Fukushima - Unit 2, what happened different to Units 1 and 3? There are recent discussions about this topic going on, so I think it deserves its own thread. According to attachement IV-2, Unit 2 is responsible for more than 90% of the overall emissions. So there's one big question: What was different at Unit 2, compared to Units 1 and 3? We know for sure that there are three total meltdowns in Units 1-3. Part of the Corium probably penetrated the RPV and is now at the bottom of the primary containment. As for Unit 1, there are probably holes in the secondary containment, but I think that those are mainly ruptured valves. So the primary containment is keeping most of the fission products in check. They would need to go through a labyrinth of pipes and valves to see fresh air. Unit 3 is a little more tricky. I'd say it's the same situation as in Unit 1, but the big bang which happened is still a mystery. So let's look at Unit 2 now. Unit 2s RPV lost pressure around one hour before midnight on March 14th, so that's probably the time when the corium penetrated the vessel. It's interesting to see that simultaneously the Drywell pressure (= primary containment) increased. Then, at 6:00 am, another interesting thing happens - the explosion near the torus, followed by a fast depressurization of both the torus and the RPV. Now I found the following pdf which deals with containment failures for Mark-I containments: http://www.osti.gov/bridge/servlets/purl/5835351-nR29Hq/5835351.pdf It says "Drywell shell melt-through would result in blowdown to torus room or second floor of reactor building". So perhaps that's what happened at Unit 2, but not at Unit 1 and 3. Molten corium attacked the primary containment walls which resulted in a blowdown to either the torus room or the second floor. Or only one of the two. And the following depressurization, maybe coupled with a hydrogen explosion, could have further damaged the building. So that the primary containment would be directly connected to the outside, not through pipes and valves. If the blowdown occured in the eastern corner of the building, the shockwave may have deflagrated in the turbine building without even coming near the refueling deck. Which would explain why the outer structure of Unit 2 is still okay. What do you think?