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On the other hand, if it is trapped steam, that might allow the formation of Hydrogen pocket there, and that could be setting itself up for another explosion ?

Beats me. I'm not seeing significant changes in drywell or torus pressure, even with the pretty decent rise in DW temperatures. The TEPCO fairies might be magically transporting the extra steam away. (aren't we seeing pretty continuous steam emission from #3? -- verhaps DW is indeed venting right out)

steam from top right side of #2 barely visible

Only way we would get steam in the D/W is because there was definitely a breach between the RPV, S/V and D/W, plus if the CAMS readings are that high, it would mean that reactor rod components are in the D/W ??

@Edano, good theory, but if that was completely the case, I would think that none of the sensors would be working inside the RPV,S/C or D/W, as they would have been torn out from the explosion. Possibly, it exploded between the top of RPV and the top concrete containment cap ???

@wtm : i really wonder if we can trust any reading after such a combustion. hard to believe that any cable could be still intact.

@Edano, I will agree on that, but seems that both the seal temp and the CAMS are going up in that D/W area ??? Where is superman and his x-ray eyes when you need him ????

In #2 we have the rising S/C CAMs. In 3 the CAMS in both DW and S/C are showing consistent, though slow, decrease, although temp is rising pretty fast.

@MarkFM, makes you wonder about the melt down tests they conducted, since after a month and half, we seem to still have active reactions going on in the lower half of the units !!!!

FWIW too...
2. COOLABILITY PHENOMENOLOGY
Several heat transfer mechanisms have been identified
through experiments that can contribute to long-term
corium coolability. These mechanisms are summarized
in Table 1, while a physical illustration is provided in
Fig. 1. When water is introduced atop an MCCI, the
question of whether or not a significant amount of the
thermal energy is initially removed will depend upon
whether a stable crust is able to form that inhibits heat
transfer from the melt to the water layer. For a stable
crust to form, two necessary conditions must be met [3]:
(i) a thermal condition, viz., the melt/water interfacial
temperature must fall below the corium freezing
temperature, and (ii) a mechanical condition, viz., the
incipient crust must be stable with respect to local
mechanical loads imposed by the agitated melt. If either
of these two conditions is violated, then stable crust
formation is precluded. In this bulk cooling regime,
efficient melt/water heat transfer occurs due to conduction
and, predominately, radiation heat transfer across the
agitated (i.e., area enhanced) melt/water interface, in
addition to entrainment of melt droplets into the water
overlayer.
article.nuclear.or.kr

@Rob, thanks on that post ! What this is all confirming is that we continue to have active reactions and corium melt. The worst may not be over, if the corium is still actively melting through the D/W, and will within a period of time hit the bedrock ???

@Rob, I would agree that the corium should have started to crust over and slowly stop. But possibly could all the smaller quakes in that region be stirring the corium back up, so it does not sit long enough to form that crust ???

@Rob, if it hits base pan, then it is into the ground water supply !!!! That would allow leakage of major radioactivity into the entire water supply in the area.

Question for somebody that knows chemistry real well ? What happens when Hot corium hits Cold groundwater ???? Instant steam explosion or what ??

@wtm Makes you wonder why they're talking about building an underground containment wall, eh?
The basemats should be 25-30 feet thick at Daiichi.
(JNES).pdf

@Rob, I think it would really be risky to try to dig underneath the reactors to put in a concrete wall, especially with the on going quakes in the region. One good quake and the reactor would settle into the excavation they were digging, and that would really mess things up !!!

@wtm Not under, but a horseshoe around them all preventing flow to the sea.

does anyone know what the negative pressure (40% & 12% atmosphere) in rpv #3 indicates ?

@Edano, there was an article by one that stated that they felt it was a reaction of steam being present, as it rose from the C/V up through the breach into the RPV that it created a negative pressure in the RPV.

40 kPa = 0.4 atm, about 6 psi. To me that's not large.

@Markfm The entire air-cooling process you describe I believe is the effort needed to prepare the area such that they can THEN go in with jumpers to seal around the reactor to restore liquid cooling on the rods down the road in some 3 months or so, no?

@Rob, if there is any rods to restore cooling too by then ??