Ignalina Source Book

5.3 Forced Circulation

The suction header supplies coolant to four suction pipes and to four pumps. In the normal power-operation mode, three pumps are used and one is kept in reserve in each loop. Two pumps per loop are employed for operation below 2400 MW(th) power. The exiting water is pumped through the pressure pipes to the pressure header.

5.3.1 Main Circulation Pumps

For the forced circulation of cooling water through the RBMK-1500 reactor at the Ignalina NPP type CVN-8 MCP are employed. These pumps belong to the “wet” stator pump group. The CVN-8 type is a centrifugal, vertical, single-stage pump with a sealed shaft. A schematic of the pump is shown in Fig. 5.9, its characteristics are listed in Table 5.6 [42]. The pump is powered by a vertical type VDA-173/99-6-2 AUChL4 electric motor, which is a three-phase asynchronous motor with a close - connected rotor (2). Motor characteristics are given in the Table 5.7. Rotary moment of inertia from the electric motor to the pump is transferred by means of an elastic coupling mechanism (4) which is provided with rubber packing (coupling type 65 GSP).

Table 5.6 Pump characteristics [42] ( type- CVN-8, manufacturer- OKBM (Special Designer Bureau of Engineering), Niznij Novgorod, Russia)

Capacity, kg/s (m³/h)	2.22± 0.05 (8000±200)
Head, MPa (m of water column)	1.962 ± 0.20 (200±20)
Temperature, ^oC	270
Absolute suction pressure , MPa (kgf/cm²)	7.06 (72)
Minimum pressure margin before cooler boiling in the suction branch pipe of the pump, MPa (m of water column)	0.226 (23)
Shaft power, kW	4300 ± 300
Rotational speed, rpm	1000
Water seal pressure range, MPa (kgf/cm²)	7.85 - 9.81 (80-100)
Water seal flow rate, kg/s (dm³/h)	0.0138 (50)
Seal leakage ( from shaft-sealing device to the atmosphere), kg/s (dm³/h)	< 0.0069 (25)
Sealing water temperature, ^oC
- at inlet	< 50
- at outlet	< 65
Cooling water flow rate though the cooler of sealing system, kg/s (m³/h)	2.22 ± 0.28 (8±1)
Cooling water excess pressure, MPa (kgf/cm²)	£ 0.981 (10)
Cooling water pressure drop at cooler, when flow rate is 2.22 kg/s, MPa (kgf/cm²)	£ 0.1962 (2)
Cooling water temperature, ^oC
- at inlet of cooler	< 40
- at outlet of cooler	< 60
Lubricant flow rate through radial-axial bearing, m³/s	8 ± 0.3
Lubricant pressure drop at the inlet of bearing, MPa (kgf/cm²)	0.147 -0.343 (1.5-3.5)
Lubricant temperature at the inlet of bearing, ^oC	40 - 50
Lubricant pressure at radial bearing, MPa (kgf/cm²)	8.83 (90)
Top (radial-axial) bearing temperature,^oC	70
Water flow rate through the hydrostatic bearing, kg/s (m³/h)	11.1 - 16.7 (40-60)
Maximum peak - to-peak amplitude of vibration in bearings, m	< 0.0001
Maximum admissible heating/cooling velocity, ^oC/min	2
Time to full rotor acceleration, s	16
Time to full rotor deceleration, s	120 - 300
Total moment of inertia (pump&motor&flywheel), kg× m²	3741
Overall dimensions
- height, m	9.85
- length, m	3.07
- width, m	2.75
Mass of pump equipment, kg	106000

Table 5.7 Electric motor characteristics

Power, kW	5600
Voltage, V	6000
Current of stator, A	620
cos f	0.9
Rotating speed, rpm	1000
Frequency of mains, Hz	50
Efficiency, %	96

A flywheel (3) is mounted on the motor shaft, which increases the rotary inertia in order to prolong the rotation of the shaft in the event the electric motor fails. The flywheel is of type 64 GSP, which has a massive 0.2 m outside diameter and 0.195 m thick steel (type ST 25) disk. An annular groove is provided in this disk for inserting balancing weights.

The MCPs are joined in groups of four pumps each (three for normal operation and one on standby). Because the MCPs are enclosed in the confinement structure, they are readily accessible for maintenance of the mechanical parts. The pumps are mounted in such a manner, that the elevation of the intake suction and pressure is lower than the branch pipe overlap. The MCP rests on the foundation frame (6) and is attached to it by locking rings. The pump is centered on the foundation frame by a locating pin, and the foundation is centered on the overlap. Verticality of the pump is obtained by concentric discs and jacks. For ease of maintenance the main zone of the pump and its supports is protected from overheating by thermal isolation. The annular gap between the overlap and the outer cylindrical surface of the pump is enclosed within a special steel plate, which is calculated to support a pressure difference of 0.4 MPa. This prevents coolant entry into the service compartments of the pump, in the event that the MCC pipelines were to rupture.

The pump shown in Fig. 5.10, consists of a shell (1) part of which can be removed. The removable part is packed with a cooper seal of trapezoidal cross-section (4), which is needed to assure leak-tightness. The shell is a welded tank fitted with intake (suction) and pressure branch pipes connected to the MCC. The inner cavity of the shell is lined with a corrosion-resistant stainless steel sheet. The tank rests on supporting legs, which are attached to the foundation frame. The removable part consists of a cover with jaws (5), an axial - radial upper bearing and shaft (14), pump rotor (3), pump stator (2), pole (6) and a lower radial hydrostatic bearing.

The upper combined axial - radial sliding bearing consists of a radial bearing and a heel (13) (axial part of bearing) with top and bottom footstep bearings. The shaft (14) is forged steel. The pump rotor (3) (having a specific speed coefficient of 102) is enclosed by double-curved blades. It is welded of two parts: one disc with blades and a covered disc. The wheel and the pump stator are manufactured from stainless steel. The inner surface of the cover (5) is also lined with stainless steel. The upper bearing and the support of the electric motor are attached to the outlet housing (6), which is manufactured from steel casting. These construction features make the maintenance of the removable part easier.

A double - acting mechanical (contact) shaft bearing (10) is used to prevent the coolant flow from entering the service compartment of the pumps. Clean sealing water is fed to the bearing the pressure of which is higher than the pressure of the MCC coolant. The distinguishing feature of this bearing is a very small (on the order of about 10^-6 m) gap between the two bearing surfaces. It reduces the leakage of water to not more than 25 liters/hour.

The rotor of the pump moves clock-wise (from intake or suction side). To avoid the rotation of the shaft in the opposite direction (which is possible when a check valve is stuck in the open position), a special anti -rotational device is used. It consists of a ratchet, which is mounted in a recess of the flywheel. The reasons for installing the ratchet are:

radial-axial oil bearing of electric motor is not adapted to work in case when the rotor turns in the opposite direction,

because the electric motor would be overloaded, it does not permit the pump to be switched on, when the rotor rotates in the opposite direction.

The following auxiliary systems are necessary for assuring proper MCP operation :

Lubrication system with an oil filter and cooler which is part of the main circulation system. It is specifically designed for each pump.

A system, which supplies water to the shaft-sealing device. This is common for all eight MCPs. A valve at the sealing-water supply in the branch pipe is used to prevent the MCC coolant flow from entering this system. In the event that the system fails, and the pressure of the sealing-water decreases, the shaft is sealed by MCC water.

A system, which supplies water to the hydrostatic bearing and is specifically adapted for each pump. The water to this system is supplied from the pressure branch pipe of the pump. Water is filtered by a multi hydrocyclone filter before being supplied to the bearings. When the system fails, water is supplied to the bearings from the sealing-system.

A system for countering the axial forces of the pump rotor, which is also designed specifically for each pump.

The tank of the pump is designed to last 25 years. The time of operation of the pump to the first inspection (at which time it is necessary to examine the removable part) is about 20000 hours.

5.3.2 Suction and Pressure Piping of the MCPs

These pipes direct the coolant from the suction header to the pump and down-stream from the pump to the pressure header. They have 282 mm outside diameters and 38 mm thick walls. Each individual pipe includes a gate valve and a branch between the gate valve and the pump for the following pipes:

piping system for countering the axial forces of the pump rotor, (89 x 5) mm,

inlet-outlet cooling heating pipes, (89 x 5) mm,

outlet pipe for the pressure water of the hydrostatic bearing, (57 x 4) mm,

draining pipe for the suction side of the main pump, external (57 x 4) mm,

supply pipe for the hydraulic system of the pump and to the forced circulation system, (25 x 3) mm.

Fig. 5.10 Schematic of the RBMK-1500 pump

1 - outlet case, 2 - pump stator, 3 - pump rotor, 4 - seal, 5 - cover of bearing seat, 6 - outlet housing, 7 - water overflow from behind the pump rotor (from system for countering the axial forces), 8 - water to hydrostatic bearing, 9 - lubricant from block and radial bearing, 10 - shaft sealing device, 11 - lubricant supply to radial bearing and block, 12 - lubricant from block, 13 - pillow block, 14 - shaft, 15 - water from cooler, 16 - outlet of water, which is penetrates through the seal, 17 - water supply to shaft sealing, 18 - venting, 19 - removal of water - lubricant emulsion, 20 - water to cooler

Each individual pipe on the pressure side of the pump contains a check valve, a throttling-regulating valve, a gate valve and a throttle disc flow rate meter. A branch pipe is connected between the check valve and the pump to supply water to the hydrostatic bearing, (108 x 7) mm. The gate valves are used to disconnect the pump during maintenance from its pressure pipes and pressure headers. The gate valve is open in the stand-by position of the pump, and its proper temperature is maintained by a small amount of water arriving to the suction header through four openings of the 10 mm diameter in the check valve. The type MA11112-800-05 gate valves, used here, are commonly used in other industrial applications. The check valves used are of type PT4409-800-01.

The power of each individual pump is governed by its throttling-regulating valve, Fig. 5.11. The throttling-regulating valve is partially closed at the start, and is gradually opened as the reactor power rises and of the flow rate increases. Table 5.8 [39] provides the characteristics of the throttling-regulating valve.

The throttling-regulating valve is controlled from a MCR.

5.3.3 MCP Pressure Header

The functions of an individual pressure header are to:

collect the water from all main pumps of one MCC loop. This water arrives through the pressure pipes,

Fig. 5.11 A schematic representation of the throttling-regulating valve

1 - electric drive, 2 - speed reducer, 3 - pivot arm, 4 -pivot axis, 5 - top cover, 6 - beam, 7 - disc, 8 - pin, 9 - bottom

supply the coolant to the twenty pipes and the twenty group distribution headers (325 x 15) mm,

supply the water to the PCS along two pipes connected to the stagnation region of the header (159 x 9) mm.

The construction of a pressure header is similar to that of a suction header, except for its wall thickness, as shown in Table 5.9 [39]. The outlets to the group distribution header contain filters for solid particles and flow rate controls which are 120 mm long Laval diffusers of cylindrical diameter of 151.1mm.

5.3.4 Pipe Connections between Suction Headers and Pressure Headers

Each suction header in a separate circulation loop is connected by six pipes (325 x 15) mm with the respective pressure header, to ensure that natural circulation can take place when the pumps are disconnected. Each of the pipes include a gate valve type C23202K-0160-300 and a check valve C20 401-160. The type C20 401-160 check valve is made in the former Czechoslovakia.

The exploitation of these by-pass pipes has been modified as a result of the Barselina study [91]. The study directed attention to the circumstance that in the event of a Design Basis LOCA (That is, the break of the pressure header), if one of the check valves would fail to close, this would lead to an increased rate of coolant loss. Studies were conducted which determined that adequate natural circulation can be maintained through the stalled rotor blades of the MCP’s [92]. It was then suggested to remove these pipes altogether. In fact, this modification has been adopted in the Leningrad plants [93]. However, these pipes are useful during maintenance shutdown. Therefore in the Ignalina NPP the procedure was adopted to close the manually operated gate valves within these pipes (see Fig. 5.1) during operating mode of the reactor and to open during maintenance.

Table 5.8 Specifications of throttling-regulating valve [39] (type - RT 96510-800)

Number per reactor		8
Capacity, kg/s (m³/h)		< 2.22 (8000)
Pressure drop, MPa		1.766
Pressure, MPa		9.81
Nominal diameter, mm		800

Table 5.9 Specifications of the pressure header of the MCP [39] (manufacturer - Izhora Plant, Russia)

Number per reactor	2
Length, m	18.204
Outside diameter, mm	1040
Wall thickness, mm	70

5.3.5 Pipe Connections between the Pressure Header and the Group Distribution Header

Water is distributed to individual group distribution header by means of 20 pipes (325 x 15) mm. Each pipe has a manual-control gate-valve, a check valve and a mixer to mix the cold water from the Emergency Core Cooling System (ECCS) and the hot water from the MCC. The type C23201-0160-300 gate valves are closed for servicing the pressure header or the isolation and control valves. The check valve prevents back-flow from the fuel channels in case of failure of the pressure header. Fig. 5.12 shows a schematic of a type C20 401-0160 check valves employed for this purpose. All check valves have are provided with guard devices (2) which prevent a disconnected valve disc (1) from closing the flow path to the respective group distribution header. The valve is partially open when the pressure on both sides is equal.

Mixers protect the MCC from thermal or hydraulic shocks. Flanges designed to prevent pipe whip in the event of a pressure surge are fixed to the structural beams of the plant and to a special framework.