atw 2018-10

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atw Vol. 63 (2018) | Issue 10 ı October DECOMMISSIONING AND WASTE MANAGEMENT 524 | | Fig. 3. Range of Hazards Considered for Robot Controlled Laser Cutting Facility. | | Fig. 4. Laser Area Classification System. The defence in depth approach adopted for the safety assessment of chemotoxic hazards is broadly to similar to that described above for radiological hazards, with indivi dual hazards identified by HAZOP studies and consequences based on industry standard Hazard Class and Category Codes, Hazard Statement Codes and Dangerous Toxic Loads (DTL). For significant chemotoxic faults (operator fatality, or serious danger to public) the designation of a passive safety measure, or two independent safety measures is required. Alternatively, it is possible to claim Operational Safety Measures, which must be carried out to prevent chemotoxic exposure. For lesser significant chemotoxic faults, (serious harm relates lower tier chemotoxic hazards, which could result in an operator being seriously injured requiring prolonged medical treatment, or, physical distress to a member of public), require the designation of one safety measure or alternatively, it is possible to in some instances to claim Operational Safety Measures, about operator actions, or plant conditions which support the safety case. There is no requirement to iden tify any safety measures in the safety case for chemotoxic hazards for which fall below the low consequence chemotoxic thresholds, with residual risk being addressed by COSHH assessments. The philosophy for the identification, designation and substantiation of chemotoxic safety measures is performed in a broadly similar way to the radiological approach described earlier. Laser assessment In the UK lasers and laser systems fall into 7 categories: 1; 1M; 2, 2M; 3R; 3B and 4, where Class 1 is the safest class and Class 4 is the most potentially hazardous. The 5kW laser used within NNL’s Robot Controlled Laser Cutting Facility is a Class 4 laser. The high energy of these lasers makes them suitable for metal cutting applications, and also renders them capable of causing severe injury to eyes and skin, and of posing a fire hazard. Exposure to direct or scattered laser light presents a direct hazard from risk of injury to the skin or eyes when Maximum Permissible Exposure (MPE) levels are exceeded. There is a possibility of life changing injury ( including blindness) from this hazard, or theoretically, death. Most high power lasers used for cutting do not utilise visible spectrum light, therefore there may be a risk to operators who could become inadvertently and unknowingly exposed to harmful quantities of invisible laser energy. In addition, laser cutting a substrate gives rise to fumes containing Area Description Comment L4 L3 L2 L1 This is an area in which laser cutting will be undertaken, with no man entry area or personnel to access the location. This is an area in which laser cutting will be undertaken where man access is required for routine operations/ maintenance, or recovery from fault conditions. This is an area in which no laser use will be undertaken. However it is in area adjacent proximity to an L3/4 area. Personnel in this area could be exposed to laser light if the barrier between the areas is compromised. This is an area at a sufficient distance from the laser that harm to personnel is not credible. | | Tab. 1. Laser Area Descriptions. particulate within the respirable size range of 0.1-10 mm, which poses a serious operator hazard of harmful parti culate inhalation. Laser cutting deployment in a nuclear decommissioning environment has the potential to result in a radiological and/or non-active chemical airborne release as discussed previously. Laser cutting is carried out with an assist gas, which itself may disturb loose contamination on the surface of the substrate. The potential for the assist gas to impact upon fixed ventilation systems should also be considered. Further indirect hazards relevant to the nuclear industry may include mis-direction of the laser beam resulting in accidental breaches to radiological or chemical containment plants. Such an event could cause a release of activity or chemical toxins into the operator’s surroundings or off-site. There is also the potential of laser damage to radiological/chemotoxic safety significant equipment and ignition of materials. The Safety Case for NNL’s Robot Controlled Laser Cutting Facility developed a unique approach to area classification for the application of high powered lasers in nuclear operations to facilitate the production of a robust and proportionate HMS. The area classification system allows potential consequences and expected controlled measures required to be readily identified in a proportionate manner. Four distinct areas are identified based upon the type of access The ‘not credible’ argument is based on specific areas which personnel are not able to access such as a radiological shielded cell. If access could occur, and prevention is reliant upon safety measures the area should be considered an L3 area. The ‘credible’ argument is mitigated by procedural control and safety interlocks to prevent operator exposure during man access area entry. The ‘credible’ argument is mitigated by safety measures to prevent operator exposure in an area adjacent to an L3/ L4 area. The ‘not credible’ argument is based on distance of the operator from the laser beam so that they are beyond the Nominal Ocular Hazard Distance (NOHD) Decommissioning and Waste Management Laser Cutting for Nuclear Decommissioning: An Integrated Safety Approach ı Howard Chapman, Stephen Lawton and Joshua Fitzpatrick
atw Vol. 63 (2018) | Issue 10 ı October required and whether there is a potential for exposure to personnel of laser light (Figure 3). An overview of the laser area classification approach is provided in Figure 4, and Table 1. The laser area classification approach is supported by a significant volume of experimental data on the vulnerability of construction and containment materials to exposure to laser light at varying distances and laser powers collated by NNL. Reference to this information is key when assessing the potential for damage to the containment structure and also consideration of the adequacy of human response times. The potential consequences arising from the use of a 5kW infra-red laser in NNL’s Robot Controlled Laser Cutting Facility range from asset damage, personnel exposure resulting in injuries, to death in the worst case. The Laser Safety Assessment uses a standard frequency/consequence matrix approach with risk scores being broadly defined as Acceptable, Tolerable, or Intolerable. Suitable and proportionate defence in depth Engineering and/or Operational Safety Measures are identified to ensure the risk is reduced to a score of Acceptable and ALARP. The philosophy for the identification, designation and substantiation of laser safety measures is performed in a broadly similar way to the radiological approach described earlier. Robot safety assessment NNL’s Laser Cutting Facility is equipped with a robot fitted with a laser which enables selective, semiautonomous controlled laser cutting for disassembly within an enclosed space [13] [14]. The robot used in NNLs Laser Cutting Facility is a proprietary KR series KUKA, with local access to an automated tool changing station, allowing the robot to switch between environment-scanning equipment (such as a point-cloud camera) and an attachable 5 kW laser cutting head (Figure 5). | | Fig. 5. View from Inside of Enclosure. Operation of this system is possible in two modes: a manual mode, where cuts are pre-planned by operators along a fixed path using KUKA’s Robot Language (KRL) and a semi-autonomous mode, where the cutting path between two operator selected points is calculated by external control software. The Robot Safety Assessment uses a standard frequency/consequence matrix approach. with risk scores being broadly defined as Acceptable, Tolerable or Intolerable. Suitable and proportionate defence in depth Engineering and/or Operational Safety Measures are identified to ensure the risk is reduced to a score of Acceptable and ALARP. The philosophy for the identification, designation and substantiation of robot safety measures is performed in a broadly similar way to the radiological approach described earlier. As the design of NNL’s Robot Controlled Laser Cutting Facility does not permit operation of the robot with operators in close proximity, the main safety concern relate to impact damage resulting in the potential for a radiological and/or chemotoxic release, or stray laser light hazards due to failure of the enclosure containment. There are multiple of safety systems implemented within the robotic system; these focus on limiting the robot’s movement to a controlled safe working area, providing additional laser firing safety inputs, and reducing the amount of human intervention required in order to reduce rig downtime. The access door to the cutting enclosure is interlocked with the laser activation and robotic movement to prevent access to the cutting enclosure during usage. The KUKA robot has physical hard-stops installed in each joint to limit robot joint range to prevent any potential damage to the enclosure structure. As an additional work-space control for laser safety, one input to the laser firing system is triggered only if the robot is physically located in a pre-defined cutting area. The robot controller uses PLC logic to determine when the laser should be enabled; these were only set during pre-defined laser cutting programs. Both the laser and robotic systems have dead-man handles that must be used to action any activity. An additional emergency stop system which completely powers down the full system provides further defence in depth. Integrated safety case For the purposes of NNL’s laser cutting trial programme, the HMS for radiological /chemotoxic hazards is mainly reliant upon claims on containment and limiting the quantity and type of materials used on test pieces to ensure any potential consequences were below the low consequence threshold. The inherent laser hazards provides the most significant risk associated with the facility, and of these, the potential for persons to be within the enclosure during cutting or maloperation of the robot/laser resulting in damage to the containment structure resulting in a subsequent radiological/chemotoxic release and/or release of stray laser light are of greatest concern. This has led to the requirement for the identification of high integrity independent safety measures and operational safety measures to be identified within the safety case purely from a laser and robotic perspective. The philosophy for the identification and designation of the laser and robotic safety measures was performed in a broadly similar way to the radiological approach described earlier. However, the substantiation of the CE&I equipment has been undertaken to meet fit for purpose industry standards for lasers and robots, rather than usual nuclear standards, because any potential radiological consequences are below the lower consequence threshold. The methods described above provide confidence to ensure that a suitable assessment methodology is in place to prepare a robust safety case for any future deployment of NNL’s Robot Controlled Laser Cutting Facility in a potential high radiological/chemotoxic consequence environment. Acknowledgments NNL collaborated with project partners, OC Robotics, TWI, ULO Optics and Laser Optical Engineering in the nuclearisation and development of this unique solution in response to the growing appetite to increase the use of robotics and automation in the nuclear decommissioning sector coupled with the a specific requirement to provide size reduction and packaging of contaminated vessels. This is the largest funded project to come out of Innovate UK, with additional funding provided by NDA and BEIS who all advocate the inclusion of small to medium enterprises in the nuclear decommissioning sector. DECOMMISSIONING AND WASTE MANAGEMENT 525 Decommissioning and Waste Management Laser Cutting for Nuclear Decommissioning: An Integrated Safety Approach ı Howard Chapman, Stephen Lawton and Joshua Fitzpatrick
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