Akio Matsumura, Japanese Diplomat, Feb 11, 2017 (emphasis added): The Potential Catastrophe of Reactor 2 at Fukushima Daiichi: What Effect for the Pacific and the US?… It is clear to us now that the radiation level in the containment vessel of the crippled Reactor 2 is much higher than experts had believed… The danger of Reactor 2 begs us to ask many new questions…

Dr. Shuzo Takemoto, professor of the Department of Geophysics, Graduate School of Science, Kyoto University — responded to Mr. Matsumura concerns: Potential Global Catastrophe of the Reactor No.2 at Fukushima Daiichi, by Professor Shuzo Takemoto — On July 28, 2016, the [Tepco] published the images of the F1 Unit 2 reactor screened by muon particles… They showed the shadow of materials equivalent to 180 – 210 tons at the lower part of the pressure vessel… It can hardly be said that the Fukushima accident is heading toward a solution. The problem of Unit 2, where a large volume of nuclear fuels remain, is particularly crucial. Reactor Unit 2 started its commercial operation in July 1974. It held out severe circumstances of high temperature and high pressure emanating [after 3/11]…  years long use of the pressure vessel must have brought about its weakening due to irradiation. If it should encounter a big earth tremor, it will be destroyed and scatter the remaining nuclear fuel and its debris, making the Tokyo metropolitan area uninhabitable. The Tokyo Olympics in 2020 will then be utterly out of the question… [Fukushima is] situated in the aftershock area of the 2011 earthquake off the Pacific coast of Tōhoku. In this area, we must foresee a number of magnitude 7 class earthquakes. Consequently, we cannot exclude the possibility of intensity 6 and intensity 7 earthquakes befalling the Fukushima Daiichi. What is most dreaded is what could happen to the Unit 2 whose pressure vessel contain a large volume of nuclear fuel debris. This pressure vessel has endured the sudden change of temperature and pressure in the accident of March 2011, but in light of its possible weakening due to irradiation, it could be seriously damaged if a new big earth tremor occurs nearby.

Dr. Helen Caldicott, Feb 13, 2017: What the photos taken by the robot did reveal was that some of the structural supports of unit 2 have been damaged… all four buildings were structurally damaged by the original earthquake… and by the subsequent hydrogen explosions, so, should there be an earthquake greater than seven on the Richter scale, it is very possible that one or more of these structures could collapse leading to a massive release of radiation as the building fell on the molten core beneath… The reactor complex was built adjacent to a mountain range and millions of gallons of water emanate from the mountains daily beneath the reactor complex, causing some of the earth below the reactor buildings to partially liquefy…

See also (from Feb 3, 2017): Collapse imminent at Fukushima — TV: Officials find large section under reactor is “unstable… about to collapse”