This study developed a debris removal robot for the Fukushima Daiichi Nuclear Power Plant. The creative robot contest for decommissioning the nuclear reactors of the Fukushima Daiichi Nuclear Power Station is an annual competition based on problem-based learning. It aims to stimulate students’ interest in decommissioning nuclear reactors while simultaneously imparting problem-solving skills. The development of our debris removal robot started with this contest. The proposed robot can travel in a pipe, has a 3 m telescopic arm, and is equipped with a debris removal device known as a spiral catcher. This report provides an overview of this robot. We expect that complex robot development will be achieved by putting forth several ideas from various fields and fostering advanced problem-solving skills by encouraging students to address difficult tasks. Therefore, we successfully fostered the social implementation skills of students and developed a debris removal robot.

This is a report on the No.2 debris retrieval robot that has been developed. While unit 1 was a machine built to demonstrate basic ideas, such as the arm mechanism, the robot that was designed and manufactured this time was developed with an emphasis on its structure and the functions that can be used at the decommissioning site. Only the ideas of a crawler that can run through a thin pipe and a telescopic arm mechanism to access debris were carried over from unit 1; all other parts for debris retrieval robot No.2 were redesigned. The crawler was made from metal with significantly increased rigidity to improve the running performance of the robot. In addition, to improve maneuverability, an omnidirectional camera unit was installed. As the view of this camera is not limited to the direction of travel of the robot, it makes it possible for the operator to have a better grasp of situations that the robot may encounter. The debris collection mechanism was changed to a more reliable mechanism that is not affected by the shape of the debris. The arm expansion and contraction mechanisms were also changed to a more reliable structure.

A change in the wettability of liquid sodium due to sodium implantation was evaluated theoretically. Interface models between sodium and a sodium-implanted substrate metal were created, and their electronic states were calculated. The discrete variational Xα cluster method was used in the calculations. The calculations show that the bond order, indicating the strength of the covalent bonds between atoms, decreases within the substrate metal and increases at the interface between sodium and the substrate metal due to sodium implantation. Their ratio, the bond order ratio, increased with the increasing sodium implantation. Previous studies have shown that wettability is better as the bond order ratio increases. Therefore, from the calculations results, wettability improves with increasing sodium implantation. The wettability of sodium-implanted metals was experimentally evaluated using the contact angle of the sodium droplets. The contact angle decreased, and the wettability improved due to sodium implantation. The change rate in the contact angle increased with the bond order ratio. The experimental and calculation results correlate well, without contradiction. In conclusion, sodium implantation changes the electronic state of the interface, affecting the wettability of liquid sodium.