Rive said:
Spalling does not necessarily requires direct concrete-fuel interaction. It requires only heat.
At the time of RPV break the whole bottom of RPv were red (yellow?) hot I guess.
After the break there was a shower of yellow (white?) hot material under the RPV.
However, i cannot say that what we see there is spalling for sure: it more looks like broken down plaster.
It's pretty much guaranteed. Concrete spalling occures anytime you have molten fuel ejection from an RPV into a dry PVC
which we know happened in all three Units. You can read a lot about the process and what potential implications Tepco are facing in many of the published journal papers and conducted experiments on loss of coolant (LOCA) and station black out (SBO) accidents. They have been modelling and testing how they progress since the 1980s.
MCCI Concrete-Fuel Interaction and Corium Coolability,
BWR containment failure analysis during degraded-core accidents and
Simulation of a BWR Lower Head RV in an Accident are all good reads.
This paper is especially good because it's a straight forward essay and compares a lot of previous experiments with a whole section on spalling in nuclear accidents.
The image from the video is good because it's above the water line so you can clearly make out the textbook morphology of heat induced damage where the material breaks up in a characteristic layered sheet-like manner. But it's just an example. The real spall site of interest is directly outside the Unit 1 pedestal doorway to the PVC. This is the presumed pathway any molten fuel would have flowed along as it left the RV and the latest data shows that the deposited material found there is too inactive to be the fuel itself. That indicates that the fuel did pass through or under that area.As for this new Tepco video... to get an idea of how the Tepco investigation team is puting together it's own data is fascinating. So many interesting images:
(00:59) straight away we see a model suggesting they presume all three PVC's were essentially dry at the time of fuel melt through. It looked that way for 1 and 2 but I thought as 3 is so flooded, it may have been so at the time of the accident but the flooding of the building seems to be something that happened later on. As far as concrete-fuel interactions, if there was no quenching at the time of core exit, it leaves the basemat more open to core damage.
(02:14) This is one of the most interesting images. As said before, RVs that have CRDs and other equipment that load from the bottom are inherently weaker by design. If you want to keep a liquid substance in a container, having holes in the bottom instantly makes that more difficult. This particular plumbing out let seems to be what Tepco suspect was the primary fuel escape route and shows how you can have gross fuel exit but still see a relatively intact CRD structure as we see. The disturbing thing from this and the other image above, is that they think the RV could have been breached even before the rest of the fuel had melted.
(02:17) This suspected outlet also goes on to explain very well the particular 'spray and splatter' pattern of the hot fuel exit and why we see the CRD room floor grating was melted away in the manner it was.
(13:42) There is the 1x1 meter central melt hole mentioned here the other day which confirms the overall scale of the damage.
(01:12) This graphic shows the molten fuel flowing straight out of the pedestal into the PVC with the splatter along the way. This is where the spalling would primarily occur.
(08:05) This seems to refer to the Unit 1 survey but should be pretty much the same across the three units with the molten fuel exiting the pedestal door into the PVC proper. That black mass is supposed to be the fuel but according to the handout report, the sample readings came back to low to be fuel which is why I am more confident of my original 'spalled concrete with fuel around or under this area' assumption.
(33:25) This is the other interesting section. Now that muon, robot investigations and site data all indicate gross fuel exit across the three units, the next question becomes where the fuel went from there and can that be used to explain the other situations on the site such as the persistent groundwater contamination on site. I had assumed that the burrowing action of the fuel into the basemat combined with explosion/earthquake damage had allowed the fuel to interact with the ingress of site groundwater to cause the contamination problem there. Basically with the fuel going under the PVC/pedestal floor. Tepco appear to be working with a different assumption.
(34:03) According to this graphic, they suspect hot molten fuel somehow made it's way into the suppression pool and burned through the bottom of the torus to contaminate and leak water from there.
(34:09) They seem to be experimenting with different concrete consistencies and mixtures so they can get a mix to pipe into the suppression and plug the suspected leaks. I do not know enough about the plant design to say if this is actually the natural progression for fuel escape or what amounts of fuel would have traveled into that area. I find it strange the molten fuel could travel through that plumbing but they think they can use the same pipework to get a decent concrete delivery.
(46:20) Apparently they think the torus leak is the main site causing the groundwater contamination and is still a major issue for the recovery effort.
Many many thanks Sotan, this was a gem of a find.