Jiji Press, Mar 5, 2015 (emphasis added): Where is the melted fuel in the stricken reactors at Fukushima No. 1? This remains a question… cosmic rays [are being used] to “see through” the reactors… [Prof. Fumihiko Takasaki of the High Energy Accelerator Research Organization calls the] nuclear disaster a “national crisis”… muon detectors were… placed last July at reactor No. 1… As detector units can’t be placed underground at reactor 1, any melted fuel in the underground part of the reactor will go undetected.
Science (AAAS), Mar 5, 2015: Fukushima Daiichi… won’t be truly safe until engineers can remove nuclear fuel… But first, they have to find that fuel… [TEPCO] thinks… fuel in the Unit 1… dropped to the bottom of the containment… engineers need much more detail about its location and condition… By the end of this month, Takasaki says, the detectors may have absorbed enough muons to confirm there is no fuel left in the reactor core… [Detectors] won’t be able to map fuel that may have flowed to the bottom of the containment vessel.
Wall St Journal: Fuel rods… melted fully out of their pressure vessel [says Tepco]… but it likely stopped as Tepco began [injecting] seawater [See: Tepco: We should have told public this sooner… water injections failed to cool melted fuel]… Edwin Lyman, a senior scientist with the Union of Concerned Scientists, says it remains unclear why the fuel rods didn’t also breach the containment wall. “Why this didn’t happen is still unknown”…
U.S. NRC — State-of-the-Art Reactor Consequence Analyses (SOARCA) Report (pdf), Oct 2012: Fukushima Daiichi… presented… many insights with potential parallels to SOARCA’s analysis of… Peach Bottom, a similarly designed plant… in the SOARCA scenarios, significant hydrogen release [begins with] failure of the containment pressure boundary, which… results from molten debris failing the drywell liner… (i.e., drywell liner melt-through)… TEPCO has announced… the fuel did not move laterally across the drywell floor [and melt through the liner]… In the analyses presented in this report, hydrogen [was] released via the failure of the drywell liner by melt-attack.
Argonne National Lab (USA), MCM (Switzerland), Sep 2014: location of 1F corium — Critical questions relate to the extent of core melt and the extent to which it has melted through RPV and penetrated into the primary containment… “hot particles”… may be throughout the reactor building [and] water collection system and even released to surroundings… Identification of hot particles… is going to be critical for safe decommissioning… [the equipment] must withstand extremely high radiation – perhaps up to greater than 200 Sv/hr.
More from MCM: Most molten core appears to be contained within the primary containment, although a very small extent of melt-through to… cannot be precluded.
Symposium sponsored by Consortium for Japan Relief — Chim Pom, published Feb 2015: “Media never reported that the whole process — meltdown, melt-through, and so-called melt-out — happened, was done within a few days after the earthquake.”