TEPCO recently published a video of the work to remove spent fuel from the unit 3 fuel pool. In this video was an unexpected finding with serious implications.
In the video of the fuel assembly removal from a fuel rack inside the spent fuel pool, was a tell tale sign of something significant. Prior to the effort to remove fuel from the pool, the pool underwent significant cleaning work. This included removing most of the debris that fell into the pool along with use of a vacuum to remove small pieces of broken concrete and dust.
What remains adhered to the side of the fuel rack appears to be the same thick white substances found inside the reactor containment of unit 3 and in the pedestal below the reactor vessel. These substances also have the same appearance as those inside containment. They are stuck to both vertical and horizontal surfaces as if they splattered then stuck to where they landed. What these may be and how they managed to end up on the fuel racks is explained further in this report.
Below are two photos from the spent fuel removal in unit 3’s spent fuel pool. The first photo shows a circle around the top of the fuel assembly in question. The fuel assemblies used in unit 3 are 9 x 9 inch standard fuel assemblies. This gives a reference point we can use to estimate the size of the white substances. The 9 inch width of the fuel assembly is marked on the second photo below in pink. Each of the white substances on the fuel rack is in a pink box.
The comparison of the fuel assembly to the white substances shows that the blobs are about 1-3 inches in size.
These thick white substances are found inside containment in multiple locations. In the photos below we explain where and what was found.
2015 Unit 3 Containment Inspection
Thick white substances can be seen on the catwalk inside containment.
The photo above shows some significant data about these substances. This photo from 2015 shows with good detail what some of these substances look like. They are thick with holes similar to what is seen in types of lava pumice.
Some of the substances appear as smooth thick splatters, indicating they were more liquid and less viscous as they adhered to surfaces.
More thick white substances in containment.
More of the thick porous white substances on the right side of the photo above.
2017 Unit 3 Reactor Pedestal Inspection
In the photo above the same thick substances can be seen. The amber color may be due to the lighting used on the swimming robot or may indicate the substances inside the pedestal have a different hue.
More white substances adhered to remaining structures in the reactor pedestal.
Erosion or pits can be seen on the concrete pedestal wall to the right in the photo above.
The metal structures in the photo above are looking up at the under side of the reactor vessel. Thick almost spherical white blobs can be seen adhered to the metal.
What appears to be a brace or piece of catwalk and other structures show more white spherical blobs.
More photos of the underside of the reactor vessel show the blobs on many of the surfaces.
A side view of the underside of the reactor vessel shows the white substances adhered to the structure.
What Might These Be?
The pumice like white substances found in containment provide a hint to their formation.
A known material from volcanic eruptions is rhyolite.
“Rhyolite is light-coloured or white – this is a clue that the rock contains a lot of silica (more than 70%) and not much iron or magnesium.” source
“Rhyolitic magmas are associated with low temperatures (750–850ºC) and are often thick, which means gases can’t escape. Some rhyolitic rocks are quite light, for example, pumice, which may still have evidence of the bubbles of gas trapped as the rock solidified.” source
photo above of white pumice rhyolite from https://www.sandatlas.org/pumice/
This behavior seems to match what we have seen with these white substances. Some appear as thick, smooth, flat residues. Others appear very thick and capable of maintaining a spherical shape.
The high silica content in rhyolite provides a clue to the formation of these white substances.
Concrete contains high amounts of silica. As melted fuel (corium) interacts with the concrete, the silica of the concrete can incorporate into the molten fuel or melt due to the heat in proximity to the fuel. Molten fuel had to have been active in significant amounts on the containment floor to create these white substances.
The formation of these white substances was active at the time unit 3 exploded. White substances of considerable size (1-3) inches were able to escape containment to the environment at the time of the explosion. A significant mass of molten fuel would have been in the pedestal or drywall floor in containment at the time of the explosion for this behavior to take place. The white substances would have had to still be in a molten form to adhere to the fuel rack outside containment at the time of the explosion.
This provides a tangible marker for the progression of the meltdown at unit 3. Further review of the event timeline for unit 3 will be needed to determine if this new finding changes any of the assumptions of the earlier meltdown events.
Large substances, not just microparticles of fuel, were able to escape containment in the explosion. The adhered white substances on the spent fuel pool fuel racks prove this out. With the understanding that a large mass of molten fuel was active in unit 3’s containment at the time this happened, it raises new questions about the amount of fuel or fuel particles released by the explosion.
This article would not be possible without the extensive efforts of the SimplyInfo research team
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