SPECIAL MOBILE ROBOT STR-1 FOR LIQUIDATION OF THE ACCIDENT CONSEQUENCES AT THE CHERNOBYL NUCLEAR POWER PLANT

A.N. Bogatchev **

V.I. Koutcherenko **

S.I. Matrossov  **

S.V.Fedoseev *

M.I.Malenkov *

S.A.Vladikin **

V.N.Kashirin *

V.S.Solomnikov *

V.N.Petriga **

** Science & Technology Rover Co. Ltd. (RCL)

* Russian Mobile Vehicle Engineering Institute (VNIITransMash)

Zarechnaya Street, 2, 198323, St. Petersburg, RUSSIA

Phone\Fax: (812) 746-1851, E-mail: rover@peterlink.ru , rcl@rcl.spb.su

http:\\www.rovercompany.ru, http:\\www.vniitransmash.ru

Abstract: An accident in the 4th power unit at Chernobyl nuclear power plant (not a nuclear explosion) in April 26, 1986 shows the necessity of mobile robots employment for liquidation of its consequences to reduce the number of people in dangerous zones. The big volume of works was made with the aid of special mobile robot STR-1 developed by VNIITRANSMASH jointly with other organizations.

The features of design and control system of self-propelled chassis STR-1 with bulldozer blade designed and manufactured due to wide experience gained in development of self-propelled chassis for planetary rovers, starting from "Lunokhod-1", are considered in this report. Also the results of STR-1 work at the roofings are given.

Robots on the STR-1 base with additional manipulator and updated motion control system are considered. These results will be used for new generation of mobile robots intended to operate in dangerous conditions.

Keywords: special mobile robot, Chernobyl nuclear power plant, chassis, roofing

1. INTRODUCTION

The accident happened at Chernobyl nuclear power plant (NPP) in April 26, 1986 was not a nuclear explosion. However, the explosion caused the reactor opening followed by throwing about of the radioactive fuel elements as well as graphite stacking, heat ducts pipes and other constructions elements with different radiation doses. The main purpose of the accident consequences elimination was to bury the radioactive materials had been thrown about the plant territory and on the roofings of the buildings by the explosion.

To fulfil the task, various remotely controlled vehicles were applied. Among them there was a special mobile robot STR-1 developed by VNIITRANSMASH together with other organizations. The robot was used to remove the obstructions and radioactive elements from the roofing of one of the plant buildings where people could not work for a long time [Kemurdjian, 1993].

The STR-1 with 6-wheeled full-drive chassis equipped with a bulldozer blade was the main part of RC including also a transporting cradle intended for the robot transportation to the roof and for recharging on-board batteries; stationary surveying TV-cameras placed on the masts; remote radiocontrolled box with picture monitor linked with stationary and on-board TV-cameras; stationary charging unit and equipment for the chassis decontamination.

For the robot design there was used a concept of mobile vehicle with high cross-country capabilities which is radiation-precipitation-transport overloads-registrant-proof and is able to be decontaminated mechanically and also with water and chemical solutions. STR-1 was expected to be able to move on the surface like a dump of waste of building materials and scrap. The chassis for such robot was designed and manufactured at VNIITRANSMASH within very short time period (less than 2 months) due to wide experience gained in development of self-propelled chassis for planetary rovers, starting from "Lunokhod-1" [Kemurdjian, 1993; Mobile laboratory, 1971; Gromov, 1997].

The results of the STR-1 and whole RC operation at Chernobyl nuclear power plant verified the high efficiency and advisability of application of remotely controlled mobile robots in the areas with high radiation. At the same time there was revealed a necessity to have robots equipped with various tools depending on the tasks to be fulfilled.

The design and control system of the STR-1, its operation peculiarities as well as new robots developed on STR-1 base that have more efficiency and reliability because of equipping with on-board manipulator, improved control system are considered bellow. The results of further tests will be used for creation of new generation of mobile robots for dangerous conditions.

2. STR-1 OPERATIONAL CONDITIONS

Diagram of the RC deployment on the roofs and inside the 3-rd power unit building of Chernobyl NPP is shown in Figure 1.

Figure 1. Diagram of robotic complex deployment.

1 – transmitting radio antenna; 2 – cradle for transporting and charging STR-1; 3 – delivery and evaluation of STR-1 in the cradle and outside the cradle; 4 – stationery TV-cameras; 5 – stationery television system monitor; 6 – STR-1 control panel; 7 – board TV camera monitor; 8 – transmitter-control panel; 9 – charge/discharge unit control panel; 10 – containers; 11 – delivery of STR-1 in the cradle with Libher crane.

The control station was located in the room situated at 52 meters height. It was equipped for simultaneous control of two robots. Receiving antennae of picture monitors, control panels (transmitters) connected to transmitting radio antennae placed on the roofing with cables were accommodated there. Buses of joining-charging unit installed in transporting cradle were connected to charger located in special compartment with cables.

Within the period from 8 August till 10 October, 1986 the STR-1 worked on the roofing of 3rd power unit building and common for two reactors ventilation unit.

The roofing disposition diagram is given in Figure 2.

Figure 2. The roofing (marked with letters) disposition diagram of the 3-rd power unit building and ventilation unit.

1 - ramp for entering the roofing M; 2 - ramps for entering the roofing L; 3 - expansion joints

The largest roofing K is that of central room of 3-rd power unit building. During the process of liquidation of the accident consequences it was joined with ramps with the roofings M and L adjoined to the emergency reactor. A large ramp compensated the difference between K and M roofings height, and small ramps were laid through expansion joints projecting above the roofing surface by 1 m. All the roofings have fences and fire pipelines (at 0.5 m height from the roofing) round the whole periphery. Besides, the lightning - discharge protectors that were in some cases insurmountable vertical obstacles for the robot were mounted there too. The roofing G was cleaned by throwing the radioactive materials down to the roofing B. As a consequence of this, large secondary obstructions were formed there. Especially large were the obstructions close to the wall. The character of the obstructions was determined by heterogeneity of materials and high temperatures accompanying the explosion and fair.

The hot elements were the cause of ruberoid and bitumen melting. In the melt the broken bricks, fragments of graphite stacking, metal heat pipes with length up to 5-7 meters as well as fragments of construction elements sank. Therefore the closest analogue of surface for the robots motion on the K, B and M roofings is the dump of wastes of construction elements and scrap.

On the largest part of the roofings where the STR-1 operated, excepting the roofing B, the radiation level exceeded 200 Roentgen/h. On the eastern side of the roofing B maximum power of the electromagnetic dose (PED) was 2800-3100 Roentgen/h and on the roofing M it was considerably higher. Up to 70% of PED were produced by hard - radiation with an energy of 3.3 MeV.

Thus, the operational conditions under which the mobile robots worked on the Chernobyl NPP roofings (dangerous for people environment, complex relief of surface, lack of information on the surface physical properties) are similar to those for planetary rovers operation. Therefore, the wide experience gained by VNIITRANSMASH in development of automotive chassis for planetary rovers has been used for development and manufacturing of the STR-1.

3 STR-1 CHASSIS DESIGN

The STR-1 consists of automotive chassis with devices, units and equipment ensuring the motion; radio television complex providing the remote control; a bulldozer blade.

Autonomy of the chassis is ensured by the use of an on-board power source as two silver-zinc batteries. To recharge the batteries, the chassis is equipped with joining charging unit allowing the connection of the battery circuits and stationary charging unit without direct participation of man. For this, the robot entered the transport cradle going backward up to the moment of mechanical contact of the charging unit and buses of the cradle connected with cables to the charger. Besides, there was a device for recharging the batteries by means of direct connection of cables of the charger and the contacts of joining charging unit of the robot (without entering the cradle).

The chassis includes the following systems and assemblies (Fig.3, Fig.4).

Figure 3. General view of STR-1.

Figure 4. Composition of STR-1.

A – side view; B – rear view; C – top view

1) supporting frame 1 with locomotion system and equipment attached;

2) individual independent lever suspensions 17 with an elastic element as torsion bar;

3) motor - wheels 3 with individual electromechanical tractive drives 2;

4) individual friction brakes of motor - wheels with electromagnetic control on the driving shaft of the motor - wheel drive;

5) electrical units 9 for the chassis control, current distribution and commutation;

6) on-board power source 20 consisting of traction battery and battery for on-board equipment;

7) unit of on-board equipment 4 for remote control of the batteries state and for their recharging (is observed by TV- camera);

8) on-board joining-charging unit 5 for recharging the batteries;

9) dose rate meter (DRM) 14;

10) container 6 for accommodation of the RC electronics and manual contactor intended for disconnecting the robot on-board net from the power source;

11) П - post 7 for course 10 and surveying TVcameras to install. The latter has the drives at the angles of azimuth and vertical angle;

12) lateral posts 8 for DRM 14 and transmitting TV- antennae 12 to install;

13) unit 19 for transporting STR-1 with the use of a helicopter;

14) receiving radio antenna 13;

15) sights (front 21, rear 22) to facilitate the robot driving by an operator by means of on-board TV-cameras. The front sight is a pin with a striped painting fixed on the frame. The rear sight is a hook for emergency towing (for towing it is put in horizontal position);

16) bulldozer blade 15 with drives for lift and fall 16. It is attached to the frame bumper and is in the field of view of the front course TV- camera;

17) manual control panel 18.

The STR-1 main parameters are given in Table 1.

Table 1. STR-1 main parameters

Parameter

Value

Mass (for various complete sets), kg 960-1100
Tractive force on surface covered with bitumen, kN 960-1100
Tractive force on asphalt concrete, kN Up to 10
Motion speed, km/h 0.46
Clearance, m 0.4
Overall dimensions without bulldozer blade (width x length x height), m 2.2x2.4x2.3
Maximum height of obstacle (step) to overcome while moving without bulldozer blade, m 0.4
Maximum angle of slope with hard surface to climb, dgr. 22
Wheel diameter, m 0.7
Wheel width, m 0.23
Turn mode Tractor-type,
R=0
Angle of longitudinal static stability, dgr. 45
Angle of transversal static stability, dgr 35
Traction battery discharge time (according to operation data on the roofing B), h 8-12
Battery for on-board equipment discharge time (according to operation data on the roofing B), h 48
Bulldozer blade parameters, m:  
Width 2
Lifting height 0.7
Radio transmitter power, mW 200
Stable radio communication range with straight visibility, m 500

The chassis wheels are welded from titanium sheet of 1,5 mm width (Fig.5). Rim transversal section is a combination of circles arcs of different diameters which provide minimal soil resistance to lateral displacement of the wheels when the robot makes a side turn. Lateral surfaces joining the rim and the wheel hub have a cone form. All the wheel elements as well as its grousers are connected by welding. To improve the wheel traction and cohesion characteristics, while turning the grousers are placed at angle with respect to the wheel axis. The wheels are sealed. Their static tests showed the high rigidity and reliability. With diameter of 0,7 m and width of 0,23 m the wheel has mass of 13 kg and bears the radial loads exceeding 7 kN. With such load the radial deformation of the shell is about 7 mm and residual deformation is not more than 3 mm.

Figure 5. Diagram of welded titanium wheel (front view).

1-spherical rim; 2-hub; 3-cone; 4-grouser; 5-rib; 6-spacer

The wheels suspensions are equipped with the levers making the lateral movements that ensures compact arrangement of rather long titanium torsion bars along the frame. The welded frame made of titanium alloys consists of two longitudinal side rails connected to each other with six lateral beams. Underneath a sheet for protecting the equipment and increasing the rigidity is welded.

Radiation resistance of the chassis is ensured due to selected materials and components, taking into consideration the possibility for decontamination. In particular, circuits of electric units base on relays and contractors, all joints both fixed and movable are sealed, radiation-proof lubricant is used in tractive drives, radiation-proof wires are applied in cable net. Main structural materials are titanium alloys and alloyed metals. Outer surfaces of the chassis units have simple forms. They are covered with radiation - proof paint that is also water - acid - alkali solutions - proof. TV - cameras are equipped with radiation -proof glasses.

The container 6 walls and covers (fig. 4) are faced with lead plates of 5 mm thick. Stationary course TV-cameras 10 are accommodated in pressurized metallic boxes. Pressurization of the surveying TV-camera 11 is ensured by means of polyethylene film bag. Structural diagram of STR-1 control system as part of RC is shown in Fig.6.

Figure 6. STR-1 control system structural diagram.

CT1, CT2 - course TV-cameras, CT3 - surveying TV-camera; MCP- manual control panel; PD - pointing device for course TV-cameras, DRM - dose rate meter; JCU - joining- charging unit; WO-1, WO-2 - left and right drives of working organ; B-1, B-2 - traction and on-board equipment storage batteries; MW - motor-wheels.

Control system (CS) ensures execution of commands for motion FORWARD, BACKWARD, TO THE LEFT, TO THE RIGHT, EMERGENCY STOP and an automatic stop at cancelled command for motion or in case of giving two commands accidentally. When the radio communication is lost, the chassis stops in order to avoid an emergency situation. The command EMERGENCY STOP is only used in case of the control system failure. The CS reset after emergency stop is executed by command TURN OFF CHARGE.

The CS provides the wheels electric motors with overcurrent protection, which may be cancelled by command PROTECTION CANCELL. After the end-switch of the joining-charging unit action, the chassis is stopped automatically. The CS commutation unit realizes the control commands for drives of bulldozer blade, i.e. its lifting and falling.

Due to using the contractors and relays instead of remote switches in the CS as well as the STR-1 control by signals which transmitting time is equal to the chassis manoeuvre time it was possible to simplify the motion control unit up to the maximum, to exclude the command STOP and also reduce mass and overall dimensions of control unit.

All the STR-1 radio control means are concentrated on the control panel-transmitter allowing the independent control of the robot motion and bulldozer blade operation control by means of two handles. The surveillance TV-camera drives control buttons and also images focusing were located there as well.

Manual control panel was intended for the chassis experimental improvement and testing and also as a reserve in case of radio control system failure.

4 STR-1 OPERATION PECULIARITIES

Owing to the use of environment images provided by stationary TV- cameras installed on masts (Fig.1) for the robot control, the efficiency of its operation was several times increased. In this case the training of operators in driving was more simple and there was reduced their psychological strain.

Special marking of TV-screens and the operators’ knack provided an effective robot control within limited area beyond the stationary TVcameras field of view.

At different stages of decontamination of the roofings two robots operated on the roofings K, L, M, B, I, V (Fig.2). The robots entered the L and M roofing from the K roofing with the use of ramps. To the K roofing the vehicles were delivered by Libher crane and helicopters and to the roofing by helicopters.

To the V roofing the robots moved from the roofing B independently. Fragments of the STR-1 operation on the V roofing are shown in Fig.7.

When the STR-1 moved on the roofing M under very high radiation, no faults of both the chassis and the entire robot were recorded. It proves that developed equipment including radio control means is efficient under radiation conditions and at presence of metallic screens and shadowings.

On the K, B and V roofings cleaning from obstructions and transportation of materials were performed by bulldozer blade. To remove the materials from under the pipes, the back side of the blade was used. The most difficult task was loosening the baked mass and transporting long (up to 5m) fragments of tubes of thermal ducts that required constant maneuvering of the robot with simultaneous changing the blade position. On the roofing M, removal of materials with water by means of special water - gun installed on the robot and connected with the main was used.

It was estimated that application of STR-1 allowed to avoid attraction of about 1000 persons to the work in dangerous for them zones. About 90 metric tons of radioactive materials were removed from the roofings of third power unit building with the use of the STR-1, after that a dose of radiation was reduced tens times.

The number of people attracted to the work with the STR-1 was minimal. They were just involved in one-time operations such as mounting the stationary TV-camera, laying the power cables to transporting cradle and antennae, fixing the robot on helicopter's external suspensions and on a crane for transportation (Fig.1).

The robot operation results showed the high reliability of the chassis systems and units which were specially developed to operate in radioactive zones. Mostly failed were units and systems of series production which were used in the RC. There was showed up a necessity to increase the radio transmitter power and agreement in operation of all the channels of the command radio line considering the cable communications of the transmitting antennae and to increase the operation reliability of TV-cameras and TV- system as a whole.

5 Conclusions

5.1. The results of application of mobile robots for elimination of accident consequences at Chernobyl NPP have proved that it is possible and advisable to use the mobile robots with remote control for emergency and repair works in the zones with high radiation.

Application of the STR-1 and other robotic systems did not play a main role in cleaning and decontamination of the roofings at the NPP. That huge and dangerous work has been performed by people: Soviet Army soldiers, pilots, drivers, members of teams servicing the equipment. Utilization of the STR-1 allowed to exclude the attraction of about 1000 persons to the work in hazardous zones and to remove about 90 metric tons of radioactive materials from the roofings and thus to reduce the radiation 10 s times.

5.2. In order to increase the efficiency of the mobile robots application, the fleet of robots of various classes equipped with multipurposed working elements corresponding to various conditions of operation in emergency zones. It is necessary to increase the reliability of the robots and eliminate a man participation in the work on deployment, service and evacuation of the robots.

For this purpose taking into account the experience of STR-1 operation, two robotic complexes RC-1 and RC-2 were developed and manufactured. Each of them consists of the updated mobile robot (STR-M1 for the RC-1 and STR-M2 for RC-2) and mobile control station (MCS-1 for RC-1 and MCS-2 for RC-2) based on a vehicle with sealed body and having high cross-country capabilities.

The difference between these two complexes is furnishing the STR-M2 with a manipulator equipped with two TV-cameras, a bed with replaceable tools, gearboxes in the motor-wheels drives (robots chassis were similar to that of STR-1).

General view of STR-M1 is shown in Fig.8, STR-2 in Fig.9.

Box with automatics for power circuits control, containers with on-board equipment and the batteries, a crossarm with dozimeter, a bulldozer blade, an evacuator (coupling), stop with interface for charging unit connection were installed on the robots platforms. The STR-M2 platform was additionally equipped with a bumper on which a manipulator and a bed with replaceable tools were fixed.

The container for on-board equipment accommodated on STR-M2 comprises a computer, interface unit, radio modem, TV-commutator, two transmitters of TV-signals, manipulator and crossarm control unit, replaceable tools control unit, DC/DC converter, current sensors unit. The commutator is based on the ORTGON SYSTEM components. The computer is equipped with micro controller 6040 compatible with IBM PC, module for discrete input-output 5600 with 96 lines, 12-BIT ADC 5710-1 having 16 single - pole or 8 differential channels of analog input and 19 lines of discrete input-output.

For reliable transmitting the commands and TM-data between MCS and the robots the radio modems produced by PACIFIC CREST which provide required communication range both outdoors and indoors as well as a good communication with on-board computer via sequential channel RS-232 are applied.

To evacuate STR lost mobility with using another STR each robot has a special mechanism (evacuator) placed on a rear part of the robot platform. The evacuator is provided with a drive for its rotation and installation in transportation or operating positions.

Each control point consist of the control desk, the operator working place and other equipment for control of the robot.

Control instructions are given by operator with using either the keyboard and screen menu or the special panels of the control desk.

Forthcoming tests of the robots STR-M1 and STR-M2 with the additionally mounted manipulator having changeable tools and the improved control system allow to obtain data necessary to develop a new generation of mobile robots for extremal conditions, also to de-bug remote control systems including joint operation of the robots.

REFERENCES

[Kemurdjian, 1993] Planetary rovers / A.L.Kemurdjian, V.V.Gromov, I.F.Kadjukalo etc. Edited prof. A.L.Kemurdjian. Moscow, Mashinostroenie, 1993 (In Russian).

[Mobile laboratory, 1971] Mobile laboratory on the Moon – Lunokhod-1. Edited acad. A.P. Vinogradov. Nauka, Moscow, 1971 (In Russian).

[Gromov, 1997] V.Gromov, M.Malenkov, S.Fedoseev, et al. Some details of the development of mobile robot platforms. In proceedings of the International Conference on Field and service Robotics. Canberra, Australia, December 1997.