The RBMK-1500 reactivity control systems reflect the general complexity of this reactor type. For example, to meet the various neutron flux and fission power control needs, seven different types of control rods are used. They differ in their structure, insertion speed, insertion direction or the control mode (automatic or manual). The individual rod types and their control systems are described in Section 6. This Section 4.3 deals with several complementary control-related aspects:
In order to identify the mentioned control rod types reference should be made to the tables and graphs presented in Subsection 6.4.3.2.
4.3.1 Special Purpose Channels
Of the 2052 channels which penetrate the reactor graphite stack, 1661 of them are purposed for fuel loading, and the remainder, including additional channels in the radial reflector, are termed "special purpose channels" and contain control rods or various types of instrumentation. A list of the number of various channel types and their purpose is presented in Table 4.6 [35]. The channels with control rods, the channels carrying vertical traverse flux distribution instruments and the fission chamber channels are all identical. A schematic of their structure and the guide tubes is shown in Fig. 4.14. The upper segment of the guide tube incorporates a compensator bellows (2) to accommodate the significant thermal expansion of the channels due to the large temperature difference between the top biological shield and the cold guide tubes. The lower section of the channel guide tube, unlike that in the fuel channel, also contains a thermal expansion compensator (6). The top and bottom sections are made of stainless steel, while the middle section is of a zirconium - niobium alloy.
Table 4.6 The channels and their number [35]
|
Type of channel |
Number of channels |
|
Channels inside graphite columns |
|
|
Fuel channels |
1661 |
|
Channels with control rods |
211 |
|
Channels with in-core sensor of axial power density monitoring |
20 |
|
Channels with fission chambers |
4 |
|
Total number of the reactor Control and Protection System (CPS) channels |
235 |
|
Radial reflector cooling channels |
156 |
|
Total number of channels inside graphite columns |
2052 |
|
Channels at the intersections of graphite columns |
|
|
Graphite temperature instrumentation channels in the core |
13 |
|
Graphite temperature instrumentation channels in the radial reflector |
4 |
|
Gas sampling channel |
1 |
|
Total number of channels at the intersections of graphite columns |
18 |
|
Channels in radial biological shielding |
|
|
Ionization chamber channels: |
|
|
for normal reactor operation |
20 |
|
for reactor start-up |
4 |
|
Total number of channels in radial biological shielding |
24 |
The channels with control rods (Fig. 4.14) are provided with a metal cover (4) which is employed for mounting the control rod drive mechanisms (5) and for providing access to cooling water. The lower ends of the ionization chamber and flux detector channels are sealed by metal caps. The ionization chamber channel caps are made of stainless steel, and also serves in supporting the ionization chambers. The caps of the other channels are made of an aluminum alloy. At the bottom of this type of a channel is a throttling device (7), which provides some resistance to water flow, and helps to ensure reliable filling of the channel.
The CPS channels are cooled by an independent water circuit provided with its own pumps and heat exchangers. The cooling water is supplied to the channels from above, and flows over the exterior and interior casings of the absorber rods. In this process, the water is heated from 40 oC to a temperature of 70 oC. During reactor operation, regardless of the position of the control rod, the inside of the channel is filled with water. When the absorber rod is withdrawn from the core, and if no special provisions are taken, its volume would be replaced by water. Because water is a moderately strong neutron absorber, most control rods have not only a boron carbide absorber, but also a graphite follower which displaces water and improves the reactor's neutron balance.
Fig. 4.14 Reactor-control and protection system channel
1 - channel opening, 2 - thermal-expansion bellows compensator, 3 - welding-support ledge, 4 - channel cover, 5 - operating mechanism, 6 - bottom bellows compensator, 7 - throttling device
The above described channels are always filled with either control rod or a graphite control rod follower and cooled with water. Post- Chernobyl modifications included the addition of 24 fast acting scram-type rods. To achieve higher insertion speeds this rod type must drop into a gas- filled channel without having to displace water. As described in Section 6, this led to a different rod design and required also some changes in the top and bottom fittings of the special purpose coolant channels. These channels are cooled by a lowing film of water and internally supplied nitrogen. Therefore, the top cover includes both access to the liquid and the gas coolants and a distributor which creates the falling liquid film. The bottom cap is provided with a liquid collector.
Cooling channels also provide cooling to the radial reflector (4) of the graphite stack shown in Fig. 4.15. These channels also cool the radial reflector reinforcing tube (5), and reduce the heat flow toward the shell of the graphite stack and its compensator. The channel is made of stainless steel. The cooling water is supplied to this channel from above through a central tube, it flows down to the bottom biological shield, then rises again to the top in the annular space between the two tubes, and finally leaves the channel.
Radial reflector cooling channels, CPS channels and fuel channels are inside the central part of the graphite columns, the openings of which are 114 mm diameter. Total number of channels inside the graphite columns is 2052.
The corners of the rectangular cross-sections of the graphite columns in the stack are hollow and incorporate 17 vertical 45 mm diameter instrumentation channels used for measuring the support and the shielding plates. Thirteen of these channels are positioned within the boundaries of the core, while four are in the reflector. Within each channel the temperature is measured at 5 vertical positions. One channel of the same type is placed in the core and is used for gas sampling. Consequently, there are 18 channels in the graphite stack of the reactor which are outside the graphite columns.
In the radial biological shielding of the reactor there are a total of 24 channels into which radial ionization chambers are positioned. Reactor startup channels are made of shell inside of which there is as suspension bracket and a convex lead shield (it mass is 1220 kg) which protects startup ionization chambers from the gamma radiation. The normal reactor operation ionization chamber channels are quite analogous to the channels mentioned above except that are no lead shields. This is because at normal reactor operation the measured neutron flux significantly exceeds the gamma radiation.
4.3.2 Fuel Handling System
One of the distinguishing features of the RBMK - type reactor is its on-line refueling capability. This complicated operation is accomplished by an especially
Fig. 4.15 Reflector - cooling channel
1 - external bushing, 2 - top biological shield, 3 - thermal - expansion bellows compensator, 4 - radial reflector, 5 - reinforcing tube, 6 - an annual double - wall tube arrangement for central flow in one direction to the end of the tube, then reversing to annual flow in opposite direction, 7 - support sleeve, 8 - bottom biological shield.
designed fuel loading and unloading machine. Besides changing fuel bundles without shutting down the reactor, the refueling machine can also be used to inspect a fuel channel by a special fuel-assembly- shaped calibration instrument, seal off a fuel channel with a standard or an emergency sealing plug, and to correct certain emergency conditions.
On line reloading operations at nominal power must ensure that the coolant flow to the fuel assembly being changed is not interrupted. The refueling machine is capable of changing up to five fuel assemblies per day when the reactor is in operation, and up to 20 when the reactor is shut down.
The principal components of the refueling machine (shown schematically in Fig. 4.16) are: the refueling machine transport mechanism (1), a container (3) which serves as a biological shield, two replaceable caskets (4) (one mounted in the machine, the other kept in the repair area), a metal frame (9), the positioning mechanisms (8), and control equipment.
The refueling machine is moved through the reactor hall by means of the transport mechanism. A bridge-type crane (2), consisting of 21 m long beams, moves on transverse tracks located in the upper section of the reactor hall with 39.6 m of travel distance. On the bridge, 11 m above the hall floor, a carriage (1) transports the refueling machine equipment along the other axis. The travel distance of the carriage is 12.5 m. The bridge and the carriage can be moved at two speeds, 9.75 and 1.2 m/min. The slow speed is used for final, accurate positioning. During this operating phase the bridge and the carriage move in 1 mm increments.
The container (3) in Fig. 4.16 is a steel cylinder, assembled from six sections. Its inside diameter is 0.77 m and the wall thickness is 0.5 m. The lower part of the container incorporates a movable biological shield (7) used to screen the gap which appears between the bottom of the container and the hall floor during refueling operations. The operator's booth and access platforms are located on the outside of the container, the inside provides space for the movable fuel casket (4).
Fig. 4.16 Refueling machine
1 - carriage of refueling machine, 2 - refueling machine bridge, 3 - container, 4 - fuel casket, 5 - closing mechanism, 6 - standpipe, 7 - bottom movable biological shield, 8 - refueling machine positioning systems, 9 - metal frame, 10 - fuel assembly receptacle, 11 - grabber, 12 - grabber control mechanism
The fuel casket (4) is the principal component of the refueling machine. It consists of a cylindrical pressure vessel together with its internal operating mechanisms. These mechanisms can perform the following functions:
The metal frame (9) above the carriage houses the equipment supplying the refueling machine with process water, feedwater, pressurized air, inspection and measurement instruments, and control-related equipment.
The mechanisms (12) for raising and lowering the fuel assembly are mounted on the top part of the casket shown in Fig. 4.16. The assemblies are lifted by means of a mechanical grabber mechanism shown in Fig. 4.17.
Fig. 4.17 Grabber
1 - movable racks, 2 - extender, 3 - jaws, 4 - shock absorber
This mechanism consists of a cylindrical body holding a pinion gear on opposite sides of which ride two movable racks (1) connected to the controlling mechanism. The top ends of the racks are welded to the chains used to lift the grabber. The bottom end of one of the racks is welded to an extender (2), which works the jaws (3) of the grabber. If both of the chains which control the grabber move in the same direction, the jaws stay closed and the fuel assembly is raised or lowered. If they move in opposite directions, a cam opens the jaws, and the fuel assembly is released. The high pressure segment of the casket contains only the grabber, the chains and their gears, all of the control mechanisms are external to the fuel casket.
The central part of the casket contains a three-section, 16.5 m long fuel assembly receptacle (10) shown in Fig. 4.16. Each section of the receptacle has four slots, which can store new fuel assemblies, the fuel channel gauge, and the emergency plug. One slot is left empty to receive the spent fuel assembly.
The bottom part of the fuel casket contains a closing mechanism (5) and a standpipe (6). The closing mechanism serves to lock and unlock the casket to the fuel channel. It also acts as a biological shield during removal of the spent fuel. This mechanism consists of two dampers, with parallel disks installed in series within one frame.
The standpipe is used in performing the connection to the fuel channel. When the refueling machine is positioned over the required fuel channel, the standpipe is lowered so that it encloses the top part of the fuel channel. The joint is sealed by means of an inflatable rubber gasket. A special key located inside the standpipe is used for activating the fuel channel seal plug when sealing or unsealing the channel.
Two methods are used for positioning the refueling machine so that it coincides with the fuel channel coordinates:
In case steam escapes from the fuel channel, and it is impossible to use either the optical or the television system to position the machine accurately over the fuel channel, a contact positioning system is utilized.
4.3.3 Control Rod Drive Characteristics
The RBMK-1500 power plant employs several types of control rods. In terms of travel direction and response time they can be divided into three categories. The "standard" control rods are inserted from the top, and drop into a water-cooled channel, the fast-acting "scram" rods drop from the top into a gas filled channel, and finally, control rods used for vertical power shaping are inserted from the bottom of the core. Section 6 lists additional control rod classification categories, but these depend on function or control mode. These three types of control rods are powered by appropriate servodrives [1,5].
The isometric view of a servodrive used for the predominant (Type 1) control rod is presented in Fig. 4.18. The control rod is withdrawn from the core by means of a metal tape which is wound on a drum (4). The housing is constructed of an aluminum alloy. The drive is powered by a DC motor (type D500MF) and is provided with a direct electromagnetic clutch for braking. Magnitude of the load is indicated by a temperature sensor. The position of the control rod is indicated on a dial for manual inspection or a selsyn indicator (5) (type BM-404NA), which is connected to the rod via a reducer. It has top and bottom position switches and a delayed dynamic- braking switch. Positions of the rods are replicated on the main control panel and the completion of either insertion or extraction of the rod is indicated by a specific sound signal.
In the earlier RBMK-1000 reactor plants the control rods were suspended by steel cables, while in the advanced RBMK-1500 pants a steel tape is used, which has similar strength characteristics but a significantly longer life. The tapes are fastened via eccentric cams to the drums, and the rods are suspended at their ends via locks and dampers. The 40 mm wide, 20 mm thick, and 7.9 m long tapes are made from specific steel (type 12Ch18N9).
Fig. 4.18 Control rod drive
1 - housing shell, 2 - DC motor, 3 -
gear-train reducer, 4 - drum, 5 - selsyn indicator with switches, 6 -
manual mechanism, 7 - electrical connection
In the neutral position the induction coil of a servodrive and the anchor of its motor are disconnected, while the DC clutch is turned on. Consequently the brakes of the motor-rotor are activated. As soon as the logic control circuit issues an order for the removal of a rod, the induction coil of the motor is activated and the clutch is disconnected. Voltage is supplied to the anchor in about 0.2 s, the motor starts to extract the rod and continues until the limit microswitch turns off the power supply from the motor to the clutch. Each rod can be inserted either manually or automatically. In manual operation the gravitational fall of the rod is damped. Simultaneously as the rod is released, voltage is supplied to the induction coil, this operation is referred to as dynamic breaking. In this mode the electric motor serves as a generator. In case of an automatic scram the rod is in free fall for 5 s, the induction coil is reactivated in 5 s and dynamic braking is established to cushion the final segment of rod travel. If the electric current supply to the CPS rods fails, the rods are automatically released to fall into the reactor core.
The shortened absorber rods (Type 2) are drawn upwards into the core from the bottom. This requires the modification of three specific features of the servodrives. Firstly, the direct electromagnetic clutches are replaced by clutches acting in the inverse direction, secondly, an inverse calibration is provided on the dial located on the selsyn beam, and third, the length of the suspension tapes is increased to 8.035 m.
Dynamic breaking is provided during removal of the rods in the downward direction. In case of electric current failure, the clutches lock and the rods are maintained in their positions.
The fast-acting scram rods (Type 3) drop into a gas-filled, water-film-cooled channel. This design feature requires several specific design modifications. Thus, the servodrive mechanism is provided with a valve which admits the gas coolant and includes a float-operated lever which closes the entrance in the event if the channel should become flooded by coolant water. Because the drop height into an RBMK reactor core is about 8 m, a completely free drop could generate excessive speeds and lead to damage. To mitigate this situation the drive is equipped with a tachometric generator which provides breaking when the rod achieves excessive speeds. To allow higher speeds the gear train of the drive is modified so that the inertial resistance is reduced.
Most of the Type 1 rods are directly controlled by the operator and used to flatten the radial power distribution. Some are controlled by Local Automatic Control (LAC) or Local Emergency Protection (LEP) zone signals, i.e. they are controlled by the Power Density Distribution Monitoring System (PDDMS). Four other Type 1 rods are controlled by the lateral ion chambers of the automatic control system. Four Type 2 rods (shortened absorber rods) are also part of the automatic control system, controlled by the lateral ionization chambers. The rods can be divided into seven groups according to their function. The breakdown is provided in Table 4.7 [37].
Table 4.7 Reactivity control system rods [37]
|
Rods |
Number |
Time to fully insert rod (automatic shutdown), |
Rod insertion speed shutdown |
||
|
s |
automatic, m/s |
manual, m/s |
|||
|
Type 1 rods |
|||||
|
1 - Manual Control Rods (MCR) |
107 |
12-14 |
0.4± 0.1 |
0.4± 0.1 |
|
|
2 - Local Automatic Control Rods (LACR) |
12 |
12-14 |
0.2± 0.05 |
0.2± 0.05 |
|
|
3 - Local Scram Rods (LSR) |
24 |
12-14 |
0.4± 0.1 |
0.4± 0.1 |
|
|
4 - Automatic Control Rods (ACR) |
4 |
12-14 |
0.2± 0.05 |
0.2± 0.05 |
|
|
Total type 1 rods: |
147 |
||||
|
Type 2 rods |
|||||
|
5 - Shortened Automatic Control Rods (SACR) |
4 |
12-14 |
0.4± 0.1 |
0.4± 0.1 |
|
|
6 - Shortened Absorbers Rods (SAR) |
36 |
12-14 |
0.4± 0.1 |
0.4± 0.1 |
|
|
Total type 2 rods: |
40 |
||||
|
Type 3 rods |
|||||
|
7 - Fast - Acting Scram rods (FASR) |
24 |
2.0-2.5* 5.0-7.0** |
- - |
- 1.15± 0.2** |
|
|
Total number of control rods |
211 |
||||
* Fast-acting scram
** AZ-1 scram