radiation tolerance of components and materials in nuclear robot applications

ELSEVIER

Reliability Engineering and System Safety 53 (1996) 291-299 © 1996 AEA Technology pie, Published by Elsevier Science Limited

Printed in Northern Ireland. All rights reserved P I I : S 0 9 5 1 - 8 3 2 0 ( 9 6 ) 0 0 0 5 4 - 3 0951-8320/96/$15.00

Radiation tolerance of components and materials in nuclear robot applications

Richard Sharp" & Marc Decr6ton b

aAEA Technology, BlO.2 Harwell, Didcot, Oxfordshire, 0)(11 ORA, UK bSCK/CEN, Boeretang200, B-2400 Mol, Belgium

This paper describes the implications of radiation effects on the reliable and safe operation of robotic and manipulator systems in nuclear environments. Radiation effects on typical components are outlined and examples of two systems to which radiation tolerant design rules have been applied are illustrated. © 1996 AEA Technology pie, Published by Elsevier Science Limited.

1 INTRODUCTION

The nuclear industry has used remotely controlled manipulators from its very beginning. In recent years, an evolution towards advanced teleoperation has been observed, where robotic features are added to the basic manual control. The control system helps the operator in many ways: by monitoring for collisions; maintaining orientation,, distance and applied force; carrying out repetitive tasks automatically; and managing auxiliary systems such as tools, cameras or waste baskets. Even radiation tolerant versions of industrial robots have even successfully introduced to nuclear environments. Fault tolerance of industrial robots has been the subject of many studies, of which a general survey has :recently been published) In nuclear applications, a significant cause of malfuncti- oning is radiation-induced damage. Although the first mechanical master-slave manipulators were intrinsi- cally tolerant to the ,damaging effects of ionising radiation, the advanced features of modern robots, with sensors, drives and electronic circuits, have increased their sensitivity to radiation. The implications of this for system reliability and the means by which to overcome the limitations are considered here.

2 DESCRIPTION OF THE NUCLEAR ENVIRONMENT

The nuclear power industry has many types of facility in which direct, manual operations are impossible, generally because of the constraints imposed by the

291

environmental conditions. These facilities encompass the whole nuclear fuel cycle, from fuel fabrication, through nuclear reactors and irradiated fuel handling, to storage, reprocessing, waste handling and decom- missioning. The main features that characterise the environments within these facilities are shown in Table 1. 2

Operating reactors present a special case because of the extreme temperatures and radiation levels involved; in most cases remote operations are not attempted during reactor operation, the associated equipment being withdrawn before start-up. If this special case is excluded, temperatures are normally around ambient, although some facilities may be slightly warmer. The main heat sources are normally either irradiated fuel (if in close proximity to the equipment) or dissipation from the equipment itself. Some operations, such as sintering for isotope production, may generate additional heat but these are relatively uncommon. Air pressure is normally held just below ambient to maintain safe ventilation conditions. Humidity is not normally controlled, except where high moisture levels might lead to excess corrosion (e.g., magnox stores) or a criticality risk. Atmospheric composition, again, is not normally controlled but is important because oxygen can be a deciding factor in the rate of degradation of organic materials. Many strongly acidic or caustic chemicals are used during reprocessing and for decontamination and some materials can even emit such chemicals when irradiated.

In many instances, particularly for robotic operations, these conditions are compounded by cutting, welding or drilling tasks, and can be more stringent than in similar, non-nuclear robotic environments,

292 R. Sharp, M. Decr~ton

Table 1. In-cell environmental conditions

Temperature 0-50°C Pressure ambient Humidity 0-100% RH Atmospheric composition 0-20% oxygen Chemicals 8M acids, alkalis etc. Types of radiation 7,/3, X, a, neutron Dose rate 0-10 kGy/hr Total dose 0-1 MGy

where the ambient constraints can more easily be separated, and where a higher degree of automation reduces the impact of human errors.

The principal types of radiation of concern to equipment designers and operators are gamma, beta, X-ray, alpha and neutron. The first four are essentially in order of penetrating power, although alpha radiation is of particular relevance to surface coatings, paints and cable materials. Neutron radiation can be especially serious for certain types of electronics but levels are generally very low outside operating reactors. Dose rates, and thus total doses, vary widely over the range of plant described. In many cases, man entry is only just precluded, while, at the other extreme, the dose rate at the surface of a vitrified high level waste container may be 10 kGy/hr. Maintenance in future fusion reactors may involve total doses as high as 100 MGy.

Given these environmental constraints, it can clearly be seen that remote operations are necessary for many tasks. Some of the more interesting and challenging of these include repair, refurbishment and dismantling tasks.

The selection of cost-effective equipment and its level of environmental qualification for these tasks is a complicated process. The handling of contaminated items, for example, does not require a high level of radiation tolerance but does require attention to surface finish and possible contamination traps. At the other extreme, the handling of highly irradiated fuel or certain types of waste can entirely prohibit the use of standard electronics and many organic materials. It should be noted that reliability will depend on both dose rate and total integrated dose, as well as the other factors. 3-5

Further constraints on operating in nuclear facilities include the requirement for radioactive confinement, i.e., there must be a clear separation between the hazardous area and the control systems and man-machine interface located remotely in a safe zone. Long cables, up to several hundreds of metres, and recurrent connectors required to pass through leak-tight feed-through passages, bring particular difficulties for signal transmission. Maintenance concerns relate to the inaccessibility of in-cell equipment and the need to maintain remotely.

3 C O M P O N E N T S SENSITIVE TO R A D I A T I O N EFFECTS

The sensitive components installed on advanced manipulators can be divided into three categories: 1) the drives (usually electrical actuators with bearings, gear boxes and position feedback devices); 2) the sensors (distance and force sensors, viewing systems and microphones); and 3) the cables and other communication devices (including line drivers, multiplexing circuits, analogue to digital converters, radio links and even the preamplifiers needed for some sensors). For each category, the radiation hardening level required will depend on their location with respect to the radiation sources (near the end effector or near gantry tracks or walls) and on their frequency of use (e.g., a tool being used a small number of times, compared with protection systems in use permanently).

In order to analyze the impact of radiation-induced degradation on the reliability of the whole system, three questions must be addressed: 1) how critical is a failure in terms of the global reliability of the system; 2) how does the component degrade (sudden failure, progressive decalibration); and 3) what degree of radiation tolerance is available for special versions of the components. The following paragraphs will briefly present these issues for the most commonly installed components.

3.1 Drive mechanisms

The radiation induced decrease in performance or total failure of electrical motors can be caused by several mechanisms: a loss of insulation in the motor coils or in the connection wires; embrittlement of the connections; hardening of the lubricant in the bearings and gearbox; and degradation of the commutation electronics. Degradation can also be caused indirectly by radiation, e.g., gamma heating causing too high an internal temperature or halogen release from polymeric materials and lubricants inside the motor leading to the corrosion of critical parts.

Motors are usually vital parts of the manipulator and failure can lead to the locking of joints in a configuration where retrievability is very difficult. A combination of fail-safe brake mechanisms and the ability to release gear boxes is an essential safety feature. However, higher reliability will be obtained by using radiation hardened motors, specified with radiation hardened cables for coils and leads, radiation hardened lubricants in the bearings (or grease-free bearings) and careful design of the connections to remove any source of fatigue. Tests on advanced prototypes have shown that total dose limits of several tens of megagray are achievable. 6 Of course, no electronics should be located at the motor

Radiation tolerance in nuclear robot applications 293

level, and low power density is preferred to permit better internal cooling. The same attention must be paid to positioning feedback devices, often part of the drive mechanism. Optical shaft encoders are quite sensitive, although radiation hardened versions have been designed and are successfully used on nuclear robots up to a total dose of 1 MGy. 7 Resolvers, however, are normally to be preferred. They can be built in a similar fashion to the motors, with remote electronics. For both motors and resolvers, most tests have shown that the failure behaviour is usually abrupt, and caused by broken connections after thermal fatigue, but gradual loss of insulation, progressive lubricant degradation and erratic encoding faults can lead to unreliable operation when approaching the end of life.

3.2 Sensors

Sensors are central when advanced computer control and improved help to the operator in perceiving the remote environment are implemented. Both distance and force must be measured to detect obstacles, to locate or recognise targets and to feel the applied contact forces. Some of these sensors are used in safety related functions, where they prevent collisions or excess tool gripping. Advanced robotic developments tend to use so-called smart sensors with built-in signal processing. This e, legant approach is, however, unsuited for nuclear applications, where passive and robust transducers with remote electronics are to be preferred. This require,~ additional precautions and greater sophistication of the remote processing hardware and software. 8

3.3 Distance sensors

For distance sensing, three main types of transducer are used: electromagnetic for short distances, ultrasonic for large distances arid wide angular coverage, and optical systems for accurate angular resolution. 9

Electromagnetic transducers based, for instance, on the measurement of eddy currents, can be obtained with remote electronics, leaving only a passive component in the radiation field. Using carefully designed systems, a very high radiation tolerance, of up to 20 MGy at high dose rates, can be obtained. Only minor decalibration has been observed, and abrupt failure is usually due to connection problems. Such sensors can typically be used for short distance measurement at the tool level to provide accurate mating or surface following. In some instances, they can be used as safety limit switches. Similar systems, based on capacitive effects, are used for collision

avoidance applications, as they are well suited to providing large area coverage over the manipulator body. Work is presently in progress to achieve radiation hardening up to representative dose levels, but the need for on-board, front end electronics will limit the tolerable doses.

For larger distances, ultrasonic sensors offer advantages, often as navigation aids during the positioning of equipment. Thei r main advantage is wide coverage, both in angle and range, making them best suited for obstacle and target detection. Accuracies of around 1% can be achieved, but care must be taken into account for the influence of environmental parameters, such as air turbulence or temperature. For nuclear applications, transducers with remote electronics are preferred. These exist in two designs: the capacitive membrane type, used mainly on mobile robots where very wide angular coverage (30 ° ) is needed, and the piezoelectric crystal type mounted on a metallic membrane, allowing narrower angular resolution (10°). Both types have been shown to be resistant to high gamma doses (up to 10 MGy), as long as suitable polymers are used for the membrane and the cabling. No impedance matching material is used. One major advantage of such systems is their high reliability under radiation as no progressive decalibration is observed. The measurement principle is based on time of flight, i.e., independent of the radiation, and only the maximum range is affected as the accumulated dose increases.

Contrary to ultrasonic sensors, where reliable distance accuracy is achieved with poor angular knowledge, optical reflection type sensors are very directive. However, target surface characteristics greatly affect the measurement. They are in general used when reproducible conditions are to be expected or when a sharp contrast is to be located (edge, change in material, etc.). One main application is for close-range collision detection where a series of such sensors can provide a virtual safety zone around a part of a manipulator. 1° Their use in nuclear environments leads to a measurement performance degradation, as the efficiency of light emitters and receivers degrades significantly with radiation dose. This directly affects the calibration. Even the use of hardened optoelectronics would only slow down the decalibration process and not eliminate it completely.

Several ways to overcome this drawback have been considered. One is to use optical fibres and to remove the emitter/receiver from the environment. Pre- irradiated, hardened fibres actually show fairly stable attenuation characteristics, exhibiting only a negligible decalibration. Data fusion is another approach. Here, one complements the optical measurement with that from another type of sensor, e.g., ultrasonic. These sensing techniques are complementary due to their respective selective accuracy (range for the ultrasonic


technique, angular discrimination for the optical type). Combined sensor systems have been proposed and have been shown to be virtually independent of the radiation-induced degradation of individual components. Even more improvement can be obtained by making the system independent of the reflected amplitude; for instance by a triangulation technique, where angle is measured instead of amplitude, or by digitally encoding the signal. Optical fibres can also be used here to remove the optoelectronic components from the radiation field.

3.4 Force sensors

Force sensors are used at the wrist of a manipulator to input the correct signals to a force refecting joystick. They can also be used for automated sub-tasks using force control (e.g., to ensure a constant force in a particular direction or to follow curved surfaces), or to protect manipulators and tools from excessive loads. All force sensors are based on the measurement of a deformation. Strain gauges measure, for instance, the resistance change in a strained structure and can be considered as quite radiation hard, at least in principle. All-metal, weldable gauges have been used in highly radioactive environments, including in-core applications during reactor operation. More conven- tional gauges must be carefully chosen to cope with substrate and bonding embrittlementJ ~ Tests performed on strain gauges, load cells and 6-axis force/torque sensors have shown that hardened versions can withstand up to 1 MGy without significant decalibration effects. 12

3.5 Viewing systems

Remote viewing is a key feature in reliable teleoperation, but cameras are particularly sensitive to the radiation environment due to their optical elements (lenses), drive mechanisms (pan/tilt, focus and zoom), image sensors (tube, CCD, etc.), image transport system (prisms, periscope, optical fibre bundle or cables) and, of course, the front-end electronics (scan circuits, power supply, etc.). Normally, cameras are placed relatively far from the components being handled by the robot, and they do not receive as high a total dose. The 'throwaway' option of cheap, classical CCD devices is then a workable solution. However, some close-up mono and stereo cameras are fixed close to the manipulator tool, leading to a higher dose build-up, typically up to 1 MGy.

Radiation hardened cameras are offered by some specialised manufacturers. The majority of these cameras have a tube as the light sensitive device. CCDs, even in radiation hardened versions, have much lower total dose limits, and rarely go beyond

10-100 Gy. 13 They also suffer from temporary picture degradation when submitted to only moderate dose rates.

The electronics certainly form the key part with regard to the radiation tolerance of cameras. Many cameras can be obtained in versions where most of the electronics are located remotely. However, some front-end electronics cannot be avoided and must be made tolerant. Several options exist, as presented below. The option of using discrete components in a special, radiation tolerant design has been followed, yielding a tolerance in excess of 1 MGy of gamma radiation. TM

For some applications, the whole camera can be removed by using a fibrescope, but radiation hard versions are also limited to the 1 MGy levelJ 5 For all systems, the image quality tends to degrade progressively. Care must be taken to account for reliability problems where operators become used to poor images of well-known environments and then overlook unusual events. For advanced systems based on computer image analysis, such degradation must clearly be monitored to avoid sudden process failure.

3.6 Audio feedback

It has been experienced that audio feedback can be one of the most important features of efficient and reliable teleoperation. Work performed at CERN, for instance, using the MANTIS machines, have shown that with audio feedback alone, complex tasks can be performed without actual force reflection. 16 Audio feedback means the presence of microphones inside the cell, not only for ambient noise, but also for close-up listening. For ambient noise, the microphones can be placed far from the radiation sources. For close-up monitoring, either remotely located, directive microphones can be used, or microphones may be placed on the manipulator itself. Microphones are actually capacitive or piezoelectric sensors, very similar in construction to ultrasonic transducers. No real data on the radiation resistance of commercial microphones has been found in the literature but, in principle, hardening would be feasible up to very high values of accumulated dose.

3.7 Communication systems

Experience has shown that one of the most critical parts of remotely operated equipment is the connection between on-board components and the control station. This is true both for services (electrical, pneumatic) and for signal lines. Its criticality with respect to the global system reliability arises from a number of considerations: 1) umbilical management is difficult with mobile systems and a large number of cables; 2) the unavoidable presence


of a large number of connectors (due to mounting and maintenance constraints in a confined cell), some of them leak-tight; 3) multiplexing options and wireless solutions only transfer the problems to radiation hardening of the electronics; 4) the long distance between transducers and processing electronics causes a degradation of the signal-to-noise ratio and leads to issues of analogue to digital conversion, balanced line drivers, front-end preamplification, etc; 5) the fact that the umbilical is subject to constant bending or tension leads to additional mechanical stress on the materials used.

3.8 Electrical cables and connectors

Great experience has been gained on cable lifetime prediction in nuclear power plants) 7 However, most cables based on polymeric insulation are installed in areas with a very low background radiation level. For near-core and in-core instrumentation, mineral insu- lated cables have been used, although some modern polymeric materials are beginning to show very high levels of radiation tolera:ace.

Sensors and actuators can be placed on the manipulator arm and on the end effector itself. At these locations, higher doses are accumulated. At the same time, all connections around the manipulator must retain a high level of flexibility and are subject to frequent bending stress. Cables also pass through the manipulator arm itself and through the tool interface. An alternative is to rou~te the cables outside the arm around the wrist. The problem is therefore to find cables that tolerate a fairly high level of total dose, i.e., 1 MGy, and still retain their electrical and mechanical characteristic, s when submitted to constant bending.

Flexible polymeric insulation, such as PVC (polyv- inyl chloride) and PE (polyethylene), are not resistant to radiation at this orde:r of total dose, and lose their electrical and mechanical properties even at lower doses. 2 For better radiation resistance, Radox (polyolefin), PEEK (polyetheretherketone) or Kapton (polyimide) materials are preferred. These cables are usually more rigid and this causes greater stress at connectors. A high reliability cable assembly must therefore tend to reduce the stress that the cables receive by proper mechanical support.

The presence of polymeric materials near radiation sources can also lead to chemical problems, due to halogen gases released by radiation-induced molecular scission. Experience has shown that, in unfavourable circumstances, electrical sockets can be corroded sufficiently to cause failure.

Remotely operated connectors with PEEK insulation have been shown to be very resistant, both mechanically and electr!ically, up to high total doses (10 MGy) and even under thermal stresses (120°C).

Global reliability will, however, depend greatly on the care with which connectors have been mounted and protected from excessive loads and vibrations.

3.9 Electronics for signal communications

Although electronics are usually avoided in radioactive environments, advanced teleoperation requires some on-board signal processing functions to be carried out on the manipulator itself. The multiplica- tion of signal lines from sensors makes multiplexing or wireless communication very attractive and even critical for good umbilical cable reliability.

Some sensors require front-end preamplification or digital conversion. Optical communication requires on-board optoelectronics. For mobile robotics, even more sophisticated circuits are located in the radiation field, including microprocessors and memories. TM

Most semiconductors will be affected by ionisation processes creating free carriers. 19 One example is the transport and subsequent trapping of mobile charges in thin film oxide dielectrics operated under high applied voltage fields. The presence of such an undesirable charge sheet in thin oxide films can completely prevent the operation of metal-oxide- semiconductor (MOS) devices, unless the charge is dispersed in some way. 2

Different methods have been used to enhance the radiation tolerance of semiconductor-based equipment. One is a thorough knowledge of the degradation of the functional characteristics of standard devices, allowing a special, tolerant circuit design to be applied. Digital switch operation for a circuit designed by this method has been shown to operate up to a total dose of 6 MGy. z°

The radiation hardening of individual devices at the process level is achieved mainly by insulation techniques, reducing parasitic photocurrents. Tolerant CMOS devices, produced mainly for space applications, are available up to total doses of 1 to 10 kGy. 19 To go beyond this limit, the most promising technologies are SOl (silicon on insulator) and GaAs (gallium arsenide). Recent tests have shown the feasibility of obtaining devices tolerant to 1 MGy by both approaches. 21"22 Advances are also being made with silicon and a few processes have been shown to yield devices tolerant to the 1 MGy level. 23 In any case, the reliability issues associated with electronics will depend greatly on a good knowledge of the degradation behaviour and the use of predictive techniques. 24

3.10 Summary of this section

Table 2 summarises each of the areas described in detail above, giving the limits of usability for example sub-systems.


Table 2.

Sub-system Limit

Drive mechanisms Distance sensors Force sensors Viewing systems Audio feedback Electrical cables and connectors Electronics for signal communications

10 MGy 20 MGy 1 MGy 1 MGy 1 MGy 10 MGy 1 MGy

4 EXAMPLES

Two examples follow to illustrate applications where the principles of nuclear engineering and radiation tolerant design have been used to ensure the safe and reliable operation of equipment in radioactive environments.

4.1 Example 1. INGRID

INGRID (Intelligent Nuclear Gantry Robot Integr- ated Demonstrator) is one of the five projects of the European Community Teleman Phase II Programme, aiming at demonstrating advanced telerobotic technologies developed primarily for the nuclear industryY The INGRID facility, located at BNFL's Sellafield site, consists of a gantry mounted robot with remote control provided by a man-machine interface. In the cell, the robot deploys tooling packages under telerobotic and robotic control to undertake a variety of remote handling tasks. The remote handling workstation provides the operator with the spatial, functional and procedural support required to conduct the tasks. In order to prove the capabilities of the facility, scenarios applicable to the nuclear industry have been demonstrated. INGRID has proved the capabilities of an integrated telerobotic system for hazardous and unstructured environments, representative of many aspects of nuclear remote operation, particularly in the context of maintenance, refurbish- ing and dismantling in large cells. The project was, however, limited to a demonstration in a simulated environment, based on a mock-up in real size, but without the actual radiation environment. Neverthe- less, it provided a representative set-up for the evaluation of the necessary components, as well as the impact of their reliability on the global performance of the system.

The demonstration cell uses a 6 degree of freedom (dof) NEATER 760 robot 7 capable of conducting semi-autonomous and telerobotic tasks, mounted on a 4 dof gantry (Fig. 1). The robot and the gantry are electrically driven. Sensors are used to calculate the position of the gantry and the robot links. An additional hoist is manually operated without position

feedback. There are no on-board electronic circuits connected with these sensors, all cabling being brought out of the cell via the inside of the robot, the telescopic mast and along the gantry. The typical distance between the robot end-effector and the cell wall is 50 metres and line drivers are used to ensure proper communication. There is no force sensing on the drivers. Possible torque excursions are detected only by excess current supply.

Teleoperation control is based on stereo and mono viewing facilities, placed along the cell walls for global viewing and on the robot itself for close angle perception. The cameras have pan and tilt capabilities and two mono cameras can be controlled in closed loop mode for automatic target following. The stereo camera is used to compensate for the difficulties experienced by an operator in spatially interpreting mono camera views during complex tasks. This stereo camera incorporates a zoom lens and instrumented pan and tilt, and is further capable of acting as a measurement system enabling photogrammetric data to be obtainable. A radiomicrophone is mounted on the robot, providing audio feedback to the operator.

An advanced man-machine interface, with a BSP force reflecting joystick, 26 is used for manipulator control. A collision avoidance sensing system protects the robot wrist. This system is based on ultrasonic rangers and non-contact, optical proximity switches using optical fibres. The signals from this multi-sensor system are multiplexed before being sent to the remote control system. The multiplexing electronics is located on the robot forearm. To perform the different tasks, several tools are accessible through a tool change rack, complete with connectors for power and signals to the tools. The tools consist of several grippers, a reciprocating saw, several drill and tapping devices, and a decontamination head. The project focuses on four reference scenarios, typical of remote handling in nuclear cells. These scenarios are: the repair of piping, involving cutting and welding operations; the removal of heavy components, involving disassembly, cutting and transfer tasks; surface monitoring and decontamination; and the dismantling of equipment, involving cutting, waste collection and transfer.

A reliability analysis has been made for the INGRID machine, 25 based on the components used on the mock-up and an assumption of the eventual availability of radiation hardened versions of them. This analysis, together with the experience gained during the execution of the scenarios, has led to a number of recommendations in respect of reliability. The first is to use radiation hardened components for all end-effector locations. A study showed that it would be beneficial to have a shielded parking place for the robot at one end of the cell and to keep the highly radioactive equipment at the opposite end.


Fig. 1.

Another recommendation was to allow for modularity and easy remotely operated replacement of the most sensitive components, for example, the electronic modules for the force sensors and the anti-collision system. This replacement should be planned on the basis of on-line measurement of the total integrated dose, using semiconductor or optical dosimeters.

4.2 Example 2. WindscaJe vitrification plant

The Windscale Vitrification Plant (WVP), operated by BNFL at Sellafield, converts liquid high level waste into a solid, vitrified product, contained in stainless steel flasks. The waste is highly radioactive and the dose rate at the surface of a flask can reach 10 kGy/hr. The system uses a NEATER 760 robot, operating in teach-and-repeat mode, to pick up a swab, move it at constant pressure over the entire surface of a flask and then transfer the swab to a monitoring station (Fig. 2). Hence, the wrist and forearm of this robot accumulate a high total dose

over a relatively short period of time. The radiation tolerance of the arm is thus critical to the reliability of the swabbing process.

The NEATER 760 robot is based upon the St~iubli Unimation Puma 760 robot, modified to make it radiation tolerant. This exercise covered all non- metallic parts, including motors, brakes, optical shaft encoders, seals, bearings, limit switches, greases, adhesives and paint, as well as electronic components. Where radiation effects data on these components were not available, radiation testing was carried out, using test facilities at Harwell. The performance of each component with increasing total dose was assessed to ensure its reliability in service. The most challenging task was the design of the 16-bit optical shaft encoder, especially as it had to be rated to operate over the temperature range from 0 to 80°C. The techniques described earlier in this paper were applied and recent operation at WVP has borne out the reliability of this approach.

The first robot has operated in WVP for 17,687

298 R. Sharp, M. Decrdton

Fig. 2.

hours and has carried out in excess of 10,000 swabs on some 700 containers. During this time, its performance has considerably exceeded both expectations and the design life. The total dose estimated to have been received by the wrist and forearm areas of the robot is approximately 1.8 MGy.

These results show that radiation tolerant design techniques can successfully be applied to robotic equipment to give reliable and safe solutions for tasks that cannot be carried out manually.

5 CONCLUSIONS

The use of robotic equipment in nuclear environments adds a range of new constraints to those normally considered in industrial applications. The effects of radiation form one of the most important of these new constraints. This paper has described these effects on the susceptible components and materials found on

typical robots and manipulators and has then given two examples of applications where the basic principles of designing for radiation tolerance have been applied. Experience with these applications has shown that the design rules are successful and yield systems that match or better their industrial counterparts for reliability and safety.

ACKNOWLEDGEMENT

The INGRID project was funded by the European Commission (contract FI2T-CT92-0025) and co- ordinated by BNFL--Sellafield, with AEA Technology--Harwell, SCK.CEN Mol, CRIF-- Brussels, FZK--Karlsruhe, RisO---Roskilde and the universities of Hannover, Newcastle and Oxford as partners. Additional funding was provided by the UK Department of Trade and Industry via the Active Handling Programme. In particular, thanks are due to


Bill Webster and Myrian Bishop of BNFL and Kurt Lauridsen of Riso for their help in providing information on the chosen components and the reliability analysis. We are also grateful to Ian Milgate of BNFL for operating data on the WVP systems. The participation of Jan De Geeter and Simon Coenen of SCK.CEN--Mol and Lee', Pater of A E A Technology in radiation tolerance studies was also greatly appreciated.

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radiation tolerance of components and materials in nuclear robot applications

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