Fukushima Unit 4 is in the news lately. Tepco had scheduled to remove fuel assemblies from the #4 spent fuel pool starting tomorrow, though this has been pushed back a couple of weeks. This is ostensibly due to Tepco making more tests, but the recent announcement that the US is moving in to Fukushima Daiichi for decommissioning probably has more to do with it.
We always hear that the Unit 4 reactor was empty of nuclear fuel at the time of the earthquake and tsunami. All the fuel was in SFP#4, which went dry and burned, according to the FOIA documents, widespread news reports, and the IAEA.
But there is evidence that there was indeed fuel in Unit 4 at time of the explosion and fire. A good summary is in the following Japanese posts (here) and (here). The cap of the reactor was off for the next refueling cycle:
The reactor was in the center of the building, while the spent fuel pool was over on the south side. Infrared images show significant heat emanating from the center.
The reactor cap was off, so this meltdown occurred in the open air. This picture from March 24, 2011 shows steam or smoke emanating from the center of the building, as well as the right side, where the pool is.
The official Japanese atomic energy website shows 4,250 sieverts/hour of radiation inside the containment vessel at reactor 4 on April 25. This could only have been possible fuel had been loaded into it.
And this video from May 23 sure looks like something is on fire in the middle of Unit 4:
What does it mean? I’m not sure. What is significant is that apparently Tepco lied again. If they were unloading the rods, the assemblies in the reactor would have been spent fuel. If they were loading them (which is more likely), they would have been less radioactive fresh fuel. If the fresh rods came from the SFP, then either way there is the same amount of radiation involved… both of them burned. Though we don’t know for sure if the fresh rods were from the #4 SFP.
Could they be covering something else up? Some kind of plutonium separation activity in the basement? Who knows. But it looks like they are doing something before the Americans arrive.
And what is this goop coming out of Unit 4 on March 16? This is the north side of the building… the fuel pool is on the other side. Tepco claims there was a lube oil fire, and also solvents were stored in the building. It doesn’t look like lube oil or solvents to me.