China Syndrome, a bad thing

Eben Harrell has a post at Time.com about Fukushima:

The China Syndrome refers to a scenario (and a movie) in which a molten nuclear reactor core could fission its way through its containment vessel, melt through the basement of the power plant, and then down into the earth. While a molten reactor core would not burn "all the way through to China", it could enter the soil and water table and cause huge contamination in the crops and drinking water around the power plant. It's a nightmare scenario,the stuff of movies. And it just might have happened at Fukushima.
Last week, plant operator Tepco sent engineers in to recalibrate water level gauges in Reactor Number One. They made an alarming discovery: virtually all the fuel in the core had melted down. That means that the zirconium alloy tubes that hold the uranium fuel and the fuel itself lies in a clump, either at the bottom of the pressure vessel, or in the basement below, or possibly even outside the containment building. Engineers don't know for sure, though current temperature readings suggest that fission inside the reactor core has definitely ceased for good (i.e., there will be no further melting).
Anecdotal evidence doesn't bode well for how far the fuel melted: Tepco has been pumping thousands of tons of water onto Reactor One to try to cool it, yet the water level in the containment vessel is too low to run an emergency cooling system. That means the water is escaping somewhere on a course cut by molten fuel; probably into the basement of the reactor building, though it's also possible it melted through everything into the earth.
Many experts say a full-blown China syndrome is unlikely, in large part because the fuel from the type of reactors at Fukushima is designed in such a way that it probably won't sustain "recriticality" once meltdown occurs. What's more, boron, which slows nuclear reactions, was pumped into the cooling water of the reactor after the initial accident to prevent the core from going "critical" again.
But, assuming a worst-case scenario hasn't occurred, having so much highly radioactive water sloshing around the basement is going to make cleanup even more difficult. Tepco says it will come up with a new plan to stabilize the reactor, and their main task will be to find a way to suck up the water and store it while simultaneously ensuring the reactor core remains cool. It's unclear how this will be achieved, but according to press reports, a giant water-storage barge, a Megafloat,  has been dispatched to Fukushima as a possible storage site for contaminated water, and will arrive at the end of the month.
Tepco also said that it has started preparatory work for the construction of a cover for Unit One's reactor building, which had its roof blown off by a hydrogen explosion on 12 March. The cover is to be built as a temporary measure to prevent the release of radioactive substances until further measures can be put in place, Nature News reported.
Meanwhile, around 5,000 residents in two towns, Kawamata and Iitate, some thirty kilometers from the power plant and well beyond the the twenty kilometer exclusion zone, were evacuated. More evacuations are expected in the coming days, as Tepco continues to struggle with the crisis. Around 3,400 cows, 31,500 pigs, and 630,000 chickens will soon be slaughtered inside the Fukushima exclusion zone, as feeding them has proven to be impossible.
It's difficult to say for sure just how bad things are at the plant itself; high radioactive levels mean that engineers can't get close to the reactor cores themselves and can only make inferences, deductions, and guesses about the extent of the damage. As Alexis Madrigal of The Atlantic has pointed out, we've faced this uncertainty— and troubling surprises— before: eight months after the Three Mile Island accident, "an Oak Ridge National Laboratory scientist declared: 'Little, if any, fuel melting occurred, even though the reactor core was uncovered. The safety systems functioned reliably.' A few years later, robotic sorties into the area revealed that half the core, not 'little, if any', had melted down."
I and Time's Kiev-based stringer recently published a piece for Time from Chernobyl in Ukraine, where clean-up efforts continue a full 25 years after the accident. Whatever the end game at Fukushima, get your head around this, folks: it is going to be a huge mess for a long time yet.