Japan Earthquake: nuclear plants Fukushima part 2

[Astronuc]

I hope that little piece of debris is not being considered representative for the whole deposit at the bottom of the PCV - what was it, 200 tons plus?

Fukushima Daiichi Unit 2 was a BWR/4 type with a gross electrical power generation of 784 MWe gross, 760 MWe net (so about 24 MWe used by the plant). Units 2, 3, 4 and 5 were much the same design, and apparently not uprated.
https://en.wikipedia.org/wiki/Fukushima_Daiichi_Nuclear_Power_Plant

The Fukushima BWR/4s would have 560 assemblies in the core, and I believe each assembly has about 180 kg of fuel (which could be in terms of U metal), about about 200 kg of fuel (UO2) + structural materials like Zircaloy-2 (~46 kg Zr-2, not including channels/shrouds), stainless steel (~6.7 kg), and Inconel (incidental).

Estimating about 270 kg/assy x 560 assy/core, one has 151,200 kg (or 151 tonne) in the core, excluding core support structures.

The limited sampling can only give a clue as to what might have happened. It there is little or no U (and Np, Pu), then one is left with mostly stainless steel and/or Zr. If there is little or no Zr, then one is left with stainless steel. All metals are probably oxidized. I think they will look for metals that are characteristic of fuel, cladding and stainless steel structures, in order to determine where the sample originated, or what it represents. If they find some fission products, e.g., Ru, Rh, . . . . , then that would be from the fuel.

More samples are needed, but at least, TEPCO was successful in retrieving a sample, however small.



FYI - Comprehensive Analysis and Evaluation of Fukushima Daiichi Nuclear Power Station Unit 2
https://www.tandfonline.com/doi/full/10.1080/00295450.2019.1704581#abstract

 

[Sotan]

Two new posts on the "Fuel Debris Portal" site, dated March 19:

- Status of preparations for the second fuel debris trial retrieval from Unit 2 (pdf)
- Training at the Kobe Factory for the second fuel debris trial retrieval from Unit 2 (video)

 

[Sotan]

They completed the second "fuel debris sampling" operation.
Link to report

I don't want to sound disappointed but... This time it seems they retrieved 0.2g.

 

[Sotan]

New report on Tepco's Fuel Debris Portal site, about the first results of the analysis of the second sample (May 29).
>> Link

 

[Sotan]

New post on the "Fuel Debris Portal" site.
"2025.7.29 Fukushima Daiichi Nuclear Power Station Unit 3 Design Deliberation for Fuel Debris Retrieval Method"
There are two links at the bottom of that article, one is the report of a sub-committee regarding fuel debris retrieval (published in March, unfortunately only in Japanese) and the second is TEPCO's own report on this matter, submitted to the sub-committee on July 23 (in English).

"At present, it is estimated that approximately 12 to 15 years will be necessary to prepare the top and side access points regardless of which option (North-South work platform or the East-West framework) is employed. (However, the results of this deliberation are based on the assumption that progress will proceed as expected, even though there remain issues that require additional review.)"

 

[Sotan]

Hello again and may you all have a great new year in 2026!

I thought I'd point out an interesting new post on the "Fuel Debris Portal" site:

Fukushima Daiichi Nuclear Power Station Unit 3 PCV Internal Investigation (non-submerged area) using Micro drones

A new investigation device designed to insert two small drones into the PCV of Unit 3 through the X-53 penetration stumbles because of a slight misalignment / difference in level between the internal and external side of the penetration.

 

[gmax137]

Thanks for that link, @Sotan !
Very interesting and the graphics in the report are excellent.

 

[Charles Smalls]

Greetings all, I hope everyone is well.

Just passing by to mark the anniversary and add a few small comments. It’s remarkable that despite the huge amount we all read and researched for multiple years, there can still be new surprises about fukushima.

Many years ago, Kei Sugaoka alleged General Electric weren’t disclosing the fact that American GE staff were at Fukushima inspecting the units when the earthquake occurred. The new HBO documentary features GE technician Carl Pilliterri who confirms they were indeed present and describes the scene and their departure that day.


More interesting was this 2026 NHK video:
https://biz.jibtv.com/programs/young_engineers_decommissioning_fukushima_daiichi/

@19:15 they discuss units 5 and 6 saying: “the 2011 earthquake followed by the tsunami damaged the buildings, allowing ground water to enter them. The ground water must be pumped out regularly to prevent the building basements flooding”

During this, they show some shots of the flooded lower levels inside the building. I had never heard of this before and I find it terribly interesting.

 

[Sotan]

Three new reports on the Fuel Debris Portal site regarding the inspection of Unit 3 PCV using flying micro-drones.

The first 2 reports, dated March 5 and March 12, are PDFs explaining the examination procedure and the first results.

Most spectacular, most impressive is the video presented in 3rd report, with clear views from inside the PCV, including images from the pedestal.

 

[Rive]

Most spectacular, most impressive is the video presented in 3rd report, with clear views from inside the PCV, including images from the pedestal.

Quite strange to see this environment without the usual radiation artifacts...
I wonder whether they have some software for that, or it's the situation what improved so much.

 

[Sotan]

New report in which they announce finding a structure that "appears to be the bottom of the PCV". The "structure" though looks like a beam, or - in another photo - like a very long cut or edge to me. Not as the original convex shape of the PCV if it was seen intact from that direction. Is it possible the hole at the bottom of the PCV is that large?

*There are still some radiation artifacts in the photos posted in this report.

 

[Sotan]

They added two more videos:
Vicinity of the bottom of the Reactor Pressure Vessel (RPV)
Inside the pedestal

Might have to download them to be able to view them full screen.

I would love to hear from people with knowledge, what they see in these images.
To me it looks like most of the bottom of the RPV must have melted/fallen, so it's not so much a "hole in the bottom of the RPV" but more like "most of the bottom of the RPV is gone". or call it a hole several meters wide...

 

[Rive]

To me it looks like most of the bottom of the RPV must have melted/fallen, so it's not so much a "hole in the bottom of the RPV" but more like "most of the bottom of the RPV is gone".

Yes, looks like that
As I recall something less dramatic was expected...

 

[Charles Smalls]

Wow. Okay, a couple of things here. Within the actual micro-drone videos, I'm seeing some new info, some confirmations of things we already deduced, and at least one genuinely new question.

New info: They have the water level in unit 3 PCV a good 2-3 meters lower than it was in 2017. They did the same inspection in 2017 using a Toshiba underwater 'sunfish' robot and it swam the exact same route the new drone is now flying. (Entered x-53, swam above x-6, followed CRD replacement rail into pedestal and swam up to fully submerged CRD drives). You can see the 2017 water surface above the CRDs at 18 seconds here. The new drone shots are the same area but much clearer.

Confirmations: Unit 3 RPV bottomhead failure was total. Here's how the RPV bottom head was;

You can also see a quick up-skirt of Fukushima Daiichi's actual RPV during installation here.
The penetration holes are for control rod drives, neutron detectors and water drainage etc. Due to the nature of spheres, the penetrations increase in size away from the centre. They also decrease the thermal tolerance of the bottom of the vessel, can increase fracture risks and are capped with different materials. At 00:14 and a few other places of the drone footage, we see smooth semi-circles which were penetrations traversing the steel of the now failed bottom head. IRID says the Unit 3 RPV is 5.5 meters wide so we see a ~2 meter hole in the RPV. That number is a very rough guess but you could use penetration hole distance to calculate more accurately.


New questions:
Full failure of unit 3 RPV bottom was always a given, but I still don't know how or why Unit 3 water levels are so much better/higher than the other 2 units. The answer may be here. At 01:11 in the second video the drone flies behind a fallen penetration tube inside the pedestal. Just beneath the water surface is a huge mass of light brown material that appears to be filling much of the pedestal. It looks multiple meters tall. If the PCV was flooded before the meltout happened, then it may be re-solidified corium halted partially there within the pedestal. (Unit 1 pedestal is comparitively empty, probably since the PCV was dry and the fuel headed off to the torus). If that material inside unit 3 pedestal isn't corium, then it could be the remains of structural materials but I wouldn't know where from. People more familiar can probably estimate what the mass is by observing radiation sparkle noise vs distance and the amount of water shielding. I can't recognize the other pedestal components seen after that. Would have to compare to the unit 5 videos.

They will want a plan to remove the cables and other debris from the x-6 penetration. Would be good if they perform similar flights in unit 1 and 2 too.

 

[Sotan]

Thank you very much Charles Smalls! Even though I knew of all the rods that pierce the bottom of the PCV, that sieve-like image makes it much less of a surprise to hear the bottom has failed on a large scale.

Let me link the latest report on the Fuel Debris Portal. It contains a summary of the micro-drone investigations of the PCV of Unit 3 and some of the results.

One interesting plan going forward:
"Point cloud conversion of footage: The footage acquired from inside and outside the pedestal will be used to generate a point cloud."