The Tokyo Electric Power Company (TEPCO) confirmed Friday that all or nearly all of the fuel at the Fukushima Daiichi nuclear power station's Unit 1 reactor melted during the catastrophic events of March 2011, when a 9.0-magnitude earthquake and subsequent tsunami event crippled the six-reactor facility.
TEPCO said muon scans showed no shadows at the reactor's core, which is where the dense nuclear fuel would be during normal operations. With none showing up at the core, it is presumed the fuel melted and fell down to the bottom of the containment vessel.
"While our previous analysis has already strongly suggested that fuel rods had melted down, the latest study provided further data that we like to regard as a progress in our effort to determine the exact locations of the debris," said a TEPCO spokesman.
While the muon tomography did not show anything unexpected, it is still critical for TEPCO to identify what happened in the reactor, so it can design clean up and decommissioning accordingly.
The images indicate that the molten curium flowed through the reactor pressure vessel and into the pressurized containment vessel. This was previously considered uncertain, because radiation measurements and water tests from the reactor were inconsistent with expectations of what would be found if the corium completely settled in the concrete of the PCV. According to Extreme Tech, one theory postulates that the seawater pumped into Unit 1 after the tsunami event was able to stop the meltdown before it burned through the reactor pressure vessel.
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