Fukushima Unit 3 has been in the news because Toshiba, working with the International Research Institute for Nuclear Decommissioning, "a government-funded consortium," created an amazingly innovative robot, labeled Little Sunfish, that has been sent into this reactor, which had been running MOX fuel at the time of the disaster and had, by far, the largest explosion.
Here are some interesting screenshots from unit 3:
Spent fuel pool unit 3: https://cryptome.org/2013/04/daiichi-13-0422.pdf and https://cryptome.org/2012/09/daiichi-12-0924.pdf
Little Sunfish is now investigating the reactor conditions:
Mari Yamaguchi (July 19, 2017). Swimming Robot Captures Underwater Images of Damaged Fukushima Nuclear Reactor. Time, http://time.com/4864939/little-sunfish-robot-fukushima-nuclear-plant-pictures/I recommend visiting the Time link because it provides a history of robotic efforts to investigate the ruined reactors at Daiichi. Sunfish is a major achievement because it can withstand up to 200 sieverts of radiation. Congratulations to Toshiba and their collaborators.
... The robot used Wednesday was designed to tolerate radiation of up to 200 sieverts — a level that can kill humans instantly. Kimoto said the robot showed that the Unit 3 reactor chamber was "clearly more severely damaged" than Unit 2, which was explored by the scorpion probe.
TEPCO needs to know the reactor conditions in order to move forward with "decommissioning." The best way to decommission these reactors is to have them filled with water. However, the bad news is that it appears most, if not all 3 of the reactors, cannot hold water (which is why TEPCO must add up to 300 tons of water a day to the reactors and also to the damaged spent fuel pools).
TEPCO is planning an alternative approach to decommissioning but this alternative approach is "dirtier" in the sense that its going to spread more radioactive particles into the environment. You can read what has been reported about the proposed plan here:
Fuel debris extraction plan for crippled Fukushima reactors to be revealed soon: sources. (July 5, 2017) The Japan Times http://www.japantimes.co.jp/news/2017/07/05/national/group-mulls-fukushima-no-1-melted-fuel-debris-extraction-without-filling-containment-vessels-water/#.WWZceVHdnwk
The Nuclear Damage Compensation and Decommissioning Facilitation Corp., tasked with providing technical support for decommissioning the complex, may propose a method to remove nuclear debris without completely filling the reactor containment vessels with water, the sources said Tuesday.
The plan means the debris inside reactors 1, 2 and 3 at the crippled Fukushima No. 1 complex is likely to be shaved off gradually with a drill or laser equipment as a shower of water is poured remotely, the sources said.It is indeed unfortunate that the reactors cannot contain water because the proposed process of shaving off graduation with a drill or laser equipment under a shower of water sounds very, very messy. Will the process be encased in plastic or something to contain the resulting radioactively contaminated spray?
The Chairman of Japan's nuclear regulator recently decried TEPCO's lack of urgency and described a sense of danger.
I echo that sense of urgency. Yesterday's earthquake left Daiichi looking worse than it has been and recently its not been looking great.
Unit 3 appeared to be glowing this morning: