That last part leaves me clueless. Do they think the meltdown has stopped? Because even I know that there cannot be an “unmeltdown”

It’s really just supposed to sound mollifying – technically, any meltdown is ‘temporary’: the fission will only continue until all of the available fuel is used up. They suspect that the meltdown is only partial, i.e. that only part of the fuel in the reactor is affected (either because the rods have partially lost their cladding, or parts of the fuel rods have broke off and collected at the bottom of the containment vessel), so fission will cease once that amount of fuel is spent – but since there seems to no idea what amount were actually talking about, it’s really more or less a complete non-statement.

I may well be that the final net harm other than injury to the plant workers will be land set off limits for a period of time, plus a zillion dollars in cleanup costs.

One of the favorable assumtions that seems implied in these conversations, and one that never ceases to frustrate me, is that the radioactive materials will disperse evenly though whatever medium they are introduced to.

‘On average’ sounds nice– but there is no reason to think the average levels of any isotope in any quantity of water is a meaningful metric. That’s just not how things mix, flow and disperse in nature. Both in the air and in water, materials move in ‘plumes’, remainingly suprisingly concentrated because of differences in temperature, salinity, and so forth.

An example is the Amazon river–

‘The river pushes a vast plume of freshwater into the ocean. The plume is about 400 kilometres (250 mi) long and between 100 and 200 kilometres (62 and 120 mi) wide. The freshwater, being lighter, overrides the salty ocean, diluting the salinity and altering the color of the ocean surface over an area up to 1,000,000 square miles (2,600,000 km2) large. For centuries ships have reported freshwater near the Amazon’s mouth yet well out of sight of land in what otherwise seemed to be the open ocean.’

(from: http://en.wikipedia.org/wiki/Amazon_River)
(see also: http://www.ouramazingplanet.com/amazon-river-plume-spied-by-satellite–0519/)

The mechanics of how these materials disperse in the environment will not be an average of readings taken at various locations; they will travel in plumes, and settle into areas of greater and lesser concentration (local ‘hot’ and ‘cool’ spots) Why? Becasue any time substances come into contact and mix in nature, that’s how they operate. Accordingly, we can expect that the massive discharge will remain a fairly cohesive mass, traveling as a plume according to the primary currents around Japan’s eastern shore:

(from: http://en.wikipedia.org/wiki/Kuroshio_Current)

“Satellite images of the Kuroshio Current illustrates how the current path meanders and forms isolated rings or eddies on the order of 100â??300 km. Eddies retain their unique form for several months and have their own biological characteristics that depend on where they form. If the eddies are formed between the current and coastline of Japan, they may impinge on the continental shelf and their high kinetic energy has the effect of drawing large volumes of water off the shelf on one side of the ring, while adding water to the other side. Eddies size and strength decline with distance from major ocean currents. The amount of energy decreases from the rings associated with the major currents and down to eddies remote from those currents.”

Note on the map (if you follow the link) of the southward current closer to shore, with the northward current further in the Pacific, and fairly large circular currents just east of the area that includes both Fukushima and Tokyo.

The assumption of ‘dilution to harmless levels’ because the Pacific is really really big, and the radioactive materials will behave nicely and spread evenly, is wishful thinking. Similarly, the idea that concentrations will diminish in an even way over greater distances presupposes that ‘average dispersal rates’ are of any use in the real world. They’re not. Some places will find much lower levels of radioctivity, others much higher. You don’t want to be in the places that are much higher, and they are going to be found much further from Daiichi than would be predicted by averaging.

OC: “At this point the metric of harm is going to be how much land ends up off limits. The goal at this point (aside from worker safety) should be to minimize that.”

Omega, with all due respect, the metric of harm remains what it always has been– how many people become seriously ill, how many suffer and die, how many children are born with birth defects, and how many people’s lives are simply destroyed because they can never return home. There’s an abstraction in talking about ‘land off limits’ that borders on callousness.

How many people would you estimate– as we see things today– are going to die because of the radiactive material already introduced into the environment? If you claim just a few of the workers at the plant, you’re simply not acknowledging what’s been happening.

http://www.xkcd.com/radiation

The crack remains a mystery.

The dose given therapeutically to kill the thyroid dead is 550MBq of I-131. You are not meant to sleep next to anyone after that treatment for weeks if not months.

The dose in the liquid in reactor 2 is 13 MBq/ml of I-131 and 2900 MBq/ml of I-134. These things are hard to work out directly as a dose to the patient, but I suspect more than a sip would be very dangerous.

For the workers there, the radiation in the water is multiples higher than in reactor 3, where the workers got very high doses and were hospitalised. Of course, they were hospitalised because they werent wearing waterproof shoes, and got radioactive stuff all over their feet … so maybe that isnt the most reasonable context!

More concerningly I-134 has a half life of 52 minutes, so unless some other fission product is decaying into it in a big way, it may mean fission is occuring in the damaged fuel rods