After the passage of more than one year from the accident at the Fukushima I nuclear power station, a challenge now is to develop radiation-resistant equipment for removal of fuel debris (damaged fuel) from the stricken units. A radiation dose rate of 72 Sieverts (Sv) per hour was recorded in the unit 2 pressure containment vessel, in which operations are difficult not only for humans but also for robots. In particular, it is essential to improve the durability of semiconductors, which are known to be vulnerable to radiation.
The national government and Tokyo Electric Power Company (TEPCO) plan to start removing fuel debris in 2020. Even with the shielding effect provided by filling the container vessels with water, ensuring the radiation hardness of equipment that directly gets in contact with fuel is a serious challenge.
The components most vulnerable to radiation are semiconductor devices. Some mechanisms of malfunctions, failures and degradation in irradiated semiconductors have already been clarified; the most concerned mechanism is the “total dose effect,” which is generation of defects by gamma irradiation in the surface oxide layers of devices. There are two approaches to preventing such defects: surrounding semiconductors with lead or other shielding materials, and developing new semiconductors without oxide layers.
A research team of Japan Atomic Energy Agency (JAEA) developed a semiconductor device using silicon carbide (SiC), a material receiving attention in the field of power electronics. Not having an oxide layer, it is resistant to radiation as well as high voltage and high temperature. In an irradiation test, the device remained unchanged up to a dose of 100,000 Sv and showed a voltage change of less than 1 V even at a very high dose of 10,000,000 Sv. This means the device can withstand continuous irradiation at a dose rate of 100 Sv per hour for more than 10 years. The combination of this kind of device and lead shields is considered to be effective for designing and developing equipment and systems for fuel debris removal.
Research on the radiation hardness of semiconductors is more advanced in the United States and other Western countries than in Japan. Because many of the relevant technologies are military related, it is difficult to import devices and share technologies. At present, it appears more practical to conduct decommissioning using domestic equipment, which requires further research and development.
