Last Friday [Feb. 3] Orcas George published a diary on the then-latest news from Fukushima that reached the rec list -
Fukushima Reactor #2 pressure vessel breached, rising to "unimaginable" levels of radiation.
The issue was news reports in the Japanese press and The Guardian about a video probe entry into the unit-2 containment’s pedestal at the CRD rail - a metal grate catwalk leading from a containment vessel entry hatch called the "hot cell" that leads to the pedestal supporting the reactor vessel. The upper portion of the pedestal houses the control rod drive [CRD] machinery. The probe was able to enter the pedestal at the CRD inspection level floor (also metal grating) 10.5 feet above the containment floor, and managed to reach the middle before radiation interference prevented further usable video. Before the video failed it captured a 2-meter wide melt-hole in the CRD floor grating, as well as a lot of corium residue littering the entire area. Estimating the radiation level over the hole in the center of the pedestal by the amount of radiation interference with the probe's video, a level of 530 Sv/hr is what they came up with.
Previous instrument penetration of the unit-2 containment vessel in 2013 from a higher level measured radiation levels of between 17 and 36 Sv/hr along the CRD catwalk but outside the pedestal, leading to an estimate of what the highest level "should" be in the pedestal of 73 Sv/hr. So the robot development consortium created for this kind of work at Fukushima — the International Research Institute for Nuclear Decommissioning [IRID] — created an instrumentation and tool-wielding robot for the planned pedestal entry that could withstand a 75 Sv/hr radiation level for at least 10 hours. Unfortunately, it cannot withstand a 530 Sv/hr radiation level for more than a couple of hours (and I think they’re wildly overconfident on that).
TEPCO and IRID announced they might have to wait for a tougher robot, but then decided that investigation of the hole and what might be underneath it was too important in the quest to find out where the melted fuel went when it came out of the bottom of the reactor vessel during the meltdown. So they decided that they'd send the new 75 Sv/hr-hardened robot in anyway late this week or early next week to do what it can before the radiation kills it. In order to give that robot — "Scorpion" — as extended a working life as possible in that extreme environment, they sent a 'cleaning robot' into the pedestal overnight (today in Japan, which is now into tomorrow) to water-blast, hammer and remove as much corium debris from the floor grating as possible.
Fukushima Unit 2 High Radiation Damages New Robot
The poor cleaning robot was gravely 'injured' by the high radiation level after working only 1 meter into its planned 5 meter cleaning routine. Again, it was a radiation-caused camera failure. TEPCO was able to retrieve the robot, so it was luckier than other robots used in various operations at the facility's three total reactor meltdowns, which had to be abandoned as just more radioactive debris.
TEPCO is again re-thinking the Scorpion robot's planned entry, questioning how much useful data it can return before it too dies, and even whether they could retrieve it at all - making whatever data it might have gathered unaccessible forever.
It would appear that the bulk of unit-2's corium still remains inside the containment. Specifically, it is for the most part inside the pedestal. Because TEPCO found corium residue (and an undetermined amount of corium) in/on one of the torus downcomers in 2012 and high contamination of the water flooding the torus room (basement level, below containment), it would appear some of the corium on the pedestal floor exited the floor-level opening and flowed to the nearest downcomer vent and through it into the suppression pool. This would be the big "thump" TEPCO described as having occurred very early in the disaster from deep in the bowels of the plant. The "bowels" of the plant is the torus suppression pool.
To get an idea of what this would look like, and how the molten corium is likely distributed, check out a "model meltdown" experiment conducted by the SimplyInfo.org research team back in 2014. In this experiment the stand-in core material is lead. While it's not too hard to get lead to melt, it's not precisely like nuclear reactor fuel because it doesn't carry its own heat source around with it in the form of elemental decay. So there will be differences in viscosity and distance traveled. Still, this experiment actually does provide a fair model of the unit-2 conditions now 'discovered' by TEPCO. The lead you see flowing out of the 'pedestal door' of the model would at Fukushima have been more viscous (more like glass than lead) and very well could have reached the wall of the containment structure where the downcomer vents are located.
Hopefully at some point TEPCO will have a good idea of where unit-2's corium went, and find that there's little or none still missing in action. That would make cleanup of this particular plant easier to plan, if cleanup is what they plan to do. Whether or not they ever actually DO any cleanup is a whole other question, the answer to which I am not investing the least bit of confidence in.
At any rate, there were more than 500 comments to Orcas George's rec-listed diary last Friday, so I thought there might be some folks interested in the followup to last week's near-panic inducing "Surprise" radiation level. Those interested will probably not be surprised again that the actual level inside the pedestal - NOT close to the middle - is quite a bit higher than the 530 Sv/hr estimate too. One of these days TEPCO might even figure out their little formula for estimating radiation levels by radiation interference with their probe/robot cameras does not come usefully close to reality.
More Links for the Interested -
SimplyInfo.org Unit-2 Extended Report 2015
Fukushima Corium Research Experiment Results (Glass Model)