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. These three types of control rods are powered by appropriate servodrives. Characteristics of the control rods are summarized at the end of this page.
The predominant control rod is withdrawn from the core by means of a metal tape which is wound on a drum (4).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) 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.
ln the earlier RBMK-1000 reactor plants the control rods were suspended by steel cables, while in the advanced RBMK- 1500 plants 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.
ln 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 on 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 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 thirdly, the suspension tapes are made 8.035 m long. Dynamic breaking is provided during removal of the rods in the downward direction. ln case of electric current failure, the clutches lock and the rods are maintained in their positions.
The fast-acting scram rods 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 becomes flooded by the 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 predominant 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 power level protection (LPLP) zone signals, i.e. they are controlled by the power density distribution control system (PDDCS). Four other predominant rods are controlled by the lateral ion chambers of the automatic control system. Four shrtened 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 as shown in the table below.
Reactivity control system rod
Time (s) Rod insertion speed
Rod type Number to full shutdown (m/s)
insertion automatic manual
Type 1 rods
1-manual control rods (MCR) 107 14 0.4 0.4
2-local automatic control
rods (LACR) 12 14 0.4 0.2
3-local scram rods (LSR) 24 14 0.4 .4,.2***
4-automatic control rods (ACR) 4 14 0.4 0.2
Total type 1 rods 147
Type 2 rods
5-shortened automatic control
rods (SACR) 4 8 0.4 0.4**
6-shortened absorber rods (SAR) 36 8 0.4 0.4**
Total type 2 rods 40
Type 3 rods
7-fast-acting scram rods (FASR) 24 6-8 1.15 -
2.5* 3.15* -
Total number of control rods 211
* Fast-acting scram
** Rods, which drop in the top part of the core
*** Rod removal speed