Did a second meltdown occur at Fukushima Daiichi Unit 3?

Photo taken in the first moments of the Fukushima Daiichi Unit 3 hydrogen explosion on March 14thThe Asahi Shimbun has reported that Fumiya Tanabe, a former senior researcher at the government-affiliated Japan Atomic Energy Research Institute (JAERI) and an expert on atomic meltdowns, has argued that a second meltdown occurred at Fukushima Daiichi Unit 3. He theorizes that the first meltdown occurred on March 14th, associated with the violent hydrogen explosion (photo at left) that largely rubblized the secondary containment building, but that a second meltdown occurred on March 21st. He provides several lines of evidence. If, as he purports, Unit 3's melted down core has melted through the reactor pressure vessel and now rests on the primary containment structure's concrete floor, this could hamper Tokyo Electric Power Company's plan to bring the molten core into "cold shutdown."