High power channel reactor. RBMK high-power channel reactor Metal structure of the “KZh” circuit
1. Introduction
2. Control and protection system in the RBMK-1000 reactor
3. Control rods
4.Reducing the positive effect of reactivity during dehydration of CPS CO
5. Differential and integral characteristics of the rod
6. Block diagram of RBMK reactor control
Control and protection system in the RBMK-1000 reactor
For continuous operation of the reactor, the core must be in a critical state. Therefore, for the reactor to operate, it is necessary that the core have excess reactivity to compensate for the gradual decrease in the amount of fissile material during the burnup process, as well as to compensate for changes in reactivity due to the accumulation of fission products. This excess reactivity must be compensated for at all times to keep the reactor in a critical state when operating at steady-state power levels. This problem is solved with the help of regulatory bodies that use materials that are strong neutron absorbers. Regulatory bodies perform the following tasks:
Regulate the energy release in the core;
Carry out a quick shutdown of the reactor;
Compensates for rapid and slow changes in reactivity caused by temperature fluctuations, accumulation of fission products and depletion of fissile material.
In reactor engineering, the most widely used method for changing the neutron flux is one in which the amount of substances that absorb neutrons is regulated. It should be noted that a very large absorption cross section will lead to rapid depletion of the absorbing material due to the transformation of its nuclei into other nuclei that are not strong neutron absorbers. For this reason, strong neutron absorbers are used mostly as burnable absorbers, the amount of which in the core must be gradually reduced to compensate for the decrease in the amount of fissile material during the burnup process. To operate successfully under reactor conditions, the materials of the control bodies must have such properties as mechanical strength, high corrosion resistance, chemical stability at operating temperature and irradiation, relatively low density so that the regulator can move quickly, accessibility and relatively low price, good machinability.
In the RBMK-1000 control system, the neutron flux is controlled by introducing absorber rods containing boron into the core. Natural boron consists of two isotopes (19% 10B and 81% 11B) and has a lower absorbency than 10B. Boron is rarely used in pure form, for the manufacture of rods, boron carbide (B4C) is mainly used - a refractory material with a melting point between 2340 and 2480 ° C. Powder metallurgy methods are mainly used to manufacture boron carbide products. The main problem when using boron carbide is its swelling as a result of the formation of helium gas through the following neutron reactions: 10 3 4 B H 2 He n + → +⎡ ⎤ ⎣ ⎦; 10 7 4 B Li He. n + → + The movement of the absorber rod is carried out using an actuator. The actuators operate in conjunction with rod position indicators in the core, equipped with synchronous sensors, and rod travel limiters in extreme positions. Pointer accuracy ±50 mm. Information about the position of the rods is provided to synchronous indicators operating in indicator mode in conjunction with synchronous sensors and placed on the control panel mnemonic display in the main control room and on the reactor plateau in the central hall. The absorber rod and the actuator form the actuator.
The structure of the control system includes executive bodies.
Executive bodies RR designed for manual regulation of the energy release field, USP– for manual control of the energy release field in the lower half of the core. Their distinctive features are entry from the bottom of the active zone and half the length relative to the length of the RR rods. The executive bodies of the AR and LAR are part of the reactor power auto-regulators, which are represented by the following automatic regulators: AWS– low power level regulator;
AR– two regulators of the main power range, only one regulator can be in operation, the second in standby mode;
LAR– local automatic reactor power regulator, used in the main power range; with the help of LAR, the power of 9–12 zones is controlled, into which the reactor core is conventionally divided.
Executive bodies LAZ perform the function of preventive protection, are inserted into the core until the alarm signal is removed in case of emergency exceeding a given power level in the LAR control zones. LAZ executive bodies can be used for manual regulation. To enable the executive bodies of the LAZ to perform their protective functions, the logical circuit of the LAZ imposes restrictions on their position in the core. LAZ executive bodies are also used to implement the overcompensation mode (PK-AZ). The PC mode is intended for additional automatic input of negative reactivity during an emergency power reduction of AZ-1, AZ-2, controlled power reduction (CPR), carried out by the switched on autoregulator LAR or 1(2)AR. The need for additional input of negative reactivity is due to the fact that the executive bodies of the autoregulator cannot provide the required speed of emergency power reduction. The executive bodies of the BAZ are intended only for emergency shutdown of the reactor. To perform their functions they must always be cocked. The control and protection system in the RBMK reactor is practically the only means of operational reactivity control, including shutting down the reactor and ensuring subcriticality. That is, it is a very important element from the point of view of ensuring the nuclear safety of the reactor plant. Let us consider in more detail some elements of the control system.
Control rods
Currently, four types of control rods are used in reactors.
RR rods (AR, LAZ, LAR) Their design was formed as a result of improving the design of the control rods of the first-stage reactors during the implementation of measures to improve safety. Distinctive feature from previous designs is that the length of the control rods is increased to 6.55 m (at the first stages they have a length of 5.5 m, at the second - 6.2 m) and when the rods are positioned on the VC, the absorbing part is located at the upper edge of the core , and the bottom of the displacer is on the lower section of the active zone. This ensures the input of negative reactivity throughout the entire range of movement and eliminates the input of positive reactivity in all situations, which was not excluded with the previous design. Design and location of the PP rod in the CPS channel. The disadvantage of rods of this design is the presence of a large column of water (~ 2.5 m) between the displacer and absorber in the area of the telescopic connection. This is the reason for the great positive effect of dehydration of CPS in critical condition. In order to reduce this drawback with further improvement of these control rods, a design with a thickened telescope and a skirt design of lower absorbers has been developed. Rods of this design were introduced at SAPP.
Design and location of the rod in the PP channel of the control rod:
1 – servo drive; 2 – pressure pipeline; 3 – channel head; 4 – protective plug; 5 – absorbent rod; 6 – telescopic displacer rod; 7 – displacer; 8 – drain pipeline
After installing 25 rods, the dewatering effect of critical control system CP, measured in a cold reactor, decreased by 0.1 β. After installing 50 rods on blocks 1 and 2, the magnitude of the dehydration effect of the CPS system decreases by β. Rods of this design are dialed into PP and LAZ modes. The speed of insertion of rods into the core according to a signal from the control key is 17-18 s, and according to an emergency protection signal - 12 s. Rapid emergency protection rods (BAZ) They differ from the previous ones in that they do not have a displacer and the diameter of the absorbing elements is larger than that of RR rods. In addition, the channels for the BAZ rods are film cooled. The speed of insertion of the BAZ rods from the control key is 6-7 s, according to the BAZ signal - 2.5 s. The efficiency of BAZ rods is ∼ 2 β. Having such characteristics, the BAZ rods, together with other rods, provide a sufficient rate of input of negative reactivity (1 β/s) according to the BAZ signal and are guaranteed to shut down the reactor. Shortened USP absorber rods USP rods consist of the same structural elements as RR rods: an absorber of four links 4088 mm long and a displacer of six links 6700 mm long. The stroke of the USP rods is 3500 mm. USP rods, unlike all other types of rods, are inserted into the active zone from below. Instead of a telescopic load-bearing element, a fixed load-bearing element is installed between the absorber and the displacer. Throughout the entire path of movement of the USP rod, a constant gap between the absorber and the displacer is maintained; the gap size is 150 mm. The presence of USP in the reactor core is due to the following design features RBMK-1000 reactor, as:
The presence of steam in the upper part of the core, leading to the fact that the upper parts of the DP of fully submerged control rods are more efficient than the lower ones;
The reactivity margin on the partially submerged rods RR and AP is realized in the upper part of the core;
Columns of water between the absorbers and displacers of the control rods located on the VC absorb neutrons better than the displacers.
All these features lead to the energy release field shifting to the lower part of the core. To maintain its shape, close to symmetrical, USP is provided. They have an absorbent part length of 4 m and are inserted from below. Layout of the control rod actuator rods along the height of the RBMK reactor core
As a fuel element in the RBMK-1000 reactor, a zirconium tube with a diameter of 13.9 mm, a wall thickness of 0.9 mm and a length of about 3.5 m, closed at both ends, is used, filled with fuel pellets with a diameter of 11.5 mm and a height of 15 mm. To reduce the thermal expansion of the fuel column, the tablets have holes. The initial medium under the shell is filled with helium under a pressure of 5 kgf/cm 2. The fuel column is fixed with a spring. Maximum temperature in the center of the fuel pellet can reach 2100ºС. In reality, this temperature is not higher than 1600ºС, the helium pressure is up to 17 kgf/cm 2, and the temperature of the outer surface of the fuel rod shell is about 300°С.
Fuel elements (fuel rods) are arranged into fuel assemblies (FA) of 18 pieces each; 6 pieces around the circumference with a diameter of 32 mm and 12 pieces with a diameter of 62 mm. In the center there is a supporting rod (see Fig. 2.14, section B-B). The fuel rods in the assembly are fastened every half meter with special spacer grids.
The main fuel block of the reactor is the fuel (or working) cassette; it consists of two fuel assemblies connected by a common support rod, a rod, a tip and a shank. Thus, the part of the cassette located in the active zone has a length of about 7 m.
The cassettes are washed with water, and there is no direct contact of the fuel with the coolant during normal operation of the reactor.
To obtain an acceptable coefficient useful action A nuclear power plant needs to have the highest possible temperature and pressure of the steam generated by the reactor. Therefore, a housing must be provided to hold the coolant at these parameters. Such a vessel is the main structural element of VVER-type reactors. For RBMK reactors, the role of the housing is played by a large number of durable pipelines, inside of which the cassettes are placed. Such a pipeline is called a technological channel (TC), within the core it is zirconium and has a diameter of 88 mm with a wall thickness of 4 mm; in the RBMK-1000 there are 1661 technological channels.
Rice. 1.14. RBMK reactor fuel assembly
The technological channel (see Fig. 1.13) is designed to accommodate fuel assemblies and organize the coolant flow.
The channel body is a welded structure consisting of middle and end parts. The middle part of the channel is made of zirconium alloy, the end parts are made of stainless steel. They are connected to each other by steel-zirconium adapters. The channel body is designed for 23 years of trouble-free operation, however, if necessary, when the reactor is shut down, the defective channel body can be removed and a new one installed in its place.
The fuel cassette is installed inside the channel on a suspension that holds it in the core and allows, with the help of rare earth metals, to replace the spent cassette without shutting down the reactor. The suspension is equipped with a locking plug that seals the channel.
In addition, the reactor contains control and protection channels. They contain absorber rods and energy release control sensors. Placement of control channels in graphite masonry columns is independent from process channels.
The space between the graphite and the channels is filled with gas, which has good thermal conductivity, low heat capacity and does not have a significant effect on the course of the chain reaction. The best gas from this point of view is helium. However, due to its high resistance, it is not used in its pure form, but in a mixture with nitrogen (at the rated power level 80% helium and 20% nitrogen, at lower power there is more nitrogen, at 50% rated there may already be pure nitrogen).
At the same time, contact of graphite with oxygen is prevented, i.e. its oxidation. The nitrogen-helium mixture in the graphite stack is blown from the bottom up, this is done to achieve the third goal - monitoring the integrity of the technological channels. Indeed, when the TC leaks, the humidity of the gas at the exits from the masonry and its temperature increases.
To improve heat transfer from graphite to the channel, a kind of labyrinth is created when the gas moves (see Fig. 1.15). Split graphite rings, each 20 mm high, are alternately placed on the channel and the holes of the blocks over a section of 5.35 m in the center of the core. Thus, the gas moves according to the following scheme: graphite – ring cut – channel wall – ring cut – graphite.
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, interconnected by a point resistance 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 heated to each other high temperature parts made of zirconium alloy and stainless steel. 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. Developed electron beam welding modes, methods and modes of mechanical and heat treatment welds and heat-affected zones made it possible to obtain reliable vacuum-tight steel-zirconium welded joints.
Ministry of Education and Science Russian Federation National Research Nuclear University "MEPhI" Obninsk Institute of Nuclear Energy
A.S. Shelegov, S.T. Leskin, V.I. Slobodchuk
PHYSICAL FEATURES AND DESIGN OF THE RBMK-1000 REACTOR
for students of higher educational institutions
Moscow 2011
UDC 621.039.5(075) BBK 31.46ya7 Sh 42
Shelegov A.S., Leskin S.T., Slobodchuk V.I. Physical features and design of the reactor RBMK-1000: Study guide. M.: National Research Nuclear University MEPhI, 2011, – 64 p.
The principles of physical design, safety criteria and design features of a nuclear power reactor of the standard RBMK-1000 design are considered. The design of fuel assemblies and core fuel channels, principles and means of controlling the reactor installation are described.
The main features of the physics and thermal hydraulics of the RBMK-1000 reactor are outlined.
The manual contains basic technical specifications reactor installation, reactor control and protection systems, as well as fuel elements and their assemblies.
The information presented can be used only for training and is intended for students of specialty 140404 “Nuclear Power Plants and Installations” when mastering the discipline “Nuclear Power Reactors”.
Prepared within the framework of the Program for the creation and development of National Research Nuclear University MEPhI.
Reviewer: Dr. Phys.-Math. sciences, prof. N.V. Shchukin
Introduction
The creation of nuclear power plants with RBMK channel uranium-graphite reactors is a national feature of the development of domestic energy. The main characteristics of power plants were chosen in such a way as to make maximum use of the experience in the development and construction of industrial reactors, as well as the capabilities of the mechanical engineering and construction industries. The use of a single-circuit design of a reactor installation with a boiling coolant made it possible to use mastered thermomechanical equipment with relatively moderate thermophysical parameters.
The first Soviet industrial uranium-graphite reactor was put into operation in 1948, and in 1954, a demonstration uranium-graphite water-cooled reactor of the world's first nuclear power plant began operating in Obninsk electrical power 5 MW.
Work on the project of the new RBMK reactor was launched at the Institute of Atomic Energy (now RRC KI) and NII-8 (now NIKIET named after N.A. Dollezha-
la) in 1964
The idea of creating a high-power channel boiling energy reactor was institutionalized in 1965. It was decided to develop a technical design for a 1000 MW(e) channel boiling energy reactor according to the technical specifications of the Institute of Atomic Energy named after. I.V. Kurchatov (the application for a method of generating electricity and the RBMK-1000 reactor with priority dated October 6, 1967 was submitted by IAE employees). The project was initially called B-19), and its construction was first entrusted to the design bureau of the Bolshevik plant.
In 1966, on the recommendation of the NTS ministry, work on technical project the high-power channel boiling water reactor RBMK-1000 was entrusted to NIKIET. By Resolution of the Council of Ministers of the USSR No. 800-252 of September 29, 1966, a decision was made to build the Leningrad Nuclear Power Plant in the village of Sosnovy Bor, Leningrad Region. This resolution identified the main developers of the plant and reactor project:
KAE – scientific director of the project; GSPI-11 (VNIPIET) – general designer of LNPP; NII-8 (NIKIET) – chief designer of the reactor plant.
At the IV UN Geneva Conference in 1971, the Soviet Union announced the decision to build a series of RBMK reactors with an electrical power of 1000 MW each. The first power units were put into operation in 1973 and 1975.
CHAPTER 1. Some aspects of the safety concept of RBMK reactors
1.1. Basic principles of physical design
The concept for the development of channel uranium-graphite reactors, cooled by boiling water, was based on design solutions proven by the practice of operating industrial reactors, and assumed the implementation of the RBMK physics features, which together were supposed to ensure the creation of safe power units of large unit capacity with a high installed capacity utilization factor and economical fuel cycle.
Among the arguments in favor of the RBMK were the advantages due to the better physical characteristics of the core, primarily the better neutron balance due to the weak absorption of graphite, and the ability to achieve deep burnup of uranium due to continuous fuel refueling. The consumption of natural uranium per unit of energy generated, which at that time was considered one of the main criteria for efficiency, was approximately 25% lower than in VVER.
The initial idea that the physical problems of the RBMK do not require significant adjustments to the developed methods of physical research of industrial reactors, but are associated only with the use of zirconium instead of aluminum as the main structural material of the core, had to be abandoned almost immediately. Already the first assessments of neutronic (and thermophysical) characteristics showed the need to solve a wide range of problems to optimize the physical parameters of the reactor and develop methodological and software:
The main problems in determining the optimal physical characteristics of the RBMK are the safety and efficiency of the fuel cycle. The nuclear safety of a reactor is ensured by the ability to monitor and control reactivity in all operating modes, which requires the determination of safe ranges for changes in effects and reactivity coefficients. Particularly important are the physical characteristics that determine the passive safety of the reactor installation, as in
conditions of normal operation, as well as in emergency and transient modes. No less important characteristics that ensure nuclear safety are the efficiency and speed of the working parts of the safety control system, which ensure damping and maintaining it in a subcritical state.
The technical and economic performance of a reactor installation is also largely determined by such physical characteristics as the burnup and nuclide composition of the discharged fuel, the specific consumption of natural and enriched uranium and fuel assemblies per unit of electricity generated, and the components of the neutron balance in the core.
1.2. Basic principles and criteria for ensuring safety
The main safety principle underlying the design of the RBMK-1000 reactor plant is not exceeding the established doses for internal and external exposure. service personnel and the population, as well as standards for the content of radioactive products in the environment during normal operation and accidents considered in the project.
The set of technical means for ensuring the safety of the RBMK-1000 reactor installation performs the following functions:
reliable control and management of energy distribution throughout the core volume;
diagnosing the state of the core for timely replacement of structural elements that have lost their functionality;
automatic power reduction and reactor shutdown in emergency situations;
reliable cooling of the core in the event of failure of various equipment;
emergency cooling of the core in case of ruptures of circulation loop pipelines, steam pipelines and feed pipelines.
ensuring the safety of reactor structures during any initiating events;
equipping the reactor with protective, localizing, control systems for safety and removal of coolant emissions in the event of depressurization of pipelines from the reactor premises to the localization system;
ensuring the maintainability of equipment during operation of the reactor plant and during the liquidation of the consequences of design basis accidents.
During the design process of the first RBMK-1000 reactor plants, a list of initial emergency events was compiled and the most unfavorable paths of their development were analyzed. Based on the experience of operating reactor plants at the power units of the Leningrad, Kursk and Chernobyl nuclear power plants and as the requirements for nuclear power plant safety become more stringent, which is taking place
V world energy in general, the initial list of initiating events has been significantly expanded.
The list of initiating events in relation to RBMK-1000 reactor installations of the latest modifications includes more than 30 emergency situations, which can be divided into four main principles:
1) situations with changes in reactivity;
2) accidents in the core cooling system;
3) accidents caused by pipeline ruptures;
4) situations involving equipment shutdown or failure.
The design of the RBMK-1000 reactor plant, when analyzing emergency situations and developing safety equipment, includes the following safety criteria in accordance with OPB-82:
1) the rupture of a pipeline of maximum diameter with unimpeded two-way flow of coolant when the reactor is operating at rated power is considered as a maximum design basis accident;
2) the first design limit for damage to fuel rods for normal operating conditions is: 1% of fuel rods with defects such as gas leakage and 0.1% of fuel rods with direct contact of the coolant and fuel;
3) The second design limit for damage to fuel rods in the event of ruptures in the circulation circuit pipelines and activation of the emergency cooling system sets:
fuel cladding temperature− no more than 1200 °C;
local depth of fuel cladding oxidation− no more than 18% of the original wall thickness;
proportion of reacted zirconium− no more than 1% of the mass of the fuel element cladding of the channels of one distribution manifold;
4) the possibility of unloading the core and the removability of the process channel from the reactor after the MPA must be ensured.
1.3. Advantages and disadvantages of channel uranium-graphite power reactors
The main advantages of channel power reactors, confirmed by more than 55 years of experience in their development and operation in our country, include the following.
Disintegration of the structure:
absence of problems associated with the manufacture, transportation and operation of the reactor vessel and steam generators;
easier accidents in case of ruptures of coolant circulation circuit pipelines compared to pressure vessel reactors;
large volume of coolant in the circulation circuit.
Continuous refueling:
small reactivity margin;
reduction of fission products simultaneously present
in the core;
the possibility of early detection and unloading of fuel assemblies with leaking fuel rods from the reactor;
the ability to maintain a low level of coolant activity.
Heat storage in the core (graphite stack):
the possibility of heat flow from the channels of the dehydrated loop to the channels that have retained cooling, when organizing a “chessboard” arrangement of the channels of various loops;
reducing the rate of temperature rise during dehydration accidents.
A high level of natural coolant circulation, which allows for a long period of time to cool the reactor when the power unit is de-energized.
Possibility of obtaining the required neutronic characteristics of the core.
Fuel cycle flexibility:
low fuel enrichment;
the ability to burn spent fuel from VVER reactors after regeneration;
opportunity to develop wide range isotopes. Disadvantages of channel water-graphite reactors:
complexity of organizing control and management due to the large size of the active zone;
the presence in the core of structural materials that worsen the neutron balance;
assembly of the reactor on installation from separate transportable units, which leads to an increase in the volume of work on the construction site;
branching of the reactor circulation circuit, which increases the scope of operational control of the base metal and welds and dose costs during repair and maintenance;
generation of additional waste due to the graphite stack material when the reactor is decommissioned.
CHAPTER 2. Design of the RBMK-1000 reactor
2.1. General description reactor design
The RBMK-1000 reactor (Fig. 2.1) with a thermal power of 3200 MW is a system that uses light water as a coolant and uranium dioxide as fuel.
The RBMK-1000 reactor is a heterogeneous, uranium-graphite, boiling-type, thermal neutron reactor designed to produce saturated steam with a pressure of 70 kg/cm2. The coolant is boiling water. The main technical characteristics of the reactor are given in table. 2.1.
Rice. 2.1. Section of the block with the RBMK-1000 reactor
A set of equipment including nuclear reactor, technical means that ensure its operation, devices for removing thermal energy from the reactor and converting it into another type of energy, as a rule, are called a nuclear power plant. Approximately 95% of the energy released as a result of the fission reaction is directly transferred to the coolant. About 5% of the reactor power is released in graphite from moderating neutrons and absorbing gamma rays.
The reactor consists of a set of vertical channels inserted into the cylindrical holes of graphite columns, as well as upper and lower protective plates. A lightweight cylindrical body (casing) closes the cavity of the graphite stack.
The masonry consists of graphite blocks of square cross-section assembled into columns with cylindrical holes along the axis. The masonry rests on a bottom slab, which transfers the weight of the reactor to the concrete shaft. Fuel and control rod channels pass through the lower and upper metal structures.
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 control and protection system (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 others 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 core radius 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 cylinders low pressure.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 (SHP), 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 presented high demands reliability throughout the 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.
