Here's what I can harvest this morning focused on volatile releases from melted nuclear fuel at the Fukushima 1 nuclear complex. The prognosis looks dreadful for the plant site itself but not a dire as Chernobyl for the downwind areas:

Union of Concerned Scientists teleconference transcript Friday, March 25

"REPORTER:

I'm seeing these sort of ominous reports from the Japanese Prime Minister, you know, warning of a breach in the reactor vessel of Unit 3, can you give us some kind of context to understand why, you know, how risky that is and just give me some sense of, you know, we've been hearing about this for weeks now and I actually, I mean, just give me a sense of what you guys know about this breach and what it could mean.

MR. LOCHBAUM: This is Dave Lochbaum, I'll start and then Ed can supplement. First of all, we're hearing reports of a reactor vessel breach on Unit 3, but the data is inconclusive as to whether that's actually what's going on. There could be other factors that could cause the reactor vessel integrity to be lost, for example, one of the relief valves could have stuck open. If it is, indeed, a reactor vessel breach, the consequences, or the most likely reason for that would be that the core has been damaged to the point of melting. Some of the molten mass fell down to the bottom of the reactor vessel and caused a hole, burned a hole or created a hole in the bottom of the reactor vessel, which became the breach. If that's the case, it's bad, because first of all, it indicates that the level of fuel damage went beyond blistering and cracking of the fuel rods, to the point of melting of fuel, which is a more severe form of fuel damage. And it was followed by a loss of integrity of the reactor vessel. If the reactor vessel remains intact, you always have the option or the ability to put water back into it to cool the fuel, even if it's damaged. If the reactor vessel has been breached, you face challenges in putting enough water in it and keeping the water in it rather than having it just drain back out through a hole in the bottom.

In the emergency procedures world, if you do lose the integrity of the reactor vessel, the option is to fill up the entire containment above the point where the breach has occurred so that you try to still cover up the fuel, even if the fuel has been relocated, part of the fuel may have relocated, and the reactor vessel integrity has been lost. So, you still have methods to deal with that, if that were to occur, but they're obviously much more severe than if you keep the reactor vessel intact and allow the fuel to remain where it was placed originally. Ed, is there anything to add to that?

DR. LYMAN: Just that, you know, until the core starts to degrade, as I mentioned at the beginning, a relatively small amount of radioactivity is actually released to the atmosphere, or to the reactor vessel and the coolant, and as the core starts to degrade, much higher quantities can be released. So, it's already been reported that the core has been exposed up to halfway for many days, I think that the expectation is that at least the part that was above the water line experienced extensive damage, and so the reactor vessel itself now probably might have up to, I don't know, 30 or 40 percent of some of the more volatile radionuclides like iodine and cesium in the vessel, then if the core melts through the bottom of the reactor vessel, and falls on the floor of the containment, it then can react to the concrete basement and that can generate additional gasses which will help sweep some of that material up and potentially out through a breach.

So, when all's said and done, something like 70 or 80 percent of the iodine, cesium and the fuel could actually be released to the containment atmosphere, and if the containment is breached, that's available for release. So, as bad as the releases have been so far, they could increase by several fold, ultimately, if this proceeds any further.

(SNIP)

REPORTER: What would be the worst case scenario if we have a reactor breach, and, you know, all this plutonium should just go to the sea of the ocean because it affects the sea life and the atmosphere. So, what can we impose -- what can we see as the worst?

DR. LYMAN: This is Ed Lyman.
I mean, worst case meaning that there's a breach of the reactor vessel, the core falls into the containment, it spreads out across the floor, and this would require the containment floor to be completely dry, which I'm not sure that would be the case, but if it were completely dry, it would spread out to the corners of the containment, or it could actually contact the containment liner and melt through the liner, and then you have a pathway directly from the core material to the environment.

So, then it depends on essentially how much of the radioactive isotopes that were contained in the core enter the atmosphere of the containment and then how much leak out from the containment.

There are numerous modeling and simulations over the years show that a high fraction of the isotopes like cesium and iodine would be released from the core material in this situation, and enter the atmosphere in the containment. There are a range of other isotopes, radioactive barium, tellurium, and strontium, all that have varying properties, and would be released to varying extents less than 100 percent. It could be on the order of five, ten, 20 percent, it depends.

Then there are the lowest volatile elements that include plutonium and certain lanthanides, and certain other actinides, like americium and uranium. Uranium actually under certain conditions could be released on the order of one to 10 percent, that was demonstrated in experiments over the last ten years, plutonium and the lowest volatile isotopes would be less than one percent, probably.

The ultimate consequences could exceed those at Chernobyl, because of the total inventory of radioactive material in the three reactors and potentially three spent fuel pools is many times what was in the core at Chernobyl. But the key is how much, what are the released fractions, and that's still highly uncertain.

But in this case, which is essentially a late containment failure, very late, weeks after the reactors originally scrammed, analyses typically show that there would be some -- well, first of all there's radioactive decay, like I said at the beginning, iodine, some other short list isotopes, significantly reduced, and to the extent that other parts of the reactor cooler, you might have played out, but it really has to do with when the timing of the containment failure in relation to the vessel breach.

So, if the vessel breaches and the containment failure is still delayed significantly, then you have more played out and less environmental release."

Much of this is still guesswork but at least it's guesswork by professionally trained nuclear engineers and scientists.

Charley Noble