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

It’s Saturday 3:00 pm EDT; it’s Sunday 4:00 am in Japan.

The news on Saturday was discovery of a large crack in the concrete floor of a staging area “pit” outside Fukushima Daiichi Reactor Unit 2. See story and video from NHK World. New York Times coverage is here.

Cracks in concrete floors are rarely a crisis, but since this crack was in a low spot outside Unit 2, and radioactive water from the Daiiche units was apparently leaking through the it, the crack is a pathway for leaking radioactivity into the nearby ocean, where higher readings were again recorded. They’ll try to seal the crack with concrete, but do they know where the water is coming from?

[Update: Unfortunately, per CNN, initial efforts to seal the leak have failed to stop radioactive water from leaking into the sea. ]

The Japanese utility, TEPCO, also continued efforts Saturday to pump contaminated water out of the turbine buildings basements and outside trenches and into any storage facility that still had room. Removing this water is necessary to be able to work in the area and to continue hooking up electrical equipment they hope will eventually allow a restart of cooling water pumps, values and related equipment. That’s still the main objective for stabilizing these reactors and the fuel in their storage pools.

To remove this water, they’ve filled condenser units inside the turbine buildings and are trying to make room in condenser storage tanks on site. To do that, and to make way for new fresh water brought on by US Navy barges, they’re trying to decide where to put excess contaminated water, such as on barges or an offshore floating “island” that might be able to store, temporarily, up to 10,000 tons of water. They’ll need more than that, as they keep pouring more fresh water into the reactors and spent fuel storage pools each day.

It’s important to keep in mind Unit 4′s storage pool has a full load of “non-spent” fuel that was removed from the core last December. The pool has already suffered a fire and/or explosion and may be damaged/leaking. Most important, the storage pool is outside the containment structure, and even the external building walls/roof have been destroyed. Unlike the damaged fuel in the reactor cores, which are still, we hope, inside one or more layers of containment, whatever happens to that still vulnerable fuel in the storage pool has a direct path to the environment.

Sources:

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

____________
The AREVA Presentation

A few days ago, a number of folks began seeing a PowerPoint presentation by Dr. Matthias Braun at the French nuclear firm, AREVA. Then on Thursday, an apparently approved version of the presentation was posted at the blogsite, EnergyFromThorium.

[Update: This is the same AREVA presentation described by the New York Times in an article posted Saturday evening. That story confirms the analysis relies on computer simulations based on observed hydrogen and types of fission products -- e.g., iodine, cesium -- and models of how the fuel reacts at various temperatures to release them. This allows analysts to understand what's probably happening in the core or pools even without other on-site measurements.]

The presentation is very helpful for its pictures, descriptions and accident time lines and sequence. So read through that link, and if possible, open that presentation in a separate tab. I’ll add a few comments here and refer to specific pages.

Slides 4, 5 and 6: The reactor design we’ve used in our posts here is on slide 4, along with a picture of an actual reactor of this type under construction at Browns Ferry. Slide 5 has a picture we haven’t used before of the reactor “service floor.” This service floor is above the reactor and containment structure and is used for loading/unloading the fuel, to/from the reactor or storage pool, using the crane.

In slide 6, we see a crane from the service floor lifting the dome cap off the containment vessel below. In the right panel of that slide the dome is sitting to the left. I believe the reactor vessel is down below, in the dark hole to the mid-upper right, and the spent fuel storage pool could be lower right, beneath the crane.

Slide 7′s schematic of the plant design identifies the key features and illustrates how fresh water is introduced to the reactor core (“main feedwater”) and how steam is drawn from the reactor (“fresh steam line”) to eventually drive the turbines.

Slide 8 begins the accident sequence, with the quake on March 11, 14:46. The term SCRAM = the automatic shutdown sequence that each reactor follows to bring the reactor to a safe cold shutdown. The figures for continuing heat generation after the SCRAM of 6%, 1% after a 1 day, and 0.5% after 5 days represent, I believe, what is supposed to happen, but didn’t. As slide 9 shows, in a normal shutdown, the reactor is sealed off from the outside systems, and even if electricity from the grid is lost, the backup generators kick in to power the cooling system.

Slide 10, the tsunami happens at 15:41, just 55 minutes after the quake. It floods the backup generators and cooling pumps/equipment in the turbine building. We have a station blackout, but an emergency, battery-operated cooling system is still available, as long as it lasts.

Slide 11 illustrates that emergency cooling system. Steam is still being produced from boiling water inside the reactor, and that steam can be used to drive a turbine which in turn drives a pump. The steam then goes into the “wetwall” structure where it condenses to water, and the somewhat cooler water is pumped back into the reactor. This works as long as the batteries continue to control this system and the pumps work, but since this is a closed system, gradual heat buildup can’t be avoided. It’s designed to be only a temporary solution until power is restored, but that didn’t happen.

In slide 12, either the batteries or the steam-driven pumps fail. At unit 1, the batteries fail within an hour after the tsunami. For some reason, in Unit 3, they fail 35 hours after the tsunami (given the short 8-hour expected life for the batteries, did this system not begin sooner?) And in Unit 2, the pump fails before the batteries give out, almost 3 days after the tsunami.

Slides 12 through 17 then follow the consequences of the emergency cooling system failures. Each successive slide shows the water level in the core falling until parts of the core containing the fuel become uncovered.

Slide 17 shows what they would predict as the core becomes exposed (I don’t believe these are actual measurements of the event). At 50% exposure, the core may still be undamaged, but at 2/3 exposure the fuel rod cladding starts to break down and release of radioactive fission products begins.

In slide 18, at 3/4 of core exposure, the zirconium cladding begins to burn, which produces hydrogen gas. Pressure pushes hydrogen out of the reactor vessel, down into the wet well (doughnut at bottom), where it escapes up into the containment structure. At this point, the hydrogen and fission products radiation are still partially “contained,” though outside the reactor vessel.

However, from slide 19, the core continues to heat up, the fuel rod cladding begins to melt and that melts adjoining steel structures. Even higher heat can destroy the fuel rods and leave molten debris on the reactor vessel bottom.

According to slide 19, the “restoration of water supply stops the accidents.” I assume this means it stopped the accident from getting even worse. The author notes that the core was “27h w.o. water” at Unit 1, which I take to mean that the core was at least partially (or totally?) uncovered for 27 hours in Unit 1. It was uncovered for 7 hours in units 2 and 3 each.

Slides 20 – 22 illustrate the progression that led to the release of radiation (“fission products”) and hydrogen from the containment structure surrounding the reactor vessel. From the melting core, it moves down into the wet wall and then up into the space contained by the dry well. Pressure inside the dry well containment rises — up to 8 atmospheres in a structure designed for 4-5 atmospheres. And the reactor core is still melting, creating more steam and pressure. So TEPCO decides to relieve this pressure within a day or two after the quake/tsunami. Slide 22 shows steam/hydrogen being released outside the containment — it goes first into the service floor area at the top of the building and gathers there. Of course, the hydrogen is flammable and can explode.

Slide 23 depicts the resulting hydrogen explosion at the tops (service floor) of Units 1 and 3. These destroy much of the outer buildings and equipment on the service floor.

But as slide 24 shows, Unit 2′s fire or explosion occurs below, down at/by the wet well/condenser, not in the upper service floor. The outer building remains mostly intact, but there is presumably some damage to the emergency cooling system. The author doesn’t know why Unit 2 was different.

Finally, slides 30-32 illustrate the problems at Unit 4′s spent fuel storage pool. Down for maintenance, it had a full non-spent fuel load recently removed from the reactor core to the storage pool. All storage pools are outside containment. So any fire or explosion associated with that fuel, if it becomes uncovered by cooling water, has an unrestricted direct path to the environment.

These are just initial comments without the benefit of actually hearing the presentation. So if folks see errors in my reading, let us know in the comments.