Ignalina Source Book

5. Main circulation circuit

This Section describes the characteristics of the Main Circulation Circuit (MCC) and the associated thermal-hydraulic systems. This includes the steam pipes delivering steam to the turbine, the feedwater system, the control rod cooling system and the associated water purification systems.

A simplified overview of one of the coolant loops is provided in Fig. 5.1, volumetric data for the various components is listed in Table 5.1. Starting at the inlet to the core (11) the coolant is forced upwards through the reactor core block in a large number of individual fuel channels. Flowing through the core it acquires about 95% of the energy emitted by the fuel elements. The coolant reaches saturation temperatures in the lower part of the channel, starts boiling and exits as a from 23 to 29.1% quality steam-water mixture (mass-fraction).

The MCC consists of two loops, whose components are arranged symmetrically with respect to the vertical axis of the reactor. Each loop has two separator drums (1), which separate the steam from the steam-water mixture exiting from the core block. The separator drums are horizontal cylindrical steel vessels 2.6 m inside diameter and 33.76 m long with elliptical ends. Wall thickness of the shell is 115 mm. The drums are interconnected both at the lower, liquid filled, and the upper, steam filled elevations. In the water-filled zone the drums in the original design were connected by six pipes, each with a 325 mm outside diameter and a wall thickness of 16 mm (325 x 16) mm and in the steam zone the drums are joined by five (325 x 19) mm pipes. In the bottom section of each separator drum, a feedwater header is mounted, which through special mixers provides feedwater to the downcomer pipes. The separated water mixed with the returning feedwater, reaches the suction header (3) through 24 downcomer pipes on each loop (2). From the suction header it flows through four pipes (4) to the four Main Circulation Pump (MCP). During normal reactor operation, only three pumps are operating in each loop, the fourth pump is a reserve. The MCPs are of a vertical, centrifugal, single-stage configuration. The MCP assembly consists of a tank, a removable pump section, and an electric motor. The steel pump tank (5) is covered on the inside with an anti-corrosive mixture. The nominal capacity of the pump is 2.22 m³/s at a head of 1.962 MPa, speed - 1000 rpm, electric motor power - 5600 kW.

From the MCP, water flows through pressure header pipes (6) to the pressure header (8). The suction and pressure headers are connected by six bypass lines (7), each of which is provided with a gate and a check valve. The bypass (7) ensures that natural circulation of the coolant takes place in case the main circulation pumps are shut-off.

From the pressure header (8), water continues through twenty pipes to twenty group distribution headers (9). The outside diameter of a group distribution header is 325 mm, wall thickness is 15 mm, and the length is ~6 m. Mechanical filters are provided inside the pressure header, while, at the upstream end of the group distribution header, there is a flow limiter, a check valve, and a mixer for water from the reactor emergency core cooling system.

Fig. 5.1 Schematic representation of one loop of the main forced circulation circuit

1 - separator drum, 2 - downcomers, 3 - suction header, 4 - suction piping of the MCP, 5 - MCP, 6 - pressure piping of the MCP, 7 - bypass between headers, 8 - pressure header, 9 - group distribution header with flow limiter, check valve and mixer, 10 - bottom water piping, 11 - fuel channel before the core, 12 - fuel channel within the core, 13 - fuel channel above the core, 14 - steam-water pipes, 15 - steam pipelines

Each group distribution header is connected to 40-43 bottom water pipes (10) leading to fuel channels. The flow in each pipe, and therefore in each fuel channel (12), is set by isolation and control valves and is measured by a ball flow-meter. The steam-water mixture generated in the fuel channel flows through the steam-water pipes (14) to the separator drums (1).

The elevations for the most important components of the MCC are presented schematically in Fig. 5.2. The Figure shows that the total, top-to-bottom elevation of the primary system is over 30 m. The elevation driving the natural circulation loop, that is from the bottom of the core to the bottom of the separator drums is ~21m. These large elevation heads determine the flow parameters of the system under natural circulation conditions.

From the separator drums the generated steam is directed to the turbines. Discharge steam from the turbines is accumulated in condensers, from there the condensate flows down through filters, heaters and deaerators to the main feed water pump and is finally returned to the separator drums. This condensate retrieval system is known as the water feedback system.

The purification and cooling of the water is performed by the Purification and Cooling System (PCS) which is an equivalent of Chemical and Volume Control System (CVCS) in Western LWRs. Part of the water is taken from the MCC, cooled down and filtered by a mechanical filter and an ion-exchanger in the purification bypass. The treated water then joins the feedwater flow.

The reactor also contains a number of channels for control rods and metering devices. These are cooled by a separate circulation system, which is called the Control Rod Cooling Circuit (CRCC). More detailed information regarding the various components and systems which make up the MCC is provided in the subsequent sections of this chapter.

Table 5.1 Water and steam volumes of one loop of the MCC [39]

Component*		Outside diameter x	Number	Volume, m³		Total volume,
	wall thickness, mm		per loop	Water	Steam	m³
(1) Separator drum	2830 x 115		2	162.0	173.6	335.6
(2) Downcomers	325 x 16		24	61.5	-	61.5
(3) Suction header	1020 x 60		1	13.4	-	13.4
(4) MCP suction pipes	828 x 38		4	61.5	-	61.5
(5) MCP tanks	-		4	-	8.0	8.0
(6) MCP pressure pipes	828 x 38		4	72.9	-	72.9
(7) Bypass between headers	325 x 15		6	4.2	-	4.2
(8) Pressure header	1040 x 70		1	11.8	-	11.8
(9) Group distribution headers and inlet pipes	325 x 15		20	32.6	-	32.6
(10) Bottom water pipes -each fuel channel	57 x 3.5		1	0.0412	-	0.0412
-total for 830 channels	57 x 3.5		830	34.2	-	34.2
(11) Fuel channel, before the core -each fuel channel	-		1	0.0189	-	0.0189
-total for 830 channels	-		830	15.7	-	15.7
12) Fuel channel within the core -each fuel channel	-		1	0.0089	0.0070	0.0159
-total for 830 channels	-		830	7.5	5.8	13.3
(13) Fuel channel above the core -each fuel channel	-		1	0.0067	0.0371	0.0438
-total for 830 channels	-		830	5.6	30.7	36.3
(14) Steam-water pipes -each fuel channel	76 x 4		1	0.0152	0.0844	0.0996
-total for 830 channels	76 x 4		830	12.7	70.0	82.7
(15) Pipes from separator drums to turbine	-		-	-	404.7	404.7
Total volume of one circulation loop (830 channels)	-		-	503.6	684.8	1188.4
Total volume of primary circuit (1661 channels)	-		-	1007.3	985.4	1992.7

* Component numbers below correspond to the component numbers in Figs. 5.1 and 5.2

Fig. 5.2 Elevations of the MCC