The MCC provides the following types of coolant flow for the proper operation of the core:
Table 5.2 Coolant operating conditions at 4200MW(th) power operation [2,35]
|
Discharge steam flow rate of the separator drum * , kg/s (t/h) |
2055-2125 (7400-7650) |
|
Feedwater flow rate *, kg/s (t/h) |
2055-2125 (7400-7650) |
|
Flow rate in the core *, m3/s (m3/h) |
10.83-13.33 (39000-48000) |
|
Saturated steam pressure in separator drums (absolute pressure), MPa (kgf/cm2) |
6.47-6.96 (66-71) |
|
Fluid temperature entrance of the core, oC |
260 - 266 |
|
Steam content in the steam-water mixture at core exit (mass fraction), % |
(23.0 - 29.0) |
|
Water content in the separated steam (mass fraction), % |
about 0.1 |
* Data applies to a reactor
The coolant is supplied to the 1661 fuel channels from 40 group distribution headers. It exits the core as a steam-water mixture and is directed to four drums separators by means of individual steam-water pipes.
5.1.1 Group Distribution Header, Water Piping, Isolation and Control Valve
The coolant is supplied to the individual fuel channels via group distribution headers (Fig. 5.3), which are horizontal cylinders with 325 mm outside diameter and 15 mm thick walls. The Group Distribution Header (GDH) are securely fastened to support structures to prevent any sliding in case of failure. Each header distributes coolant to 40-43 bottom water pipes (57 x 3.5) mm. These pipes are provided with isolation and control valves between the GDH outlet and the entrance to the fuel channel. Isolation and control valves are used to adjust channel flow on the basis of channel power. Flow rates can be controlled by varying the flow-area of the valves. This is achieved by manual operation from a separate room in the vicinity of the reactor block. The operating life of the isolation and control valves is estimated as 50000 hours. Ball type flow rate meters are mounted downstream of the valves, their indications are transmitted to the Main Control Room (MCR). Construction of a isolation and control valve is represented in Fig. 5.4 and its operation parameters are presented in Fig. 5.5.
Fig. 5.3 Group distribution header
1 - isolation and control valve, 2 - ball type flow-rate meter, 3 - coolant water pipe leading to the fuel channel, 4 - group distribution header
Fig. 5.4 Isolation and control valve
1 - upper housing, 2 - pressurized ring, 3 - copper seal, 4 - bushing, 5 - lower housing, 6 - group distribution header
|
Curve number |
Valve steam position, mm |
Flow cross-section area, mm2 |
|
1 |
2 |
54 |
|
2 |
4 |
102 |
|
3 |
6 |
153 |
|
4 |
8 |
206 |
|
5 |
10 |
263 |
|
6 |
12 |
319 |
|
7 |
16 |
437 |
|
8 |
24 |
691 |
Fig. 5.5 Operation parameters of the isolation and control valve [94,95]
5.1.2 Fuel Channels: Operation Parameters
A detailed description of the mechanical characteristics of an individual fuel channel is provided in Subsection 4.2.4.2. The design thermal-hydraulic parameters of a channel are given in Table 5.3. The Table provides two sets of values, the first column represents the original design parameters [35], the second, best estimate values determined for the current (1997) operating conditions [89].
The axial distribution of thermal-hydraulic parameters along a fuel channel at the maximum power 4.5 MW [35] are shown in Fig. 5.6. Flow rates in the individual fuel channels are based on predictions of fluid and thermal dynamics of the core, and include the variations in local power generation rates. The gradual reduction of power because of nuclear fuel burn-up requires periodic re-adjustment of the flow rate in each channel. Standard adjustments are performed at 30% and 60% burn-up.
Table 5.3 Parameters of the fuel channels
|
Parameter |
Design parameter [35] |
Best estimate values [89] |
|
|
Channel power, MW |
4.5 |
2.53 |
|
|
Coolant flow rate, kg/s (t/h) |
6.67 (24) |
5.51 (19.8) |
|
|
Maximum quality at channel exit, % |
36.1 |
23 |
|
|
Coolant inlet temperature in the channel, oC |
260 |
263 |
|
|
Steam-water temperature at channel exit, oC |
288 |
283 |
|
|
Pressure drop in the channel, MPa |
1.18 |
0.5 |
|
Fig. 5.6 Vertical variation of coolant parameters along the maximum designed power 4.5 MW fuel channel [35]
p - pressure, i - enthalpy, w - velocity, q - heat flux density, y - steam content in the steam-water mixture (volume fraction), t - temperature, x - steam quality
5.1.3 Steam-Water Piping
The steam-water flow from the top of the fuel channels is conducted by way of individual steam - water pipes (76x4) mm to the separator drums. The bottom water pipes leading into the reactor block and the exiting steam-water pipes include several bends, this aids in reducing gamma radiation streaming.