Emergency response to the nuclear accident at the Fukushima Daiichi Nuclear Power Plants using mobile rescue robots

Abstract

On March 11, 2011, a massive earthquake (magnitude 9.0) and accompanying tsunami hit the Tohoku region of eastern Japan. Since then, the Fukushima Daiichi Nuclear Power Plants have been facing a crisis due to the loss of all power that resulted from the meltdown accidents. Three buildings housing nuclear reactors were seriously damaged from hydrogen explosions, and, in one building, the nuclear reactions became out of control. It was too dangerous for humans to enter the buildings to inspect the damage because radioactive materials were also being released. In response to this crisis, it was decided that mobile rescue robots would be used to carry out surveillance missions. The mobile rescue robots needed could not be delivered to the Tokyo Electric Power Company (TEPCO) until various technical issues were resolved. Those issues involved hardware reliability, communication functions, and the ability of the robots' electronic components to withstand radiation. Additional sensors and functionality that would enable the robots to respond effectively to the crisis were also needed. Available robots were therefore retrofitted for the disaster reponse missions. First, the radiation tolerance of the electronic componenets was checked by means of gamma ray irradiation tests, which were conducted using the facilities of the Japan Atomic Energy Agency (JAEA). The commercial electronic devices used in the original robot systems operated long enough (more than 100 h at a 10% safety margin) in the assumed environment (100 mGy/h). Next, the usability of wireless communication in the target environment was assessed. Such tests were not possible in the target environment itself, so they were performed at the Hamaoka Daiichi Nuclear Power Plants, which are similar to the target environment. As previously predicted, the test results indicated that robust wireless communication would not be possible in the reactor buildings. It was therefore determined that a wired communication device would need to be installed. After TEPCO's official urgent mission proposal was received, the team mounted additional devices to facilitate the installation of a water gauge in the basement of the reactor buildings to determine flooding levels. While these preparations were taking place, prospective robot operators from TEPCO trained in a laboratory environment. Finally, one of the robots was delivered to the Fukushima Daiichi Nuclear Power Plants on June 20, 2011, where it performed a number of important missions inside the buildings. In this paper, the requirements for the exploration mission in the Fukushima Daiichi Nuclear Power Plants are presented, the implementation is discussed, and the results of the mission are reported.

Ab initio study of radiation effects on the Li4Ti5O12 electrode used in lithium-ion batteries

“…21 Furthermore, Vanderbilt ultrasoft pseudopotentials 22 were used to describe the core electrons and nuclei. A 1 × 1 × 3 supercell (a total of 168 atoms for Li 4 Ti 5 O 12 and 192 atoms for Li 7 Ti 5 O 12 ) and a Gaussian smearing with a small broadening width of 0.003 Ry were employed in all simulations. The wave functions and the electron density were expanded in a plane wave basis with cutoff energies of 60 Ry and 600 Ry, respectively.…”

“…In our calculation, we allowed the atomic positions and the cell volume to relax until the forces at each atom converged to within 1.5 × 10 −3 eV/Å. In the literature, two different structures have been proposed for Li 4 Ti 5 O 12 depending on how the lithium atoms occupy the 16d sites: the first was proposed by Ouyang et al 16 in 2007, where the arrangement of the Li atoms partially occupying the 16d sites was optimized using cubic spinel-type cells, and the second was proposed by Tsai et al 18 in 2014. In order to determine the energetically preferable structure to use for our study, we ran simulations using both types of structures.…”

“…In one defect calculation, a 16d lithium atom was removed (Defect 1), and in another calculation, a 32e oxygen atom was removed (Defect 2). The defects were introduced to the spinel Li 4 Ti 5 O 12 and are shown in Figure 1. Following the introduction of the defects, the atomic positions and the cell volume were relaxed.…”

“…In Fukushima, Japan, in 2011, robots powered by lithium-ion batteries were engineered to perform sampling, recovery, and rescue missions in the high radiation environments, but were found to fail quickly. 4 Furthermore, high-energy protons in space may affect the performance of NASA's electronic devices as well as their battery power sources. Therefore, a careful analysis of the performance of these batteries under high radiation environments is required.…”

“…8 They used a Co-60 source with a dose rate of 100 Gy/min for 24 h, and reported that radiation affects the cobalt valence charge and increases the impedance 9 The material exhibits minimal change in cell volume upon intercalation and de-intercalation (less than 1%), which helps maintain its structural stability, 10 and it also has good thermal stability. 11 In addition, the charge discharge plateau of Li 4 Ti 5 O 12 is approximately 1.55 V, which is higher than the reduction decomposition potential of most organic solvents; 12 therefore, it is a safer option compared to the graphite anode. 13 In this study, we investigate the radiation effects on the lithium titanium oxide electrode and determine its potential impacts on the battery performance.…”