So it turns out that nadinbrzezinski was correct re Fukushima

[FBaggins]

478. Some things to consider

What you've asked for necessarily involves taking the words of experts, who pretty much universally agree with the posters that you view as biased toward nuclear power - and thus may dismiss them.

So let's begin with things unrelated to Fukushima that you can confirm for yourself. Go ahead and do a search for "corium" and "Chernobyl" and look at the photos. Most of the photos that will rise to the top of the search were taken within the first year after the disaster.

Ask yourself "what stopped that core?"

Things to consider:
* The Fukushima reactors were shut down roughly an hour before power was lost, so the core was producing somewhere around 2% of the heat that it produced at full power.
* By comparison, Chernobyl's design flaws caused the core to power UP. As it melted down, it was actually producing much more heat than at full power. This means that the corium was producing scores of times as much heat (available for burning through concrete) as at Fukushima
* They had trouble keeping water on the core at Fukushima... but at Chernobyl the core exploded and then burned in the open air. They tried to dump sand/water from helicopters, but that was necessarily less effective.
* There was no ongoing water spray (and possibly ground water) reaching the core (obvious from the photos). Yet somehow the corium failed to burn through the foundation or remain molten.

So perhaps the better question would be "on what basis would anyone think that the cores at Fukushima would continue to burn their way through much more concrete?"

If so wouldn't the cooling water be turned into steam?

The amount of heat produced by the corium is a knowable (and continually declining) figure (certainly within a reasonable margin of error). At this point (as with Chernobyl's corium), the heat produced is likely below that which would be carried away by air circulation at temperatures well below the boiling point of water. Thus even if they weren't spraying water, it would be unlikely to be able to flash water to steam. There just isn't enough heat. With the water spray, the temperature on the surface of the corium is probably quite warm to the touch, but not "hot".

how the containment can hold the molten corium and how long it will hold it.

The easiest answer is that it's precisely what it was designed to do. Note that from early on, there were retired experts who claimed that this design couldn't contain a total loss of cooling accident because the core would melt down so quickly that it would still be producing too much heat when it reached the bottom of the primary containment... so the corium would remain too liquid and (rather than burning through the base of the containment) would spread to the outer edges of the upside-down metal "bulb" and burn through the sides and into the torus room.

Those concerns were one of the main reasons that the design was replaced with better containment options in later reactors... but the folks who continued to claim from this that the Mk1 design "couldn't" contain the corium missed the critical distinction that Fukushima did not have that absolute loss of cooling accident while running. The hour of cooling plus decay of short-half-life elements made a huge difference... and it's why the cores tool many hours to melt down.

Where can I get this information other than from the Japanese government?

Old NRC reports and design reviews. Nuclear physics and reactor textbooks. Decay heat curves, etc.