High power channel reactor. RBMK high-power channel reactor Basic principles of repair technology
This article, which should give a general idea of the design and operation of the reactor, which has become one of the main ones for our nuclear energy today, serves as an explanatory text for the drawings showing the RBMK-1000 reactor, and for diagrams explaining the operation of the unloading and loading machine (REM) ).
The main building of the nuclear power plant with the RBMK reactor consists of two power units with an electrical power of 1000 MW each, with a common turbogenerator room and separate rooms for the reactors. The power unit is a reactor with a coolant circulation circuit and auxiliary systems, a system of pipelines and equipment through which water from turbine condensers is directed to the coolant circulation circuit, and two turbogenerators with a capacity of 500 MW each.
The coolant is water, circulates through two parallel systems. Each system includes two separator drums, 24 drop pipes, 4 suction and - pressure manifolds, - 4 circulation pumps, three of which are operational, and one is in reserve, 22 group distribution manifolds, - as well as shut-off and control valves .
From the distribution group collectors, water with a temperature of 270°C is distributed through individual pipelines using shut-off and control valves into process channels. Washing the fuel elements, it is heated to saturation temperature, partially evaporates, and the resulting steam-water mixture also enters separator drums through individual pipelines from each channel. Here the steam-water mixture is separated into steam and water. The separated water is mixed with feed water and through downdrafts. pipes is sent to the main circulation pumps. Saturated steam with a pressure of 70 kgf/cm2 is sent through eight steam lines to two turbines. Having worked in the high-pressure cylinders of the turbines, the steam enters intermediate separators-superheaters, where moisture is separated from it and it is overheated to a temperature of 250 ° C . Having passed the low-pressure cylinders, the steam enters the condensers. The condensate undergoes 100% purification on filters, is heated in five regenerative heaters and enters the deaerators. From there, water at a temperature of 165°C is pumped back into the separator drums. In just an hour, the pumps pass through the reactor They drive about 38 thousand tons of water. The rated thermal power of the reactor is 3140 MW; per hour it produces 5400 tons of steam.
The reactor is located in a square-section concrete shaft measuring 21.6 X 21.6 m and 25.5 m deep. The weight of the reactor is transferred to the concrete using welded metal structures, which at the same time serve as biological protection. Together with the casing, they form a sealed cavity filled with a mixture of helium and nitrogen - the reactor space, in which the graphite stack is located. The gas is used to maintain the temperature of the masonry.
The upper and lower metal structures of the reactor are covered with protective material (serpentinite rock) and filled with nitrogen. Water tanks are used as lateral biological protection.
The graphite stack is a vertically located cylinder assembled from graphite columns with central holes for process (steam-generating) channels and channels of the control and protection system (they are not shown in the diagram).
Since approximately 5% of the thermal energy is released in the graphite moderator during reactor operation, an original design of solid contact rings was proposed to maintain the required temperature conditions of the graphite blocks and improve heat removal from the graphite to the coolant flowing in the channels. Split rings (20 mm high) are placed along the height of the channel close to each other in such a way that each adjacent ring has reliable contact along the cylindrical surface either with the channel pipe or with the inner surface of the graphite masonry block, as well as at the ends with two other rings. The effectiveness of the proposed design was tested by experiments on a thermal bench. The operating experience of power units of the Leningrad NPP has confirmed the possibility and simplicity of installing a channel with graphite rings into the technological path and removing it from it.
A technological channel is a welded pipe structure designed to install fuel assemblies (FA) in it and organize the coolant flow.
The upper and lower parts of the channel are made of stainless steel, and the central pipe with a diameter of 88 mm and a wall thickness of 4 mm within the core, which is 7 m high, is made of a zirconium alloy with niobium (2.5%). This alloy is smaller than steel, absorbs neutrons, and has high mechanical and corrosion properties. Creating a reliable hermetic connection between the central zirconium part of the channel and steel pipes turned out to be a difficult task, since the linear expansion coefficients of the materials being connected differ by approximately three times. It was possible to solve it with the help of steel-zirconium adapters made by diffusion welding.
A cassette with two fuel assemblies is placed in the technological channel (there are 1693 such channels); Each such assembly consists of 18 fuel rods. The fuel element is a zirconium alloy tube with an outer diameter of 13.6 mm, a wall thickness of 0.9 mm with two end plugs, inside which uranium dioxide pellets are placed. In total, about 190 tons of uranium containing 1.8% uranium-235 isotope is loaded into the reactor.
Three types of power reactors have been developed and are successfully operating in our country:
channel water-graphite reactor RBMK-1000 (RBMK-1500);
pressure water pressure vessel reactor VVER-1000 (VVER-440);
fast neutron reactor BN-600.
The following types of power reactors have been developed and operated in other countries:
Pressurized water reactor PWR;
Pressurized boiling water reactor BWR;
channel heavy water reactor CANDU;
gas-graphite vessel reactor AGR.
The number of fuel rods loaded into the reactor core reaches 50,000 pieces. For ease of installation, reloading, transportation and cooling, the fuel rods of all power reactors are combined into fuel assemblies - FAs. For reliable cooling, fuel rods in a fuel assembly are separated from each other by spacer elements.
Fuel elements and fuel assemblies of the RBMK-1000 and RBMK-1500 reactors
In the core of the RBMK-1000 and RBMK-1500 reactors with a square grid pitch of 250 mm, there are 1693 and 1661 process channels. The fuel assemblies are located in the supporting pipe of each channel. To channel pipe F 80x4 mm made of Zr+ 2.5% Nb alloy in a re-crystallized state, tips made of OKH18N10T steel are attached on both sides by diffusion welding, allowing each channel to be tightly connected to the coolant collector.
This channel design makes it possible to easily load and reload fuel assemblies using a reloading machine, including when the reactor is running. A cassette is loaded into the channel of the RBMK-1000 reactor, consisting of two separate fuel assemblies, located one above the other, connected into a single whole by a hollow supporting rod made of Zr+ 2.5% Nb alloy ( f 15x1.25 mm). In the cavity of the supporting rod, in a separate tubular shell made of zirconium alloy, energy release monitoring sensors or additional neutron absorbers are located, which serve to level out the energy release in the reactor core.
Fig.1. FA of the RBMK-1000 reactor
Each upper and lower fuel assembly (Fig. 1) is formed by a parallel bundle of fuel rod rods of 18 pieces, arranged in concentric circles with a fixed radius step, which creates a stable heat removal throughout the entire service life of the fuel rods. Fixation of fuel rods is ensured by a frame formed by a supporting central rod and ten spacer grids evenly spaced along the height of each fuel assembly. Spacer grids are assembled from individual shaped cells, welded together at points and fastened externally with a rim. Each cell has internal protrusions 0.1 - 0.2 mm long: four in the cells of the outer row and five in the cells of the inner row of fuel rods, firmly, with tension, fixing the fuel rods passed through the cells. This prevents radial movements of fuel elements in the cells, which can be excited by vibration of the structure under the influence of turbulent coolant flow. In this way, the occurrence of fretting corrosion in places where the fuel element cladding touches the metal of the cells is eliminated. The gratings are made of stainless austenitic steel (work is underway to replace the material with a zirconium alloy). The spacer grids have freedom of movement along with the support rod fuel rod bundle, but rotation of the grid relative to the rod axis is excluded.
The fuel rods are attached at one end to the supporting grid using ring locks, crimped into the cutouts of the shaped tips. The other ends of the fuel rods remain free. The supporting grid (end) is rigidly attached to the axial half of the supporting rod.
A general view of the fuel rod is shown in Fig. 2. The total length of the fuel rod is 3644 mm, the length of the fuel core is 3430 mm.
The material of the cladding and end parts of fuel rods is a Zr+1% Nb alloy in a recrystallized state. Shell diameter 13.6 mm, wall thickness 0.9 mm. The fuel is pellets of sintered uranium dioxide with a height close to their diameter and having holes at the ends.
The average mass of the fuel column is 3590 g with a minimum density of 10.4 g/cm 3 .
The spread of the diametrical gap between the tablet and the shell is 0.18-0.36 mm. In the shell, the fuel pellets are compressed by a coiled spring located in a gas collector, which reduces the pressure of gaseous fission products. The ratio of the free volume under the shell to the total volume at average geometric parameters is 0.09.
Fig.2. RBMK reactor fuel rod: 1 - plug, 2 - fuel pellet, 3 - shell, 4 - spring, 5 - bushing, 6 - tip
Designs of channels of uranium-graphite reactors of nuclear power plants
Fuel-generating part of the RBMK-1000 channel
(Fig. 2.31) consists of two fuel assemblies, a supporting central rod, a shank, a rod, and a tip. The fuel assembly is assembled from 18 rod-type fuel rods with a diameter of 13.5x0.9 mm, a frame and fasteners; FAs are interchangeable. The frame consists of a central pipe on which one end and ten spacer grilles are fixed. Spacer grids serve to ensure the required
location of fuel elements in the cross section of the fuel assembly and are mounted in the central tube. The fastening of the spacer grids allows them to move along the axis by a distance of 3.5 m during thermal expansion of the fuel elements. The outermost spacer grid is mounted on a key to increase rigidity against torsion of the beam.
The spacer grid is a honeycomb structure and is assembled from a central one, an intermediate pole, twelve peripheral cells and a rim, connected to each other by spot welding. The rim is provided with spacer projections.
Rice. 2.31. FA RBMK-1000:
1 - suspension; 2 - adapter; 3 - shank; 4 - fuel rod; 5 - supporting rod; 6 - bushing; 7 - tip; 8 - nut
The central tube of the fuel assembly at the end has a rectangular cut of half the diameter for joining the fuel assemblies to each other in the channel. This ensures the necessary alignment of the fuel rods of the two fuel assemblies and prevents their rotation relative to each other.
Fuel elements are rigidly fixed in the end grids of the fuel assembly (at the upper and lower boundaries of the core), and when the reactor is operating, the gap in the center of the core is selected due to thermal expansion. Reducing the distance between fuel rods in the center of the core reduces the heat surge and reduces the temperature of the fuel and structural material in the fuel rod plug zone. The use of two fuel assemblies at the height of the core allows each assembly to operate in the zone of both maximum and minimum energy release in height.
All parts of the fuel assembly except the rod and spacer grids are made of zirconium alloy. The rod, which serves to connect the assembly with the suspension, and the spacer grids are made of X18N10T stainless steel.
An analysis of the thermal-hydraulic and strength characteristics of the RBMK-YOO reactor revealed the available reserves for increasing the power of the installation. An increase in the critical power of the process channel, i.e., the power at which a heat transfer crisis occurs on the surface of the fuel elements, accompanied by an unacceptable increase in the temperature of the zirconium cladding, was achieved by introducing heat transfer intensifiers into the fuel assembly. The use of intensifier grids with axial swirl of the coolant flow made it possible to increase the capacity of the RBMK-1000 process channel by 1.5 times. The design of the RBMK-1500 fuel assembly differs from the design of the RBMK-1000 fuel assembly in that spacer intensifier grids are used in the upper fuel assembly; otherwise, the design of the fuel assembly has no fundamental differences. Maintaining the resistance of the circulation circuit is achieved by reducing the coolant flow.
An increase in the power of the fuel assembly causes a corresponding increase in the linear power of the fuel elements to 550 W/cm. Domestic and foreign experience shows that this level of linear power is not the limit. At a number of US stations, the maximum linear powers are 570-610 W/cm.
For installation and replacement of the housing of the technological channel during operation, as well as for organizing reliable heat removal for the graphite masonry to the channel, there are “hard contact” rings on its middle part (Fig. 2.32). Split rings 20 mm high are placed along the height of the channel close to each other in such a way that each adjacent ring has reliable contact along the cylindrical surface either with the channel pipe or with the inner surface of the graphite masonry block, as well as at the end with each other. The minimum permissible gaps channel-ring and ring-block are determined from the condition that the channel is not jammed in the masonry as a result of radiation shrinkage of graphite and an increase in the diameter of the channel as a result
creep of pipe material. A slight increase in the gaps will lead to a deterioration in heat removal from the graphite of the masonry. Several bushings are welded on the upper part of the channel body, designed to improve heat removal from the metal structures of the reactor to ensure radiation safety and create technological bases for the manufacture of the channel body.
Rice. 2.32. Installation of a technological channel in graphite masonry:
1- pipe (Zr+2.5% Nb alloy); 2 - outer graphite ring; 3 - inner graphite ring; 4 - graphite masonry
As already noted, zirconium alloys are used mainly for the manufacture of reactor core elements, which take full advantage of their specific properties: neutron
“transparency”, heat resistance, corrosion and radiation resistance, etc. For the manufacture of other parts of the reactor, a cheaper material is used - stainless steel. The combination of these materials is determined by the design requirements, as well as economic considerations regarding materials and technology. The difference in physical, mechanical and technological properties of zirconium alloys and steels causes the problem of their connection.
In industrial reactors, it is known to connect steel with zirconium alloys mechanically, for example, in the Canadian Pickering-2, -3 and -4 reactors, the connection of channel pipes made of zirconium alloy with end fittings made of tempered stainless steel (Fig. 2.33) was made using rolling. However, such compounds work satisfactorily at temperatures of 200-250 °C. Joints between steel and zirconium by fusion welding (argon-arc) and solid-phase welding were studied abroad. Argon-arc welding is carried out at higher temperatures than solid-phase welding, which leads to the formation of layers of brittle intermetallic compounds in the joint zone, which negatively affect the mechanical and corrosion properties of the weld. Among the methods being studied for joining zirconium alloys with steel in the solid phase are explosion welding, joint forging, stamping, pressure welding, joint pressing, resistance brazing, friction welding, etc.
However, all these connections are not applicable for the pipes of the process channel of the RBMK reactor, since all of them are intended
to work under other parameters, and they cannot provide the required density and strength.
The middle zirconium part of the RBMK channel, located in the reactor core, is connected to the stainless steel end assemblies using special steel-zirconium adapters. Steel-zirconium adapters are produced by diffusion welding.
Welding is carried out in a vacuum chamber as a result of strong pressing of parts made of zirconium alloy and stainless steel heated to a high temperature against each other. After mechanical processing, an adapter is obtained, one end of which is a zirconium alloy, the other is stainless steel. To reduce the stresses arising in a connection with a large difference in the linear expansion coefficients of zirconium alloy (a = 5.6 * 10 -6 1/°C) and steel 0Х18Н10Т (a = 17.2 * 10 -6 1/°C), a bandage made of bimetallic hot-pressed pipes is used (steel grade 0Х18Н10Т + steel grade 1Х17Н2) (a=11*10 -6 1/°С).
The connection of the adapter with a zirconium pipe with an outer diameter of 88 and a wall thickness of 4 mm is carried out by electron beam welding. The welds are subject to the same requirements for strength and corrosion properties as the main pipe. The developed modes of electron beam welding, methods and modes of mechanical and thermal treatment of welds and heat-affected zones made it possible to obtain reliable vacuum-tight steel-zirconium welded joints.
Second life of channel-type reactors
Next year will mark 70 years since the launch of the first channel-type reactor plant. Why is technology denied development today and who disagrees with this? Alexey Slobodchikov, chief designer of power channel reactor plants, department director of JSC NIKIET, explains and answers.
First, a few words about the history of channel reactors. Their appearance was closely connected with the emergence of the nuclear industry itself, both the military-industrial complex and the energy sector.
The first channel reactor was launched on June 19, 1948 in the Chelyabinsk region. The development of industrial reactor A was carried out by the chief designer Nikolai Antonovich Dollezhal, and the scientific project was led by Igor Vasilyevich Kurchatov. Of course, the main purpose of the reactor was to produce weapons-grade plutonium, and the first stage of development of the channel reactor industry is inextricably linked with defense issues.
The first reactors were purely utilitarian. They are based on a flow diagram and the absence of a closed loop. In the process of developing operational solutions, it became possible to move on to using the reactor in the classical industrial sense - as part of an energy complex. The reactor of the Siberian Nuclear Power Plant, built in 1958, was the first to realize this task. During that period, prospects for using nuclear energy for peaceful purposes began to open up.
The first nuclear power plant with a channel uranium-graphite reactor was built in Obninsk. By energy standards, the AM reactor had a low power - only 5 MW. But nevertheless, its creation, design and operation (largely in a research mode) made it possible to resolve issues related to the study of materials and their behavior during the generation of electricity by a nuclear reactor.
Starting point
After the commissioning of the nuclear power plant in Obninsk, the next stage is the Beloyarsk station. This project was bold not only for its time, but also for reactor engineering in general. At the Beloyarsk NPP, the technology of nuclear steam superheating was implemented, which made it possible to significantly increase the efficiency of the power plant and get closer to those indicators that are typical for power plants with fossil fuels. After this, at the turn of the 1960–1970s, the opportunity arose to begin the development and construction of the RBMK-1000 reactor.
The launch of the RBMK-1000 reactor became the starting point for the large-scale use of nuclear energy in the national economy. It was the first million-plus block, which remained the only one with such capacity for quite a long time.
The first power unit with RBMK reactors was launched in December 1973 at the Leningrad Nuclear Power Plant. Then, throughout the 1970s–1980s, 17 power units with RBMK reactors were successively commissioned.
Today in Russia there are 11 such power units in operation at the sites of the Leningrad, Kursk and Smolensk nuclear power plants. Four power units were built in Ukraine, and two more on the territory of the Lithuanian SSR. The power of the latter was increased 1.5 times - up to 1500 MW (nominal electrical power). These power units were the most powerful at that time, and in the foreseeable future for the Russian nuclear industry they still remain the limit on the power of an individual power unit.
Biography
Alexey Vladimirovich SLOBODCHIKOV
born in 1972. Graduated from Moscow State Technical University. N. E. Bauman with a degree in Nuclear Power Plants.
Since 1995 he has been working at JSC NIKIET. Currently he holds the position of chief designer of power channel reactor plants, director of the department.
For his contribution to the work on restoring the resource characteristics of RBMK reactors, A. Slobodchikov, as part of the team of authors, was awarded the Prize of the Government of the Russian Federation. The creation and industrial implementation of this unique technology, developed by NIKIET together with leading enterprises in the industry, Russian science and industry, make it possible to maintain nuclear power plants with such reactors in the unified energy system of Russia until replacement capacities are commissioned.
About the present, past and future of RBMK
If we talk about the share of RBMK reactors in the energy balance, then this figure, depending on the year, fluctuates around 39–41%. So far, only units built in the 1970s–1980s continue to be used. The first of them was launched in 1973, and the youngest - the third block of the Smolensk station - in 1990. Taking into account the operating experience of uranium-graphite reactors, the service life of the RBMK was determined at the design stage - 30 years.
It’s worth making a small note here. The history of the development of the entire channel sector - speaking specifically about RBMK reactors - is a process of its improvement and modernization in accordance with the latest technology at a certain moment. For example, it is impossible to compare the technical condition of a reactor in 1973 (such as at the Leningrad Nuclear Power Plant) with what we have today. Over more than 40 years, significant changes have occurred in control systems, safety, the fuel cycle itself, and the physics of the core.
The Chernobyl accident became a black page in the history of the development of both channel and world reactor construction in general. But after it, appropriate conclusions were drawn. Now the RBMK reactor is called a “Chernobyl-type reactor,” but this is not an entirely correct definition. It is impossible to compare what was with what we have today. The continuous modernization process that I spoke about made it possible at the turn of the 1990–2000s to raise the question of extending the service life of reactors to 45 years. Thus, the extended service life of the first unit of the Leningrad NPP will end in 2018, and the operation of the third unit of the Smolensk station will end in 2035.
About graphite elements and curvature prediction
There are different types of channel reactors. For example, in Canada, the basis of nuclear energy is CANDU reactors with heavy water. In our country, only uranium-graphite channel reactors are operated. Graphite is a non-trivial material; its properties are not similar to steel or concrete. The study of graphite as an element of the active zone began from the first day of operation of industrial devices.
Even then it was clear that under the influence of high temperatures and high-energy flows this material was subject to degradation. At the same time, changes in the physical and mechanical properties of graphite and its geometry affect the state of the core as a whole. Not only Soviet scientists studied this issue in detail. Changes in the states of graphite were also of interest to our American colleagues.
One of the main problems is changing the geometry of graphite elements. The RBMK reactor core consists of graphite columns. Each column is 8 meters high and consists of 14 graphite blocks - parallelepipeds with a height of 600 mm and a cross-section of 250x250 mm. There are 2.5 thousand such columns in total.
The core itself has a height of 7 meters, the length of the fuel assembly that is located in it is also 7 meters, and the total length of the fuel module is 16 meters.
It is necessary to understand that the active zone is a single whole, therefore changes in one element along the chain - as a cumulative effect - are first transmitted to nearby areas, and subsequently can cover the entire geometry of the active zone. One of the most negative factors in changes in graphite blocks is the curvature of the columns and, as a consequence, deflections of the fuel channels and control rod channels.
During installation, all columns are, of course, vertical, but during operation this verticality is lost. If we turn again to history, we can see that for industrial devices and the first uranium-graphite reactors this process began in the first years of operation. At the same time, the mechanisms of this phenomenon were understood. During the development of the RBMK reactor, some processes were prevented by design solutions.
It is impossible to completely get rid of changes. It is difficult to predict their appearance. With a 45-year reactor lifespan, it was assumed that the process of change would enter an active phase at the turn of 43–44. But it turned out that we encountered a problem at the turn of the 40th year of operation. That is, the forecast error was about three years.
In 2011, at the first power unit of the Leningrad station, changes in geometry were recorded: curvature of process channels (nuclear fuel - fuel assemblies are installed in them), channels of control and protection rods. I would like to draw your attention to the fact that the operation of the RBMK requires constant monitoring of parameters that determine safety. With the help of ultrasonic testing, the diameter of the channels and the curvature, integrity, and mutual state of the elements are monitored, which determine the performance under various (both nominal and transient) modes. When, during planned monitoring, the beginning of the change process was discovered, it became clear: once the process has begun, its speed will be quite high; operation of a reactor plant under such conditions requires additional solutions.
Main indicators of RBMK reactors
Finding the right solutions
When process channels and control rods are bent, it is first necessary to ensure the unconditional operability of the actuators of control and protection systems, as well as fuel assemblies under conditions of changing geometry.
It is also necessary to confirm the ability of technological channels operating under deflection conditions to maintain strength properties. At the first block of the Leningradskaya station, the number of technological channels is 1693, and not a single one of them, when operating under curvature conditions, is at risk from the point of view of its performance.
Another important point: all technological operations associated with loading and unloading fuel assemblies must be ensured. A distinctive feature, which is also an advantage, of the RBMK reactor is the ability to operate it under conditions of continuous overload. The design allows overloading during operation directly at power. This provides a flexible fuel cycle, core shaping and increased burnup. Actually, this determines the economy: the reactor does not operate in campaigns, it operates in constant overload mode.
In 2011, a number of works were carried out at the Leningrad station that confirmed the operability of the reactor plant elements under conditions of deflection of up to 100 mm. After this, the first power unit of the Leningrad NPP was put into operation for a short time under enhanced control of parameters. Seven months later, it was stopped again for extended geometry control: the development of a process associated with a change in the shape of the graphite stack was recorded. Then it became clear that further operation of the reactor was impossible. In May 2012, the first power unit of the Leningrad station was stopped.
At the same time, the beginning of changes was recorded at the second power unit of the Leningrad NPP and at the second power unit of the Kursk Nuclear Power Plant. The identified deflections indicated that the process was approaching the active phase.
A solution was required that would be applicable to all power units of the Leningrad, Kursk and Smolensk nuclear power plants with RBMK reactors. Several ways were considered. It was possible to use a passive method of controlling curvatures, but it became obvious that the processes of graphite degradation and, as a consequence, shape changes are associated with the level of damaging factors. First of all, with temperature and fast neutron flux.
Accordingly, passive methods of controlling this process could be as follows: a radical, up to 50%, reduction in the power of power units in order for a significant effect to appear; or their operation in seasonal mode. That is, the unit is in operation for four months, then sits for several months. But these methods were only suitable for those reactors where the process of change had not gone far.
The second direction - active, as we called it then - is the development and implementation of repair technologies. Their periodic use would make it possible to operate the reactor plant longer.
Why did we even talk about the possibility of repair? In answering this question, we need to return to the experience of industrial devices, since for them the problem of shape change has existed for many decades. Significant channel deflections were recorded in the reactor of the Siberian nuclear power plant EI-2. If for the RBMK reactor the deflection was 100 mm, then the deflections of the process channels in the EI-2 reactor reached 400 mm.
Using various technological techniques, using the example of industrial devices, the possibility of partial repair of graphite masonry was shown. Even the experience of the RBMK reactor itself indicated that the graphite stack is a complex, large element, but to some extent repairable. At each power unit with RBMK, technological channels were replaced - this, among other things, was due to the impact on the graphite masonry.
The extensive experience accumulated in design institutes and directly at plants in the field of repairs in the core has made it possible to create and implement new repair technologies.
An analysis of the technological methods used on industrial devices showed that their use for the RBMK reactor is impossible for various reasons. Some operations are ineffective under RBMK conditions; others are impossible from the point of view of design features. Engineers and designers began to look for new solutions. A technology was required that would make it possible to directly influence the cause of the shape change and change in the geometry of an individual graphite block, that is, it would reduce its transverse size.
The scale of the problem required the gradual decommissioning of RBMK reactors. In 2012 - the first, in 2013 - the second block of the Leningrad station; in 2012 - the second block of the Kursk station; During 2012–2014, half of the RBMK reactors were to be decommissioned - 20–25% of all nuclear power generation in Russia!
Most experts understood that the methods applicable to industrial devices would not give the desired effect in the case of reactors due to various features.
Revenue of NPPs with RBMK by year
Cumulative revenue of NPPs with RBMK (2014–2035)
Determining decision
Finally, in June 2012, an interesting technical proposal appeared. A month later, in July, a meeting was held at the Leningrad NPP under the leadership of Sergei Vladilenovich Kiriyenko, as a result of which a decision was made to develop and implement a draft repair program.
At that time, no one could give guarantees of success. The proposed technological method was complex; First of all, this was due to the fact that all work had to be carried out by robotic systems at a depth of about 18 meters, in a hole with a diameter of 113 mm. Plus, repairs were made not to one specific column, but to the entire reactor.
Work on the first power unit of the Leningrad station began in the first ten days of January 2013.
It turns out that in six months the entire complex of operations was thought out. It was intense and multifactorial work, in which three alternative developers of the technical complex were involved: JSC NIKIMT-Atomstroy and two organizations outside Rosatom.
The development of technical means was the beginning of solving the problem. In parallel, a whole complex of computational, scientific, and experimental work was carried out to confirm and study the possibilities of operating all elements of the core under conditions of curvature, in combination with the influence of repair technology.
Before entering the reactor facility, even for trial operation of the devices being developed, large-scale testing of the technology was required. Of course, the priority principle was “do no harm,” because any action was irreversible. Therefore, it was necessary to verify every step at the development stage of both technology and equipment.
At the ENITs Research Institute, in Elektrogorsk, on a stand created earlier for other tests, full-scale tests of equipment both for cutting graphite columns and for applying force to elements of graphite masonry were carried out. Particular attention was paid to issues of ensuring radiation safety. When carrying out any mechanical operations to remove graphite (which is a radioactive material), it must be taken into account that it should not come into contact with the environment.
All this was thoroughly tested in the test bench conditions. Let me emphasize once again: we had no experience in such work, so all the preparatory processes were carried out gradually. All technical materials underwent a thorough examination by Rostechnadzor. If necessary, adjustments were made and additions were made. Only after all these procedures we received permission and began work at the Leningrad station. They were carried out in several stages: the first nine cells, one row, then three rows, five rows, and only after that a decision was made about the effectiveness of the technology and the possibility of its application for the entire apparatus.
Technology as it is
The root cause of the change in the shape of the graphite masonry is a change in the geometry of the graphite block. After prolonged use, graphite enters the so-called “swelling” stage: its layers, most exposed to temperature and fluence, increase density. And the outer layers of the graphite block continue to shrink. Internal stress arises, leading to the formation of cracks.
The width of a vertical crack in a graphite block increases over time. Thus, the geometric dimensions of the graphite block, originally 250x250 mm, increase to 255x257 mm. Since there are thousands of graphite blocks in contact with each other in the masonry, the appearance of a large number of cracks in them and an increase in their geometric dimensions lead to the fact that they begin to push each other and gradually move from the center to the periphery, determining changes in geometry.
The appearance of curvatures is also associated with the neutron flux, which looks like a shelf with a decline at the periphery. Actually, this whole shelf behaves the same way. There are 24 graphite blocks in one row, and each pushes away its neighbor: let’s say the first block pushed by 2 mm, the next by another 2, all this is added up, and the result is quite high deflection arrows on the periphery.
The mechanics of this process were confirmed during measurements of the first power unit of the Leningrad station, which made it possible to develop a repair technology. Repulsion associated with the formation of cracks and an increase in geometry are the root causes of changes in the shape of the entire graphite masonry. Hence the conclusion: as a relief measure, it is necessary to reduce the transverse dimensions of the graphite block.
The whole technology is based on the fact that if a negative factor is an increase in size, then a positive factor will be its reduction. This technology includes, without stopping at intermediate stages, three operations for one cell, which at first glance look quite simple. First: using a cutting tool, graphite blocks are cut vertically. The cutting width changes sequentially from 12 to 36 mm - the graphite block is cut on both sides, and the “excess” is removed in the process. The second operation is the bringing together of cut graphite blocks that have been machined. The third operation is to restore the hole.
To restore the geometry of the reactor as a whole, a scheme is being developed that takes into account the influence of cells located on the periphery on the center, and vice versa. This mutual influence is the determining factor when choosing a repair scheme, which in turn affects the amount of work. Thus, for the first block of the Leningradskaya station, the volume of repairs in 2013 amounted to 300 cells out of a total number of 1,693.
Basic principles of repair technology
For repairs, the design and geometric position of those cells are selected that will reduce the overall curvature, which will allow the reactor to continue to be operated.
Along with the development of repair technology and its implementation, a whole scientific, technical and computational set of measures is being carried out to confirm the possibility of operating all elements of the reactor plant after the work has been completed and under conditions of ongoing deformation.
Many industry enterprises participated in the work to substantiate the possibility of operating the reactor plant after repair: NIKIET, VNIIAES, VNIIEF, OKBM im. I. I. Afrikantova, ENITs, NIKIMT.
General coordination was carried out by NIKIET. He also served as a general contractor in the development, feasibility study and repair of the power unit of the Leningrad Nuclear Power Plant.
General task
With such a large number of participants in the process, there were no problems in the interaction between them. Work at the Leningrad Nuclear Power Plant has become one of the striking examples of a common cause, achieving a result formulated as follows: develop and implement technology, carry out repairs and justify the possibility of further operation, determine optimal conditions. When performing all operations, further degradation of graphite and subsequent shape changes were also taken into account.
The launch of the first block of the Leningrad station took place in November 2013. A little over a year passed between the moment the decision was made and the start-up of the power unit. As a result, we have developed a technical solution that allows us to restore the functionality of the graphite stack and extend the life of the reactor by repeating a similar operation.
Another feature of the procedure for restoring resource characteristics (this is what such repairs are called) is that it is impossible to make a new reactor out of this operation. That is, the process of shaping will continue: a limited number of cells are cut, leaving cells that cannot be repaired, so the process of shaping and, accordingly, curvature will continue. Its speed is fixed through sequential control.
The methodology implies the following: with a controlled process, its numerical forecasting determines the repair time, frequency of its implementation and service intervals between repairs. Of course, this process must be repeated cyclically. To date, the restoration of the resource characteristics of graphite masonry has been carried out at two power units of the Leningrad station: the first and second - and at the first stage of the Kursk station (also the first and second power units).
From 2013 to 2017, the technology was significantly modernized. For example, the time required to complete the work has been reduced, technological operations have been optimized, and the cost has been significantly reduced - almost multiple times compared to the power units of the Leningrad NPP. We can say that the technology has been introduced into industrial operation.
General design of the RBMK-1000 reactor
The “heart” of a nuclear power plant is a reactor, in the core of which a chain reaction of fission of uranium nuclei is maintained. RBMK is a channel water-graphite reactor using slow (thermal) neutrons. The main coolant in it is water, and the neutron moderator is the graphite masonry of the reactor. The masonry is composed of 2488 vertical graphite columns, with a base of 250x250 mm and an internal hole with a diameter of 114 mm. 1661 columns are intended for installation of fuel channels in them, 211 - for the control and protection system channels of the reactor, and the rest are side reflectors.The reactor is single-circuit, with boiling coolant in the channels and direct supply of saturated steam to the turbines.
Core, fuel rods and fuel cassettes
The fuel in the RBMK is uranium dioxide-235 U0 2, the degree of fuel enrichment according to U-235 is 2.0 - 2.4%. Structurally, the fuel is located in fuel elements (fuel elements), which are zirconium alloy rods filled with sintered uranium dioxide pellets. The height of the fuel element is approximately 3.5 m, diameter 13.5 mm. Fuel rods are packaged into fuel assemblies (FA), containing 18 fuel rods each. Two fuel assemblies connected in series form a fuel cassette, the height of which is 7 m.Water is supplied to the channels from below, washes the fuel rods and heats up, and part of it turns into steam. The resulting steam-water mixture is removed from the upper part of the channel. To regulate the water flow, shut-off and control valves are provided at the inlet of each channel.
In total, the core diameter is ~12 m, the height is ~7 m. It contains about 200 tons of uranium-235.
CPS
The control rods are designed to regulate the radial field of energy release (PC), automatic power control (AP), rapid shutdown of the reactor (A3) and control of the altitude field of energy release (USP), and the USP rods with a length of 3050 mm are removed from the core downwards, and all the rest with a length of 5120 mm, up.To monitor the energy distribution along the height of the core, 12 channels with seven-section detectors are provided, which are installed evenly in the central part of the reactor outside the network of fuel channels and control rods. The energy distribution along the radius of the core is monitored using detectors installed in the central tubes of the fuel assembly in 117 fuel channels. At the joints of the graphite columns of the reactor masonry, 20 vertical holes with a diameter of 45 mm are provided, in which three-zone thermometers are installed to monitor the graphite temperature.
The reactor is controlled by rods evenly distributed throughout the reactor containing a neutron-absorbing element - boron. The rods are moved by individual servos in special channels, the design of which is similar to technological ones. The rods have their own water cooling circuit with a temperature of 40-70°C. The use of rods of various designs makes it possible to regulate the energy release throughout the entire volume of the reactor and quickly shut it down if necessary.
There are 24 AZ (emergency protection) rods in the RBMK. Automatic control rods - 12 pieces. There are 12 local automatic control rods, 131 manual control rods, and 32 shortened absorber rods (USP).
1. Core 2. Steam-water pipelines 3. Drum-separator 4. Main circulation pumps 5. Dispensing group manifolds 6. Water pipelines 7. Upper biological protection 8. Unloading and loading machine 9. Lower biological protection.
Multiple forced circulation circuit
This is a heat removal circuit from the reactor core. The main movement of water in it is provided by the main circulation pumps (MCP). In total, there are 8 main circulation pumps in the circuit, divided into 2 groups. One pump from each group is a reserve pump. The capacity of the main circulation pump is 8000 m 3 /h, the pressure is 200 m of water column, the engine power is 5.5 MW, the pump type is centrifugal, the input voltage is 6000 V.
In addition to the main circulation pump, there are feed pumps, condensate pumps and safety system pumps.
Turbine
In a turbine, the working fluid - saturated steam - expands and does work. The RBMK-1000 reactor supplies steam to 2 turbines of 500 MW each. In turn, each turbine consists of one high-pressure cylinder and four low-pressure cylinders.At the turbine inlet the pressure is about 60 atmospheres; at the turbine outlet the steam is at a pressure less than atmospheric. The expansion of steam leads to the fact that the flow area of the channel must increase; for this, the height of the blades as the steam moves in the turbine increases from stage to stage. Since steam enters the turbine saturated, expanding in the turbine, it quickly becomes moistened. The maximum permissible moisture content of steam should usually not exceed 8-12% in order to avoid intense erosive wear of the blade apparatus by water drops and a decrease in efficiency.
When the maximum humidity is reached, all steam is removed from the high-pressure cylinder and passed through a separator - steam heater (SPP), where it is dried and heated. To heat the main steam to saturation temperature, steam from the first turbine extraction is used, live steam (steam from the separator drum) is used for superheating, and the heating steam drains into the deaerator.
After the separator - steam heater, the steam enters the low pressure cylinder. Here, during the expansion process, the steam is again moistened to the maximum permissible humidity and enters the condenser (K). The desire to get as much work as possible from every kilogram of steam and thereby increase efficiency forces us to maintain the deepest possible vacuum in the condenser. In this regard, the condenser and most of the low-pressure cylinder of the turbine are under vacuum.
The turbine has seven steam extractions, the first is used in the separator-superheater to heat the main steam to saturation temperature, the second extraction is used to heat water in the deaerator, and extractions 3 – 7 are used to heat the main condensate flow in, respectively, PND-5 - PND- 1 (low pressure heaters).
Fuel cassettes
Fuel rods and fuel assemblies are subject to high reliability requirements throughout their entire service life. The complexity of their implementation is aggravated by the fact that the length of the channel is 7000 mm with a relatively small diameter, and at the same time, machine overload of the cassettes must be ensured both when the reactor is stopped and when the reactor is running.
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Loading and unloading machine (RZM)
A distinctive feature of the RBMK is the ability to reload fuel cassettes without stopping the reactor at rated power. In fact, this is a routine operation and is performed almost daily.The installation of the machine over the corresponding channel is carried out according to coordinates, and precise guidance to the channel using an optical-television system, through which you can observe the head of the channel plug, or using a contact system in which a signal is generated when the detector touches the side surface of the top of the channel riser.
The REM has a sealed case-suit surrounded by biological protection (container), equipped with a rotary magazine with four slots for fuel assemblies and other devices. The suit is equipped with special mechanisms for performing overload work.
When reloading fuel, the suit is compacted along the outer surface of the channel riser, and a water pressure is created in it equal to the coolant pressure in the channels. In this state, the stopper plug is released, the spent fuel assembly with suspension is removed, a new fuel assembly is installed and the plug is sealed. During all these operations, water from the rare earth metal enters the upper part of the channel and, mixing with the main coolant, is removed from the channel through the outlet pipeline. Thus, when reloading fuel, continuous circulation of the coolant is ensured through the overloaded channel, while water from the channel does not enter the rare earth metal.
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