Japan's Nuclear Regulatory Agency (NRA) is losing patience with robots sent into Fukushima's reactor buildings as high radiation levels prevent their operation:
Kohei Tomida and Masanobu Higashiyama.) Nuke watchdog critical as robot failures mount at Fukushima plant THE ASAHI SHIMBUN March 24, 2017 at 17:50 http://www.asahi.com/ajw/articles/AJ201703240064.html
Units 1 and 3 are particularly hot. Efforts to investigate Unit 1 have been unsuccessful and unit 3 is altogether too hot for any robotic investigation.
Consequently, reactor conditions remain "mysterious" and TEPCO continues to express uncertainty as to the location of melted reactor fuel. The condition of spent fuel in the fuel pools from units 1 and 3 is also a mystery.
Early press releases from TEPCO in 2011 indicated that Daiichi unit 1 had only ‘mildly radioactive releases.’[i] Later, it was reported it suffered a 70 percent fuel meltdown and was experiencing localized nuclear criticalities.[ii]
The problem is that there are over a thousand tons of fuel at Daiichi that must be kept cooled until TEPCO can remove it, which is emerging as an impossible task.
According to a November 16 report by TEPCO titled, ‘Integrity Inspection of Dry Storage Casks and Spent Fuel at Fukushima Daiichi Nuclear Power Station,’ as of March 2010 the Daini site held 1,060 tons of spent uranium fuel. The total spent uranium fuel inventory at Daiichi in March 2010 was reported as 1,760 tons:
Integrity Inspection of Dry Storage Casks and Spent Fuels at Fukushima Daiichi Nuclear Power Station (16 November 2010), http://www.nirs.org/reactorwatch/accidents/6-1_powerpoint.pdf.
It is worth noting that although this report was produced on 10/2 6/2010, the file properties indicate the document was modified on 3/13/2011
The 2010 report asserts that approximately 700 spent fuel assemblies are generated every year.[iv] The report specifies that Daiichi’s 3,450 assemblies are stored in each of the six reactor’s spent fuel pools. The common spent fuel pool contains 6291 assemblies. The amount of MOX fuel stored at the plant has not been reported.
All that fuel is degrading and its particulates are being swept by ground water and TEPCO's injections into the aquifer, sea, and atmosphere.
[i] Y. Hayashi and R. Smith (12-13 March 2011) ‘Radiation Leaks at Damaged Plant’, The Wall Street Journal, A6.
[ii] The report of a partial meltdown of unit 1 can be found here: K. Hall and C. Williams (15 March 2011) ‘Fire Erupts Again at Fukushima Daiichi's No. 4 Reactor; Nuclear Fuel Rods Damaged at Other Reactors’, The Los Angeles Times (2011, March 15), http://articles.latimes.com/2011/mar/15/world/la-fgw-japan-quake-reactor-fire-20110316, date accessed 15 March 2011. The report of criticalities in unit 1 in April here: J. Makinen and T. Maugh (1 April 2011) ‘Radioactivity Surges Again at Japan Nuclear Plant’, Los Angeles Times, http://articles.latimes.com/2011/apr/01/science/la-sci-japan-reactor-damage-20110331, date accessed 5 April 2011.