Japanese officials announced early Wednesda they have managed to stop the serious leak of water with high radiation levels that had been flowing into the Pacific Ocean from the pit and sea-water intakes near Unit 2.

That was the good news. The bad news is what’s still unsolved and may be getting worse.

[Update: David Dayen reports Congressman Markey claims NRC officials told him some portion of the core in Unit 2 has "probably" melted through the reactor vessel, into the containment floor. That would be very bad news if confirmed.]

[Update II: The NRC disputes this interpretation even though Markey received an e-mail from an NRC staffer saying some NRC experts had speculated there might have been a reactor vessel breach]

The bad news, compiled by the New York Times, listed the very serious threats still facing the Fukushima Daiichi Units 1-4, as the Japanese stuggle with what are, as lobster points out, the potential for four simultaneous INES level 7 accidents. Be sure to read lobster’s post on how the situation at any of these Units could be credibly rated in the same category as Chernobyl.

From the New York Times:

United States government engineers sent to help with the crisis in Japan are warning that the troubled nuclear plant there is facing a wide array of fresh threats that could persist indefinitely, and that in some cases are expected to increase as a result of the very measures being taken to keep the plant stable, according to a confidential assessment prepared by the Nuclear Regulatory Commission.

Among the new threats that were cited in the assessment, dated March 26, are the mounting stresses placed on the containment structures as they fill with radioactive cooling water, making them more vulnerable to rupture in one of the aftershocks rattling the site after the earthquake and tsunami of March 11. The document also cites the possibility of explosions inside the containment structures due to the release of hydrogen and oxygen from seawater pumped into the reactors, and offers new details on how semimolten fuel rods and salt buildup are impeding the flow of fresh water meant to cool the nuclear cores.

Paraphrasing the Times’ list of serious causes for concern:

– Continuing potential for exposure of the cores in Units 1-3, all of which have suffered varying degrees of “meltdown” from loss of cooling water, leaving the possibility of further fuel rod cladding breakdown and release of radioactive materials, plus hydrogen gas. Hence the need for injecting nitrogen gas to inhibit explosions and boron to prevent recriticality.

– Continuing potential for further hydrogen explosions that could damage/breach containment and/or cause further radiation releases into the environment.

– Growing concern about the non-spent fuel stored in Unit 4′s storage pool, which is outside containment and thus open to the outside environment and has already suffered fire/explosion after the pool’s cooling water was partially or fully lost for some period.

– Concern that earlier explosions, especially from Unit 4′s exposed spent fuel storage pool, could have spewed radioactive materials up to a mile away.

– Growing concerns about possible containment vulnerability at Units 1-3 from the weight of massive water injections already made into the containment structures. These structures surrounding the reactor vessel are not designed to hold that much water, increasing the potential for containment breach in the event of further quakes.

– Concerns about the ability to continue, without fail, essential fresh water injections indefinitely, with no near-term solution yet available for how to get beyond this stage.

– Growing concerns about sea-water salt deposition on fuel assemblies, restricting water flow around them and thus reducing any water’s ability to carry away heat.

Because slumping fuel and salt from seawater that had been used as a coolant is probably blocking circulation pathways, the water flow in No. 1 “is severely restricted and likely blocked.” Inside the core itself, “there is likely no water level,” the assessment says, adding that as a result, “it is difficult to determine how much cooling is getting to the fuel.” Similar problems exist in No. 2 and No. 3, although the blockage is probably less severe, the assessment says.

It’s good that TEPCO was able to stop the flow of contaminated water into the Pacific Ocean, though radiation levels remain high as of Wednesday night. It’s at least better for workers who must function outside the reactor buildings. Still, radiation levels inside the reactor buildings are so high they’re literally off the scale.

Stopping the sea leak won’t change the decision to dump other supposedly less contaminated water from storage areas directly into the ocean, to make room for pumping out somewhat worse contaminated water from each unit’s turbine buildings and outside trenches. But I wonder what stopping the leak to the sea means.

After failing to stop the leak with concrete, shredded paper and other mixtures, they apparently succeeded with liquid glass — sodium silicate.

On Tuesday, the plant operator drilled a hole into a layer of gravel around the pit, and poured a hardening agent called liquid glass, or sodium silicate, to stop the leak of highly radioactive water into the sea.

TEPCO says the flow was confirmed to have stopped on Wednesday morning, and that there has since been no change in the water level in the pit and the nearby turbine building.

But where was the water coming from? And if the flow to the sea has been stopped, where is that water going or pooling now?

Sources:

NHK World
Kyodo News
Hi-res photos
IAEA Updates
Union of Concerned Scientists