Fukushima Unit 3 – New Unanswered Questions

The new inspections at Fukushima unit 3 by Japan’s nuclear regulator (NRA) has raised new questions and brought some old ones back into the spotlight. NRA’s recent inspection involved a limited walk through inside unit 3. Inspectors took a brief walk through of a section of the 2nd floor and about half of the third floor.

We have compiled documentation of this inspection along with our analysis at the link below. This includes the original NRA inspection video and English translations of the NRA reports.

The original documents from NRA (in Japanese)

We explain below, our new questions and the context for these issues along with the applicable sections of the NRA inspections.

Not One Explosion

This has been a long term question since the 2011 disaster. Unit 3’s explosion was unusual in magnitude and the characteristics of how it took place.

The first frames of the video of unit 3’s explosion show a significant horizontal explosion to the north, towards unit 2. A solid slab of the north refueling floor wall can be seen lifting up then falling away. Then the vertical explosion shoots upward out of the refueling floor and reactor well areas. The horizontal explosion doesn’t spread much higher than the roof line of the building. These early frames of the explosion appear to show two separate events.

fukushima unit 3 explosion

fukushima unit 3 explosion

fukushima unit 3 explosion

All of the frames of the unit 3 explosion can be found here.

Asymmetrical Explosion

Along with the explosion images showing a horizontal initial blast, the remains of the reactor building provide additional evidence. The 5th floor (refueling floor) is mostly gone except for some of the concrete framing on the east (sea facing) side. The 4th floor north and west corner is blown out. The north facing side where the horizontal explosion began lacks any remaining concrete frame work. The 5th floor floor and 4th floor ceiling in this area is largely destroyed or missing. The tool pit remains largely intact including the north facing outer wall. This appears to indicate that the tool pit itself was not included in the worst of the initial portion of the blast.

unit 3 reactor building post explosion

The blast also shows directional damage throughout the building.

The basement torus room shows outward blast damage toward the north end of the building. In the one location documented, a door from the torus room is bowed out. Directional but minor blast damage (a steel door was found wrapped around a stairwell support) was found in the torus room. The torus room and basement was otherwise in unusually good shape at the time of the inspections in 2012 and 2013.

The first floor shows directional damage towards the north and east in the three locations identified. In the south side of the building, the TIPS room door was blown into the TIPS room hallway in a north bound manner. The large concrete access hatch plug on the north side of the building was pushed out of the hatch. The door into the MSIV room was similarly pushed into a hallway in a directional fashion These show an asymmetrical blast pattern on the first floor.


Visual evidence on the 2nd floor is limited. TEPCO has more evidence they have not released to the public. Radiation readings were taken in the north side of the 2nd floor around 2014. This can be confirmed by the radiation readings on the floor plan from 2014 radiation reading reports. The visual evidence of conditions in the building for this area is still being withheld by TEPCO without explanation. NRA’s inspection discovered what appears to be directional debris and damage from the south side of the 2nd floor pushing north.

The diagram for the 3rd floor shifts orientation. Instead of north being the top side of the floor plan, this one has north as the left side of the floor plan. Directional damage for the 3rd floor again shows north pushing/pulling blast damage along the south side of the building. The entry to the south hallway from the larger area of the west side 3rd floor shows an east blast damage. This may indicate that the blast rolled down this hallway and came from the larger portion of the 3rd floor. The larger area of the 3rd floor in the west side of the building shows downward damage. The ceiling slab is collapsed downward in many areas. Equipment that had been hung below the ceiling is found in piles on the floor. The 3rd floor north and west walls are damaged but largely intact. NRA took radiation readings along various unidentified pipes that run up to the 4th floor through the 3rd floor ceiling. Results of those tests have not been made public. The 3rd floor appears to have suffered some sort of downward blast from the 4th floor that centered in this north west area.


Based on the limited visual evidence collected from outside the 4th floor and damage found to the ceiling of the 3rd floor we estimate directional blasts on the 4th floor to be outward out the north wall and downward in the area marked. This area is the location of the MG sets. These are motor generators that help power the cooling pumps inside containment. If they are involved or if other equipment in this area caused a portion of the blast remains an open question. Other components tied to the MG sets within the reactor building hadn’t shown high levels of radiation as was found in unit 1.

4th Floor Damage

The existing evidence shows an even either began in the north west corner of the 4th floor or a significant portion of the initiating event took place in the 4th floor in addition to the 5 floor in the same location. The area above on the 5th floor lacks any known equipment that could have concentrated hydrogen or otherwise initiated the horizontal portion of the explosion. This area is where the containment and reactor caps are stored during refueling, so this area is largely a blank slate. In the photo below the containment caps and reactor cap can be seen in the left background during pre-disaster refueling work.

Hydrogen Leaks

Hydrogen leaks may have been a contributing factor to the initiation of the blast outside of containment. The route of such a leak still needs to be identified. Details of this area in the form of diagrams or detailed floor plans have never been published by TEPCO. We do know of two potential leak routes in this area of the building. In unit 1, a reactor well line cuts through the containment wall to the outer portion of the reactor building. TEPCO recently used this to run a dust monitor into the reactor well at unit 1. This pipe penetration exists on the 4th floor of unit 1 and likely exists in a similar location at unit 3. The reactor cavity differential pressure adjustment line at unit 1 was used to install the dust monitor in the reactor well. This pipe provides an easy route from the reactor well to the reactor building.


Another location in this same general area is the connection of the tool pit to the reactor well. After the explosion at unit 3, this location was discovered to be both damaged and significantly leaking steam. We do not know if this was potentially damaged in the earthquake or only suffered damage from the explosions. The tool pit itself was largely undamaged. If this location cracked and provided a route from the reactor well to the reactor building 4th floor it could have allowed significant amounts of hydrogen to escape into this area of the 4th floor. The tool pit gate shows a new gap between the gate and the outer wall. If this includes damage or gaps into the reactor building at the 4th floor level will require further inspections.

unit 3 reactor well

There may be other factors in play, but determining the mechanism of the initiating horizontal blast is critical towards understanding the failure and explosion at unit 3.

Boron Injection

While not directly related to the new inspections at unit 3, the boron injection at unit 3 has always been a bit of an anomaly. Discussion related to the NRA’s inspections caused this question to come back around. This effort only took place at unit 3. Unit 2’s condition was still being combated and was not considered a lost cause at this point. Why boron was injected into unit 3 and not unit 2, where water was being injected at the same time, is an existing unanswered question. We speculate that TEPCO knew the danger of the MOX fuel inside unit 3 and hoped the boron injected with the sea water would help prevent a criticality inside unit 3’s reactor vessel and containment.
The unit 3 timeline shows these events in chronological order.


Unit 2’s timeline

Add these to the long list of unanswered questions regarding the failures at Fukushima Daiichi.

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4 thoughts on “Fukushima Unit 3 – New Unanswered Questions

  • January 31, 2020 at 2:57 am

    What does the 3rd floor where they did not go look like? Is Containment concrete wall or penetrations blown out? Is it higher radiation in this area? Was there too much debris to prevent walking around other part of third floor? Is next floor up accessible or is it the refuelling floor (ie 4th floor)?

    • January 31, 2020 at 9:34 pm

      Next floor up is 4th and it is inaccessible, the stairwell to 4 is destroyed. They did not explore further into the areas of the 3rd floor with more damage. They could conceivably use a small quadcopter drone to look in that area. Similar drones have been used in other reactor and turbine building inspections on site.

  • January 31, 2020 at 9:17 pm

    I believe there were 3 blow out explosions. The horizontal blow out to North combination steam H2 explosion. Then vertical blow out starts probably steam explosion due to prompt critical. There was then a bright flash of a H2 explosion at base of vertical plume and somewhat behind it. Maybe the SBGT system exploding. You can hear 3 booms during this event in rapid fire.

  • January 31, 2020 at 9:24 pm

    The heat and force of Unit 3 explosions we’re much higher than Unit 1, given the totally burnt and pulverized concrete in plume and given there were melted structural support beams on outer walls. These forces and instantaneous heat
    cannot be solely attributed to 750 kg of H2 exploding.


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