The RBMK-1500 is a graphite moderated, channel-type, boiling water nuclear reactor. Its design thermal rating is 4800 MW. However, for safety reasons, these reactors are currently running at a reduced capacity of maximum 4200 MW. The most important reactor parameters are presented in Table 1.3. The position of the reactor core and its main components in a RBMK-1500 plant is shown in the schematic cross-section of the main reactor building provided in Figure 1.7. The core is 11.8 m in diameter with an active length of 7 m. It consists of stacks of graphite blocks penetrated with 2052 channels, 1661 of which are pressure tubes. The remaining core channels contain control rods or various types of instrumentation. Each fuel channel contains a stack of two fuel bundles. Each fuel bundle is approximately 3.5 meters long and consists of 18 fuel rods arranged in two concentric circles around a central carrier rod, Figure 1.8. The fuel rods are made up of enriched uranium dioxide pellets contained within zirconium alloy tubes. The principal technical parameters of a fuel assembly are summarized in Table 1.4. Each pressure tube is located within graphite blocks, which provide neutron moderation. At operating conditions, approximately 5% of the reactor power is deposited directly in the graphite. This heat is removed from the graphite primarily by conduction through the graphite back to the pressure tube wall, where the heat is convected to the reactor coolant. The graphite blocks are also cooled by a moderator cooling system, consisting of a helium-nitrogen gas mixture flowing within gaps between the blocks and between the blocks and the pressure tubes. The core of the reactor is housed in a 25 m deep, 21 x 21 m cross-section concrete vault, Figure 1.9. The core volume is dominated by a large cylindrical graphite stack. The graphite stack is located in a hermetically sealed cavity consisting of cylindrical walls and top and bottom metal plates. In the radial direction as well as above and below the reactor it is surrounded by the primary biological shield structures. The graphite stack of the RBMK-1500 reactors serves several functions.
Fig. 1.7 Schematic cross-section through the main reactor building [4]:
1 - graphite stack; 2 - fuel channel feed pipes; 3 - water pipes; 4 - group 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/water pipes; 14 - steam pipes; 15 - refueling machine; 16 - separator drum
Table 1.3 Principal parameters of the RBMK-1500 reactor [3,4]
|
|
water (steam-water mixture) |
|
single circuit |
|
|
|
thermal (design) |
4800 |
|
thermal (actual) |
4200 |
|
electrical (design) |
1500 |
|
7 |
|
11.8 |
|
|
|
end |
0.5 |
|
side |
0.88 |
|
0.25 x 0.25 |
|
|
|
fuel |
1661 |
|
control and shutdown system |
235 |
|
reflector-cooling |
156 |
|
pellets of uranium dioxide |
|
2.0 |
|
21.6 |
|
|
|
maximum fuel center temperature, °C |
2100 |
|
maximum graphite stack temperature, °C |
750 |
|
maximum fuel channel temperature, °C |
350 |
|
channel inlet temperature, °C |
260-266 |
|
feedwater temperature, °C |
177-190 |
|
6.47-6.96 |
|
10.83-13.33 |
|
2056-2125 |
|
23-29 |
|
|
|
channel power, kW |
4250 |
|
coolant flow rate, m3/s |
0.011 |
|
void fraction at fuel channel outlet, % |
36.1 |
|
8 |
|
1.805-2.22 7.5/7.0 |
Table 1.4 RBMK-1500 fuel assembly parameters [3,4]
|
Fuel pellet |
|
|
Fuel |
Uranium dioxide |
|
Fuel enrichment, % of U235 |
2 |
|
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 fuel center temperature, oC |
2100 |
|
Fuel element |
|
|
Cladding material |
Zr + 1%Nb |
|
Outside diameter, mm |
13.6 |
|
Length, m |
3.64 |
|
Active fuel length in cold state, m |
3.4 |
|
Cladding thickness, mm |
0.825 |
|
Pellet/clad gap, mm |
0.22-0.38 |
|
Mass of fuel pellets, kg |
3.5 |
|
Helium pressure in the cladding, MPa |
0.5 |
|
Maximum permissible operating temperature, oC |
700 |
|
Average linear heat generation rate, W/cm |
218 |
|
Maximum linear heat generation rate, W/cm |
485 |
|
Fuel assembly |
|
|
Number of bundles |
2 |
|
Number of fuel rods per bundle |
18 |
|
Total length, m |
10.015 |
|
Active length, m |
6.862 |
|
Diameter (in the core), mm |
79 |
|
Mass without bracket, kg |
185 |
|
Total mass with the bracket, kg |
280 |
|
Total steel mass, kg |
2.34 |
|
Total mass of zirconium alloy, 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, MWd/assembly |
2500 |
|
Authorized fuel assembly lifetime, year |
6 |
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.
Fig. 1.8 Fuel assembly:
1 - suspension bracket; 2 - top plug; 3 - adapter; 4 - connecting rod; 5 - fuel rod; 6 - carrier rod; 7 - end sleeve; 8 - end cap; 9 - nut. (Dimensions in mm)
Fig. l.9 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 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. 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 350°C. The temperature of the bottom support plates reaches 440°C, while the maximum calculated graphite temperature is 750°C.
All of the structures surrounding the core region contribute to some extent to biological shielding. 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.
The radial shield 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.
Design criteria for shielding below the core include the requirement to reduce gamma radiation during shutdown in order to allow personnel access for maintenance, and the necessity to minimize activation of the metal structures and coolant feeder pipes. The bottom shield consists of the 0.5 m thick graphite reflector, 0.2 m of steel blocks, and the bottom biological shield, filled with a mixture of serpentinite chips and gallium. The density of the mixture is 1700 kg/m3. There are 0.1 m thick steel screens under the annular water tank (above the bottom water piping) and between the reactor vault and the water tank.
The maximum allowable pressure within the reactor cavity is 0.31 MPa. This pressure just offset the weight of the reactor top metal structure. To avoid this in case of pressure tube ruptures, the concept of confinement is used with reactor cavity overpressure protection system, which is a part of ACS. The reactor overpressure protection system vents the steam/gas mixture into the pressure suppression pools where steam is condensated, and gas is retained in the leaktight spaces. The main functions and limitations of the accident confinement system and reactor cavity overpressure protection system are discussed in Chapter 3.