Melted Nuclear Fuel Below Fukushima Located by Underwater Robot Six Years Later
The nuclear disaster at Fukushima in 2011 was the most significant since Chernobyl 25 years earlier. Cleanup efforts at the Japanese plant have been slowed by technical difficulties and hardware breakdowns. Recently, however, the Japanese have built a small underwater robot, dubbed “Little Sunfish,” that can see into the water below the melted Reactor #3 inside the Fukushima Daiichi nuclear power plant.
The robot appears to have answered some critical questions about what happened to Reactor #3’s fuel supply in the hours following the earthquake and tsunami that struck the plant on March 11, 2011. Previous attempts to see into the reactors themselves have been largely unsuccessful; the amount of radiation pouring off the melted-down reactor would turn your average human into a burned-out screaming meat puppet within a second, and the overall radiation level was high enough to burn out robots within a matter of minutes, as we discussed earlier this year.
Little Sunfish’s ability to penetrate the water pooled at the bottom of Reactor #3 is a major leap forward for the cleanup effort. It allows scientists to determine what radioactive material ended up where within the structure.
We’ve previously discussed Reactor #2 at Fukushima in some depth, so let’s take a look at where the two reactors diverged. Both were boiling water reactors, and both were subject to the same type of damage — an earthquake plus tsunami that disrupted power to their cooling systems. At this point, the only thing that could’ve stopped the fuel in Reactors 1-3 from melting down was to begin immediate seawater injection. But this would have destroyed the reactors, and the Japanese government didn’t immediately want to take that step.
To bring the temperature down at Reactor #3, and to prevent the possibility of further hydrogen gas explosions, the Japanese government hurled huge amounts of boric acid-infused water at the problem. From the date of the accident through March 22, approximately 988,532 gallons of water were sprayed or injected into Reactor #3 alone. An Olympic swimming pool contains about 660,000 gallons of water, to give some idea of scale. Reactor #3 seems to have had temperature stabilization problems that weren’t encountered as severely at the other reactors.
This graph shows how high Reactor #3’s Temperature readings were and how long it took to bring them down. The light orange line refers to temperature readings taken at the water feed, as compared to readings taken from under (the exact word is “lower” so I’m uncertain if the usage is best read one way or the other) the pressure vessel flange ). The line below the graph states: “False readings possible,” and what I believe means “Handled sensors were damaged,” Either way, take this data with a grain of salt.
Here’s the BBC:
Tokyo Electric Power Company (Tepco) said that the images – revealed on Friday – were the first ‘high likely’ sighting of melted fuel since the 2011 disaster. ‘There is a high possibility that the solidified objects are mixtures of melted metal and fuel that fell from the vessel,’ a spokesman said…Some of the objects appeared like icicles hanging around a control rod mechanism, which is attached to the bottom of the reactor’s pressure vessel holding the core, the company said.
Finding the missing fuel in Reactor #3 is critical to long-term clean-up efforts. But the sheer difficulty of fixing the meltdown problem is part of why it’s taken us six years just to get to this point. The radioactivity in that water is going to be difficult to deal with.