: ... quite banal, but nevertheless I never found the information in a digestible form - how a nuclear reactor BEGINS to work. Everything about the principle and operation of the device has already been chewed and understood 300 times, but here's how the fuel is obtained and from what, and why it is not so dangerous until it is in the reactor and why it does not react before being immersed in the reactor! - after all, it warms up only inside, nevertheless, before loading the fuel rods are cold and everything is fine, so what causes the elements to heat up is not entirely clear how they are affected, and so on, preferably not scientifically).
Of course, it is difficult to arrange such a topic not “according to science”, but I will try. Let's first understand what these very TVELs are.
Nuclear fuel is black tablets with a diameter of about 1 cm and a height of about 1.5 cm. They contain 2% uranium dioxide 235, and 98% uranium 238, 236, 239. In all cases, with any amount of nuclear fuel, a nuclear explosion cannot develop , because for an avalanche-like rapid fission reaction characteristic of a nuclear explosion, a concentration of uranium 235 of more than 60% is required.
Two hundred nuclear fuel pellets are loaded into a tube made of zirconium metal. The length of this tube is 3.5m. diameter 1.35 cm. This tube is called TVEL - fuel element. 36 TVELs are assembled into a cassette (another name is "assembly").
The device of the fuel element of the RBMK reactor: 1 - plug; 2 - tablets of uranium dioxide; 3 - zirconium shell; 4 - spring; 5 - bushing; 6 - tip.
The transformation of a substance is accompanied by the release of free energy only if the substance has a reserve of energies. The latter means that the microparticles of the substance are in a state with a rest energy greater than in another possible state, the transition to which exists. Spontaneous transition is always hindered by an energy barrier, to overcome which the microparticle must receive some amount of energy from the outside - the energy of excitation. The exoenergetic reaction consists in the fact that in the transformation following the excitation, more energy is released than is required to excite the process. There are two ways to overcome the energy barrier: either due to the kinetic energy of the colliding particles, or due to the binding energy of the acceding particle.
If we keep in mind the macroscopic scales of the energy release, then the kinetic energy necessary for the excitation of reactions must have all or at first at least some of the particles of the substance. This can only be achieved by increasing the temperature of the medium to a value at which the energy of thermal motion approaches the value of the energy threshold that limits the course of the process. In the case of molecular transformations, that is, chemical reactions, such an increase is usually hundreds of degrees Kelvin, while in the case of nuclear reactions it is at least 107 K due to the very high height of the Coulomb barriers of colliding nuclei. Thermal excitation of nuclear reactions has been carried out in practice only in the synthesis of the lightest nuclei, in which the Coulomb barriers are minimal (thermonuclear fusion).
Excitation by the joining particles does not require a large kinetic energy, and, therefore, does not depend on the temperature of the medium, since it occurs due to unused bonds inherent in the particles of attractive forces. But on the other hand, the particles themselves are necessary to excite the reactions. And if again we have in mind not a separate act of reaction, but the production of energy on a macroscopic scale, then this is possible only when a chain reaction occurs. The latter arises when the particles that excite the reaction reappear as products of an exoenergetic reaction.
For control and protection nuclear reactor control rods are used, which can be moved along the entire height of the core. The rods are made from substances that strongly absorb neutrons, such as boron or cadmium. With the deep introduction of the rods, the chain reaction becomes impossible, since the neutrons are strongly absorbed and removed from the reaction zone.
The rods are moved remotely from the control panel. With a small movement of the rods, the chain process will either develop or decay. In this way, the power of the reactor is regulated.
Leningrad NPP, RBMK reactor
Reactor start:
At the initial moment of time after the first loading with fuel, there is no fission chain reaction in the reactor, the reactor is in a subcritical state. The coolant temperature is much lower than the operating temperature.
As we have already mentioned here, in order to start a chain reaction, the fissile material must form a critical mass - a sufficient amount of spontaneously fissile material in a sufficiently small space, the condition under which the number of neutrons released during nuclear fission must be greater than the number of absorbed neutrons. This can be done by increasing the content of uranium-235 (the number of loaded fuel elements), or by slowing down the speed of neutrons so that they do not fly past the uranium-235 nuclei.
The reactor is brought to power in several stages. With the help of the reactivity regulators, the reactor is transferred to the supercritical state Kef>1 and the reactor power increases to a level of 1-2% of the nominal. At this stage, the reactor is heated up to the operating parameters of the coolant, and the heating rate is limited. During the warm-up process, the controls keep the power at a constant level. Then the circulation pumps are started and the heat removal system is put into operation. After that, the reactor power can be increased to any level in the range from 2 to 100% of the rated power.
When the reactor is heated, the reactivity changes due to changes in the temperature and density of the core materials. Sometimes, during heating, the mutual position of the core and the control elements that enter the core or leave it changes, causing a reactivity effect in the absence of active movement of the control elements.
Control by solid, moving absorber elements
In the overwhelming majority of cases, solid mobile absorbers are used to quickly change the reactivity. In the RBMK reactor, the control rods contain bushings made of boron carbide enclosed in an aluminum alloy tube with a diameter of 50 or 70 mm. Each control rod is placed in a separate channel and cooled with water from the CPS circuit (control and protection system) at an average temperature of 50 ° C. According to their purpose, the rods are divided into rods AZ (emergency protection), in RBMK there are 24 such rods. Automatic control rods - 12 pieces, Local automatic control rods - 12 pieces, manual control rods -131, and 32 shortened absorber rods (USP). There are 211 rods in total. Moreover, shortened rods are introduced into the AZ from the bottom, the rest from the top.
VVER 1000 reactor. 1 - CPS drive; 2 - reactor cover; 3 - reactor vessel; 4 - block of protective pipes (BZT); 5 - mine; 6 - core baffle; 7 - fuel assemblies (FA) and control rods;
Burn-out absorbing elements.
Burnable poisons are often used to compensate for excess reactivity after fresh fuel has been loaded. The principle of operation of which is that they, like fuel, after the capture of a neutron, subsequently cease to absorb neutrons (burn out). Moreover, the rate of decline as a result of the absorption of neutrons, absorber nuclei, is less than or equal to the rate of loss, as a result of fission, of fuel nuclei. If we load into the reactor core fuel designed for operation during the year, then it is obvious that the number of fissile fuel nuclei at the beginning of work will be greater than at the end, and we must compensate for the excess reactivity by placing absorbers in the core. If control rods are used for this purpose, then we must constantly move them as the number of fuel nuclei decreases. The use of burnable poisons makes it possible to reduce the use of moving rods. At present, burnable poisons are often incorporated directly into fuel pellets during their manufacture.
Liquid regulation of reactivity.
Such regulation is used, in particular, during the operation of a VVER-type reactor, boric acid H3BO3 containing 10B nuclei absorbing neutrons is introduced into the coolant. By changing the concentration of boric acid in the coolant path, we thereby change the reactivity in the core. In the initial period of the reactor operation, when there are many fuel nuclei, the acid concentration is maximum. As the fuel burns out, the acid concentration decreases.
chain reaction mechanism
A nuclear reactor can operate at a given power for a long time only if it has a reactivity margin at the beginning of operation. The exception is subcritical reactors with an external source of thermal neutrons. The release of bound reactivity as it decreases due to natural causes ensures that the critical state of the reactor is maintained at every moment of its operation. The initial reactivity margin is created by building a core with dimensions that are much larger than the critical ones. To prevent the reactor from becoming supercritical, k0 of the breeding medium is artificially reduced at the same time. This is achieved by introducing neutron absorbers into the core, which can be subsequently removed from the core. As in the elements of chain reaction control, absorbent substances are included in the material of rods of one or another cross-section, moving along the corresponding channels in the core. But if one, two or several rods are sufficient for regulation, then the number of rods can reach hundreds to compensate for the initial excess of reactivity. These rods are called compensating. Regulating and compensating rods are not necessarily different structural elements. A number of compensating rods can be control rods, but the functions of both are different. The control rods are designed to maintain a critical state at any time, to stop, start the reactor, switch from one power level to another. All these operations require small changes in reactivity. Compensating rods are gradually withdrawn from the reactor core, ensuring a critical state during the entire time of its operation.
Sometimes control rods are made not from absorbent materials, but from fissile or scatter material. In thermal reactors, these are mainly neutron absorbers, while there are no effective fast neutron absorbers. Such absorbers as cadmium, hafnium and others strongly absorb only thermal neutrons due to the proximity of the first resonance to the thermal region, and outside the latter they do not differ from other substances in their absorbing properties. An exception is boron, whose neutron absorption cross section decreases with energy much more slowly than that of the indicated substances, according to the l / v law. Therefore, boron absorbs fast neutrons, although weakly, but somewhat better than other substances. Only boron, if possible enriched in the 10B isotope, can serve as an absorbent material in a fast neutron reactor. In addition to boron, fissile materials are also used for control rods in fast neutron reactors. A compensating rod made of fissile material performs the same function as a neutron absorber rod: it increases the reactivity of the reactor with its natural decrease. However, unlike an absorber, such a rod is located outside the core at the beginning of the reactor operation, and then it is introduced into the core.
Of the scatterer materials in fast reactors, nickel is used, which has a scattering cross section for fast neutrons somewhat larger than the cross sections for other substances. Scatterer rods are located along the periphery of the core and their immersion in the corresponding channel causes a decrease in neutron leakage from the core and, consequently, an increase in reactivity. In some special cases, the purpose of controlling a chain reaction is the moving parts of the neutron reflectors, which, when moving, change the leakage of neutrons from the core. The control, compensating and emergency rods, together with all the equipment that ensures their normal functioning, form the reactor control and protection system (CPS).
Emergency protection:
Nuclear reactor emergency protection - a set of devices designed to quickly stop a nuclear chain reaction in the reactor core.
Active emergency protection is automatically triggered when one of the parameters of a nuclear reactor reaches a value that can lead to an accident. Such parameters can be: temperature, pressure and flow rate of the coolant, level and rate of power increase.
The executive elements of emergency protection are, in most cases, rods with a substance that absorbs neutrons well (boron or cadmium). Sometimes a liquid scavenger is injected into the coolant loop to shut down the reactor.
In addition to active protection, many modern designs also include elements of passive protection. For example, modern versions of VVER reactors include the "Emergency Core Cooling System" (ECCS) - special tanks with boric acid located above the reactor. In the event of a maximum design basis accident (rupture of the primary cooling circuit of the reactor), the contents of these tanks are by gravity inside the reactor core and the nuclear chain reaction is quenched by a large amount of a boron-containing substance that absorbs neutrons well.
According to the "Nuclear Safety Rules for Reactor Installations of Nuclear Power Plants", at least one of the provided reactor shutdown systems must perform the function of emergency protection (EP). Emergency protection must have at least two independent groups of working bodies. At the signal of the AZ, the working bodies of the AZ must be actuated from any working or intermediate positions.
The AZ equipment must consist of at least two independent sets.
Each set of AZ equipment must be designed in such a way that, in the range of neutron flux density changes from 7% to 120% of the nominal value, protection is provided for:
1. According to the density of the neutron flux - at least three independent channels;
2. According to the rate of increase in the neutron flux density - by at least three independent channels.
Each set of AZ equipment must be designed in such a way that, in the entire range of process parameter changes established in the reactor plant (RP) design, emergency protection is provided by at least three independent channels for each process parameter for which protection is necessary.
The control commands of each set for AZ actuators must be transmitted over at least two channels. When one channel is taken out of operation in one of the AZ equipment sets without this set being taken out of operation, an alarm signal should be automatically generated for this channel.
Tripping of emergency protection should occur at least in the following cases:
1. Upon reaching the AZ setpoint in terms of neutron flux density.
2. Upon reaching the AZ setpoint in terms of the rate of increase in the neutron flux density.
3. In the event of a power failure in any set of AZ equipment and CPS power supply buses that have not been taken out of operation.
4. In case of failure of any two of the three protection channels in terms of the neutron flux density or in terms of the rate of neutron flux increase in any set of AZ equipment that has not been decommissioned.
5. When the AZ settings are reached by the technological parameters, according to which it is necessary to carry out protection.
6. When initiating the operation of the AZ from the key from the block control point (BCR) or the backup control point (RCP).
Maybe someone will be able to explain briefly even less scientifically how the power unit of a nuclear power plant starts working? :-)
Recall a topic like The original article is on the website InfoGlaz.rf Link to the article from which this copy is made -
The invention relates to the field of atomic energy and can be used for the manufacture of fuel rods for power reactors. The technical objective of the present invention is to create a fuel rod design in which plutonium or highly enriched uranium in the form of alloys or dioxides can be used without diluting them with depleted or natural uranium or thorium while providing the required load, the ratio of fissile and fertile nuclides, increasing the resource and increasing the reliability of operation, including in emergency situations. In a fuel element, a part of the core with a mass fraction of fissile nuclides from 200 to 100% is enclosed in one or more sealed ampoules of various geometric shapes, made of the same or different structural material as the fuel element cladding. The ampoules have a free volume to compensate for the swelling of nuclear fuel and to collect gaseous fission fragments. The rest of the fuel core contains nuclear fuel with a mass fraction of fissile nuclides from 0.715% and fertile nuclides from 0.01 to 100%. 5 z.p.f-ly, 4 ill.
The invention relates to nuclear engineering and can be used in the manufacture of fuel elements (fuel rods) with nuclear fuel from plutonium or highly enriched uranium for thermal neutron power reactors. Thermal and fast neutron reactors operate in the world nuclear power industry, however, 85% of the electricity of all nuclear power plants is generated in light water thermal neutron reactors, most of which use container-type fuel rods. Such fuel elements are a cylindrical metal shell with a diameter of 7 - 15 mm with end caps, inside which is placed a core in the form of tablets or vibro-compacted granules of uranium dioxide or a mixture of uranium and plutonium dioxides, while, as a rule, the mass fraction of fissile nuclides uranium-235, plutonium-239 and plutonium-241 is less than 6% of the total content of uranium and plutonium in nuclear fuel. The fuel elements have a free volume to compensate for volumetric changes in nuclear fuel and to collect gaseous fission fragments. To reduce the temperature level of fuel rod cores, holes are sometimes made in tablets, free volumes are filled with helium or low-melting materials, such as sodium, sodium-potassium alloy, lead-bismuth alloy, etc. /1/. In addition to container-type fuel rods, in nuclear power reactors, and, even in more , in research reactors, dispersion-type fuel elements are used, characterized in that their core consists of nuclear fuel particles uniformly distributed in an inert matrix. Such a structure of the fuel rod core localizes fission fragments in the particles of nuclear fuel and the thin layers of the matrix adjacent to them, therefore, there is no free volume in the fuel rods for collecting gaseous fission fragments /2/. Container-type fuel rods are easy to manufacture and operate reliably at stationary reactor power levels during a 2-, 3-, and less frequently 4-year campaign at a high conversion factor of new nuclear fuel (up to 0.5). The energy production of such fuel rods is limited by volumetric changes in nuclear fuel from accumulated fission fragments, mass transfer of nuclear fuel from a hot (up to 2000 o C) to a cold zone (about 300 o C), corrosive effect of aggressive fission fragments on the cladding, and maneuvering the reactor power - by thermomechanical stresses in shell and core associated with the difference in their temperature levels and coefficients of thermal expansion of materials. In addition, the high temperature level of the fuel rod core, the thermal energy accumulated in it, and the residual heat release in emergency situations can lead to burning through the cladding. Regardless of the reason for the depressurization of the fuel rod, accidental, exhaustion of the fuel rod resource or an emergency situation, fission fragments released from nuclear fuel enter the coolant, while its radioactivity may exceed the maximum allowable values. For dispersion fuel elements, with good thermal conductivity of the matrix, which ensures reliable thermal contact between nuclear fuel and cladding, the temperature level of the fuel core is significantly reduced, for example, the temperature drop in the core with an aluminum alloy matrix in the fuel element of the VVER-1000 reactor can be reduced by about one and a half orders of magnitude ( from 1500 o C to 100 o C). This makes it possible to successfully operate fuel rods in maneuvering modes, make them less safe in emergency situations, and, in the event of a fuel rod depressurization, reduce the degree of coolant contamination, since it will come into contact with nuclear fuel only at the site of the defect. In addition, at low temperatures, nuclear fuel is less subject to volumetric changes from accumulated fission fragments and it becomes possible to use other types of nuclear fuel, for example, uranium silicide, an alloy of uranium with molybdenum, etc. However, a lower concentration in the core of a dispersive fuel rod of a nuclear fuel requires an increase in the mass fraction of fissile nuclide, which accordingly reduces the conversion factor of the new nuclear fuel. The power output of dispersive fuel elements is limited by the allowable increase in the diameter of the fuel element or the allowable deformation of the cladding material. As a result of the orientation of the world nuclear power industry towards light water reactors with container-type fuel elements and dioxide fuel, several hundred tons of plutonium have accumulated, which has a polyisotope composition with mass numbers of 238, 239, 240, 241 and 242. The problem of storing plutonium and its further use. The most effective use of plutonium as a nuclear fuel is in fast neutron reactors, but their number in the world is limited, and the program for building new reactors has been delayed for several decades. To the problem of using polyisotopic plutonium was added the problem of the prompt destruction of the released uranium and plutonium as a result of disarmament. The most common solution for using plutonium in thermal reactors is to dilute it with depleted or natural uranium, since for thermal reactors the mass fraction of plutonium should be about 5%. Such fuel is called uranium-plutonium or mixed fuel. It should be noted that only odd isotopes of plutonium are fissile in thermal neutron reactors. The plutonium-241 isotope, whose concentration in polyisotopic plutonium reaches 14% wt., has a half-life of about 14 years, forming americium 241 with hard gamma radiation, which complicates work with polyisotopic plutonium when it long-term storage. In addition, there are losses of power-grade plutonium (about 9% over 10 years). Unlike polyisotopic plutonium, weapons grade plutonium mainly contains the 239 isotope and can be considered monoisotopic. The main difficulty in the manufacture of mixed dioxide nuclear fuel is the creation of a homogeneous mixture of plutonium and uranium dioxides, from which pellets are pressed. The possibility and expediency of using mixed microspherical dioxide fuel either directly for the manufacture of fuel elements with a vibro-compacted core, or for the manufacture of pellets from them, is also being considered. The advantage of using microspheres over powders is a more convenient form for handling at all stages of the technological process and significantly less dust generation, which ensures safer work for operators. The technology for manufacturing pellets from powders containing about 5% plutonium dioxide, equipping fuel rods with pellets or microspheres from mixed dioxide fuel, and fuel rod designs are similar to those used for uranium fuel. However, there fundamental difference in the organization of the production itself for the manufacture of fuel elements with mixed dioxide nuclear fuel, especially when using polyisotopic plutonium. To create a normal radiation environment in industrial premises all equipment must be placed in securely sealed chambers, and all technological process should be as automated as possible, including control operations. All this leads to an increase in the cost of manufacturing fuel elements. Closest to the claimed design of the fuel element is the design of the fuel element of the container type. The fuel element consists of a cylindrical shell and end caps made of a zirconium-based alloy, inside which is placed a core in the form of sintered pellets of uranium dioxide or mixed fuel with a content of fissile isotopes of about 5% wt. and free volume to compensate for its swelling and collection of gaseous fission fragments. To improve the transfer of heat from nuclear fuel to the shell, the internal free volume is filled with helium /1, p. 45/. The disadvantage of such a fuel element with mixed fuel is the rise in the cost of manufacturing a fuel element by 4-5 times compared to a fuel element with uranium fuel, associated with ensuring the homogeneity of the mixture of dioxides and pressing pellets while observing the requirements for radiation safety and sanitation rules. It should also be noted that 20 times more plutonium-containing materials have to be processed to prepare a mixture with 5% plutonium dioxide. The main technical objective of the present invention is the creation of a fuel element design for thermal neutron power reactors, in which poly - or monoisotopic plutonium or uranium with a mass fraction of fissile nuclides up to 100% could be used as nuclear fuel. Unlike the well-known design of a container-type fuel element, the core of which consists of a homogeneous mixture of uranium and plutonium dioxide, the solution of the set technical problem is achieved by concluding a part of the fuel element core with a mass fraction of fissile nuclides from 20 to 100% in one or more sealed ampoules of various geometric shapes, made from the same or different structural material with the fuel rod cladding. The ampoules have a free volume to compensate for the swelling of the nuclear fuel of the ampoule core and to collect gaseous fission fragments. The rest of the fuel core contains nuclear fuel with a mass fraction of fissile nuclides up to 0.715% and fertile nuclides from 0.01 to 100%. To ensure heat removal from the ampoules and nuclear fuel of the fuel rod core, the voids formed by the ampoules and nuclear fuel inside the fuel rod cladding are filled with contact material. The technical result achieved by the claimed invention is that, in addition to reducing the complexity and volume of processed plutonium-containing materials, the introduction of ampoules into the fuel rod core, inside which more than 70% of fission fragments are concentrated, and a contact material that reduces the temperature level of the fuel rod core, ensures reliable operation. fuel rod in maneuverable modes of operation of the reactor, creates an additional two stages of protection for the main source of radioactivity in case of depressurization of the fuel rod, which makes the fuel rod less dangerous in emergency situations. The proposed design of the fuel element makes it possible to increase its energy production, since the rate and magnitude of volumetric changes in the part of the fuel element core with fertile nuclides will be significantly reduced compared to the fuel element core of the old design from mixed fuel, since the volumetric changes in the cores of the ampoules, in which the main part of fission fragments accumulate, are compensated in ampoules, in addition, the fuel rod core has a significantly lower operating temperature. The proposed technical solution makes it possible to vary the designs and materials of ampoules, the materials and shape of nuclear fuel of the cores of ampoules and fuel rods, the ratio of the number of fissile and reproducing nuclides, the use of the same or different contact materials in the cores of ampoules and fuel rods, the use, if necessary, in the cores of ampoules and fuel rods and in the structural material of ampoules of burnable absorbers, using getters in ampoules. In the fuel cores of ampoules, it is most expedient to use nuclear fuel in the form of particles of arbitrary (grain) or repeating (granules) form of plutonium dioxide or in the form of wire, ribbons or granules from plutonium-gallium alloys using monoisotopic plutonium, and in the fuel rod core - chemical compounds or alloys of uranium or thorium, for example, dioxides, silicides, nitrides, an alloy of uranium with 9% molybdenum, etc., while the geometric shape and dimensions of the nuclear fuel in the cores of the ampoules and the core of the fuel rod can be the same, for example, grits-grains, granules- granules or different, for example, semolina-granules, granules-blocks, etc. Structurally, ampoules can be made in the form of balls, disks, rings, polyhedral or shaped plates, straight, twisted about the longitudinal axis or wound in the form of various spirals of tapes or rods with a round, oval, triangular, square, rectangular, polyhedral, three- or multi-lobed or other cross section, including those with self-spacer ribs in the fuel core. The length of the fuel core of the ampoules can correspond to or be a multiple of the length of the fuel rod core. The compensatory volume of the ampoules can be wholly located in the core of the fuel element or partially moved outside it with the same or modified geometry of the ampule. In addition, a getter can be placed in the compensation volume. If it is necessary to load uneven loading of fissile isotopes along the length of the fuel rod core, it can be provided by the number and spacing of ampoules, by loading nuclear fuel into ampoules with a core length that is a multiple of the length of the fuel rod core and a variable cross section, a pitch of twisting or winding a spiral with a length of ampoule cores corresponding to the length fuel rod core. As contact materials in the core of the fuel element and the cores of the ampoules, materials that are under the operating conditions of the fuel element in a solid state, for example, magnesium, aluminum alloys, etc., or in a liquid state (an alloy of lead with bismuth, sodium, etc.) can be used, and in any combination of states (liquid-liquid, solid-liquid, solid-solid, liquid-solid) and chemical compositions. The material of the shell of the fuel rod and the ampoule can be the same, for example, zirconium alloy E-110 - zirconium alloy E-110, stainless steel EI-847 - stainless steel EI-847 or different, for example, zirconium alloy E-110 - stainless steel EI-847 , zirconium alloy E-110 - zirconium alloy E-125, stainless steel EI-844BU-ID stainless steel EI-852 and others. particles of a burnable absorber with particles of nuclear fuel from a fuel rod and ampoules, and/or into the structural material of ampoules, while they are the same or different in chemical composition and/or concentration of the absorbing isotope. For example, in the core of a fuel rod there is gadolinium oxide in the composition of nuclear fuel particles, in the ampoule core - gadolinium oxide in the form of particles mixed with nuclear fuel particles, in the ampoule material - boron in a zirconium alloy. Comparative analysis of the proposed technical solution with the known allows you to establish the compliance of the proposed technical solution with the requirements for inventions. The invention is illustrated by drawings. Figure 1 shows a fuel rod with three cylindrical ampoules having cores with a length corresponding to the length of the fuel rod core, the contact material in the fuel rod core, which is in the solid state under operating conditions of the fuel rod. In FIG. 2 shows a fuel rod with cylindrical ampoules with cores having a length that is a multiple of the length of the fuel rod core, and contact materials of the ampule and fuel rod cores that are in a liquid state under operating conditions. Figure 3 shows a fuel rod with one ampoule in the form of a twisted tape with a core length corresponding to the length of the fuel rod core, with a gas collector placed outside the fuel core of the fuel rod. In FIG. 4 shows a fuel rod with one ampoule in the form of a profile tape, twisted into a cylindrical spiral, with a core length corresponding to the length of the fuel rod core, a gas collector placed outside the fuel rod core. The design of the fuel rod (see figure 1) is a shell (1), sealed at the ends with plugs (2), inside which there is a core (3), consisting of a vibro-compacted mixture of nuclear fuel pellets containing fertile nuclides (4), and burnable pellets absorber (5), in the gaps between which there is a contact material (6), which is in the solid state under operating conditions of the fuel rod. In the core of the fuel rod through 120 o are three cylindrical ampoules (7). Between the ampoules and the shell there is a gap of at least 0.1 of the diameter of the ampoules, and the minimum diameter of the granules is at least 1.2 times the gap. The ampoule is a cylindrical thin-walled tube (8), sealed at the ends with plugs (9), inside which there is a core (10) consisting of a vibro-compacted mixture of porous nuclear fuel granules containing divisible nuclides (11) and a getter (12). The maximum size of the granules is not more than 0.3 of the inner diameter of the ampoule. The compensation volume in the ampoule (13) is intergranular and intragranular porosity. To align the beginning of the fuel rod core and the ampoules, the lower plug has a washer (14) with slots for the ampoules, the thickness of which is equal to the distance from the end of the ampoule to the beginning of the ampoule core. Above the layer of the fuel rod core there is a plug (15) made of inert material, the height of which is greater than the protruding part of the ampoule above the fuel rod core. The material of the shell and plugs of the fuel element is a zirconium alloy, for example, E-110, and the material of the ampoule and plugs is stainless steel, for example, steel EI-844BU-ID. Alloys and compounds of depleted or natural uranium or thorium with molybdenum, zirconium, nitrogen, silicon, aluminum, etc. can be used as nuclear fuel of the fuel rod core, depending on the required ratio of fissile and fertile nuclides in the fuel rod, and as nuclear fuel the core of the ampoules is plutonium dioxide or highly enriched uranium. Gadolinium oxide, boron carbide, gadolinium titanate, etc. can be used as a burnable absorber. Magnesium or aluminum alloys can be used as a fuel rod core contact material. As a getter material, barium-containing compounds with zirconium, aluminum, nickel. As a cork material - particles of sintered aluminum oxide (grinding grain). The design of the fuel rod (see figure 2) is a shell (1), sealed at the ends with plugs (2), inside which is a core (3), consisting of nuclear fuel containing fertile nuclides (4) and having the form of cylindrical blocks with six grooves through 60 o along the generatrices of the cylinder, and contact material (6) placed in the gaps between the blocks and the shell of the fuel element and being in the liquid state under operating conditions of the fuel element. The level of the contact material is 3-5 mm higher than the level of the last block. Cylindrical ampoules (7) are located in the grooves of the blocks. The ampoule is a cylindrical thin-walled tube (8), sealed at the ends with plugs (9), inside which there is a core (10), consisting of nuclear fuel containing fissile nuclides (11), in the form of granules with a diameter of not more than 0.3 or a wire with a diameter of not more than 0.7 of the inner diameter of the ampoule, and contact material (16), which is in the liquid state under operating conditions of the fuel element. The level of the contact material is higher than the level of the nuclear fuel of the ampoule by 2 - 3 mm. The compensation volume in the ampoule (13) is the free volume located above the level of the contact material. To align the beginning of the fuel rod core and the ampoules, there is a washer (14) on the lower plug of the fuel rod, repeating the profile of the blocks, the thickness of which is equal to the distance from the end of the ampoule to the beginning of the ampoule core. Ampoules along the length of the fuel rod are located so that in the grooves of each block, except for the first, cores and compensation volumes of ampoules alternate after 60 o. This is achieved by the fact that the length of the ampoules is equal to the height of an even number of blocks (in Fig. 1 it is equal to two blocks), the length of the blocks of the fuel core is equal to the length of the core of the ampoules, and in the first block in three grooves simulators of ampoules (17) are installed with a length equal to half the length ampoules. To distance the ampoules and blocks between themselves and the shell, on the outer surface of the ampoules there is a wire wound in a spiral (18) with a diameter of at least 0.1 of the ampoule diameter, the ends of which are welded into the ends of the ampoules. To compensate for volumetric changes in the fuel rod core and to collect gaseous fission fragments released in it, there is a free volume (19) above the level of the contact material. The materials of the shell and plugs of the fuel rod and ampoules can be the same as for the fuel rod shown in Fig.1. The nuclear fuel material of the fuel rod core can be alloys and compounds of depleted or natural uranium or thorium with molybdenum, zirconium, silicon, aluminum, etc., and the nuclear fuel material of the ampoule core can be an alloy of plutonium with gallium or an alloy of highly enriched uranium with molybdenum. The contact material of the fuel rod core can be a lead-bismuth alloy, and the contact material of the ampoule core can also be a lead-bismuth alloy or sodium. The design of the fuel rod (see figure 3) is a shell (1), sealed at the ends with plugs (2), inside which there is a core (3), consisting of a vibro-compacted mixture of nuclear fuel pellets (4) containing fertile nuclides, and a burnable absorber (5), in the gaps between which there is a contact material (6), which is in a solid state under operating conditions. An ampoule (7) is located in the center of the fuel rod core. The ampoule is a hollow tape (8), sealed from the lower end with a plug (9) and twisted about the longitudinal axis, inside which there is a core (10) consisting of vibro-compacted granules of nuclear fuel containing fertile nuclides (11) with a maximum diameter of granules not more than 0 ,3 core thickness, and in the upper part of the ampoule, outside the fuel rod core, there is a getter (12). To align the beginning of the cores of the fuel element and the ampoule, there is a washer (14) with a slot for the ampoule, the thickness of which is equal to the distance from the end of the ampoule to the beginning of the ampoule core. Above the layer of the fuel rod core there is a plug (15) made of inert material, the height of which is equal to the distance from the fuel rod core to the gas collector (20). The compensation volume of the ampoule (13) is the intergranular porosity and the gas collector (20). The fuel core of the ampoule is separated from the gas collector by a gas-permeable wad (21). All materials of this fuel rod design are similar to those of the fuel rod design shown in Fig. 1. However, aluminum alloys can also be used as the ampoule shell material for this fuel element. The design of the fuel rod (see figure 4) is a shell (1), sealed at the ends with plugs (2), inside which is a core (3), consisting of vibrocompacted granules containing nuclear fuel with fertile nuclides (4) and a burnable absorber ( 5), in the gaps between which there is a contact material (6), which is in a solid state under operating conditions. An ampoule (7) is located in the fuel rod core. The ampoule is a profile tape wound in the form of a cylindrical spiral, on the outer surface of which a rib is formed, providing a gap between the cylindrical part of the ampoule and the cladding of at least 0.15 mm, and the minimum diameter of the fuel core granules is 1.2 times greater than the gap. In the lower part, the ampoule is sealed with a plug (9). Inside the ampoule there is a core (10) with a length corresponding to the length of the fuel rod core, consisting of nuclear fuel containing fissile nuclides (11). To align the beginning of the cores of the fuel element and the ampoule, there is a washer (14) with a slot for the ampoule, the thickness of which is equal to the distance from the end of the ampoule to the beginning of the ampoule core. Above the layer of the fuel rod core there is a plug (15) made of inert material, the height of which is equal to the distance from the fuel rod core to the gas collector (20). The compensation volume of the ampoule (13) is the intergranular porosity and the gas collector (20). The fuel core of the ampoule is separated from the gas collector by a gas-permeable wad (21). All materials of the fuel element are similar to those of the fuel element shown in Fig. 1, taking into account that in this design of the fuel element, the ampoule shell material can be aluminum alloys. The fabrication of the fuel rod shown in Fig. 1, tested in laboratory conditions. The shell (1) with a diameter of 9.15 x 7.72 mm and a length of 950 mm and plugs were made of E-110 zirconium alloy. Ampoules (7) were made from capillary tubes (8) with a diameter of 1.5 x 1.26 mm. EI-844BU-ID steel was used as the material for the ampoules and their plugs. The ampoules contained a core (10) of a vibrocompacted mixture of granules of uranium dioxide 98% wt. and an alloy of barium with zirconium 2% wt. Granules of uranium dioxide had an internal porosity of 12-15%. The fractional composition of the mixture of granules was -0.4+0.08 mm. The total intragranular and intergranular porosity, which is the compensation volume (13), according to the calculation - 50 - 55%. The length of the ampoule core was 900-5 mm. To align the cores of the ampoules (10) and the fuel rod (3), a washer (14) 4 mm thick was installed, made of zirconium alloy E-110. A vibrocompacted mixture of uranium dioxide granules (4) 95% wt. was used as the material of the fuel rod core (3). and gadolinium oxide (5) 5% wt. fractional composition -0.5 + 0.315 mm, impregnated with a contact material (7) - aluminum alloy with 12% wt. silicon. The length of the fuel rod core was 900–5 mm, and the volume filling with granules was 60–65%. Above the fuel rod core layer, a plug (15) was created from particles of sintered aluminum oxide of a rounded shape (grinding grain) with a fractional composition of 0.5–0.6 mm, which was also impregnated with a contact material. The ampoules in the fuel rod core were placed at intervals of 120 o with a gap between the ampoules and the cladding of 0.2 mm. The production of ampoules was carried out in the following sequence. Cutting the pipe to size, sealing one end of the ampoule, vibrating, filling the ampoule with helium and sealing the second end of the ampoule, checking the ampoule for tightness and uniform distribution of nuclear fuel along the length of the ampoule. The manufacture of fuel rods included the following technological operations. Cutting the pipe to size and sealing one end, installing washer and ampoules, vibrating the fuel rod, filling the plug and impregnating the fuel rod core and plug with molten aluminum alloy, sealing the second end of the fuel rod, pressurizing the fuel rod with helium and checking tightness, monitoring the distribution of nuclear fuel in the fuel rod, impregnation quality contact material and appearance. The results of manufacturing laboratory samples of fuel rods showed that the uneven distribution of nuclear fuel in ampoules does not exceed 7%, and in a fuel rod - 10%. The quality of impregnation of fuel rod cores is satisfactory and appearance fuel rods corresponds control samples. The technology for manufacturing other variants of fuel rod designs is similar to that given above, only in versions with ribbon fuel rods, tube profiling is also carried out and the filled ampoules are given the required shape. Thus, the real possibility of creating fuel elements of the proposed design is shown, and a combination of selected compositions of nuclear fuel, structural, contact and other materials and designs of ampoules provides an increase in the resource and an increase in the reliability of fuel elements in maneuvering modes under specific operating conditions of the reactor. When implementing a fuel element according to the claimed invention, others can be used that are not considered in examples, shapes, sizes and geometries of granules, structural, nuclear, burnable materials and getters and their placement in the fuel core. The use of fuel rods according to the claimed invention in power reactors is more economical than fuel rods that use mixed fuel, and to a greater extent meets the requirements for ecology, sanitation and radiation safety. Used sources of information 1. "Development, production and operation of fuel elements of power reactors", book 1. Moscow, Energoatomizdat, 1995 (Prototype on p. 45). 2. A. G. Samoilov, A. I. Kashtanov, V. S. Volkov. "Dispersion fuel elements of nuclear reactors", volume 1. Moscow, Energoizdat, 1982
Although nuclear power is not completely safe today, more reactors and power plants around the world are being built than shut down. So in the United States of America, the number of operating reactors has just exceeded a hundred, in France (the second largest number of peaceful atoms on the planet) - about 60, and they provide about 80% of the electricity generated in the country.
Fuel for a nuclear reactor is TVEL. This is the element in which the controlled chain reaction takes place directly. How is the “firewood” of a nuclear boiler arranged, how is it made, and what happens to the fuel in the heart of the power plant?
What is a nuclear chain reaction
It is known that the nuclei of atoms consist of protons and neutrons. For example, the nucleus of a uranium atom contains 92 protons and 143 or 146 neutrons. The repulsive force between positively charged protons in the nucleus of uranium is simply enormous, about 100 kgf in one single (!) atom. However, intranuclear forces do not allow the nucleus to scatter. When a free neutron enters the uranium nucleus (only a neutral particle can approach the nucleus), the latter is deformed and scatters into two halves plus two or three free neutrons.
These freest neutrons attack the nuclei of other atoms, and so on. Thus, the number of collisions increases exponentially and in a fraction of a second the entire mass of radioactive metal decays. This decay is accompanied by the scattering of fragments at near-light speeds in all directions, their collisions with molecules environment cause heating up to several million degrees. This is a picture of a conventional nuclear explosion. TVEL directs this phenomenon in a peaceful direction. How does this happen?
Controlled nuclear reaction
In order for a nuclear reaction to be able to maintain itself, to become a chain reaction, a sufficient amount of radioactive fuel (the so-called "critical mass") is needed. IN nuclear weapons this issue is solved simply: two ingots of weapons-grade metal (uranium 235, plutonium 239, etc.) with a mass of each slightly less than critical are combined into one whole using an explosion of ordinary TNT.
This method is not suitable for the peaceful use of the atom. The figure schematically shows the device of the simplest nuclear reactor. Each fuel element (fuel element - uranium fuel) is less than the critical mass, while their total weight exceeds this mark. Being in close proximity to each other, fuel rods "exchange" free neutrons. Thanks to this mutual neutron bombardment, a nuclear chain reaction is maintained in the reactor. Graphite rods play the role of a kind of "brake" of the nuclear process. Graphite is a good neutron absorber, and the reaction dies out when rods of this material are placed between fuel elements. This completely stops the exchange of free neutrons.
Thus, the reaction is under constant control of automation. The decay is accompanied by the movement of fragments of uranium nuclei in the coolant medium, which heat it up to the required temperature.
How electricity is generated
The further arrangement of a nuclear power plant is not much different from a conventional thermal power plant operating on gas, fuel oil or coal. The difference lies in the fact that in CHPPs heat is obtained by burning fossil hydrocarbons, while in nuclear power plants the coolant is heated by fuel elements of nuclear reactors.
The coolant brought to a temperature of 500-800 ° C (superheated water, molten salts, and even liquid metals can act as its role) in a special heat exchanger heats the water, turning it into dry steam. Steam rotates a turbine, planted on one shaft with a generator, in which it is produced electricity.
What are they
The first nuclear reactors were homogeneous devices. They were boilers in which there was nuclear fuel (more often liquid, less often gaseous). This is a melt of salts of uranium or slightly enriched uranium, sometimes suspensions of uranium dust, etc. The process was controlled by introducing a moderator into the core in the form of plates or rods made of a material that moderates free neutrons well. Heat was transferred to the water through heat exchangers located directly in the core, like grates in a coal furnace.
Our figure shows a heterogeneous nuclear reactor, which is now the vast majority in the world. Such "nuclear boilers" are easier to maintain, change the fuel in them, repair, they are safer and more reliable than the old homogeneous ones.
Another bonus of using uranium fuel rods is the generation in them, as a result of neutron irradiation of uranium nuclei, of such an element as plutonium 239, which is then used as fuel for small nuclear reactors, as well as as a weapon metal.
Where does the fuel for nuclear power plants come from?
Uranium is mined in many countries of the world by open (quarry) or mine methods. Initially, the ore contains not even uranium itself, but its oxide. Isolation of metal from oxide is the most complicated chain of chemical transformations. Not every country in the world can afford to acquire enterprises for the production of nuclear fuel.
The next task is to enrich the mined uranium. Less than 1% of uranium 235 is found in natural material, the rest is the 238 isotope. It is extremely difficult to separate these two elements. Uranium enrichment centrifuges are the most complex devices.
In order for uranium to become highly enriched (the content of the 235 isotope has increased to 20%), after turning into a gas, it will have to go through up to a thousand stages of processing.
How TVEL works
Enriched uranium falls into the hands of engineers, but it is still for nuclear fuel. The production of this fuel is akin to powder metallurgy. Powdered metal (or its chemical compounds) is pressed into small tablets about a centimeter in diameter.
Products made from uranium metal are better able to withstand the hellish conditions inside a reactor, but the pure element is very expensive to manufacture. Uranium dioxide is much cheaper, but in order for it not to crumble from enormous pressure and heat, it is necessary to bake it under enormous pressure at a temperature of more than 1000 ° C.
TVEL is a set of such washers about 2-4 meters long, placed in a tube made of steel or iron-molybdenum alloys. The fuel rods themselves are collected in a bundle of several tens or even hundreds. Such a set is called a fuel assembly (FA).
Fuel assemblies are installed directly in the heart of a nuclear reactor. In one reactor, their number can reach several hundred. As uranium decays, fuel elements lose their ability to produce heat, then they are replaced. But one kilogram of technical uranium, enriched to 4% isotope 235, manages to produce the same amount of energy during its life in a nuclear reactor as would be obtained by burning 300 standard two-hundred-liter barrels of fuel oil.
A fuel element (FE) is the main structural part of heterogeneous cores, which largely determines their reliability, size and cost.
The fuel cladding is designed to prevent direct contact between the coolant and fuel in order to prevent the release of radioactive fission products of the fuel into the coolant, as well as corrosion and erosion of the fuel core. The cladding is a structural element that gives the fuel element the necessary shape and takes on all the loads that tend to destroy the fuel element. Fuel claddings are the most critical structural parts of cores, operating in the most difficult conditions. To reduce the absorption of neutrons in shells, it is desirable to make them as thin as possible. The thickness of metal shells, determined by the conditions of strength and manufacturing technology, is usually 0.3 - 0.8 mm.
One of the main requirements for the cladding material for thermal neutron reactors is a small thermal neutron absorption cross section, which is necessary to reduce neutron losses.
At present, claddings made of zirconium and its alloys are widely used in power pressurized water reactors on thermal neutrons, which is explained by the small absorption cross section of thermal neutrons in zirconium (0.18 barn). However, zirconium has relatively low strength properties at temperatures of 360–400°C.
Along with zirconium alloys in power reactors, shells made of stainless chromium-nickel austenitic steels are used, which, compared with zirconium, have significantly higher heat resistance, corrosion resistance, good processability and, moreover, lower cost. However, the main fundamental disadvantage of steels compared to zirconium is their large thermal neutron absorption cross section (2.7 - 2.9 barn), which requires a more highly enriched fuel. A major disadvantage of austenitic stainless steels is also the tendency to corrosion cracking, which occurs when there are tensile stresses in the metal, and chlorides and oxygen in the cooling water. In connection with this, careful maintenance of the extremely low content of chlorides and oxygen, as well as other impurities, in water is of great importance in the operation of reactors.
For high-temperature reactors, of particular interest are the refractory metals niobium (melting point 2415°C), molybdenum (2622°C), tungsten (3395°C), tantalum (2996°C), as well as their alloys, which can be used for fuel cladding at temperatures up to 800 - 1200°C in the case of using helium or liquid metals as a coolant. It should be noted that in oxygen-containing gases (air, carbon dioxide and water vapor) the resistance of these metals is very low already at a temperature of 500–600°C.
During the operation of reactors, deep changes occur in fuel element materials under the influence of irradiation, cyclic temperature changes, coolant exposure, etc., which can cause their destruction. The complete destruction of fuel elements is an extremely large and completely unacceptable accident, as it leads to severe contamination of the primary circuit with radioactive fission fragments.
The most frequently observed loss of tightness of fuel rods is due to the occurrence of cracks in the cladding or in the place of welding of sealing plugs. Loss of tightness leads to the release of gaseous fission products into the coolant. The penetration of the coolant into the shell, the resulting corrosion and washing out of the fuel, in turn, increase the release of fission fragments, resulting in an even more significant increase in the radioactivity of the coolant in the circuit.
Cracks in shells can occur as a result of the following reasons:
The appearance of unacceptable internal stresses associated with the action of static, dynamic and vibrational loads, thermal stresses due to the presence of sharp temperature gradients both along the radius and along the length of the fuel elements;
Volumetric changes in the fuel due to radiation growth, swelling, phase transformations of the fuel and leading to the appearance of forces tending to break the shell; unacceptable increase in pressure inside the fuel elements of gaseous fission products;
Changes in the structure and physical and mechanical properties of the shell material under the influence of irradiation or as a result of diffusion interaction of fuel and coolant materials with the shell, for example, saturation of the shells with hydrogen;
Long-term corrosive and erosive effects of the coolant, as well as as a result of trans- and intergranular corrosion under stress in the presence of chlorine and free oxygen ions (in water-cooled reactors when stainless steel shells are used);
Defects made during the manufacture of fuel elements (heterogeneity of the cladding material, the presence of scratches on the cladding surface, poor welding quality, etc.).
In some cases, under the influence of the same reasons, a change in the shape and size of fuel elements is observed, for example, bending, which can lead to significant general and local changes in the distribution of fuel and coolant along the technological channel and, as a result, local overheating and destruction of fuel elements.
Due to the fact that fuel rods are bodies with internal heat sources and operate at high temperatures and high specific energy releases, the greatest danger to them arises when cooling is suddenly stopped. Termination of coolant supply to the core leads, as a rule, to melting of fuel elements due to residual energy release (energy release in the process of radioactive decay of accumulated fission fragments of nuclear fuel). In a stopped reactor, due to the release of energy from the radioactive decay of fission fragments accumulated in fuel elements, it is necessary to cool the latter for a long time after shutdown. Otherwise, it is possible to melt the core in the shutdown reactor.
Particular attention during the operation of the PPU should be paid to the organization of control and maintenance of the required water-chemical regime.
Not so long ago, on my blog, I already told how and where the most expensive metal in the world, California-252, is produced. But the production of this super-expensive substance is not the only occupation of the Research Institute. nuclear reactors(RIAR) in Dimitrovgrad. Since the 1970s, the Department of Fuel Technologies has been operating in the scientific center, where they are developing environmentally friendly methods for producing granular uranium oxide and processing already irradiated nuclear fuel (including weapons-grade plutonium).
In addition, fuel assemblies (FA) are also manufactured there - devices designed to generate thermal energy in a reactor due to a controlled nuclear reaction. In fact, these are batteries for the reactor. About how and from what they are made, I want to tell in this article. We will look into the very inside of a "hot" chamber with a high level of radiation, see what nuclear fuel uranium oxide looks like, and find out how much a double-glazed window can cost in an unusual window.
I will not go into the details of the device and the principle of operation of a nuclear reactor, but to make it easier to understand, imagine a household water heater into which cold water enters and hot water flows out, and it is heated by an electric coil (heater). In a nuclear reactor, there is no electric spiral, but there are fuel assemblies - long hexagons, consisting of many thin metal tubes - fuel elements (fuel elements), in which there are pellets of compressed uranium oxide.
(photo source - sdelanounas.ru)
Due to the constant fission of uranium nuclei, a large amount of heat is released, which heats water or other coolant to a high temperature. And then according to the scheme:
(source - lab-37.com)
Typically, the fuel assembly is a hexagonal bundle of fuel elements 2.5-3.5 m long, which approximately corresponds to the height of the reactor core. Fuel assemblies are made of stainless steel or zirconium alloy (to reduce neutron absorption). Fuel elements (thin tubes) are assembled into fuel assemblies to simplify the accounting and movement of nuclear fuel in the reactor. One fuel assembly usually contains 18-350 fuel elements. 200-1600 fuel assemblies are usually placed in the reactor core (depending on the type of reactor).
This is how the lid of the reactor (boiler) looks like, under which the fuel assemblies are in a vertical position. One square - one assembly. One assembly - approximately 36 tubes (for the RBMK reactor, which is shown in the photo below, on other reactors there are more tubes, but fewer assemblies).
(photo source - visualrian.ru)
And this is how the fuel tube is arranged, which make up the fuel assemblies:
The device of the fuel element of the RBMK reactor: 1 - plug; 2 - tablets of uranium dioxide; 3 - zirconium shell; 4 - spring; 5 - sleeve; 6 - tip.
Fuel rods (tubes) and fuel assembly housing:
And everything would be fine if the magic tablets of uranium oxide did not decompose into other elements in the process of a nuclear reaction. When this happens, the reactivity of the reactor weakens, and the chain reaction stops by itself. It can be resumed only after the replacement of uranium in the core (fuel rods). Everything that has accumulated in the tubes must be unloaded from the reactor and buried. Or recycle for reuse, which is more attractive, since everyone in the nuclear industry is committed to zero-waste production and regeneration.
walkie-talkie. Why spend money on storage of nuclear waste, if you can make them, on the contrary, earn this money?
It is in this department of RIAR that they are engaged in technologies for the regeneration of spent nuclear fuel, separating radioactive manure into useful elements and into something that will never be useful anywhere.
For this, chemical separation methods are most often used. The simplest option is processing in solutions, but this method produces the largest amount of liquid radioactive waste, so this technology was popular only at the very beginning of the nuclear era. Currently, RIAR is improving the so-called "dry" methods, in the process of which much less solid waste is obtained, which is much easier to dispose of, turning it into a glassy mass.
All modern technological schemes for reprocessing spent nuclear fuel are based on extraction processes called the Purex process (from the English. Pu U Recovery EXtraction), which consists in the reductive stripping of plutonium from a mixture of uranium with its fission products. Plutonium separated during reprocessing can be used as fuel mixed with uranium oxide. This fuel is called MOX (Mixed-Oxide fuel, MOX). It is also being obtained at RIAR, in the Department of Fuel Technologies. This is a promising fuel.
All research and manufacturing process are carried out by operators remotely, in closed chambers and protective boxes.
It looks something like this:
With the help of such electromechanical manipulators, operators control special equipment in "hot" cells. The only thing that separates the operator from the high radioactivity is a meter-thick lead glass, consisting of 9-10 separate plates, 10 cm thick.
The cost of only one glass is comparable to the cost of an apartment in Ulyanovsk, and the entire camera is estimated at almost 100 million rubles. Under the influence of radiation, the glasses gradually lose their transparency and they need to be replaced. Can you see the "hand" of the manipulator in the photo?
It takes years of training and experience to learn how to masterfully control the manipulator. But with their help, sometimes it is required to perform operations from the category of unscrewing and tightening small nuts inside the chamber.
On the table, in the hall of "hot" cells, you can see samples of nuclear fuel in glass capsules. Many laboratory guests constantly look sideways at this suitcase and are afraid to come closer. But this is just a dummy, although very realistic. This is what uranium dioxide looks like, from which magic fuel pellets are made - a shiny black powder.
Uranium dioxide has no phase transitions, it is less susceptible to those undesirable physical processes that occur with metallic uranium at high core temperatures. Uranium dioxide does not interact with zirconium, niobium, stainless steel and other materials from which fuel assemblies and fuel rod tubes are made. These properties allow it to be used in nuclear reactors, obtaining high temperatures and, consequently, high efficiency of the reactor.
The manipulator control panel is a slightly different modification. There are no glasses in this cell, so the observation is carried out with the help of cameras installed inside.
What is this?! The man in the "hot" cell?! But...
It's okay, it's a "clean" camera. During Maintenance the level of radiation in it does not exceed the permissible values, so you can work in it even without special means of radio protection. Apparently, it is in this chamber that the final assembly of fuel assemblies is carried out from fuel elements already loaded with uranium pellets.
With such a not very cozy neighborhood with open nuclear fuel, the level of radiation in the laboratory does not exceed natural values. All this is achieved through strict radiation safety techniques. People have been working as operators for decades without harm to health.
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