The melted nuclear fuel within the No. 1 unit at the Fukushima Daiichi power plant was of such intensity that it eroded through 2 meters of the 2.6 meter (8.5 feet) concrete base, plant operator Tokyo Electric Power Co. said in a report issued on Wednesday.
Yesterday here at the office, we huddled with a representative of TEPCO to get a better understanding of the report, and share some additional facts that puts this information into the proper context. What's been reported is a very conservative mathematical analysis that has yet to be physically confirmed. In other words, this is a worst case scenario. And as we've seen in our industry, even in the worst case scenario, there is still a very significant safety margin.
A quick read of the article could give one the impression that the melted core was a little more than half a meter -- about 2 feet -- from reaching the external environment. I think it’s important to note that according to the TEPCO analysis only .7 meters (a little more than two feet) of concrete was actually eroded. In addition, as we've written before, plants have multiple redundant safety systems in place to protect the public, and that's exactly the case with Fukushima Daiichi Unit 1.
In addition to the 2.6 meters (about 8.5 feet) of steel reinforced concrete inside the containment vessel, underneath the steel shell of the containment vessel lies another 7.6 meters (about 25 feet) of basemat reinforced concrete and steel. Altogether, that means there was 10.2 meters (about 33.5 feet) of reinforced concrete and steel standing between the reactor core and the outside of the plant before the accident.
Even if 2 meters (about 6.5 feet) of that structure has been eroded, another 8.2 meters (almost 27 feet) of reinforced steel and concrete lies between the melted fuel and the external environment.
It’s also important to note that according to tests of air samples from inside containment, it appears that the process of erosion – called corium interaction – has essentially ceased and no further damage is occurring at this time. If that process is still continuing, it is doing so at such a slow rate that TEPCO has more than enough time to develop a mitigation strategy.