World Nuclear Association, Feb. 7, 2014: [...] Different computer models have been used by Tepco and the Japan Atomic Energy Agency to analyse the positions of the cores, coming to the same conclusion for unit 1. Both models predict that all 77 tonnes of the fuel from unit 1 melted and passed from the reactor vessel to the drywell area immediately below. [...] Tepco also noted that the steel structures below the reactor vessel are very complex. Each vessel also has up to 200 penetrations [...] An extra difficulty is the need to seal the containment, which was compromised during the accident. Tepco has not yet been able to find the points from which water injected to cool the cores escapes [...]
IRSN (France) — Initial analyses of the accident (pdf): DAMAGE SUFFERED BY THE CORES OF REACTORS 1, 2 AND 3 — TEPCO published the results of simulations of the accident [...] According to the analysis, the entire mass of corium dropped to the bottom of the reactor containment vessel and came into contact with the concrete of the foundation basemat, initiating a corium-concrete interaction. Although the corium ate away a 70 cm-deep hole into the basemat, TEPCO states that it would have had to melt through more than 1 m more of concrete before reaching the metal wall of the containment vessel. For TEPCO, the injection of water is enough to cool the corium. IRSN considers that these results should be interpreted with precaution as a number of assumptions, including the mass of corium that dropped onto the basemat and how it ate through the basemat (radial or axial erosion), can have a very significant influence on them. In particular, the raft could be eroded more extensively than announced by TEPCO.
More links here: Did China Syndrome (Fuel Outside Containment) Occur?
Published: February 10th, 2014 at 10:12 am ET