The position of the reactor core and its main components in a RBMK-1500 plant is shown in the schematic cross-section through the main reactor building provided in Fig. 4.1.
The core of the reactor is housed in a 25 m deep, 21x21 m cross-section concrete vault. The core volume is dominated by a large cylindrical graphite stack (1), an isometric drawing of this structure is shown in Fig. 4.2. The graphite stack is constructed of closely packed graphite blocks stacked into columns and provided with an axial opening. Most of the openings contain fuel channels. A number of them also serve other purposes (e.g. instrumentation, reactivity regulation). Therefore, adapting the nomenclature of the designer, these will be referred to as "special channels".
Fig. 4.1 General view of the reactor
1 - graphite stack, 2 - fuel channel feeder pipes, 3 - water pipes, 4 - distribution header, 5 - emergency core cooling pipes, 6 - pressure pipes, 7 - main circulation pump, 8 -suction pipes, 9 - pressure header, 10 - bypass pipes, 11 - suction header, 12 - downcomers, 13 - steam and water pipes, 14 - steam pipes, 15 - refueling machine, 16 - separator drum
Fig. 4.2 General view of the graphite stack and the water-steam piping
The graphite stack is located in a hermetically sealed cavity consisting of cylindrical walls and top and bottom metal plates. The entire reactor cavity is filled with a helium (about 40 % by mass) and nitrogen mixture which prevents graphite oxidation and improves heat transfer from the graphite to the fuel channels. In order to prevent loss of helium, the space surrounding the cylindrical graphite stack is filled with nitrogen at a pressure of about 0.29 - 0.98 kPa greater than that of the helium-nitrogen mixture [62]. In the radial direction as well as above and below the reactor it is surrounded by the primary biological shield structures.
The coolant channels penetrating the reactor core are divided into two essentially independent cooling loops: one containing 830, the other 831 vertical channels with fuel assemblies. Each loop is provided with four main circulation pumps, one of which is kept in reserve during normal operation. The circulation of the coolant in each of the reactor cooling loops can be traced with the help of Fig. 4.1. Water from the main pump pressure header (9) is distributed first to 20 group distribution headers (4) from there to the feeder pipes (3), these then lead to the individual fuel channels. The water rises past the fuel assemblies, attains its saturation temperature, partly vaporizes (average steam quality is 23 %) and, in the form of a steam-water mixture, flows through the header pipes (13) to the separator drums (16). Here the two-phase mixture is separated, and the steam continues to the turbines. The condensate passes through deaerators, returns to the separator drums where it mixes with the unvaporized water. From there the condensate flows via standpipes to the pump suction header (11), and then to the main circulation pumps (7), which return it to the fuel channels.
As noted in previously, one of the important characteristics of RBMK reactors is their online refueling capability. Refueling at full reactor power is accomplished by means of the fueling machine (15). Under normal operation and nominal reactor power, it is feasible to change up to two fuel assemblies per day (24-hours). The maximum capacity of this machine is 5 fuel assemblies per day.
The reactor is provided with instrumentation systems monitoring the following parameter groups:
These systems provide integral information regarding the operation of the entire reactor and information regarding specified reactor core segments and individual fuel channels. Because of the very large size of the core and the resulting weak neutronic coupling of distant core segments, reactor operation requires a detailed spatial resolution of the main operational parameters.
4.2.1 The Graphite Stack
An isometric view of the graphite stack is presented in Fig. 4.2. The graphite stack of the RBMK-1500 reactors serves several functions. The primary one is neutron moderation and reflection, but it also provides structural integrity and in the event of a temporary cooling malfunction, a relatively large heat capacity.
The graphite blocks are assembled within the inner cavity of the reactor on a supporting metal structure. The stack can be visualized as a vertical cylinder, made up of 2488 graphite columns, constructed from various types of graphite blocks. Fig. 4.3 illustrates how these blocks fit together in the lower and upper regions of the stack. The blocks are rectangular parallelepipeds, with a base of 0.25 x 0.25 m, and heights of 0.2, 0.3, 0.5 and 0.6 m of which the 0.6 m blocks are most common. The short blocks are used only in the top and bottom end reflectors, as required to provide a staggered fit to neighboring columns. The total mass of graphite is about 1700 tons. The material must meet stringent purity requirements and has a density of 1650 kg/m3.
The outer edge of the graphite stack is covered by a metal liner. The four rows of columns at the outer edge make up the radial reflector, and a 0.5 m thick layer at the top and bottom make up the end reflectors. The blocks possess a 0.114 m diameter bore opening through the vertical axis. This provides a total of 2044 channels which are used for placing fuel clusters, reactivity regulating control rods and several types of instruments into the core. In the remaining 444 columns located within the radial reflector the central holes are filled by graphite rods, increasing the density and neutron reflecting effectiveness of this part of the graphite stack. As shown in Fig. 4.3, the graphite columns rest on a steel support plate (5) which, in turn, is supported by a steel bushing (4). The bushing is welded to the top plate of the bottom biological shield. At the top of the stack, the columns are fastened and centered with respect to the guide pipes (9) welded into the top biological shield, by means of shield plates (7) and junction sleeves (8).
Fig. 4.3 Segment of the graphite stack
1 - top ring, 2 - diaphragm, 3 - bottom ring, 4 - bushing, 5 - steel support plates, 6 - graphite rods, 7 - shield plates, 8 - standpipe, 9 - guide pipe, 10 - reinforcing tube (reflector cooling channel), 11 - outer steel shell, 12 - ring plate
The shield and support plates have a similar purpose: namely, they consist of steel and, in addition to their fundamental function of joining the intermediate elements of the graphite stack, also ensure thermal insulation of the top and bottom metal structures, and in part serve as biological shielding. A diaphragm, indicated as (2) in Fig. 4.3 is fastened to the support bushings by means of a special rings (1), (3). Its primary purpose is to channel the helium-nitrogen flow coming through the bottom biological shield into the spaces between the channels and the graphite blocks. Secondly, it is intended to reduce the radioactive heat transfer between the support plates and the top plate of the bottom biological shield. The diaphragm is a 5 mm thick stainless steel sheet. The radial span between the diaphragm and the inside of the stack shell (11) is covered by a ring (12).
Radial creep of the graphite stack is restrained by 156 hollow reinforcing bars (10). These bars are positioned in the peripheral columns of the radial reflector. At the bottom, the reinforcing bars are welded to the support plate, while at the top they fit loosely into the guide tubes welded to the bottom plate of the top biological shield. This connection at the top allows freedom for thermal expansion. Since the reinforcing bars are hollow, they also serve as reflector cooling channels. Cooling water to these channels is supplied from above. The reinforcing bars are made from stainless steel tubes, with outside diameters of 0.110 m and wall thickness of 5 mm.
The corners of the rectangular cross-sections of the graphite columns in the stack are hollow and incorporate 17 vertical 45 mm diameter instrumentation channels used for measuring the temperatures of the graphite stack itself as well as the support and the shielding plates. Thirteen of these channels are positioned within the boundaries of the core, while four are in the radial reflector. Within each channel the temperature is measured at 5 vertical positions.
The graphite stack, including its hermetically sealed cavity, is called the sealed reactor space. This space is filled with a circulating helium-nitrogen mixture at a pressure of 0.49 - 1.96 kPa. During normal operation, the gas is supplied by means of 0.3 m inside diameter tubes, and removed through the fuel channel integrity monitoring system. Four drainage tubes (inside diameter of 0.15 m) are provided in order to guard against accidental releases.
When the reactor is in operation, all the components listed above are subjected to conditions of high temperature and intense neutron/gamma radiation. For example, the temperature of the support structures in the top part of the bottom biological shield reaches 350oC. The temperature of the bottom support plates reaches 440oC, while the maximum calculated graphite temperature is 750oC.
4.2.2 Reactor Metal Structures
Fig. 4.4 provides a schematic overview of the principal metal components surrounding and supporting the reactor core. They consist of welded metal structures which transmit the weight of the reactor core and its components to the concrete foundations, and ensure the leaktightness of the inner reactor cavity. These structures also contribute to biological shielding.
Fig. 4.4 Cross-section of the reactor vault
1 - top cover, removable floor of the central hall, 2 - top metal structure filled with serpentinite, 3 - concrete vault, 4 - sand cylinder, 5 - annular water tank, 6 - graphite stack, 7 - reactor vessel, 8 - bottom metal structure, 9 - reactor support plates, 10 - steel blocks, 11 - roller supports
The graphite stack is surrounded by a water-filled biological shield tank, where the water is contained in an annular metal tank (5). It has an outside diameter of 19 m, an inside diameter of 16.6 m, the plate wall thickness is 30 mm. Internally this reservoir is divided into 16 water-filled sealed vertical sections. Water is supplied to these reservoirs from the bottom, and is removed from the top. This shield component also contains the startup and operating range ion chamber channels, and instrumentation piping for thermocouples assigned to monitor shield water temperature. The space between the wall of the concrete vault and the shield tank (4) is filled with sand.
The most complicated heavy components are the top (2) and bottom (8) metal structures. The top cover is a 17.65m diameter cylinder, 3 m high, an isometric representation of which is shown in Fig. 4.5. The top and bottom of this cylindrical structure is made from a 40mm thick steel plate. Along the outside periphery these plates are hermetically welded and internally they are joined together by means of rigid vertical plates. Axial holes through this structure are positioned to match the openings in the graphite stack. Tubes are welded into these holes, to serve as guides for the fuel channels and other components of the control and instrumentation system. The inside cavities of this metal structure are filled with serpentinite (a mineral containing bound crystalline water). The quality of the welds must be adequate to meet helium leak-tightness requirements. The entire metal structure rests on 16 rollers, which in turn rest on the top of the reinforced structure of the radial biological shield tank (see Fig. 4.4). This structural component supports the weight of the loaded fuel and control channels, that of the floor segment extending to the central refueling hall, and the weight of the water pipes.
The construction of bottom metal structure (8) is very similar to top metal structure. The diameter of this structure is 14.5 m, its height - 2 m. This structure supports the weight of the graphite stack and the feeder pipes supplying coolant water to the fuel channels. The number and distribution of the openings is the same as those of the top biological shield. The leak-tightness of the structure is tested with an air-helium mixture at a pressure of about 0.125 MPa. The remaining internal spaces of the structure are filled with serpentinite and are pressurized with nitrogen.
Fig. 4.5 Top metal structures
The bottom metal core support (9) shown in Fig.4.4 supports the weight of the entire graphite stack, the bottom biological shield, and the coolant water feeder pipes. The design of this support structure is rather simple: it consists of two heavy plates, which intersect at right angles along the center-line of the reactor and are in turn reinforced by 5.0 m high fins. These plates are welded to the bottom of the biological shield plate (8).
The cylindrical shell (7) of the reactor core (Fig 4.4) is constructed from a 16 mm thick plate, it has an outside diameter of 14.52 m and a height of 9.75 m. To compensate for axial thermal expansion, the shell is provided with a bellows compensator. The shell, together with the top and bottom metal structures, forms the sealed reactor core compartment.
The topmost structure, located above the coolant channel banks which pass through the core vertically and exit horizontally, is the upper shield cover. This structure is shown as item (1) in Fig. 4.4 and can also be identified readily in the isometric drawing of Fig. 4.2. The cover serves several purposes: it is a component of the biological shield, provides thermal insulation and controls the access to the fuel channels. The top surface of this cover is the floor of the refueling hall, its central part consists of individual plugs which can be removed for accessing the fuel channels and the special purpose channels. A schematic cross-section of this structure is shown in Fig. 4.6. It shows the block segments which bear against the tops of the vertical extensions (guide pipes) of the fuel channel tubes. The blocks are made of iron-barium-serpentinite concrete having a density of 4000 kg/m3. They are constructed of two parts: the top segment is removable to provide accessibility to the upper
Fig. 4.6 Segment of the top cover
1 - removable blocks, 2 - top cover of the control rod channel, 3 - bottom block, 4 - top cover of the fuel channel, 5 - top cover of the temperature instrumentation channel, 6 - top cover of the reflector cooling channel, 7 - peripheral part of the cover
segment of the channel during refueling, the bottom layer segments are larger - one of these blocks covers three channels. All of the blocks are supported by the guide pipes of the fuel channels and the reflector cooling channels. The top and bottom block are staggered so that the gaps between blocks are covered, and the amount of direct radiation is reduced. The peripheral sections of the top shield cover consist of 0.70 m high metal containers, filled with a mixture of cast iron fragments (86 %) and serpentinite.
Air is continuously drawn from the refueling hall down through the gaps of the cover plates, and out into the ventilation system. This provides cooling for the cover and impedes the transport of radioactive material from the steam-water pipes to the refueling hall.
The gaps between the top and bottom plates and the blocks are used for positioning the wiring for the control system servomotors, the flux distribution instrumentation, and the temperature-monitoring instrumentation (thermocouples). All of the reactor metal structures, which are in a gas and steam environment, are covered by an anti-corrosive material.
4.2.3 Biological Shielding
All of the structures surrounding the core region contribute to some extent to biological shielding. The principal structural components have been described in the previous Section, they are complemented where required by additional material. The principal structures serving the shielding function include - the graphite reflectors, the internal spaces of the metal structures, the gap between the concrete vault and the outer surface of the core support metal structures. With respect to the center of the core, the biological shields can be divided into three parts: top shield (in the direction of the refueling hall), bottom shield (in the direction of the lower coolant channel banks), and radial shielding.
Biological shielding in the direction of the refueling hall encompasses the 0.5 m thick upper graphite reflector, 0.25 m high steel shielding blocks, the upper metal structure which is filled with a mixture of serpentinite chips and gallium (weight ratio of 3:2), and the top shield cover. The density of the fill material is 1700 kg/m3, its height is 2.8 m, and the thickness of the steel foundation plates of the structure is 40 mm.
A number of special design features are incorporated into these structures in order to reduce direct streaming of radiation along the gas-filled channels (temperature, neutron flux instrumentation and ion chamber channels) and the fuel channels which in the upper region of the core are filled with a steam-water mixture. The fuel channels are capped with special steel-graphite plugs (Fig. 4.7) which incorporate spiral passages for the flow of the two-phase coolant. The ring-shaped gaps between the channels and the guide tubes are covered with shielding sleeves (Fig. 4.8). Graphite followers are employed in the control channels to reduce direct neutron and gamma streaming into the spaces underneath the reactor. Whenever possible, the gas and coolant pipes which penetrate the shielding structures are bent so that direct streaming is reduced.
The radial shield (Fig. 4.4) consists of the radial graphite reflector (average thickness 0.88 m), the shell of the core, the annular water-filled steel tank, sand filling between the tank and the walls of the reactor vault, and the 2 m thick concrete walls of the vault. The walls of the vault are made from ordinary construction concrete with a density of 2200 kg/m3.
Fig. 4.7 Fuel channel shielding plug
1 - steel sleeve, 2 - steel helical plug, 3 - channel pressure tube, 4. serpentinite filling
A summary of the compositions and dimensions of the components used for biological shielding is presented in Table 4.1 [35].
During reactor operation the biological shielding limits the radiation dose rate in the refueling hall and in the areas adjacent to the reactor to levels not exceeding 2.8 10-5 Sv/h. During refueling operations the gamma dose in selected locations close to the refueling machine can range up to 1.0 10-3 Sv/h.
It is reported that the tests of the biological shielding effectiveness, conducted in the first unit of the Ignalina NPP with the RBMK-1500 reactor operating at nominal power, confirmed that the radiation field in the reactor service areas meets health standard requirements. For
Fig. 4.8 Shielding sleeves in the top reflector
1 - graphite sleeves, 2 - steel shielding block, 3 - graphite reflector
example, the average equivalent dose rate in the central refueling hall was found to be (11 - 18) x 10-6 Sv/h, while in the access chamber to the coolant flow control valves it did not exceed 4 Sv/h, with the reactor operating at thermal power 3850 MW. These tests were performed by the Moscow Research and Development Institute of Power Engineering (RDIPE - Russian abbreviation of NIKIET) [38].
The walls of the compartments of the main coolant circuit equipment are made from ordinary concrete (density of 2200 kg/m3 ). Measurements of the effectiveness of biological shielding were performed on the second unit of Ignalina NPP in December, 1992. The measured dose rates are shown in column 3 of Table 4.2, and the calculated dose rates - in column 2 [3].
Table 4.1 Composition and dimensions (in meters) of principal biological shield components [35]
|
Material |
Shielding Direction |
||
|
Top |
Bottom |
Radial |
|
|
Graphite (reflector) |
0.5 |
0.5 |
0.88 |
|
Steel (shielding plates and metal structures) |
0.29 |
0.24 |
0.045 |
|
Serpentinite filling (1700 kg/m3) |
2.8 |
1.8 |
- |
|
Water (annular tank) |
- |
- |
1.140 |
|
Steel (metal structure) |
0.04 |
0.04 |
0.03 |
|
Sand (1300 kg/m3) |
- |
- |
1.3 |
|
Heavy concrete (4000 kg/m3) |
0.89 |
- |
- |
|
Construction concrete (2200 kg/m3) |
- |
- |
2.0 |
Table 4.2 Biological shielding parameters of the office premises which are adjacent to the operating equipment
|
Source - equipment |
Thickness of concrete shielding, m |
Calculated* dose rate, mR/h |
Measured** dose rate, mR/h |
|
Water-steam separators: |
|||
|
side walls and bottom covering |
1.4 |
1.4 |
2.5 |
|
end walls |
1.0 |
- |
0.9 |
|
top covering |
0.9 |
- |
0.26 |
|
Pipes between separators and main circulation pumps |
0.9 |
1.4 |
1.2 |
|
Main circulation pump premises: |
|||
|
wall adjacent to suction header |
0.9 |
0.4 |
0.6 |
|
top covering |
0.8 |
- |
0.9 |
|
walls between compartments |
0.6 |
- |
1.6 |
|
Lower water pipes |
0.5 |
0.7 |
3.0 |
|
Pipes from separator to turbine |
0.7 |
1.4 |
0.4 |
|
Low pressure re-heaters |
0.6 |
1.4 |
0.6 |
|
Deaerators |
0.24 |
0.8 |
0.15 |
* Data compiled from [3]
** Measurements taken at the Ignalina NPP, December 1992
By the project, the compartments, which are part of the accident localization system of the Ignalina NPP unit 2, were reinforced with 5 mm thick stainless steel liners on the floors and 4 mm thick liners on the walls.
During nuclear fission 95 % of the generated energy is released in the fuel element and an additional 5 % is released in the graphite during neutron moderation and gamma absorption. A helium-nitrogen mixture circulates around the fuel channels and between the graphite blocks. This gas retards the oxidation process, and the humidity and temperature readings of the gas are monitored to indicate leaks of the fuel channels.
Under normal reactor operating conditions the biological shielding makes it possible to perform certain repair and maintenance tasks. This applies to piping, which serves the various channels and is located below the bottom and above the top biological shields. The non-service compartments are accessible only during the reactor shutdown. The list of these compartments is given in Table 4.3 [36].
Table 4.3 List of non-service compartments [36]
|
Compartment |
Description |
|
125 |
Compartment below reactor |
|
208/1-2 |
ECCS compartments |
|
210 |
Reactor vault |
|
215 |
Connecting corridor between ACS tower |
|
408/1-2 |
Corridors |
|
409/1-2 |
Compartments of downcomers, pressure and suction headers |
|
506/1-2 |
Separator drum and ACS tower compartment |
4.2.4 Fuel Assembly and Fuel Channel
One of the principal distinguishing characteristics of the RBMK-type reactor is that each core fuel assembly is housed in an individual pressure tube. As was noted previously, the RBMK-1500 core contains 1661 fueled channels separated from its nearest neighbors by the walls of the pressure tubes and graphite blocks. Each pressure tube has considerable autonomy. For example, the coolant flow rate to the tube is controlled online by an individual isolation and control valve. This valve make it possible to de-couple it from the primary cooling system while the reactor is operation. This makes it possible to change fuel clusters online and also has a significant impact on the potential consequences of loss-of-coolant accidents.
4.2.4.1 Fuel Assembly
The nuclear fuel used in the Ignalina NPP is slightly enriched uranium (2% initially, converted to 2.4%) in the form of uranium dioxide. This is a chemically-stable and heat-resistant ceramic material. It is prepared in powdered form, pressed into small, 11.5 mm diameter and 15 mm long pellets and sintered in the presence of a binder. The pellet shape is adapted to an intensive, high-temperature operating mode. For example, the pellets have hemispherical indentations, in order to reduce the fuel column's thermal expansion and thermo-mechanical interaction with the cladding. The 2 mm diameter hole through the axis of the pellet reduces the temperature at the center of the pellet, and helps release the gases formed during operation.
The pellets are placed into a tube with an outside diameter of 1.3 cm, a wall thickness of 0.9 mm and an active length of 3.6 m. Tube material is an alloy of zirconium with one percent niobium. This alloy has good anti-corrosive properties and a low neutron absorption coefficient. The initial clearance between the UO2 pellets and the wall of the tube varies from about 0.22 to 0.38mm.
The tubes are pressurized with helium at 0.5 MPa and sealed. In the radial direction the fuel clad is augmented by retaining rings which help to withstand the pressure of the fuel channel and improve the heat transfer from the pellet to the zirconium tube. In the axial direction, the fuel pellets are held in place by a spring.
The design of the RBMK reactor fuel described so far differs little from fuel elements manufactured for standard BWR-type reactors. For example, a typical BWR fuel tube in the United States is also manufactured from a zirconium-niobium alloy, has a similar wall thickness and an outside diameter ranging from 12 to 13.5 mm. The uranium enrichment is also similar: namely, 2 % to 2.4% in the case of Ignalina NPP, 2.2% to 3% in the case of the BWR. More significant design differences are present in the manner in which the fuel elements are mounted into a structurally integral fuel assembly (or fuel cluster). The shape of the assembly is determined by the geometric characteristics of the core fuel channel. In the case of a BWR this results in a square-shaped (usually 8x8) fuel cluster which fits into the square core spaces between the control rod blades. For an RBMK reactor, the fuel assembly must fit into a circular channel having an inside diameter of 80 mm and an active core height of 7 m. In order to achieve the required height, two fuel elements must be joined end-to-end. The radial special restriction determines the arrangement and the number of the fuel rods which can fit into a fuel assembly.
A schematic representation of the principal features of a fuel assembly is shown in Fig. 4.9. The assembly contains 18 fuel elements arranged within two concentric rings in a central carrier rod. The carrier rod is a 15 mm diameter tube with a 1.25 mm wall thickness and is made of a zirconium (2.5 % Nb) alloy. The complete fuel assembly is made up of two segments which are joined by means of a sleeve (7) at the central plane. Thus, along the axis of the core there is a region in which fission does not take place. This generates a flattening of the fast neutron flux and a dip of the thermal neutron flux at this location and influences the neutron kinetic characteristics of the core.
The lower segment of the fuel assembly is provided with an end grid and ten spacing grids. The central tube and the end spacer are also made from the zirconium (2.5% niobium) alloy. The remaining spacers are made from stainless steel and are rigidly fixed (welded) to the central tube and are positioned 360 mm apart. The top segment has 10 spacing grids placed 360 mm apart, and in addition, at every 120 mm this segment is provided with specifically designed spacers which act as turbulence enhancers to improve the heat transfer characteristics. The fuel tubes are mounted so that axial expansion of the upper or lower segments takes place in the direction towards the center of the core.
Fig. 4.9 Fuel assembly
1 - suspension bracket, 2 - top plug, 3 - adapter, 4 - connecting rod, 5 - fuel element, 6 - carrier rod, 7 - end sleeve, 8 - end cap, 9 - retaining nut
For ease of manipulation, the fuel assembly is provided with appropriate fittings at both ends. The principal technical parameters of the fuel assemblies are summarized in Table 4.4 [2,35].
Table 4.4 Fuel assembly parameters [2,35,36]
|
Fuel pellet |
|
|
Fuel |
Uranium dioxide |
|
Fuel enrichment in 235U, % |
2 & 2.4 |
|
Edge pellet enrichment, % |
0.4 |
|
Fuel pellet density, kg/m3 |
10400 |
|
Fuel pellet diameter, mm |
11.5 |
|
Fuel pellet length, mm |
15 |
|
Pellet central orifice diameter, mm |
2 |
|
Maximum temperature at the center of the fuel pellet, 0C |
2100 |
|
Fuel element |
|
|
Fuel element cladding material |
Zr +1 % Nb |
|
Outside diameter of fuel element, mm |
13.6 |
|
Length of fuel element, m |
3.64 |
|
Active length of fuel element (height of fuel pallet column in cold state), m |
3.4 |
|
Cladding wall thickness, mm |
0.825 |
|
Clearance between fuel pellet and tube, mm |
0.22-0.38 |
|
Mass of fuel within fuel element, kg |
3.5 |
|
Helium pressure in the cladding, MPa |
0.5 |
|
Maximum permissible temperature of fuel element, 0C |
700 |
|
Average linear thermal flux, W/cm |
218 |
|
Maximum linear thermal flux, W/cm |
485 |
|
Fuel assembly |
|
|
Number of segments per fuel assembly |
2 |
|
Number of fuel elements per segment |
18 |
|
Total length of fuel assembly, m |
10.015 |
|
Active length of fuel assembly, m |
6.862 |
|
Fuel assembly diameter (in the core), mm |
79 |
|
Mass of fuel assembly without bracket, kg |
185 |
|
Total mass of fuel assembly with the bracket, kg |
280 |
|
Total steel mass of fuel assembly, kg |
2.34 |
|
Total mass of zirconium alloy within assembly, kg |
40 |
|
Mass of uranium within fuel pellet, kg |
111.2 |
|
Mass of uranium within edge fuel pellet, kg |
1.016 |
|
Maximum permissible power of fuel channel, MW |
4.25 |
|
Authorized fuel assembly capacity, MWdays/assembly |
2500 |
|
Authorized lifetime of fuel assembly, year |
6 |
The fuel assemblies described in this Section can be of two types: regular fuel assemblies and instrumented fuel assemblies which contain a neutron flux detector. In an instrumented fuel assembly the detector is contained within a tube which replaces the main carrier rod. This tube has an outside diameter of 15 mm and a wall thickness of 2.75 mm.
4.2.4.2 Fuel Channel
The top, center, and bottom segments of a typical reactor fuel channel are shown schematically in Fig. 4.10. The main component of the channel is the coolant-carrying tube constructed from separate end and center segments. The center segment (9) is an 88 mm outside diameter (4 mm thick wall) tube, made from a zirconium-niobium alloy (Zr + 2.5 % Nb). The top (3) and bottom (11) segments are made from a stainless steel tube. The choice of zirconium-niobium for the center part was made because of the relatively low thermal neutron absorption cross-section s = (0.2 - 0.3) × 10-29 m2) of the material and its adequate mechanical and anti-corrosive properties at high temperatures (up to 350 oC). The center and end pieces are joined by special intermediate couplings, made from a steel-zirconium alloy.
Fig. 4.10 Fuel channel
1 - steel biological shield plug, 2,10 - top and bottom metal structures, respectively, 3 - top part of the fuel channel, 4 - welding-support ledge, 5 - fuel assembly support bracket, 6 - encasement cylinder, 7 - seal plug, 8 - graphite cylinder, 9 - central part of the channel, 11 - bottom part of the channel, 12 - thermal expansion bellows compensator, 13 - stuffing box
The fuel channel tubes are set into the circular passages which consist of the aligned central openings of the graphite blocks and the stainless steel guide tubes of the top and bottom core plate structures described in Subsection 4.2.2. The channel tubes are welded to the top (2) and bottom (10) metal-structure plates to maintain the core region hermetically sealed. The tube is welded to a support ledge (4) at the top, and at the bottom to the guide tube of the metal structure (11). A bellows (12) is used to compensate for the differences in thermal expansion between the reactor metal core plates and the fuel channel. In case of failure of the bellows, additional sealing is provided by a pressure seal (13). The design life of the channel tube is about 20-25 years. If necessary, the channel tube can be replaced by removing the top and bottom welds.
As noted in Subsection 4.2.1, the fuel coolant tubes also provide cooling for the energy deposited in the graphite moderator of the core region. In order to improve heat transfer from the graphite stack, the central segment of the fuel channel is surrounded by the 20 mm high split graphite rings (8). These rings are arranged next to one another in such a manner that one is in contact with the channel, and the other with the graphite stack block. The minimum clearance between the fuel channel and the graphite ring is 1.15 mm, and between ring and graphite stack 1.385 mm (Fig. 4.11). These clearances prevent compression of the fuel channel tube due to radiation and/or thermal expansion of the graphite stack. Due to graphite shrinkage and the expansion of the pressure tube, the thickness of this gap is gradually reduced during plant operation. This phenomenon has a potential impact on plant operability and is therefore discussed in more detail in Subsection 4.2.4.3.
The fuel assembly is suspended in the center of the channel by means of a bracket (5, Fig. 4.10). The bracket is provided with a seal plug (7), which hermetically seals the fuel channel tube after the fuel assembly is installed. Since all work related to sealing, unsealing and fuel changing is complished by remote control via the refueling machine, the seal plug must have an appropriately simple design which is shown in Fig 4.12. The main parts of this plug are the bolt (4) and ring (9). When the bolt (4) is fully loosened, the fuel assembly, which is supported by the bracket (13) together with the seal plug, is lowered into the fuel channel tube by the refueling machine. The ball bearings (8) then drop into the groves of the expansion bushing (10), and are confined by the outside diameter of the retaining ring (9). The bolt (4) is then tightened with a special key of the refueling machine to seal the channel. As the bolt tightens, the expansion bushing pushes the bearing balls into a ring-shaped groove in the body of the plug. As the bolt moves further, the now enclosed bearings are tightened within the groove thus preventing upward motion of the bracket, and the compression bushing (11) compresses the sealing gasket (12).
The seal plugs described in the previous paragraphs pertain to the plugs used in unit 1 of the Ignalina NPP. The seal plugs used in unit 2 have a somewhat different construction. They have a spiral shape and the hanger of the fuel assembly is fixed not by bearing balls, but by special retaining nuts.
Fuel channels described in this Section may also contain supplementary absorbers. They may also be devoid of structural elements and just filled with cooling water.
Fig. 4.11 Graphite and zirconium interaction zone
Fig. 4.12 Fuel channel seal plug (for unit 1)
1- support handle, 2- flange, 3- retaining ring, 4- bolt, 5- support ring, 6- plug encasement tube, 7- brazing, 8- bearing ball, 9- retainer ring, 10- expansion bushing, 11- compression bushing, 12- sealing gasket, 13- support bracket of fuel assembly.
A - operating position (channel sealed),
B - position of the plug before sealing the channel
4.2.4.3 Pressure Tube - Graphite Gap
The interaction of fast neutrons can lead to dimensional changes in various materials. For example, in graphite moderated reactors, initial accumulation of the fast neutron dose produces a gradual shrinkage of the graphite blocks. For the RBMK reactors this results in a decrease of the bore diameter through which the fuel channel passes. For the pressure tube made of a Zr + 2.5 % Nb alloy, the effect is opposite, due to thermal and irradiation effects the tube diameter increases. As a result, the gap between the pressure tube and the graphite, which has a nominal thickness of 1.5 mm (Fig. 4.11) is gradually diminished leading to an eventual closure of the gap. The measured and the projected rate of gap closure for unit 1 of the Ignalina NPP as a function of the integral power per channel is shown in Fig. 4.13. For the high power channels in the Ignalina NPP this translates to a net operating time before the first gaps close of about 16 years.
The consequences of gap closure have not been thoroughly analyzed. However, it is in general agreed that operation after the gaps are closed may cause one or more of the following:
In order to preempt the listed problems the RBMK designers recommend the replacement of pressure tubes before a large fraction of the gas gaps have closed. This has been carried out in Leningrad units 1 and 2. It should be noted that after re-tubing a renewed gap closure is not expected to occur [96].
For the Ignalina NPP the gap-closure issue has received considerable attention. It was considered in some detail in the SAR [62] and was the subject of a 1997 workshop conducted at Encinitas, California [97]. An important on-going contribution to the resolution of this issue is the expanded measurement program initiated at the Ignalina NPP in 1997.
Two types of measurements are made. In one series of measurements ultra-sound techniques are employed to measure the dimensions of the metal pressure tube. The instrument used is developed by ABB TRC AB, Sweden [98]. It is designed for fast measurement of
Fig. 4.13 Change of hole diameter in graphite bricks and equivalent diameter of pressure tubes (pressure tube channel & graphite rings) during operation of Ignalina NPP unit 1 [62]
inner diameter and wall thickness, and is capable of inspecting 30 fuel channels per day. The measurement of the inside bore diameter of the graphite is more laborious. It requires the removal of the metal tube and an insertion of specially calibrated instruments. Both sets of measurements are performed during regularly scheduled maintenance shutdowns. The proposed measurement program is outlined in Table 4.5. As the table shows, it is expected that by 2001 a total of 1200 pressure tubes will have been measured. This constitutes the entire population of high to medium power channels present in the core. The number of projected measurement of graphite bore diameters is smaller, and should reach a statistically significant total of 130 measurements by 2001. These field measurements will make it possible to achieve a considerably more accurate depiction of the true state of the graphite-pressure tube gap in unit 1 of the Ignalina NPP.
Table 4.5 Fuel channel inspection program for unit 1 of the Ignalina NPP
|
Year |
No. of fuel channels for which the diameter of the pressure tube is measured |
No. of fuel channels for which inside diameter of the graphite bore hole is measured |
|
|
1 |
1997 |
150 |
- |
|
2 |
1998 |
150 |
20 |
|
3 |
1999 |
300 |
30 |
|
4 |
2000 |
300 |
30 |
|
5 |
2001 |
300 |
50 |