Fuel channels heat the coolant water to boiling temperature and discharge a steam-water mixture. The steam quality (steam content in mass fraction) of the steam-water mixture at core exit varies from 23 to 29%. The mixture arrives via the steam-water mixture flow pipes at the separator drums, where the steam and water are separated. The steam (included a water content of up to 0.1% by mass) is directed to the turbines, and the liquid fraction flows by means of the downcomer pipes to the MCP suction headers.
5.2.1 Separator Drums
Separation of steam in the RBMK plant occurs in large horizontal separator drums which contain submerged perforated sheets and upper liquid de-entrainment structures. Industrial-scale tests performed on the RBMK-1000 design drum separator suggested that an increase in efficiency by a factor of 1.5 is possible with relatively minor modifications. These modifications were implemented at Ignalina NPP. The modifications resulted in an almost 3 m longer drum with hardly any change in the diameter. The effect was a cheaper construction, a saving of transportation and material cost and the extension of equipment life.
Fig. 5.7 Separator drum
1 - vessel, 2 - cover, 3 - impact plates for steam-water flow, 4 - submerged perforated sheet, 5 - top perforated shield, 6 - feed water distribution header, 7 - jet spray nozzle, 8 - nozzle of steam-pipe, 9 - nozzle of steam-water piping, 10 - nozzle of feed water, 11 - nozzle of connection in the steam zone, 12 - nozzle of connection in the water zone, 13 - nozzle of the downcomers
Each of the reactors at the Ignalina NPP is provided with four separator drums. They perform the following functions:
Construction parameters of the separator drum are represented in Fig. 5.7. This is a horizontal 33.76 m long cylindrical vessel with a 2.6 m i nside diameter. It rests on two supports which are located close to the ends, and a mid-level support which prevents longitudinal displacement.
The steam-water mixture arrives at the separator drum, through inlet pipes (9), and a part of the steam becomes separated in the distribution compartments because the flow looses its kinetic energy on impact of the special plates (3). This steam then penetrates the submerged perforated sheet (4) and the barbotage layer above it. Final separation occurs because of gravity force. The separated steam goes through the perforations of the upper shields (5) into the steam-flow piping, the separated liquid flows downward from pipes at the bottom (13). The feed water line has a nominal diameter of 500 mm (10). It enters the separator drum at a 45 degree angle, and extends to a distribution header in the lower part of the drum. The feed water pipe includes a peronite insulation chamber. From the header (6) the feed water is injected at (7) into the downcomer flow (13) to facilitate cooling of the water to be supplied to the MCP. As has been noted, both the steam and liquid containing regions of the two separator drum are connected by a number of pipes.
Thermocouples are installed in both the upper and the lower part of the separator drums. Additional thermocouples are placed in the feed water pipe. Samples of materials used in the piping are stored in a special receptables, so that the degree of corrosion can be checked. The design specifications of the separator drums are listed in Table 5.4 [39,40].
The non-uniform generation of power in fuel channels can lead to an in homogeneous steam-water distribution in the steam drum. This requires design features which serve
Table 5.4 Specifications* of the separator drum [39,40] (type - SP-2100)
|
Number per reactor |
4 |
||
|
Steam generation, kg/s |
513.9-531.2 |
||
|
Steam-water flow rate, m3/s |
2.71-3.33 |
||
|
Average steam content in the steam-water mixture (mass fraction), % |
23-29 |
||
|
Operational pressure, MPa |
6.47-6.96 |
||
|
Design pressure, MPa |
7.5 |
||
|
Outlet water content in the steam flow (mass fraction), % |
>0.1 |
||
|
Feedwater temperature, oC |
177-190 |
||
|
Feedwater flowrate, kg/s |
513.9-531.2 |
||
|
Operational level above the perforated sheet, mm |
200 ± 50 |
||
|
Stored operational water volume for nominal steam generation and water level, m3 |
63 |
||
|
Reduced steam flow velocity per evaporation cross-section, m/s |
0.23 |
||
|
Velocity of steam in perforations of submerged sheet, m/s |
3.1 |
||
|
Velocity of steam in perforations of upper shield, m/s |
20.5 |
||
|
Outlet steam velocity, m/s |
18.62 |
||
|
Size of separator drum: - total length, m |
33.76 |
||
|
- inside diameter, m |
2.6 |
||
|
- distance between submerged sheet and upper shield, m |
0.95 |
||
|
Dry mass of separator drum, kg |
292000 |
||
|
Number of outlets: - steam-water piping (nominal diameter dn=90mm) |
424 |
||
|
- steam piping (dn = 300 mm ) |
16 |
||
|
- water downcomer (dn = 300 mm ) |
12 |
||
|
- connecting pipes at the water level (dn=300mm) |
6 |
||
|
- connecting pipes at the steam level (dn=300mm) |
5 |
||
|
- pressure metering outlets (dn = 10 mm) |
4 |
||
|
- level meterings (dn = 50 mm ) |
32 |
||
|
Submerged perforated sheet: - thickness, mm |
6 |
||
|
- diameter of perforations, mm |
10 |
||
|
- number of perforations |
70280 |
||
|
Upper perforated shield: - thickness, mm |
5 |
||
|
- diameter of perforations, mm |
10 |
||
|
- number of perforations |
10620 |
||
* Thermal parameters at 4200 MW(th) power
to reduce both transverse and longitudinal variations of the steam content. This is accomplished by a submerged perforated sheet (4) with a 150 mm thick downward frame. A downflow passages is provided between the frame and the drum wall for that part of water, which penetrates the perforations together with steam. The downflow passages functions as a hydraulic lock against any penetration of steam at the sides of the perforated sheet. The sink is covered by safety plates spaced at 75 mm from the frame.
Traverse and circumferential variations of pressure at the entrance of the steam pipes are reduced by a similar perforated shield in the upper part of the drum (5) and by 190 mm inside diameter bushing installed in the steam outlet pipes. The liquid accumulates in the lower part of the drum to be mixed with feed water and directed to the downcomers.
In 1988 an extensive performance-study was carried out in cooperation with RDIPE on unit 1 of the Ignalina NPP [40]. Fluid-dynamic and steam separation parameters of the separator drums were measured for a range of operational modes of the unit. Electric power was varied from 1050 to 1500 MW, this corresponds to an average steam flow rate from 423.6 to 583.3 kg/s, the level of water above the submerged perforated sheet as recorded by the level meters, varied from -50 to +300 mm. The study determined that optimum operating conditions at a 1500 MW(e) nominal power require that the water content in steam flow (mass fraction) is kept well below the 0.1% limit. In the range of power generation covered, the lowest water content in the exiting steam was observed to occur when the water level is maintained 150 to 250 mm above the perforated sheet.
There is an incentive to keep the water level in the steam drums as high as practical, because this water provides a coolant reserve in the event of a Loss Of Coolant Accident (LOCA). On the other hand, excessive liquid levels reduce the degree of de-entrainment. The tests have shown that a liquid layer 200 ± 50 mm above the perforated sheet represents a workable compromise.
5.2.2 Connections at the Liquid and Steam Level between Separator Drums
The two separator drums within each loop are inter-connected both in the liquid and steam region. There are five connecting (325 x 16) mm pipes in the steam zone, and in the original design there were six pipes (325 x 19) mm in the water zone. The length of pipes is 19.8 m in the water zone and 16.2 m in the steam zone. Presently this number has been reduced to two water region pipes. The connections ensure that equal water levels and steam pressures are maintained in both drums. One of the connection pipes has a branch pipe (325 x 15) mm to supply water to the PCS.
A schematic of steam and water inter-connections between the separator drums is presented in Fig. 5.8.
The number of the inter-connecting pipes at the water level has been reduced because operational experience demonstrated that these pipes present maintenance problems. There are two main causes for this:
Fig. 5.8 Connections at the liquid and steam level between separator drums
1 - separator drums, 2 - connecting pipes at the water level, 3 - connecting pipes at the steam level, 4 - submerged perforated sheet
Fig. 5.9 The pump equipment of the RBMK type reactor
1 - service platform, 2 - electric motor, 3 - flywheel, 4 -junction coupling, 5 - support of electric motor, 6 - foundation frame, 7 - tank of the pump, 8 - water outlet, 9 - water inlet
Table 5.5 Specifications of the suction header of the MCP [39] (manufacturer - Izhora Plant, Russia)
|
Number per reactor |
2 |
|
Length, m |
21.074 |
|
Outside diameter, mm |
1020 |
|
Wall thickness, mm |
60 |
A study was conducted [90] which led to the conclusion that liquid level equilibrium could be maintained adequately with two pipes. As a result four of the pipes were removed during the 1996 maintenance outage.
5.2.3 Downcomers
The downcomer pipes direct the water from the separator drums to the suction header of the MCP. Each of the separator drums is connected to the suction header by 12 downcomer pipes (325 x 16) mm. Thus each of the two circulation loops contains 24 downcomers.
5.2.4 MCP Suction Headers
The function of the suction headers is to:
The suction header is a cylinder with specifications shown in Table 5.5 [39]. The inlet pipes from the downcomers enter the cylinder from above. Its elliptic end covers have circular nozzles of 400 mm diameters. The header rests on two side rails and a fixed mid-level support.