For the last three and a half years, TEPCO engineers have been working to better characterize the state of the melted nuclear fuel in the Fukushima Daiichi nuclear reactors.
Now those officials believe that most of the melted nuclear fuel in the Unit 3 reactor melted through the reactor pressure vessel and collected at the bottom of the containment.
Before, engineers believed that some volume of fuel was still inside of the reactor, but now that understanding has changed.
TEPCO also updated its estimate of the time of fuel damage and exit from the pressure vessel. According to TEPCO officials, now they believe that the meltdown at the Unit 3 reactor began around 05:30 AM on the morning of March 13th – 5 hours earlier than TEPCO previously assumed, and that most of the fuel melted and escaped from the reactor pressure vessel into the containment vessel by the morning of March 14th.
These updates will force TEPCO to adjust its decommissioning schedule and plans, as the increased volume of melted nuclear fuel outside of the reactor pressure vessel will make decommissioning efforts much more difficult and costly.