A government-backed organization in charge of supporting the decommissioning of nuclear plants is considering to propose starting the removal of melted nuclear fuel debris beginning with the No. 2 reactor at Tokyo Electric Power Company Holdings Inc.'s Fukushima No. 1 nuclear plant, officials said Thursday. Nuclear Damage Compensation and Decommissioning Facilitation Corp., or NDF, believes that the No. 2 unit is the most suitable for melted fuel removal work among the three heavily damaged reactors based on the results of its investigation into radiation levels at the reactors and the conditions inside them. In January 2018, Tepco confirmed deposits of melted nuclear fuel debris inside the No. 2 reactor containment vessel of the plant in Fukushima that was damaged in the March 2011 earthquake and tsunami. In February this year, the company made physical contact with the deposits using equipment, making much more progress in investigating the No. 2 reactor than the No. 1 and No. 3 reactors. Tepco plans to further investigate the inside of the No. 2 reactor by the end of March next year, aiming to collect sample debris.
"We put in cameras and robots and obtained valuable images, though they were partial . . . "We first need to know the situation of the debris." Last month, the utility inserted a 10.5-meter rod with a camera on its tip into a hole in the No. 2 reactor's primary containment vessel and discovered black lumps sticking to the grating directly underneath the suspended pressure vessel, which holds the core. Tepco claims it is still unsure whether the lumps are really melted fuel that burned through the bottom of the pressure vessel. Although it is still years away from actually trying to remove the fuel, Tepco, the government and related parties are planning to decide on a basic strategy this summer and go into more detail next year.
The International Research Institute for Nuclear Decommissioning (IRID) carried out the experiments using a 20-meter wide, 12-meter high model of the No. 2 reactor's suppression chamber and torus room -- areas located below the reactor's containment vessel. IRID was established in 2013 by nuclear plant makers, power firms and government organizations to develop technology needed for the decommissioning of the Fukushima plant, which was damaged by the March 3, 2011 earthquake and tsunami. After the disaster, three of the plant's reactors suffered meltdowns in the world's most severe nuclear crisis since the 1986 Chernobyl disaster. The model is located at the Naraha Remote Technology Development Center, near the crippled Fukushima No. 1 plant. "We would like to continue testing until next summer, approximately, and use (the outcomes) in deciding methods to retrieve fuel debris," Atsufumi Yoshizawa, IRID executive director, said.
A plan to remove fuel debris from the primary containment vessel of a reactor at the Fukushima No. 1 nuclear power plant is expected to be further pushed back after it became apparent that Tokyo Electric Power Company Holdings Ltd. will not be able to conduct an internal probe -- a key step to start removing the fuel debris -- by the end of March as planned. The internal probe would involve using remote-controlled robots to collect fuel debris inside the No. 1 reactor so Tepco can examine its composition and form. Tepco's plan is to open three holes in both the outer and inner doors of the primary containment vessel using pressurized water mixed with a polishing agent. After it succeeded in opening three holes in the outer door, Tepco started drilling a hole in the inner door in June 2019. But that procedure caused the concentration of radioactive dust to increase temporarily, prompting staff to suspend work.