Translation of the Dec. 15, 2011 Nature Magazine article by former Prime Minister Yukio Hatoyama and legislator Tomoyuki Taira — both members of by EX-SKF (Certain expressions may be off, as article was translated English to Japanese and then back to English) [Emphasis Added]:
[Subheading:] Possibility of Nuclear Explosion
We need to answer the question of what caused the series of explosions at Fukushima I Nuclear Power Plant. Initially, they were reported as hydrogen explosions [...]
[T]his is not conclusive. Other possibilities exist, and they are nuclear explosions and gas explosions other than hydrogen gas. [...]
Nuclear Explosion “More Likely”
From two observed facts, we believe a nuclear explosion is more likely.
First, several transuranic elements have been detected several tens of kilometers away from the plant.
Second, the steel trusses in the upper part of the reactor building of Reactor 3 are twisted as if they had been melted.
Curium and Plutonium
According to the reports by the Ministry of Education and Science, curium-242 (242Cm) has been detected at a location 3 kilometers from the plant, and plutonium-238 (238Pu) has been detected at a location 45 kilometers from the plant. [...]
242Cm’s half life is short (about 163 days), and the deposition of 238Pu around the plant is far greater than normal, leading the Ministry of Education and Science to conclude these were emitted from Fukushima I Nuclear Power Plant.
If that’s the case, pieces of broken spent nuclear fuel rods may have been scattered around the plant, and it is extremely dangerous.
Too Forceful to be Hydrogen Explosion?
These transuranic elements are not carried by the radioactive plume like much lighter cesium or iodine.
Therefore, they must have been blown out by an extremely large force. [...]
It is unlikely that a hydrogen explosion generated a high enough temperature that would melt steel.
TEPCO initially announced that there was a white smoke from Reactor 3 explosion.
However, the later investigation has revealed that the smoke was black, and a hydrogen explosion is not considered to generate such a black smoke.
Our conclusion therefore is that it [explosion of Reactor 3] may have been a nuclear explosion.
Published: December 17th, 2011 at 1:00 pm ET