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Application of unmanned aerial vehicles for radiological inspection
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Application of unmanned aerial vehicles for radiological inspection Alberto Vale a,∗ , Rodrigo Ventura b , Paulo Carvalho c a
Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal Institute for Systems and Robotics, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal c Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal b
a r t i c l e
i n f o
Article history: Received 3 October 2016 Received in revised form 31 May 2017 Accepted 1 June 2017 Available online xxx Keywords: Remote handling Unmanned aerial vehicles Radiological inspection
a b s t r a c t Remote Handling Systems (RHS) are specially designed for regular operations inside and outside of the nuclear reactors for inspection and maintenance. The reactor is shutdown during the installation and operation of the RHS, which is time-consuming and expensive. Unmanned Aerial Vehicles (UAVs) are a possible solution to perform inspection missions inside the reactor before the RHS operations. This work presents possible applications of UAVs to perform inspection missions inside the reactor during its shutdown. Such UAVs are able to transport different on-board sensors to get an insight view of the blankets and other elements inside the reactor, while providing the maneuverability and endurance to perform the inspection missions. The same approach can also be used for inspection of contaminated areas, such as the fission reactors for leakage detection, storage areas of nuclear sources, or even in hazardous scenarios of nuclear disasters. The costs of producing, maintaining and operating UAVs are expected to be low when compared to the time and costs that can be saved with the valuable information acquired during the flight. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Experimental fusion reactors aim at the exploration of the nuclear fusion as a viable energy resource. RHS are specially designed for regular operations of inspection and maintenance inside the reactors, such as the In-Vessel Transporter, an extendable robotic arm deployed in the equatorial level of ITER (International Thermonuclear Experimental Reactor). The reactor is shutdown during the installation and operation of the RHS, which is timeconsuming and expensive. The UAVs, commonly known as “drone”, is an aircraft without a human pilot aboard. The UAVs were originated mostly in military applications, but it is being widely spread for the purpose of personal enjoyment, given the lack of legislation in most of the countries (there are several related press articles, for example in [1] of a drone hitting a British Airways passenger jet). The most common types of UAVs are blimp like systems, fixed-wing airplanes, and rotary-wing aircraft. The last one is well known as multicopters, such as the helicopters, quadcopters, hexacopters or octacopters, according the number of rotors [2]. Only the multicopters and the
∗ Corresponding author. E-mail addresses: [email protected] (A. Vale), [email protected] (R. Ventura), [email protected] (P. Carvalho).
blimp like systems have the powerful capability of a loiter mode navigation, i.e., the vehicle slows to a stop and hold position during the flight. The blimp like systems are slower, but good for other issues such as battery life, payload, risk to damage and crashing, and loss. However, the multicopters technology is well established in the market with different commercial off-the-shelf solutions and, thus, it is the type of UAVs addressed in this paper. Multipcopters provide the ability to navigate in space with different degrees of freedom, namely the position (latitude, longitude and altitude), orientation (roll, pitch and yaw) and velocities along the axes of position and orientation [3]. On-board sensors are used for navigation and also for inspection of its surroundings. Given the flexibility of the multicopters, the on-board sensors are easily target to relevant areas proving the additional capability of scanning the scenario. The UAVs can operate autonomously or remotely controlled. In autonomous operation, the predefined trajectory can be optimized according the dynamics of the vehicle and the target goal of the inspection mission. The operation is performed under human supervision and can be switched between autonomous and manual modes whenever necessary. The authors of this paper did not find any technology of multicopters ready for the inspection of nuclear facilities. Even though, given the versatility and flexibility of multicopters, its application for nuclear facilities is in the foreseeable future. For instance,
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the multicopters are able to perform simple inspection missions inside the reactor before the RHS operations. The information acquired by the multicopters provides the ability to previously setup the inspection and maintenance operations (the sequence of planned maintenance operations or to start by unplanned operations), yielding time and cost benefits. The multicopters are able to perform an inspection mission as soon as possible after the shutdown of the reactor. Such multicopters are able to transport different on-board sensors to get an insight view of the blankets and other elements inside the reactor, while providing the maneuverability and endurance to perform the inspection missions. The costs of producing, maintaining and operating multicopters are reduced when compared to the time and costs that can be saved with the valuable information acquired during the flight. The inspection mission is not only applicable to inside the reactor, but even along the galleries, storage and maintenance areas of the reactor buildings. In addition, the multicopters technologies can also be applied to regular or unexpected operations of inspection in fission facilities, storage areas of nuclear sources, or in hazardous scenarios of nuclear disasters. This paper presents possible applications of the multicopters technologies, the potential inspection missions in nuclear facilities where these type of UAVs can be valuable, the architecture of the vehicles and its sensing capabilities. The paper also addresses the main constrains of nuclear scenarios, such as the high doses of radiation, high temperatures, residual magnetic fields, safety restrictions, with impact to remote maintenance operations [4]. Possible modifications of the multicopters architecture and of the way of using this technology are proposed in such a way that it can be reliable in nuclear industry. This paper is organized as follows. Section 2 describes the proposed inspection missions with UAVs inside fusion reactors and in contaminated areas, such as outside of fission reactors for leakage detection. Section 3 describes a proposed architectures for UAVs, the most suitable sensors and the first results with a proof-ofconcept. Finally, Section 4 summarizes the main conclusions and the future work. 2. Inspection missions An inspection mission is considered when the UAV has to follow a predefined trajectory to get information from the environment to identify possible anomalies. The mission can be performed in a closed loop, where the UAV can take-off and land in the same place. The data acquired by the sensors can be streamed for a real-time insight view and/or recorded into a on-board storage device. The streaming functionality provides the ability to record the sensors data in a storage device installed faraway from the sources of radiation. However, the communication may have a narrow bandwidth or can even fail, loosing temporarily information. The missions can be performed in: • Online inspection – the data is acquired and streamed in real time to a central control room. A remote computer provides the ability to visualize and process the data, (the UAV has few computational power and time of flight). • Offline inspection – the data is acquired and recorded for a later visualization (to navigate along the frames in a time-line) and processing. During the flight, the UAV can operate in the following modes: • Full autonomous, following predefined trajectories with auto return mode. The current mission can be interrupted by the
operator or by an anomaly detected during the flight, such as communication failure or insufficient battery. In case of communication failure, the UAV has to be able to return to a safe position, where the communication can be reestablished. If interrupted by human, the mission can be resumed or switched to one of the next modes. • Semi-autonomous, similar to the full autonomous mode, but with the ability to change the trajectory, orientation or the following speed during the flight, and to attempt the loiter mode (maintain the current location, heading and altitude) to focus specific areas. • Manual, where the pilot is able to fly using the on-board cameras acting as his eyes, i.e., a first person view, with full control of the drone. There are two main types of inspection missions: (i) inside the vessel or inside the reactor during its shutdown and (ii) in contaminated areas where the location and the amount of radiation may be unknown. 2.1. Inspection inside fusion reactors Inspection inside fusion reactors are the most ambitious mission to achieve important information to manage the scheduled and unscheduled operations of maintenance. The aim is an insight view with a visual inspection enriched with extra information, such as radiation levels, temperature and 3D model to perform metric evaluation and comparison with the original CAD model of the reactor. The mission would be extended for blankets and divertors inspection and also for the ports. However, this type of inspection mission is a challenge, given the hazard scenario with high levels of radiation and temperature, leading to a very short time life of any UAV. The inspection mission inside a fusion reactor can only be performed during the shutdown and without vacuum, to support the propeller system. The most sensible parts to radiation exposure are made by semiconductors [5], which are able to support up to 100 Gy. Based on the study presented in [6,7] with a provisional (DEMOnstration Power Station) model with helium-cooled lithium-lead type blankets, the dose rates in vessel are 2300 Gy/h, 1500 Gy/h and 800 Gy/h for 1 week, 1 month and 1 year, respectively. Inside a port, the values become 95 Gy/h, 80 Gy/h and 15 Gy/h with respect to the same periods of time. Assuming the maximum dose rate of 2300 Gy/h and the maximum dose supported by semiconductors, 100 Gy, the UAV would be able to support approximately 2.6 min inside the reactor, which is useless. The semiconductors are inside the controller unit and inside the sensors. These devices shall be shielded using, for instance, lead sheets, in such a way that its thickness assures a protection barrier. Assuming a flight time of 20 min during the maximum dose rate after one week, the shielding protection shall provide a attenuation of 2.6 min ÷ 20 min, which is 13%. Assuming the intervention of the UAV inside the reactor only one month later, the protection shall provide a attenuation of 20%. By [8], 3 cm of lead provides 12.5% of attenuation, which becomes very heavy, while 1 cm of lead provides 50%. The controller unit, as well as the internal sensors (e.g. gyroscopes, as described in Section 3) can be completely enclosed by the lead. However, the external sensors that capture the scenario information shall be shielded, but at the same time, supported by mirrors to achieve information and avoid direct exposure to the radiation [9]. In case of failure of the external sensors, the information is not acquired, but without impact to navigation and hence safety. The lead required for shielding and the amount of equipment must be compatible with the payload of the UAV, which is defined by the architecture of the UAV, as described in Section 3. The residual magnetic field of the environment (1mT) have no impact to the navigation, since the gyroscope and Inertial measure-
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ment unit (IMU) can be calibrated or replaced by optical sensors. However, the temperature is still an issue even when the reactor, such as DEMO [13], has been shutdown for some weeks, where active cooling will be necessary to inhibit the neutron-induced decay heat from raising the temperature of components. Actually, the largest grade of electrical or mechanical devices is the military one, which is only able to operate up to 125 ◦ C. Thus, the temperature remains an open issue to be addressed in a future work, taking into account the following subjects: (i) the shutdown decay heat (for instance, for each blanket it reduces two orders of magnitude after approximately one month [14]), (ii) the drones are commonly light, with a reasonable surface area to volume ratio, and (iii) the air flow generated by the propellers must be assessed if, adjusting the body design, it can contribute for the cooling of the drone during the short period of flight.
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Fig. 1. Quadcopter Dji Phantom model (left) and the test scenario (right).
2.2. Inspection in contaminated areas Activated components can be found in expected scenarios, such in reactor outages, laboratory test facilities, storage areas with contaminated loads for fusion and fission reactors and with particular equipment used, for instance, in health physics. In addition, there are unexpected scenarios resulted from possible accidents or where dangerous material is hidden for terrorism attack with nuclear weapons. Thus, a remote inspection is important for a fast intervention by security forces. The values of radiation in contaminated areas are possibly unknown, but low when compared to the valued inside the nuclear reactor. Based on [7], the dose rate inside the ports is below to 100 Gy, allowing time flight of 1 h, which is still an ambitious time flight using the UAVs solution available in the market at the date of this paper. The application of UAVs in inspection missions of contaminated areas is the most reliable in a short term. At the end of the mission, it should be able to have maps with the scanned area, reporting hotspots in terms of radiation for a deep inspection. 3. Proposed architecture The architecture is defined by the design and configuration of the frame, the motor engines, the control unit, the on-board sensors, the communication system and the power supply. The UAVs shall be modular to achieve the maximum flexibility for different type of missions and also for the replace ability following radiation exposure. A multi-criteria optimization approach [10] is proposed to find the most appropriate modules to better satisfy the requirements of the mission. This approach aims at finding solutions that trade-off multiple criteria, in the sense that all candidate solutions that are outperformed in all criteria, called dominated solutions, are ignored. Thus, multi-criteria optimization algorithms aim at obtaining the set of all non-dominated solutions. The criteria considered here for our problem are: (i) maximum flight time, including take-off, navigation, and landing, (ii) maximum payload; and (iii) total cost. The best solutions are based on quadcopters, with low price, good time of flight and the reduced payload is enough to endow the UAVs with the most important sensors, such as: internal sensors and external sensors. The internal sensors are gyroscope and IMU, for the flight performance. The external sensors used for localization, mapping and inspection are: (i) video camera (RGB information, wide angle view, preferable with a panoramic view), (ii) laser range finder (also known as LiDAR for a 2D scanning of the environment) or a depth camera (kinect like sensor), (iii) thermal camera (additional information to the video cameras, i.e., RGB + temperature), and (iv) Geiger counter (to estimate the levels of radiation).
Fig. 2. From left to right: 3D reconstruction of the scenario depicted in the right image of Fig. 1, the original positions and intensities of sources of radiation, a test trajectory and the respective estimation.
The authors assembled a proof-of-concept drone for the inspection of indoor scenarios with local sources of radiation and with very low dose rates, when compared to the reactors. The selected UAV is a quadcopter Dji Phantom model depicted on the left image of Fig. 1, equipped with its inertial sensors, a full HD video camera, a depth sensor (Structure 3D sensor, which is a Kinect like sensor) and a Geiger counter (from SparkFun Electronics). The tested room is illustrated on the right image of Fig. 1. The map of the scenario is a cloud-of-points, i.e., M = {m1 , m2 , . . ., mI }, where mi = (x, y, x), returned by the depth sensor and processed with a simultaneous localization and mapping (SLAM) approach [11]. The I is the number of points in the map, mi is the ith point of the map and (x, y, z) are the 3D space coordinates. represents the radiological activity of the map, namely, = { 1 , 2 , . . ., I }, where i = (mi ) is the radiological activity of each point. The P will refer to the path followed by the UAV, i.e., P = {p1 , p2 , . . ., pJ }, where pj = (x, y, z, , , ). The J is the number of points of the path, pj is the jth point in the path, (x, y, z) represent the 3D position of the UAV and (, , ) are the Euler angles, which represent the orientation of the UAV along the path. For simplicity, only the heading angle is considered. The represents the radiological intensity measured in each point of the path, i.e., = {1 , 2 , . . ., J }, where j = f(pj , ). The function f represents the model of the sensor. Assuming each point of the map irradiates in a radial distribution and the radiation measured in each point of the trajectory is given by the contribution of all the points of the map, then = . To estimate the radiation map, given measurement data corrupted by white noise, a possi−1
ble solution is given by = −1 , where −1 = (T ) T , the Moore–Penrose pseudoinverse. A first result of SLAM with real data is illustrated on the 3D reconstruction on the first image of Fig. 2. A simulation with the model of the Geiger counter is illustrated on the right image of the same figure. The common multicopters, as the Phantom model of DJI, illustrated in the left of Fig. 1, are prone to crash and feature whirling propellers that can hurt people and easily damage themselves and equipment in its vicinity. However, an alternative to the common drone is the “duct drone” [12], that relies on four ducted fans, rather than open propellers, as well as a sturdy material housing to store everything in. While most of the common UAVs are reliable for inspection in large and open scenarios, the duct drone is more suitable for reactor inspection. The housing box of the duct drone are prepared to support clashes against the blankets and, in the worst
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case of failure and fall, it simplifies the mission of removing a single broken piece. 4. Conclusions and future work This paper presented a preliminary study of UAV applications for remote operations of radiological inspection. The most appropriate UAVs are the rotary-wing aircraft, named multicopters. The multicopters technology, which can operate autonomously or remotely controlled under human supervision, is well established in the market. The multipcopters provide the ability to navigate in space with six degrees of freedom, with on-board sensors used for navigation and also for inspection, proving the capability of scanning the scenario. The information acquired by the multicopters during the flight in nuclear facilities provides the ability to previously inspect and then setup and optimize the maintenance operations, yielding safety, time and cost benefits. The main conclusion is that the application of UAV for inspection inside fusion reactors is still ambitious for the current off-the-shelf technology, given the high dose rates and temperature. There are additional issues that shall be studied, such as: dust spread (specially in the vicinity of spare sources of radiation, like in the divertor level of a reactor), impact of the reactor architecture with the aerodynamics of a multicopter, risk of failure and risk of clash (the drone may crash with the wall and may fall or get stuck in the structure), risk mitigation and rescue operations (possibly with a multi-purpose deployer). However, the current technology provides the means for the inspection with UAV in contaminated areas resulted from storage of activated material or from accidents and, specially, to envisage the detection of dangerous material intended for terrorism attach with nuclear weapons. The outcomes of the inspection with UAV in contaminated areas may be valuable to proceed with the inspection inside fusion reactors in a near future. The next experiments are being prepared by the authors with a drone flying inside a room with sources of Potassium-40, where its location and dose rates are previously known.
Acknowledgments IST activities received financial support from “Fundac¸ão para a Ciência e Tecnologia” through projects UID/FIS/50010/2013 and UID/EEA/50009/2013. References [1] C. Turner, Lack of Regulation Blamed for Drone Hitting BA Jet, The Telegraph News, 2016 April. [2] G. Cai, B.M. Chen, T.H. Lee, Unmanned Rotorcraft Systems, Springer-Verlag, London, 2011. [3] G. Hoffmann, H. Huang, S. Waslander, C. Tomlin, Quadrotor helicopter flight dynamics and control: theory and experiment, Proc. of the AIAA Guidance, Navigation and Control Conf. and Exhibit (2007). [4] A. Tesini, J. Palmer, The ITER remote maintenance system, Fusion Eng. Des. 83 (December (7)) (2008) 810–816. [5] D. Makowski, The impact of radiation on electronic devices with the special consideration of neutron and gamma radiation monitoring, Zeszyty Naukowe. Elektryka/Politechnika Łódzka, 2007, pp. 73–80. [6] O. Crofts, A. Loving, D. Iglesias, M. Coleman, M. Siuko, M. Mittwollen, V. Queral, A. Vale, E. Villedieu, Overview of progress on the European DEMO remote maintenance strategy, Fusion Eng. Des. 109 (Pt. B) (2016) 1392–1398. [7] A. Loving, O. Crofts, N. Sykes, D. Iglesias, M. Coleman, J. Thomas, J. Harman, U. Fischer, J. Sanz, M. Siuko, M. Mittwollen, Pre-conceptual design assessment of DEMO remote maintenance, Fusion Eng. Des. 89 (2014) 2246–2250. [8] Code of Federal Regulations, Title 10, Part 20 – Standards for Protection Against Radiation, United States Nuclear Regulatory Commission, 2014. [9] G. Baldwin, S. Sickafoose, W. Sweatt, M. Thomas, Authentication approaches for standoff video surveillance, Sandia National Laboratories IAEA Safeguards Symposium, Vienna, Austria (2014 October). [10] R. Statnikov, J.B. Matusov, Multicriteria Optimization and Engineering, Chapman & Hall, 1995. [11] J. Biswas, M. Veloso, Depth camera based indoor mobile robot localization and navigation, in: Proceedings of IEEE International Conference on Robotics and Automation, Saint Paul, 2012, pp. 1697–1702. [12] Alec, Nano Tornado Camera Drone, the Safest 3D Printed Fan Duct Drone Launches on Kickstarter, 2015 May, URL: 3ders.org. [13] T. Todd, Diagnostic systems in DEMO: engineering design issues, AIP Conference Proceedings 1612, 9 (2014). [14] T. Eade, M. Garcia, R. Garcia, F. Ogando, P. Pereslavtsev, J. Sanz, G. Stankunas, A. Travleev, Activation and decay heat analysis of the European DEMO blanket concepts, Fusion Eng. Des. (2017) 920–3796.
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Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Application of unmanned aerial vehicles for radiological inspection Alberto Vale a,∗ , Rodrigo Ventura b , Paulo Carvalho c a
Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal Institute for Systems and Robotics, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal c Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal b
a r t i c l e
i n f o
Article history: Received 3 October 2016 Received in revised form 31 May 2017 Accepted 1 June 2017 Available online xxx Keywords: Remote handling Unmanned aerial vehicles Radiological inspection
a b s t r a c t Remote Handling Systems (RHS) are specially designed for regular operations inside and outside of the nuclear reactors for inspection and maintenance. The reactor is shutdown during the installation and operation of the RHS, which is time-consuming and expensive. Unmanned Aerial Vehicles (UAVs) are a possible solution to perform inspection missions inside the reactor before the RHS operations. This work presents possible applications of UAVs to perform inspection missions inside the reactor during its shutdown. Such UAVs are able to transport different on-board sensors to get an insight view of the blankets and other elements inside the reactor, while providing the maneuverability and endurance to perform the inspection missions. The same approach can also be used for inspection of contaminated areas, such as the fission reactors for leakage detection, storage areas of nuclear sources, or even in hazardous scenarios of nuclear disasters. The costs of producing, maintaining and operating UAVs are expected to be low when compared to the time and costs that can be saved with the valuable information acquired during the flight. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Experimental fusion reactors aim at the exploration of the nuclear fusion as a viable energy resource. RHS are specially designed for regular operations of inspection and maintenance inside the reactors, such as the In-Vessel Transporter, an extendable robotic arm deployed in the equatorial level of ITER (International Thermonuclear Experimental Reactor). The reactor is shutdown during the installation and operation of the RHS, which is timeconsuming and expensive. The UAVs, commonly known as “drone”, is an aircraft without a human pilot aboard. The UAVs were originated mostly in military applications, but it is being widely spread for the purpose of personal enjoyment, given the lack of legislation in most of the countries (there are several related press articles, for example in [1] of a drone hitting a British Airways passenger jet). The most common types of UAVs are blimp like systems, fixed-wing airplanes, and rotary-wing aircraft. The last one is well known as multicopters, such as the helicopters, quadcopters, hexacopters or octacopters, according the number of rotors [2]. Only the multicopters and the
∗ Corresponding author. E-mail addresses: [email protected] (A. Vale), [email protected] (R. Ventura), [email protected] (P. Carvalho).
blimp like systems have the powerful capability of a loiter mode navigation, i.e., the vehicle slows to a stop and hold position during the flight. The blimp like systems are slower, but good for other issues such as battery life, payload, risk to damage and crashing, and loss. However, the multicopters technology is well established in the market with different commercial off-the-shelf solutions and, thus, it is the type of UAVs addressed in this paper. Multipcopters provide the ability to navigate in space with different degrees of freedom, namely the position (latitude, longitude and altitude), orientation (roll, pitch and yaw) and velocities along the axes of position and orientation [3]. On-board sensors are used for navigation and also for inspection of its surroundings. Given the flexibility of the multicopters, the on-board sensors are easily target to relevant areas proving the additional capability of scanning the scenario. The UAVs can operate autonomously or remotely controlled. In autonomous operation, the predefined trajectory can be optimized according the dynamics of the vehicle and the target goal of the inspection mission. The operation is performed under human supervision and can be switched between autonomous and manual modes whenever necessary. The authors of this paper did not find any technology of multicopters ready for the inspection of nuclear facilities. Even though, given the versatility and flexibility of multicopters, its application for nuclear facilities is in the foreseeable future. For instance,
http://dx.doi.org/10.1016/j.fusengdes.2017.06.002 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Vale, et al., Application of unmanned aerial vehicles for radiological inspection, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.06.002
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the multicopters are able to perform simple inspection missions inside the reactor before the RHS operations. The information acquired by the multicopters provides the ability to previously setup the inspection and maintenance operations (the sequence of planned maintenance operations or to start by unplanned operations), yielding time and cost benefits. The multicopters are able to perform an inspection mission as soon as possible after the shutdown of the reactor. Such multicopters are able to transport different on-board sensors to get an insight view of the blankets and other elements inside the reactor, while providing the maneuverability and endurance to perform the inspection missions. The costs of producing, maintaining and operating multicopters are reduced when compared to the time and costs that can be saved with the valuable information acquired during the flight. The inspection mission is not only applicable to inside the reactor, but even along the galleries, storage and maintenance areas of the reactor buildings. In addition, the multicopters technologies can also be applied to regular or unexpected operations of inspection in fission facilities, storage areas of nuclear sources, or in hazardous scenarios of nuclear disasters. This paper presents possible applications of the multicopters technologies, the potential inspection missions in nuclear facilities where these type of UAVs can be valuable, the architecture of the vehicles and its sensing capabilities. The paper also addresses the main constrains of nuclear scenarios, such as the high doses of radiation, high temperatures, residual magnetic fields, safety restrictions, with impact to remote maintenance operations [4]. Possible modifications of the multicopters architecture and of the way of using this technology are proposed in such a way that it can be reliable in nuclear industry. This paper is organized as follows. Section 2 describes the proposed inspection missions with UAVs inside fusion reactors and in contaminated areas, such as outside of fission reactors for leakage detection. Section 3 describes a proposed architectures for UAVs, the most suitable sensors and the first results with a proof-ofconcept. Finally, Section 4 summarizes the main conclusions and the future work. 2. Inspection missions An inspection mission is considered when the UAV has to follow a predefined trajectory to get information from the environment to identify possible anomalies. The mission can be performed in a closed loop, where the UAV can take-off and land in the same place. The data acquired by the sensors can be streamed for a real-time insight view and/or recorded into a on-board storage device. The streaming functionality provides the ability to record the sensors data in a storage device installed faraway from the sources of radiation. However, the communication may have a narrow bandwidth or can even fail, loosing temporarily information. The missions can be performed in: • Online inspection – the data is acquired and streamed in real time to a central control room. A remote computer provides the ability to visualize and process the data, (the UAV has few computational power and time of flight). • Offline inspection – the data is acquired and recorded for a later visualization (to navigate along the frames in a time-line) and processing. During the flight, the UAV can operate in the following modes: • Full autonomous, following predefined trajectories with auto return mode. The current mission can be interrupted by the
operator or by an anomaly detected during the flight, such as communication failure or insufficient battery. In case of communication failure, the UAV has to be able to return to a safe position, where the communication can be reestablished. If interrupted by human, the mission can be resumed or switched to one of the next modes. • Semi-autonomous, similar to the full autonomous mode, but with the ability to change the trajectory, orientation or the following speed during the flight, and to attempt the loiter mode (maintain the current location, heading and altitude) to focus specific areas. • Manual, where the pilot is able to fly using the on-board cameras acting as his eyes, i.e., a first person view, with full control of the drone. There are two main types of inspection missions: (i) inside the vessel or inside the reactor during its shutdown and (ii) in contaminated areas where the location and the amount of radiation may be unknown. 2.1. Inspection inside fusion reactors Inspection inside fusion reactors are the most ambitious mission to achieve important information to manage the scheduled and unscheduled operations of maintenance. The aim is an insight view with a visual inspection enriched with extra information, such as radiation levels, temperature and 3D model to perform metric evaluation and comparison with the original CAD model of the reactor. The mission would be extended for blankets and divertors inspection and also for the ports. However, this type of inspection mission is a challenge, given the hazard scenario with high levels of radiation and temperature, leading to a very short time life of any UAV. The inspection mission inside a fusion reactor can only be performed during the shutdown and without vacuum, to support the propeller system. The most sensible parts to radiation exposure are made by semiconductors [5], which are able to support up to 100 Gy. Based on the study presented in [6,7] with a provisional (DEMOnstration Power Station) model with helium-cooled lithium-lead type blankets, the dose rates in vessel are 2300 Gy/h, 1500 Gy/h and 800 Gy/h for 1 week, 1 month and 1 year, respectively. Inside a port, the values become 95 Gy/h, 80 Gy/h and 15 Gy/h with respect to the same periods of time. Assuming the maximum dose rate of 2300 Gy/h and the maximum dose supported by semiconductors, 100 Gy, the UAV would be able to support approximately 2.6 min inside the reactor, which is useless. The semiconductors are inside the controller unit and inside the sensors. These devices shall be shielded using, for instance, lead sheets, in such a way that its thickness assures a protection barrier. Assuming a flight time of 20 min during the maximum dose rate after one week, the shielding protection shall provide a attenuation of 2.6 min ÷ 20 min, which is 13%. Assuming the intervention of the UAV inside the reactor only one month later, the protection shall provide a attenuation of 20%. By [8], 3 cm of lead provides 12.5% of attenuation, which becomes very heavy, while 1 cm of lead provides 50%. The controller unit, as well as the internal sensors (e.g. gyroscopes, as described in Section 3) can be completely enclosed by the lead. However, the external sensors that capture the scenario information shall be shielded, but at the same time, supported by mirrors to achieve information and avoid direct exposure to the radiation [9]. In case of failure of the external sensors, the information is not acquired, but without impact to navigation and hence safety. The lead required for shielding and the amount of equipment must be compatible with the payload of the UAV, which is defined by the architecture of the UAV, as described in Section 3. The residual magnetic field of the environment (1mT) have no impact to the navigation, since the gyroscope and Inertial measure-
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ment unit (IMU) can be calibrated or replaced by optical sensors. However, the temperature is still an issue even when the reactor, such as DEMO [13], has been shutdown for some weeks, where active cooling will be necessary to inhibit the neutron-induced decay heat from raising the temperature of components. Actually, the largest grade of electrical or mechanical devices is the military one, which is only able to operate up to 125 ◦ C. Thus, the temperature remains an open issue to be addressed in a future work, taking into account the following subjects: (i) the shutdown decay heat (for instance, for each blanket it reduces two orders of magnitude after approximately one month [14]), (ii) the drones are commonly light, with a reasonable surface area to volume ratio, and (iii) the air flow generated by the propellers must be assessed if, adjusting the body design, it can contribute for the cooling of the drone during the short period of flight.
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Fig. 1. Quadcopter Dji Phantom model (left) and the test scenario (right).
2.2. Inspection in contaminated areas Activated components can be found in expected scenarios, such in reactor outages, laboratory test facilities, storage areas with contaminated loads for fusion and fission reactors and with particular equipment used, for instance, in health physics. In addition, there are unexpected scenarios resulted from possible accidents or where dangerous material is hidden for terrorism attack with nuclear weapons. Thus, a remote inspection is important for a fast intervention by security forces. The values of radiation in contaminated areas are possibly unknown, but low when compared to the valued inside the nuclear reactor. Based on [7], the dose rate inside the ports is below to 100 Gy, allowing time flight of 1 h, which is still an ambitious time flight using the UAVs solution available in the market at the date of this paper. The application of UAVs in inspection missions of contaminated areas is the most reliable in a short term. At the end of the mission, it should be able to have maps with the scanned area, reporting hotspots in terms of radiation for a deep inspection. 3. Proposed architecture The architecture is defined by the design and configuration of the frame, the motor engines, the control unit, the on-board sensors, the communication system and the power supply. The UAVs shall be modular to achieve the maximum flexibility for different type of missions and also for the replace ability following radiation exposure. A multi-criteria optimization approach [10] is proposed to find the most appropriate modules to better satisfy the requirements of the mission. This approach aims at finding solutions that trade-off multiple criteria, in the sense that all candidate solutions that are outperformed in all criteria, called dominated solutions, are ignored. Thus, multi-criteria optimization algorithms aim at obtaining the set of all non-dominated solutions. The criteria considered here for our problem are: (i) maximum flight time, including take-off, navigation, and landing, (ii) maximum payload; and (iii) total cost. The best solutions are based on quadcopters, with low price, good time of flight and the reduced payload is enough to endow the UAVs with the most important sensors, such as: internal sensors and external sensors. The internal sensors are gyroscope and IMU, for the flight performance. The external sensors used for localization, mapping and inspection are: (i) video camera (RGB information, wide angle view, preferable with a panoramic view), (ii) laser range finder (also known as LiDAR for a 2D scanning of the environment) or a depth camera (kinect like sensor), (iii) thermal camera (additional information to the video cameras, i.e., RGB + temperature), and (iv) Geiger counter (to estimate the levels of radiation).
Fig. 2. From left to right: 3D reconstruction of the scenario depicted in the right image of Fig. 1, the original positions and intensities of sources of radiation, a test trajectory and the respective estimation.
The authors assembled a proof-of-concept drone for the inspection of indoor scenarios with local sources of radiation and with very low dose rates, when compared to the reactors. The selected UAV is a quadcopter Dji Phantom model depicted on the left image of Fig. 1, equipped with its inertial sensors, a full HD video camera, a depth sensor (Structure 3D sensor, which is a Kinect like sensor) and a Geiger counter (from SparkFun Electronics). The tested room is illustrated on the right image of Fig. 1. The map of the scenario is a cloud-of-points, i.e., M = {m1 , m2 , . . ., mI }, where mi = (x, y, x), returned by the depth sensor and processed with a simultaneous localization and mapping (SLAM) approach [11]. The I is the number of points in the map, mi is the ith point of the map and (x, y, z) are the 3D space coordinates. represents the radiological activity of the map, namely, = { 1 , 2 , . . ., I }, where i = (mi ) is the radiological activity of each point. The P will refer to the path followed by the UAV, i.e., P = {p1 , p2 , . . ., pJ }, where pj = (x, y, z, , , ). The J is the number of points of the path, pj is the jth point in the path, (x, y, z) represent the 3D position of the UAV and (, , ) are the Euler angles, which represent the orientation of the UAV along the path. For simplicity, only the heading angle is considered. The represents the radiological intensity measured in each point of the path, i.e., = {1 , 2 , . . ., J }, where j = f(pj , ). The function f represents the model of the sensor. Assuming each point of the map irradiates in a radial distribution and the radiation measured in each point of the trajectory is given by the contribution of all the points of the map, then = . To estimate the radiation map, given measurement data corrupted by white noise, a possi−1
ble solution is given by = −1 , where −1 = (T ) T , the Moore–Penrose pseudoinverse. A first result of SLAM with real data is illustrated on the 3D reconstruction on the first image of Fig. 2. A simulation with the model of the Geiger counter is illustrated on the right image of the same figure. The common multicopters, as the Phantom model of DJI, illustrated in the left of Fig. 1, are prone to crash and feature whirling propellers that can hurt people and easily damage themselves and equipment in its vicinity. However, an alternative to the common drone is the “duct drone” [12], that relies on four ducted fans, rather than open propellers, as well as a sturdy material housing to store everything in. While most of the common UAVs are reliable for inspection in large and open scenarios, the duct drone is more suitable for reactor inspection. The housing box of the duct drone are prepared to support clashes against the blankets and, in the worst
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case of failure and fall, it simplifies the mission of removing a single broken piece. 4. Conclusions and future work This paper presented a preliminary study of UAV applications for remote operations of radiological inspection. The most appropriate UAVs are the rotary-wing aircraft, named multicopters. The multicopters technology, which can operate autonomously or remotely controlled under human supervision, is well established in the market. The multipcopters provide the ability to navigate in space with six degrees of freedom, with on-board sensors used for navigation and also for inspection, proving the capability of scanning the scenario. The information acquired by the multicopters during the flight in nuclear facilities provides the ability to previously inspect and then setup and optimize the maintenance operations, yielding safety, time and cost benefits. The main conclusion is that the application of UAV for inspection inside fusion reactors is still ambitious for the current off-the-shelf technology, given the high dose rates and temperature. There are additional issues that shall be studied, such as: dust spread (specially in the vicinity of spare sources of radiation, like in the divertor level of a reactor), impact of the reactor architecture with the aerodynamics of a multicopter, risk of failure and risk of clash (the drone may crash with the wall and may fall or get stuck in the structure), risk mitigation and rescue operations (possibly with a multi-purpose deployer). However, the current technology provides the means for the inspection with UAV in contaminated areas resulted from storage of activated material or from accidents and, specially, to envisage the detection of dangerous material intended for terrorism attach with nuclear weapons. The outcomes of the inspection with UAV in contaminated areas may be valuable to proceed with the inspection inside fusion reactors in a near future. The next experiments are being prepared by the authors with a drone flying inside a room with sources of Potassium-40, where its location and dose rates are previously known.
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Please cite this article in press as: A. Vale, et al., Application of unmanned aerial vehicles for radiological inspection, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.06.002