It turns out that former Prime Minister Yukio Hatoyama had more than just the recriticality back in March and April to talk about in his Nature article that was published on December 15, a rather awkward day it must have been for the Noda administration who was going to declare the end of the Fuku I Nuke Plant accident.
Even though the original English version of the Nature article is only available to the subscribers, Nature Asia has the full translation made available to anyone.
In the article, he and his co-writer Tomoyuki Taira talks about the possibilities of recriticality (chlorine-38 detection), nuclear explosion of Reactor 3, and melt-through of the corium contaminating the groundwater.
No wonder Yomiuri Shinbun, when writing about Hatoyama's article in Nature magazine, decided to only mention TEPCO nationalization and recriticality in March/April - a subject safe enough to talk about now.
In concluding that it may have been a nuclear explosion at Reactor 3, Hatoyama forgoes the mechanism of how a nuclear explosion could have happened and focuses on the evidence of transuranic elements scattered far outside the plant, saying a hydrogen explosion wouldn't be powerful enough.
The original was English, and the translation (by Nature Asia?) was edited by Mr. Taira, according to Nature Asia website. The following is my quick translation back into English from the Japanese translation of the portion about Reactor 3 nuclear explosion. As such, it will be probably nowhere near the original English in terms of expression.
From "Nuclear energy: Nationalize the Fukushima Daiichi atomic plant" by Tomoyuki Taira & Yukio Hatoyama (translation into English from the Japanese translation of the original English):
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, i.e. explosions as the result of chemical reaction at a high temperature between the alloy in the cladding and the water vapor in the reactor core. However, this is not conclusive. Other possibilities exist, and they are nuclear explosions and gas explosions other than hydrogen gas.
How much, and what kind of radioactive materials were dispersed by the explosions, and how far did they spread? What is the condition of spent nuclear fuel stored in the Spent Fuel Pool of Reactor 3? To answer these questions, it is imperative that we know one way or the other if a nuclear explosion took place. 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.
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. These are extremely toxic, and if ingested they will cause internal radiation. 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 are not the radioactive fallout from the past atmospheric nuclear testing but that they 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.
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 not known to us whether a hydrogen explosion is powerful enough to disperse transuranic elements this far. 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. It is equally important to investigate whether a different explosive gas [other than hydrogen gas] was being generated at that time.
Hatoyama, as a former prime minister and a high-ranking official of the Democratic Party of Japan, has had access to more detailed information about the accident from both the government source and TEPCO, in addition to the data from nuclear experts that he invited to his study group on the Fukushima accident, though he complains in the Nature article how he was frustrated with the slow response from TEPCO. (He is referring to the 99% blacked out operation manual from TEPCO.)