Ignalina Source Book

4.5 Operational Procedures

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During a scheduled shutdown the reactor must be allowed to cool completely. This implies that the critical state of the reactor is reached with temperatures below 80 ^oC in the MCC and 100 ^oC in the graphite stack, respectively.

This Section presents a brief description of the procedures employed during the standard power plant operations as startup, shutdown and refueling [36]. Note that the descriptions must refer repeatedly to various plant systems which for the RBMK reactors can have different connotations than those for the more familiar PWR or BWR plant types.

 

4.5.1. Startup

 

Reactor startup proceeds in two main steps. During step one reactor power is increased until the minimum controllable power level is established, in step two it is taken to full power. The minimum controllable power level is reached when the full power instrumentation train is able to register and control the neutron flux levels, it constitutes about 0.3 % of the nominal thermal power and never exceeds 240 MW or 5 % of the design thermal power.

 

Step 1:

 

• Preparing for startup after a planned maintenance shutdown.
• Reaching the minimum controlled power.

 

A) Before starting the reactor, the following operations must be performed:

 

• the seals of the MCC must be tested,
• control instrumentation must be checked and made ready for operation. Special attention must be paid to the flow-rate meters of the fuel channel coolant,
• heating the MCC,
• separator drums must be filled to a liquid height from 1000 to 1400 mm from the bottom. The fill-water, must meet the following criteria:

 

a) Water pH at 25oC		5.5-7.2
b) Relative electric conductivity at 25oC, mS/cm		£ 1.5
c) Calcium hardness, mg(equiv.)/kg		£ 3*
d) Bulk iron concentration, mg/kg		£ 20
e) Bulk chloride ion concentration, mg/kg		£ 10
f) Bulk mineral oil concentration, mg/kg		£ 100

* water hardness of one mg/kg is equivalent to that hardness, which would be present in one kg of water with 20.04 mg of Ca ions or with 12.16 mg of Mg ions.

 

• the reactor nitrogen blowers must be in operation,
• emergency controls of the MCC must be checked,
• fuel channel flow-rate meters must be activated and their readings recorded.

B) At least two main coolant pumps (MCP) must be turned on, their flow rates must reach 1.805 to 1.944 m³/s (6500-7000 m³/h) with throttle-control on each pump.

C) The purification system of the cooling water must be turned on, its flow rate adjusted to meet the requirements of the water purity criteria.

D) The no-access areas must be cleared of personnel. Ignitable materials must be isolated.

E) The following parameter limits must never be exceeded:

 

• water temperature heating rate in the MCC 10 ^oC/h,
• temperature difference between the top and bottom metal walls of the separator drum 40 ^oC,
• temperature difference between the top metal wall of the separator drum and the supply water 150 ^oC,
• temperature differences between the top metal plate of the reactor and the fuel channel 50 ^oC in the central sections, and 120 ^oC in the periphery.

 

F) Excess water must be discharged into the clean-water storage tanks.

G) Before heating the main circulation loop above 100^oC, the following operations must be performed:

 

• all safety valves checked,
• the accident confinement system (ACS) set in operation,
• the necessary amount of water stored and its supply piping checked,
• deaerators filled with water at least 1 m from the bottom,
• the diesel electric power generator set in "ready" mode,
• electric power supply circuits of 6 kV and 0.4 kV checked.,
• water quality must meet the following criteria:

 

		MCC		CPS
a) The pH value at 25oC		5.5-8.5		4.5-6.2
b) Relative electric conduction at 25oC, mS/cm		£ 2		-
c) Calcium hardness, mg(equiv.)/kg		£ 50		-
d) Bulk silicium acid concentration, mg/kg		£ 2000		-
e) Bulk iron concentration, mg/kg		£ 500		£ 100
f) Bulk cooper concentration, mg/kg		£ 50		-
g) Bulk chloride ion concentration, mg/kg		< 100		£ 50
h) Bulk mineral oil concentration, mg/kg		£ 200		£ 100
i) Bulk aluminum concentration, mg/kg		-		£ 100

 

• water in the ACS compartments must conform as follows:

 

a) level in the hot condenser chamber pools 2.5 m

b) level in the condenser basins 1.05 m

c) level in the water storage chambers 2.8 m

d) There must be no water in the water-exhaust pipes.

H) Before initiation of control rod withdrawal, the following operations must be performed:

 

• the power supply system of the CPS must be checked,
• neutron flux instrumentation verified,
• emergency blocks and the emergency protection system AZ-1 prepared for operation,
• servo-drives of control rods checked by raising them manually,
• at least two MCPs in each pumping station turned on,
• coolant flow rate of at least 2.77× 10³ m³/s (10 m³/h) maintained in the fuel channels,
• fuel channel control valve operation checked,
• the PDDMS set in operation,
• the adequacy of the reactor nitrogen pressurization system verified,
• proper sealing of the fuel channels ensured,
• the ACS and CPS set in "ready" mode,
• the ECCS set in "ready" mode.

 

I) A representative of the Lithuanian Regulatory Body (VATESI) must be present, when the reactor is brought up to the minimum controlled power level.

J) The procedure for reaching the minimum controlled power is as follows:

 

• CPS rods are lifted, beginning with the LEP rods and proceeding to the automatic control and manual control rods,
• the startup procedure must be stopped if any discrepancy in the instrumentation readings is noted,
• up to four rods may be lifted simultaneously at 60 s intervals before criticality is achieved,
• if in 10 minutes time after the withdrawal of control rods is initiated the reactor does not achieve criticality, the reactor is shut down by re-inserting the control rods,
• the automated control system is initiated after a power level of about 30 MW(th) is reached,
• the power level of the reactor must not exceed 240 MW(th) during the warm-up of its main circulation circuit.

 

K) Warm-up of the MCC:

 

• when the absolute pressure within the separator drum reaches 0.1962 to 0.392 MPa (overall 1-3 kgf/cm²), the steam discharge valves and the deaerators must be initiated,
• when the separator drum absolute pressure reaches 0.3-0.7 MPa, hermetic seals of the fuel channels must be checked visually,
• water quality must now meet the following criteria:

 

	Feedwater	Condensed water after filtering
a) The pH value at 25oC	5.5-7.8	6.5-7.5
b) Relative electric conduction at 25oC, mS/cm	£ 1.5	£ 0.5
c) Calcium hardness, mg(equiv.)/kg	£ 5	£ 1
d) Bulk silicium acid concentration, mg/kg	£ 100	£ 50
e) Bulk iron concentration, mg/kg	£ 50	£ 20
f) Bulk copper concentration, mg/kg	£ 5	£ 5
g) Bulk chloride concentration, mg/kg	£ 10	£ 10
h) Bulk mineral oil concentration, mg/kg	£ 200	-

 

Step 2:

 

• Raising the reactor power above the minimal controlled power level.
• Initiating reactor operation.

 

A) Before reactor power can be increased, the following operations must be performed:

 

• final adjustment of the fuel channel coolant flow-rate control made,
• operation of the gaseous coolant loop checked,
• operational water levels in the separator drum and in the deaerator attained,
• proper water levels in the ACS and in the water supply systems assured,
• measurement and recording of operational parameters verified,
• coolant channel flow rates checked and recorded.

 

B) Water quality parameters must be rechecked.

C) A representative of VATESI must be present, when the reactor is brought above minimum controlled power level.

D) The procedure for increasing reactor power is as follows:

 

• the increase is accomplished in several stages:

 

Parameter	Number of stage
	1	2	3	4	5	6
a) Reactor power,
MW(th)	240	1000	2600	3300	4200	4800
MW(e)	-	250	750	1000	1300	1500
%	-	17	50	67	87	100
	1	2	3	4	5	6
b) Minimum time to reach the next stage, h	0.5	0.5	1.5	2.17	7.0	11.0
c) Minimum time spent at given power, h	-	2.0	2.0	3.0	1.0	-

 

E) After the operation at stage 2 is stabilized:

 

• increase the flow of the MCP by opening the throttling regulating valves,
• establish proper flow rate of the feedwater by MFWP:

 

Maximum flow rate of each MCP, m3/s (m3/h)	Supply rate of feedwater to each side of MCC, kg/s (t/h)
	3 MCP in operation	2 MCP in operation
1.94 (7000)	below 180 (450)	below 125 (650)
2.5 (9000)	180 to 350 (450-900)	125 to 250 (650-1250)
2.78 (10000)	over 350 (900)	over 250 (1250)

 

F) After the operation at stage 3 is stabilized:

 

• start the third MCP in each pumping station (if only two are in operation),
• initiate helium-nitrogen cooling of the reactor.

 

G) The transition from stage 3 to stage 4 must occur in 50 MW(e) (150 MW(th)) steps, maintained for 10 min. The operation in each step should not be less then 20 min.

H) The transition from stage 4 to stage 5 must occur in 50 MW(e) (150 MW(th)) steps, maintained for 20 min. The operation in each step should be continued not less then 1 hour.

I) Further power increases must occur in 50 MW(e) (150 MW(th)) steps. Each power increase step is maintained for 30 min. The duration of operation in each steps should be not less 3 hour.

J) If an emergency signal is generated by an energy distribution detector, the procedure must be stopped, the failure must be analyzed and eliminated. If two emergency signals are generated, the power must be reduced until both signals cease. Only then can corrective procedures be initiated.

K) The turbogenerators can be started after the drum pressure reaches 7 MPa.

 

4.5.2 Shutdown

 

A) Normal shutdown of power units must adhere to the following procedures:

 

• the power is decreased while maintaining a constant pressure in the separator drum by reducing the reactor power and the load of one turbine at a time, this is followed by a similar reduction at the next turbine,
• while reactor power is reduced to 1000 MW(th), two or three MCPs are in operation in each pumping station and their flow rate is reduced to 1.8 m³/s (6500 m³/h) using throttling regulating valves,
• the turbine is fed from the auxiliary transformer,
• power is decreased continuously by pushing of AZ-1 button.

 

B) Normal cooling of the reactor and the MCP:

 

• reduce steam pressure in the separators by discharging steam via steam discharge valves (SDV-C), then with the aid of coolers and regenerators of the cooling system. If necessary, discharge through steam discharge valves which directs excess steam to the turbine condens (SDV-A) into the ACS condensing pools,
• after the chain reaction has ceased, at least two MCPs must be in operation in each pumping station,
• after the MCC has been cooled to 180 ^oC, at least one MCP must in operation in each loop,
• after the MCC has been cooled to 100 ^oC, the main circulation pumps may be stopped,
• observe the inertia in the MCP, to note any reverse flow or failure of a check valve to close,
• the Purification and Cooling System (PCS) must be in operation until the reactor is completely cool.

 

C) During normal cooldown of the reactor observe the following criteria:

 

• the minimum cooling rate is 10 ^oC/h,
• the temperature difference between the top and the bottom of the separator drums must not exceed 135^oC,
• the temperature difference between the metal of the upper sections of the separator drums and the water supply must not exceed 150 ^oC,
• the temperature difference between the metal wall of the reactor and the fuel channel must not exceed 50^oC in the central region and 120 ^oC in the periphery.

 

D) Emergency cooling of the reactor and the main circulation circuit, which is initiated under the following circumstances:

 

• a reactivity-related shutdown of the reactor,
• a failure in the MCC, in the main steam line or in the feedwater supply line,
• the failure of a fuel channel or of CPS,
• a fire in the control room.

 

Emergency cooling rates must not exceed 30 ^oC/h.

Steam pressure in separator drums is reduced by discharging part of the steam by SDV-C. When the water temperature decreases below 180 ^oC, the purification and cooling system is switched on.

E) The main circulation circuit must be cooled down to 70 or 80 ^oC and the graphite stack down to about 100 ^oC.

 

4.5.3 Refueling Operation

 

The following refueling machine service areas are located in the main refueling hall:

 

1. The refueling machine storage area.
2. A practice and a calibration area. This area is used for adjustment and testing of the refueling machine. Practice operations include: filling the fuel casket with condensate, refueling operations, loading of new fuel into the fuel casket, decontamination of the interior of the fuel casket, and replacement of the inflatable rubber gasket surrounding the standpipe.
3. The spent fuel reception area.
4. A repair area for replacement of damaged fuel caskets. A completely prepared reserve fuel casket is kept in this area.

 

The refueling machine operates in the following modes: checkouts and preparation, loading of new fuel, online refueling, and refueling of a shut down, cool reactor.

 

Checkout and Preparation of the Refueling Machine

 

When the power is turned on, the coolant tank is automatically filled with reactor feedwater. Subsequently the machine is moved to the practice stand area, where it loads the channel gauge and the channel plug into appropriate receptacles of the fuel casket. After this is accomplished the fuel casket is filled with water from the coolant tank. Then the dampers of the closing mechanism are shut, the condensate from the standpipe is drained, and the machine is disconnected from the loading socket.

 

Loading of New Fuel

 

The refueling machine, with the casket filled with feedwater at a temperature of 30 ^oC, is positioned over the practice socket, into which a new fuel assembly has previously been loaded. The machine is joined to the upper section of the socket by means of the standpipe, and the joint is sealed. The socket and the standpipe are filled with water. The dampers of the closing mechanism are opened, the new fuel assembly is pulled into the receptacle of the fuel casket and locked in. The dampers of the closing mechanism are shut, water is drained from the standpipe and the practice socket, and the refueling machine is disconnected from the socket and is moved to the reactor channel to be reloaded.

 

Fuel Assembly Replacement

 

The operator issues directions to prepare the fuel channel for refueling. The block covering the fuel channel is removed and if a flux detector cable is present, it is disconnected. This is accomplished manually in the refueling hall.

Two stages of the operations performed by the refueling machine are illustrated schematically in Fig. 4.20. After the refueling machine is positioned over the fuel channel, the standpipe control mechanism (1) lowers the standpipe (3), which encloses the upper portion of the fuel channel (10). The joint is sealed by the inflatable rubber gaskets (5), and the standpipe is filled with water from the tank. Subsequently, the closing mechanism dampers are opened, and the fuel casket is pressurized by the machine's feed pump until it matches the pressure in the fuel channel. The grabber (7) is then lowered, and clamps on to the handle of the fuel channel seal plug. The sealing mechanism (2), using a special key (4) to turn the fuel channel seal plug (9), unseals the channel. Water is pumped in small quantities from the casque to the fuel channel. The cold water prevents steam and hot water from entering the refueling machine from the fuel channel. The coolant water is being pumped during the time when the new fuel assembly is being loaded. The grabber is used to lift the spent fuel assembly to a height of 7.5 m into the cooling zone, where it is kept for about 10 minutes. Afterwards, the fuel assembly is pulled into the casque receptacle.

To ascertain that the coolant channel is free of obstructions, it is inspected by means of a movable gauge, and then the new fuel assembly is lowered into the reactor. The fuel channel is then resealed. The coolant feed pump is disconnected, and the pressure in the fuel casket falls to atmospheric levels. The closing mechanism dampers are closed, the standpipe space is connected to a special ventilation system, and the leak-tightness of the fuel channel seal is verified. Before disconnecting from the channel, pressurized air is forced into the standpipe to displace the cooling water back into the supply tank. After this operation, the standpipe seal is released, the refueling machine is disengaged from the fuel channel and moved towards the spent fuel reception area.

As the spent fuel reception unit is already prepared to receive the spent fuel assembly into the storage pool casing, the spent fuel is loaded into one of the reception unit's sockets. The machine with the spent fuel is moved to the water-filled casing, is connected to it, and the standpipe cavity is filled with water from the supply tank. The closing mechanism dampers are opened, and the spent fuel assembly is transferred from the fuel casket to the casing. The dampers are closed, the water from the standpipe is forced back into the tank, the machine is disengaged from the casing, and is then ready to perform the next fuel-changing cycle.

 

Refueling a Cool (Shut Down) Reactor

 

Two options are available: changing two fuel assemblies at once and replacing the removed spent fuel with new ones, or, removing four spent fuel assemblies and loading new ones without the use of the refueling machine.

In either case, the operations performed by the refueling machine are greatly simplified, since the water pressure in the reactor is reduced (down to 0.2-0.5 MPa). Using the first option, the fuel-assembly-changing cycle takes 350 minutes, so the refueling machine can change 8 fuel assemblies per day in this mode. If the refueling machine is used only to remove the spent fuel from the fuel channels, the transfer cycle takes 267 minutes. In this case, up to 20 fuel channels per day can be refueled.



Fig. 4.20 Refueling machine operation

a) refueling machine positioning over the fuel channel,

b) unsealing of the fuel channel

1 - standpipe control mechanism, 2 - special sealing key control mechanism, 3 - standpipe, 4 - special key, 5 - inflatable rubber sealing gaskets, 6 - bottom biological shield, 7 - grabber, 8 - enclosing clock, 9 - fuel channel seal plug, 10 - fuel channel body