Reactor Containment & Fuel Storage from UCS; (h/t commenter lobster)

It’s Thursday, 6:00 pm EDT: Friday, 7:00 am in Japan.

Today’s update focuses not so much on anything new happening — another quake, near loss of power at another plant, broken power lines — but on further analysis of the range of concerns the Japanese are facing at Fukushima.

The Union of Concerned Scientists held a press briefing with David Lochbaum and Edwin Lyman to help reporters understand the details of the NRC report on reactor conditions at Fukushima Daiichi Nuclear Station. The details explain how dire the conditions are at Units 1-4.

The March 26 NRC report had been the basis of a New York Times story, which we summarized on our Wednesday update, noting that nuclear plant experts had serious concerns about several conditions facing the Japanese as they try to get the three reactors cores and at least two spent fuel storage pools under control.

The entire UCS briefing is worth reading, but here I focus on two aspects: the continuing concern about the spent fuel pools, particularly at Unit 4 but also at Unit 3, and the condition of the reactors in Units 1-3 and the challenge of trying to keep cooling water covering their respective cores.

Why they can’t keep the reactor cores covered.

As explained by UCS, and based on the March 26 NRC analysis, it appears they may not yet have the ability to sustain water over more than about half of the core. And they have limited ability to circulate water over portions that are nominally covered because of salt build up (or core damage).

UCS nuclear scientist Lochbaum first explains the problem created by the salt and other obstruction inside the core. That’s restricting the flow of water into the core and around the fuel rods, leaving them at least partially uncovered.

The fuel that’s or the water that’s being injected is being injected into the reactor vessel outside a device called the core shroud that’s kind of like a can within a can, and the reactor core is within the inner-most can, the core shroud. The water that’s being injected is supposed to flow down through the annulus region between those two pieces of metal, be turned around by the lower dome, and then flow up through the reactor core to cool it. There are pretty clear signs that that’s not happening and the fuel inside the core shroud is not fully covered.

Lochbam further explains that each reactor vessel has, for this discussion, two sets of water injection points where external pumps can inject water into the reactor core to maintain cooling. The pipes are sealed at the point where they enter the reactor vessel, but those seals can break down when exposed to intense heat. If the seals fail, water can leak back out of the reactor vessel, offsetting the ability to inject cooling water in.

Two points then become critical. One injection point where the seals may have failed is about half way down the reactor vessel relative to the height of the core. That means water is leaking from a point that could leave half the core uncovered if not offset.

Moreover, if the pumps available for water injection cannot keep up with the rate water is leaking out, they can never get the core fully covered. That appears to be what’s happening at least at Unit 1 and possibly Units 2 and 3 as well. Here’s Lochbaum:

If [the seals] fail, they’re deliberately designed to limit how much water they leak out through a failed seal to about 60 gallons per minute, more or less. It’s more if the pressure inside of the reactor vessel goes up to squeeze more water through that narrow opening; it’s less if the pressure in the reactor vessel drops down.

But if you assume that both recirculation pumps on each reactor has failed seals, you could have upwards of 100 to 120 gallons per minute leaking out through those failed seals. Their elevations are about half of the reactor vessel core height. If those seals are intact, you should be able to at least flood the vessel back up to two-thirds core height. At that point, water leaks out through what are called the jet pumps. Right now, the fact that they are having trouble get the water level above one-half of core height is telling them that the reactor seals have likely failed.

To answer your question, could they get out of this situation, if they can get flow rate through the reactor core greater than the leakage rate and greater than the boil-off rate, which if that’s the only losses you have, is about 120 gallons per minute for the seals and about 50 gallons per minute for evaporation right now, and if you could get 200 gallons or more makeup and it’s not being blocked or bypassed from the core, then you should be able to refill the entire vessel. Right now, that’s not happening, so water’s going somewhere.

So, to summarize: Water injected at the top and pushed down the annulus inside the reactor may not be circulating back up through the fuel rods. And they can’t keep the fuel in the core covered more than half way because some water is boiling off and other water is leaking out half way down at a rate faster than they can inject it through whatever pumping equipment they have now.

And what about the Unit 3 and 4 spent fuel pools?

As we’ve noted many times, the fact the spent fuel storage pools are outside containment creates additional risks of nuclear materials being dispersed to the environment from any explosion. Here’s Lochbaum:

It’s difficult to determine which is worse, the spent fuel pool situation or the reactor core situation. There are signs that the explosions in the Unit 4 and perhaps also the Unit 3 spent fuel pool have caused irradiated material to leave the building. That could have been the reason for the high or the reports periodically of neutron beams. That actually could be coming from decay from fuel or fuel particles that are now no longer in the spent fuel pool and were carried away by the explosion. That was already known to cause high radiation levels. It caused challenges for the workers, including even the helicopter pilots that were dropping water onto the site a week or so ago.

There’s also a discussion of concerns about other issues, including why they need to be injecting nitrogen into the containment vessel — it was displace by hydrogen and the explosions and must be present to prevent further hydrogen explosions — and the need to expand the evacuation range. As Lyman emphasizes, the zone between 20 and 30 kilometers has been exposed for a month, and there is no sign conditions are improving. Those people (and probably others further out) should have been moved long ago.

Sources:

NHK World
Kyodo News
Hi-res photos
IAEA Updates
Union of Concerned Scientists
fleep Graphing Earthquate, radiation and water readings