree of enrichment, gas centrifuges are combined into cascades consisting of tens of thousands of devices.
To convert UF 6 after enrichment into uranium dioxide U O, “wet” (dissolution in water, precipitation and calcination) and “dry” (combustion of UF 6 in a hydrogen flame) methods are used. The resulting U0 2 powder is pressed into tablets and sintered at a temperature of approximately 1750° C.
After enrichment, the two streams—enriched uranium and depleted uranium—follow different paths. Depleted uranium is stored in a diffusion plant, and enriched uranium is converted into uranium dioxide (U0 2) and sent to the plant for the production of fuel rods.
At these plants, U0 2 intended for reactors is converted into fuel pellets. The tablets are heated and sintered to obtain a hard, dense consistency (Fig. 1.11). After processing, they are placed in tubes (shells) made of zirconium, plugs are welded at the ends, and the result is fuel element. A certain number of fuel rods are assembled together into a single structure - fuel assembly(TVS).
Rice. 1.11. Fuel pellets from U0 2
Finished fuel assemblies are delivered to nuclear power plants in special containers by rail, road or sea transport. In some cases, air transport is used.
Work is underway all over the world to improve the technical and economic characteristics of nuclear fuel. The most important requirement from the point of view of economic efficiency of nuclear fuel is to increase the burnup. For more complete use of uranium, the fuel must remain in the reactor core longer (see Table 1.8). To increase the fuel life, structural materials are being improved, which must work under longer and more severe operating conditions; fuel compositions (to reduce the yield of fission products); the rigidity of fuel assembly frames increases.
Table 1.8
Modern and promising VVER fuel cycles using enriched natural uranium
|
Status as of 2014 |
Near term |
||||||
|
Fuel |
Thermal power reactor, |
Fuel |
Thermal power reactor, |
||||
|
Ball NPP 1-3 |
|||||||
|
RosAES 1,2 |
|||||||
|
Kal NPP 1-4 |
TVSA-plus |
||||||
|
type TVS-2 M |
|||||||
|
type TVS-2 M |
|||||||
|
Bulgaria |
Kozloduy 5.6 |
||||||
|
Tianwan 1.2 |
|||||||
|
Tianwan 3.4 |
|||||||
|
Temelin 1,2 |
|||||||
|
Kadankulam 1 |
|||||||
|
Kadankulam 2 |
|||||||
|
ZaNPP, South Ukraine NPP, Khm NPP, RovNPP |
|||||||
1.4. Yader new fuel
For VVER-1000 type reactors, there are two main improved types of fuel assemblies (Fig. 1.12): TVSA (developed by OKBM named after I. I. Afrikantov) and TVS-2 M (developed by OKB Gidropress),
Rice. 1.12. Fuel assemblies for the VVER reactor: A- TVSA-PLUS, b- TVS-2 M
Fuel assemblies TVSA-PLUS and TVS-2 M have identical technical and economic characteristics, providing the ability to increase the power of the reactor plant up to 104% of the nominal, 18-month fuel cycle (make-up 66 units), fuel burnup - 72 MW day/kg U, possibility of operation in maneuverable mode, protection from foreign objects.
The increasing share of electricity generation at nuclear power plants in the energy balance and the transition to a liberal electricity market will require in the coming years the transfer of some nuclear power units to operation in a flexible mode. This operating mode, which has not been used before at nuclear power plants, also presents Additional requirements to fuels and fuel cycles. A fuel must be developed that maintains high performance characteristics under variable load conditions.
- According to the joint report of the IAEA and OECD “Uranium 2011: reserves, production and demand”.
Nuclear fuel is a material used in nuclear reactors to carry out a controlled chain reaction. It is extremely energy-intensive and unsafe for humans, which imposes a number of restrictions on its use. Today we will learn what nuclear reactor fuel is, how it is classified and produced, and where it is used.
Progress of the chain reaction
During a nuclear chain reaction, the nucleus splits into two parts, which are called fission fragments. At the same time, several (2-3) neutrons are released, which subsequently cause the fission of subsequent nuclei. The process occurs when a neutron hits the nucleus of the original substance. Fission fragments have high kinetic energy. Their inhibition in matter is accompanied by the release of a huge amount of heat.
Fission fragments, together with their decay products, are called fission products. Nuclei that share neutrons of any energy are called nuclear fuel. As a rule, they are substances with an odd number of atoms. Some nuclei are fissioned purely by neutrons whose energy is above a certain threshold value. These are predominantly elements with an even number of atoms. Such nuclei are called raw material, since at the moment of capture of a neutron by a threshold nucleus, fuel nuclei are formed. The combination of combustible material and raw material is called nuclear fuel.
Classification
Nuclear fuel is divided into two classes:
- Natural uranium. It contains fissile uranium-235 nuclei and uranium-238 feedstock, which is capable of forming plutonium-239 upon neutron capture.
- A secondary fuel not found in nature. This includes, among other things, plutonium-239, which is obtained from fuel of the first type, as well as uranium-233, which is formed when neutrons are captured by thorium-232 nuclei.
From the point of view of chemical composition, there are the following types of nuclear fuel:
- Metal (including alloys);
- Oxide (for example, UO 2);
- Carbide (for example PuC 1-x);
- Mixed;
- Nitride.
TVEL and TVS
Fuel for nuclear reactors is used in the form of small pellets. They are placed in hermetically sealed fuel elements (fuel elements), which, in turn, are combined into several hundred fuel assemblies (FA). Nuclear fuel is subject to high requirements for compatibility with fuel rod claddings. It must have a sufficient melting and evaporation temperature, good thermal conductivity, and not greatly increase in volume under neutron irradiation. The manufacturability of production is also taken into account.
Application
Fuel comes to nuclear power plants and other nuclear installations in the form of fuel assemblies. They can be loaded into the reactor both during its operation (in place of burnt-out fuel assemblies) and during a repair campaign. In the latter case, fuel assemblies are replaced in large groups. In this case, only a third of the fuel is completely replaced. The most burned-out assemblies are unloaded from the central part of the reactor, and in their place are placed partially burned-out assemblies that were previously located in less active areas. Consequently, new fuel assemblies are installed in place of the latter. This simple rearrangement scheme is considered traditional and has a number of advantages, the main one of which is ensuring uniform energy release. Of course, this is a conditional scheme that only gives general ideas about the process.
Excerpt
After spent nuclear fuel is removed from the reactor core, it is sent to a cooling pool, which is usually located nearby. The fact is that spent fuel assemblies contain a huge amount of uranium fission fragments. After unloading from the reactor, each fuel rod contains about 300 thousand Curies of radioactive substances, releasing 100 kW/hour of energy. Due to this, the fuel self-heats and becomes highly radioactive.
The temperature of newly unloaded fuel can reach 300°C. Therefore, it is kept for 3-4 years under a layer of water, the temperature of which is maintained in the established range. As it is stored under water, the radioactivity of the fuel and the power of its residual emissions decreases. After about three years, self-heating of the fuel assembly reaches 50-60°C. Then the fuel is removed from the pools and sent for processing or disposal.
Uranium metal
Uranium metal is used relatively rarely as fuel for nuclear reactors. When a substance reaches a temperature of 660°C, a phase transition occurs, accompanied by a change in its structure. Simply put, uranium increases in volume, which can lead to the destruction of fuel rods. In the case of prolonged irradiation at a temperature of 200-500°C, the substance undergoes radiation growth. The essence of this phenomenon is the elongation of the irradiated uranium rod by 2-3 times.
The use of uranium metal at temperatures above 500°C is difficult due to its swelling. After nuclear fission, two fragments are formed, the total volume of which exceeds the volume of that very nucleus. Some fission fragments are represented by gas atoms (xenon, krypton, etc.). Gas accumulates in the pores of the uranium and forms internal pressure, which increases as the temperature increases. Due to an increase in the volume of atoms and an increase in gas pressure, nuclear fuel begins to swell. Thus, this refers to the relative change in volume associated with nuclear fission.
The strength of swelling depends on the temperature of the fuel rods and burnout. With increasing burnup, the number of fission fragments increases, and with increasing temperature and burnup, the internal gas pressure increases. If the fuel has higher mechanical properties, then it is less susceptible to swelling. Uranium metal is not one of these materials. Therefore, its use as fuel for nuclear reactors limits the burnup, which is one of the main characteristics of such fuel.
The mechanical properties of uranium and its radiation resistance are improved by alloying the material. This process involves adding aluminum, molybdenum and other metals to it. Thanks to doping additives, the number of fission neutrons required per capture is reduced. Therefore, materials that weakly absorb neutrons are used for these purposes.
Refractory compounds
Some refractory uranium compounds are considered good nuclear fuel: carbides, oxides and intermetallic compounds. The most common of these is uranium dioxide (ceramics). Its melting point is 2800°C, and its density is 10.2 g/cm 3 .
Since this material does not undergo phase transitions, it is less susceptible to swelling than uranium alloys. Thanks to this feature, the burnout temperature can be increased by several percent. On high temperatures ceramics do not interact with niobium, zirconium, stainless steel and other materials. Its main disadvantage is its low thermal conductivity - 4.5 kJ (m*K), which limits the specific power of the reactor. In addition, hot ceramics are prone to cracking.
Plutonium
Plutonium is considered a low-melting metal. It melts at a temperature of 640°C. Due to its poor plastic properties, it is practically impossible to machine. The toxicity of the substance complicates the manufacturing technology of fuel rods. The nuclear industry has repeatedly attempted to use plutonium and its compounds, but they have not been successful. It is not advisable to use fuel for nuclear power plants containing plutonium due to an approximately 2-fold reduction in the acceleration period, which standard reactor control systems are not designed for.
For the manufacture of nuclear fuel, as a rule, plutonium dioxide, alloys of plutonium with minerals, and a mixture of plutonium carbides and uranium carbides are used. Dispersion fuels, in which particles of uranium and plutonium compounds are placed in a metal matrix of molybdenum, aluminum, stainless steel and other metals, have high mechanical properties and thermal conductivity. The radiation resistance and thermal conductivity of the dispersion fuel depend on the matrix material. For example, at the first nuclear power plant, the dispersed fuel consisted of particles of a uranium alloy with 9% molybdenum, which were filled with molybdenum.
As for thorium fuel, it is not used today due to difficulties in the production and processing of fuel rods.
Production
Significant volumes of the main raw material for nuclear fuel - uranium - are concentrated in several countries: Russia, the USA, France, Canada and South Africa. Its deposits are usually located near gold and copper, so all these materials are mined at the same time.
The health of people working in mining is at great risk. The fact is that uranium is a toxic material, and the gases released during its mining can cause cancer. And this despite the fact that the ore contains no more than 1% of this substance.
Receipt
The production of nuclear fuel from uranium ore includes the following stages:
- Hydrometallurgical processing. Includes leaching, crushing and extraction or sorption recovery. The result of hydrometallurgical processing is a purified suspension of oxyuranium oxide, sodium diuranate or ammonium diuranate.
- Conversion of a substance from oxide to tetrafluoride or hexafluoride, used to enrich uranium-235.
- Enrichment of a substance by centrifugation or gas thermal diffusion.
- Conversion of enriched material into dioxide, from which fuel rod “pellets” are produced.
Regeneration
During operation of a nuclear reactor, fuel cannot be completely burned out, so free isotopes are reproduced. In this regard, spent fuel rods are subject to regeneration for the purpose of reuse.
Today, this problem is solved through the Purex process, consisting of the following stages:
- Cutting fuel rods into two parts and dissolving them in nitric acid;
- Cleaning the solution from fission products and shell parts;
- Isolation of pure compounds of uranium and plutonium.
After this, the resulting plutonium dioxide is used for the production of new cores, and the uranium is used for enrichment or also for the production of cores. Reprocessing nuclear fuel is a complex and expensive process. Its cost has a significant impact on the economic feasibility of using nuclear power plants. The same can be said about the disposal of nuclear fuel waste that is not suitable for regeneration.
Due to the fact that nuclear fuel is more efficient than all other types of fuel that we have today, great preference is given to everything that can work with the help of nuclear plants (nuclear power plants, submarines, ships, etc.). We will talk further about how nuclear fuel for reactors is produced.
Uranium is mined in two main ways:
1) Direct mining in quarries or mines, if the depth of the uranium allows it. With this method, I hope everything is clear.
2) Underground leaching. This is when wells are drilled at the place where uranium is found, a weak solution of sulfuric acid is pumped into them, and the solution interacts with the uranium, combining with it. Then the resulting mixture is pumped up to the surface, and uranium is separated from it using chemical methods.
Let's imagine that we have already extracted uranium at the mine and prepared it for further transformations. The photo below shows the so-called “yellowcake”, U3O8. In a barrel for further transportation.
Everything would be fine, and in theory this uranium could be immediately used to produce fuel for nuclear power plants, but alas. Nature, as always, gave us work to do. The fact is that natural uranium consists of a mixture of three isotopes. These are U238 (99.2745%), U235 (0.72%) and U234 (0.0055%). We are only interested in U235 here - since it perfectly shares thermal neutrons in the reactor, it is it that allows us to enjoy all the benefits of the fission chain reaction. Unfortunately, its natural concentration is not enough for stable and long-term operation of a modern nuclear power plant reactor. Although, as far as I know, the RBMK apparatus is designed in such a way that it can launch on fuel made from natural uranium, but the stability, longevity and safety of operation on such fuel is not guaranteed at all.
We need to enrich uranium. That is, increase the concentration of U235 from natural to that used in the reactor.
For example, the RBMK reactor operates on 2.8% enriched uranium, while the VVER-1000 reactor operates on 1.6 to 5.0% enriched uranium. Marine and naval nuclear power plants consume fuel enriched up to 20%. And some research reactors operate on fuel with 90% enrichment (for example, IRT-T in Tomsk).
In Russia, uranium enrichment is carried out using gas centrifuges. That is, that yellow powder that was in the photo earlier is converted into gas, uranium hexafluoride UF6. This gas is then fed to a cascade of centrifuges. At the exit from each centrifuge, due to the difference in weight of the U235 and U238 nuclei, we obtain uranium hexafluoride with a slightly increased content of U235. The process is repeated many times and in the end we obtain uranium hexafluoride with the enrichment we need. In the photo below you can just see the scale of the cascade of centrifuges - there are a lot of them and they extend into distant distances.
The UF6 gas is then converted back to UO2, in powder form. Chemistry, after all, is a very useful science and allows us to create such miracles.
However, this powder cannot be easily poured into the reactor. Or rather, you can fall asleep, but nothing good will come of it. It (the powder) must be brought to such a form that we can lower it into the reactor for a long time, for years. In this case, the fuel itself should not come into contact with the coolant and go beyond the core. And on top of all this, the fuel must withstand the very, very severe pressures and temperatures that will arise in it when working inside the reactor.
By the way, I forgot to say that the powder is also not just any kind - it must be of a certain size so that during pressing and sintering unnecessary voids and cracks do not form. First, tablets are made from the powder by pressing and baking for a long time (the technology is really not easy, if it is violated, the fuel tablets will not be usable). I will show the variations of the tablets in the photo below.
Holes and notches on the tablets are needed to compensate for thermal expansion and radiation changes. In the reactor, over time, the tablets swell, bend, change sizes, and if nothing is provided for, they can collapse, and this is bad.
The finished tablets are then packaged in metal tubes (made of steel, zirconium and its alloys and other metals). The tubes are closed at both ends and sealed. The finished tube with fuel is called a fuel element - a fuel element.
Different reactors require fuel elements of different designs and enrichments. RBMK fuel rods, for example, are 3.5 meters long. Fuel elements, by the way, are not only rod ones. as in the photo. They are plate, ring, sea various types and modifications.
The fuel elements are then combined into fuel assemblies - FAs. The fuel assembly of the RBMK reactor consists of 18 fuel rods and looks something like this:
The fuel assembly of a VVER reactor looks like this:
As can be seen, the fuel assembly of a VVER reactor consists of much more fuel rods than the RBMK.
The finished special product (FA) is then delivered to the nuclear power plant in compliance with safety precautions. Why precautions? Nuclear fuel, although not yet radioactive, is very valuable, expensive, and if handled very carelessly can cause many problems. Then the final control of the condition of the fuel assembly is carried out and loading into the reactor. That's it, uranium has come a long way from ore underground to a high-tech device inside a nuclear reactor. Now he has a different fate - to strain inside the reactor for several years and release precious heat, which water (or any other coolant) will take from him.
Nuclear energy consists of large quantity enterprises for various purposes. The raw materials for this industry are mined from uranium mines. It is then delivered to fuel production plants.
The fuel is then transported to nuclear power plants, where it enters the reactor core. When nuclear fuel reaches the end of its useful life, it is subject to disposal. It is worth noting that hazardous waste appears not only after fuel reprocessing, but also at any stage - from uranium mining to work in the reactor.
Nuclear fuel
There are two types of fuel. The first is uranium mined in mines, which is of natural origin. It contains raw materials that are capable of forming plutonium. The second is fuel that is created artificially (secondary).
Nuclear fuel is also divided according to chemical composition: metal, oxide, carbide, nitride and mixed.
Uranium mining and fuel production
A large share of uranium production comes from just a few countries: Russia, France, Australia, the USA, Canada and South Africa.
Uranium is the main element for fuel in nuclear power plants. To get into the reactor, it goes through several stages of processing. Most often, uranium deposits are located next to gold and copper, so its extraction is carried out with the extraction of precious metals.
During mining, human health is at great risk because uranium is a toxic material, and the gases that appear during its mining cause various forms of cancer. Although the ore itself contains a very small amount of uranium - from 0.1 to 1 percent. The population living near uranium mines is also at great risk.
Enriched uranium is the main fuel for nuclear power plants, but after its use a huge amount of radioactive waste remains. Despite all its dangers, uranium enrichment is an integral process of creating nuclear fuel.
In its natural form, uranium practically cannot be used anywhere. In order to be used, it must be enriched. Gas centrifuges are used for enrichment.
Enriched uranium is used not only in nuclear energy, but also in the production of weapons.
Transportation
At any stage of the fuel cycle there is transportation. It is carried out by all available means: by land, sea, air. This is a big risk and a big danger not only for the environment, but also for humans.
During the transportation of nuclear fuel or its elements, many accidents occur, resulting in the release of radioactive elements. This is one of the many reasons why it is considered unsafe.
Decommissioning of reactors
None of the reactors have been dismantled. Even the infamous Chernobyl The whole point is that, according to experts, the cost of dismantling is equal to, or even exceeds, the cost of building a new reactor. But no one can say exactly how much money will be needed: the cost was calculated based on the experience of dismantling small stations for research. Experts offer two options:
- Place reactors and spent nuclear fuel in repositories.
- Build sarcophagi over decommissioned reactors.
In the next ten years, about 350 reactors around the world will reach their end of life and must be taken out of service. But since the most suitable method in terms of safety and price has not been invented, this issue is still being resolved.
There are currently 436 reactors operating around the world. Of course, this is a big contribution to the energy system, but it is very unsafe. Research shows that in 15-20 years, nuclear power plants will be able to be replaced by stations that run on wind energy and solar panels.
Nuclear waste
A huge amount of nuclear waste is generated as a result of the activities of nuclear power plants. Reprocessing nuclear fuel also leaves behind hazardous waste. However, none of the countries found a solution to the problem.
Today, nuclear waste is kept in temporary storage facilities, in pools of water, or buried shallowly underground.
The safest method is storage in special storage facilities, but radiation leakage is also possible here, as with other methods.
In fact, nuclear waste has some value, but requires strict compliance with the rules for its storage. And this is the most pressing problem.
An important factor is the time during which the waste is hazardous. Each has its own decay period during which it is toxic.
Types of nuclear waste
During the operation of any nuclear power plant, its waste enters the environment. This is water for cooling turbines and gaseous waste.
Nuclear waste is divided into three categories:
- Low level - clothing of nuclear power plant employees, laboratory equipment. Such waste can also come from medical institutions and scientific laboratories. They do not pose a great danger, but require compliance with safety measures.
- Intermediate level - metal containers in which fuel is transported. Their radiation level is quite high, and those who are close to them must be protected.
- The high level is spent nuclear fuel and its reprocessing products. The level of radioactivity is rapidly decreasing. Waste high level very little, about 3 percent, but they contain 95 percent of all radioactivity.