What isotopic ratios from the Unit 3 debris fragments tell us about the explosion there.

EX-SKF is reporting that highly radioactive debris fragments have been discovered at the river mouth in Naraha-machi, around 9 miles from the Fukushima Daiichi plant. These fragments very likely came from the explosion of Unit 3 in 2011, since the degree of contamination in them resembles that found in debris near the Unit 3 building.

Tepco has published a pdf which contains analyses of radioactive isotopes and minerals in 4 samples of this debris. These analyses can tell us a lot about what happened at the Unit 3 explosion, what the state of the corium was, where it was, if it had melted through the containment at this point.

This is very important because debris went sky-high, and likely entered the stratosphere and the jet stream, and made its way to North America. The spent fuel pool fires and other reactor meltdowns caused contamination to enter the jet stream also, but not to the degree that the Unit 3 explosion did.

The Tepco pdf indicates that the samples contain an enormous amount of radioactive cesium. They also contain antimony-125, silver-110m, cobalt-60, strontium-90, curium-244, americium-241, and various isotopes of plutonium, as well as other radionuclides. This makes it the most dangerous material found at Fukushima so far. We do know from the FOIA documents that fuel rod fragments are out there too. We will see that these samples are not from the fuel rods themselves, though.

The radionuclides found in these fragments match those found in the “black substance”, or black dust, that were found by a separate analysis. So now we have a good idea of what the source of the black substance is.

Page 42 of this pdf contains a table listing the radioisotope inventories of the Unit 3 reactor. This will tell us what the ratios between the different radioactive elements should be, for any sample of pure MOX fuel. The document also contains inventories for the Unit 3 spent fuel pool. But the two inventories are so similar that the reactor inventory will be used only. We can’t tell if the radioactive contamination came from the reactor or the pool here.

I calculated the expected ratios of radioactivity from the elements in this inventory, to amounts of cesium-137, that are listed in the Tepco document. Then I calculated the observed ratios of the averages of the 4 samples. For nuclides with short half-lives, I corrected the observed ratios to July 2013.

Radionuclide   Expected ratio   Observed ratio   Observed/Expected
------------   --------------   --------------   -----------------
Sb-125              0.04             0.0034            0.085
Ag-110m             0.002            0.0001            0.05
Co-60               0.00001          0.0002           20.0
Sr-90               0.75             0.0008            0.001
Cm-244              0.011            0.0000002         0.00002
Pu-238+Am-241       0.025            0.000001          0.00004
Pu-239+Pu-240       0.01             0.0000004         0.00004

If the samples were contaminated purely from the MOX fuel rods in Unit 3, the observed/expected ratios would all be close to 1. The first thing that strikes you is how this the ratio is for cobalt-60. Co-60 is a neutron activation product. It results from neutrons interacting with structural elements like steel. The neutrons might come from neutron-emitting radionuclides like curium and plutonium. Or they could come from a criticality. Since the ratio is 20 times higher than expected, we can safely say that the cobalt-60 resulted from a supercriticality. This is yet more evidence (if any more is needed) that Unit 3 blew up in a nuclear explosion.

Cesium and iodine are the most volatile elements from the nuclear fuel. That means they get released first with increasing heat. The other nuclides beside Co-60 all have ratios far below 1. So the debris that went into the jet stream was not the fuel rods themselves, though it was contaminated with it to some degree. If the explosion blew the MOX fuel to kingdom come, the debris would contain 25,000 times as much plutonium than was found in the fragments.

Another possibility would be that a molten corium/concrete reaction led to the nuclear explosion. Here molten corium burns through the steel containment, and interacts with the concrete of the drywell, and the floor of the building. The interaction between the molten fuel and the chemical elements and water in the concrete results in a bubbling stew, with gases escaping. This is how less volatile nuclides like strontium-90 gets released from the fuel. If the explosion occurred after this reaction was underway, a certain pattern of isotope ratios would follow.

We see from the graphic that the silver ratio would be 0.97, antimony would be 0.77, curium would be 0.003. The observed ratios are far less than this. So we can conclude that the dangerous corium/concrete reaction had not yet been underway to a significant extent.

The Tepco documents states the iron (FE) was found in the samples. The photo of the wreckage at Unit 3 indicates rust (iron oxide). This argues that melted steel was involved.

There was a nuclear explosion at Unit 3 in March 11. I believe that it was triggered by a hydrogen explosion. It occurred very early in the meltdown process, which is a fortunate thing for the northern hemisphere. The corium had melted steel, but not concrete in any significant way yet. Perhaps it happened as soon as it came into contact with the concrete.

If the explosion had happened later, it might have released 25,000 times more plutonium into the jet stream. This is not to say that it did not release a large amount of plutonium. The corium-concrete reaction probably occurred after the explosion. But it didn’t blow sky-high after this reaction, but before it.