|Fig. 1: Fixed-wing unmanned aerial vehicle. (Source: Wikimedia Commons)|
UAVs, unmanned aerial vehicles, have become increasingly popular in many applications recently. The technology can provide many benefits to researchers, particularly in mapping radiation fields. A primary advantage of using UAVs is that they are low-cost. Very importantly, UAVs are much safer to use in dangerous radiation leaks or other potentially harmful conditions. These unmanned vehicles also feature semi- or full- autonomous capabilities, further removing humans from the radioactive scenes.  Additional advantages include flexibility in terms of electronic modifications, small size, and high endurance. 
UAVs both in general and specifically for nuclear applications can come in several forms. Two common models are fixed-wing (Fig. 1) and rotary-wing. The fixed-wing design is most popular and best suited for nuclear detection and mapping, as it can sustain longer flight times and works best in surveillance applications.  Several fixed-wing models have been tested, and the results indicate that this choice of model allows for high top speeds and ground coverage - though a side effect is a loss of spacial resolution. When optimizing for spacial resolution, the highest potential resolution is 250 meters per measurement. 
Within the category of fixed-wing UAVs, different sizes can be more useful for different situations. For example, large-scale radioactive releases typically warrant using large fixed-wing vehicles, while to create lower altitude high-spatial resolution maps small fixed-wing UAVs are preferable.  Each UAV has a system of crucial hardware and software enabling its flying and detection capabilities. The hardware structure is broken down into systems for navigation, communication, control, and actuators. The two main software components are autopilot software and ground control station software. 
Several methods of radiation monitoring exist that take advantage of on-ground systems. However, one objective of using UAVs in relation to nuclear energy is to create contour maps of nuclear radiation fields from above. It has been found that the airborne method using UAVs is the most time-effective way to collect data on radiation in large areas.  In order to do this, the aerial vehicles need to recognize and track levels of nuclear radiation. The three components to this mapping process include (1) locating a specific level set of radiation contours, (2) following that contour to create a representative map, and (3) translating the map into source position estimates.  The vehicles commonly use γ-ray detectors to interact with and measure the radiation.  Several different types of γ-ray detectors and recording techniques are used across different vehicles and situations. One technique for calculating the relationship between the height of the UAV and the recorded radioactive intensity is described by the Poisson distribution
where λ represents the average number of counts of nuclear decay events in a particular period of time, f(k,λ) is the probability that k counts will be actually received.  On average, the accuracy of determining the position of the source of radiation using Poisson and stochastic calculations is 1 foot within a 150 square-foot area.  Several algorithms are combined together to translate measured data into valuable maps. Depending on the goal of the map being created, different factors may be emphasized more or added into the calculations. Ultimately to create the map after the data is processed, a color-scaled intensity overlay is mapped to the two-dimensional spatial representation of the area, creating the contour map visualization commonly depicted.
There are constraints associated with using UAVs for radiation field mapping that may be approved upon in future designs. These limitations include the size of the system, the maximum payload given the size, and the necessity for quick use due to endurance limits.  Another important current constraint is the prevalence of strict aircraft regulations and restrictions.  These limited flying parameters make testing UAVs more difficult.
However, as these systems continue to be improved and learned, the potential for much safer radioactivity recording are immense. Responding to radioactive crises can and will be much quicker and safer when carried out by autonomous aerial vehicles. The mapping and recorded data can help create evacuation plans quicker and assist in harm-relief actions.  Looking forward, systems will likely be optimized to best fit particular incidents and cases. After fine tuning of algorithms and pieces, these devices will surely provide a much more efficient and safe alternative to the man-centered radioactive monitoring devices.
© Hailey Wilson. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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