Wednesday, March 23, 2011

Fukushima Daiichi Reactor Number 3 and 4 Damage Analyses

Reactor Building 3 at the Fukushima Daiichi Nuclear Power Plant has sustained significant damage from a hydrogen explosion that was much larger than the explosions that damaged the other buildings.  The question that we would like to answer is whether or not the top of the reactor vessel or any of the various levels of containment have been damaged, and whether or not the spent fuel pool has been damaged.

First, we need to understand the configuration of the spent fuel pool and the top of the reactor.  Here is the schematic of the power plant:

During refueling or inspection, the top cap of the concrete shell, the steel containment vessel, and the top of the reactor vessel can be removed by the building crane to access the fuel rods in the reactor.  Before this operation, the spent fuel pool and the top of the reactor area are all flooded with water.  This enables the fuel to be removed and placed into the spent fuel pool while still being submerged at all times.  The way the design allows this to be done can be seen more easily in the next figures.

The whole top area is flooded when transferring fuel.  Seals around the reactor keep the water from flowing into the dry well.  Gates with inflatable seals keep the water in the spent fuel pool after the operation is complete.  Except for the gates, the fuel pool is surrounded by fairly thick concrete.

The source of these diagrams is at "All Things Nuclear":  They also have a more detailed discussion about seal integrity, and the fact that leakage has occurred in other plants when the air pressure is lost and the seals deflate.  It is likely that the seals are at least leaking, or may even be blown away completely in some cases.

The following is an overhead view (also from the previous link) "of an irradiated spent fuel bundle being transferred from the reactor core (lower right) to the spent fuel pool (upper left) through what is called the 'cattle chute' at the Browns Ferry Nuclear Plant in Alabama. The spent fuel pool gate has been removed to connect the spent fuel pool water with the water in the reactor well area above the open reactor pressure vessel."

To further understand what this looks like at Fukushima, we can peer into the ruins of Reactor Building 4:

We are looking here at the SE corner of the building.  I am fairly sure that the top two "stories" of number 4 are the open area at the top of the reactor where the crane runs.  The floor of the second floor from the top, where the steam is coming from, is the top of the spent fuel pool.  In the following closeup, you can see the green building crane dolly, about the size of a Greyhound bus, which appears to have fallen from its rails and is laying on top of the spent fuel pool.  The thick and largely intact gray beam above the crane is one of the rails, I believe.

For number 4, the spent fuel pool may be intact, but the holes in the building on the right side, third floor from the top make me wonder.  Also, the gates with their inflatable seals, are probably leaking badly.  The Japanese apparently have been successful in spraying large quantities of water into this area.

In looking at the photos of the Reactor Building 3 damage, my concern is that the top of the remaining ruins are so low that the reactor top may be missing, and the spent fuel pool largely destroyed.

First let's look at the Number 1 building for some perspective on the problem.  A hydrogen explosion destroyed the top crane attic of this building too, but much less damage to the rest of the building occurred.  It appears that Number 1 had a different design, with a more fragile upper two stories.  This weakness allowed the explosion (which may have been smaller to begin with) to exit the building without building up damaging pressure in the lower floors.  The remaining top structure beams are the equivalent height as the top two stories in Buildings 3 and 4, I believe, even though the top of Number 1 has three repeating structures, i.e. three "stories".

We do not see all the way to the ground around the building, but it appears that the top one-quarter is damaged.  It also appears that a central structure exists -- perhaps the top cover to the reactor, perhaps not.  I would expect that the top of the spent fuel pool is right at the top of the undamaged section.

Reactor Building 3 sustained severe damage from a very large explosion.  The damage to the facing side of Number 4 was caused by the explosion of Number 3.  In the following video, note the large structural components falling after being blown thousands of feet into the air.

Here are views of the damage to 3:

Reactor Building 3 in foreground left.  North side of Reactor Building 4 in middle of image.

The west side of 3

The northeast corner of 3 (left upper part of the frame), looking mostly east

The southeast corner of 3

Looking down onto the west side of 3

The northwest corner of 3.  West side of 2 on left, north side of 4 on right.  Intervening materials and structures do not allow us to see the base of the buildings, making the buildings seem shorter than they are.

West side of 3. The damage on this side is extensive down to a level below that seen on the east side.  The building is basically destroyed to the tops of the support buildings nearby.

I believe that the top of the free standing skeleton wall on the east side is at the original top of the building.  The spent fuel pool at the SE corner, with the steam coming out, appears to have some damage to its concrete walls and structure, but may be largely intact.  I would suspect that the gates and seals are gone -- blown away by the explosion.  This would mean, based on the diagrams above, that the tops of the rod assemblies in the pool are at the top of the maximum possible water level with no gates.  It is likely impossible to fully cover the rods.

There is no evidence of the building crane dolly.  Not sure where it went.

The following thermal IR image shows the spent fuel pool in 3 glowing at 62 deg C.  The center of the building, around the reactor cap, is glowing at 128 deg C.  This likely represents steam leaking from the primary containment through a damaged reactor dry well top cap -- or perhaps scattered fuel assemblies.

The spent fuel pools were pretty much at or above capacity.  Those in Building 3 contained MOX, a mixture of plutonium and uranium.  Not good.  Probably worse than is being reported, if my analysis is correct.

And I am really concerned about what is happening under that hot roof of Building 2.

click for hi-res

1 comment:

  1. This is a fuel-crane of a reactor with MK I. type containment:

    It's worth to compare it with the picture of reactor 4. Maybe this crane is still in place.