On The Containment Vessel Damage at Fukushima Daiichi Unit 1

Early on Wednesday morning, the Dow Jones News Wire first reportedthe following from Japan, before the rest of the mainstream media got holdof the story today:

The melted nuclear fuel within the No. 1 unit at the FukushimaDaiichi power plant was of such intensity that it eroded through 2 meters ofthe 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 ofTEPCO to get a better understanding of the report, and share some additionalfacts that puts this information into the proper context. What's been reported isa very conservative mathematical analysis that has yet to be physicallyconfirmed. In other words, this is a worst case scenario. And as we've seen inour industry, even in the worst case scenario, there is still a verysignificant safety margin.

A quick read ofthe article could give one the impression that the melted core was a littlemore than half a meter -- about 2 feet -- from reaching the externalenvironment. I think it’s important to note that according to the TEPCO analysisonly .7 meters (a little more than two feet) of concrete was actually eroded.  In addition, as we've written before, plantshave multiple redundant safety systems in place to protect the public, andthat's exactly the case with Fukushima Daiichi Unit 1.

In addition to the2.6 meters (about 8.5 feet) of steel reinforced concrete inside the containmentvessel, underneath the steel shell of the containment vessel lies another 7.6meters (about 25 feet) of basemat reinforced concrete and steel. Altogether,that means there was 10.2 meters (about 33.5 feet) of reinforced concrete andsteel standing between the reactor core and the outside of the plant before theaccident.

Even if 2 meters (about6.5 feet) of that structure has been eroded, another 8.2 meters (almost 27feet) of reinforced steel and concrete lies between the melted fuel and the externalenvironment.

It’s also important to note that according to tests of air samplesfrom inside containment, it appears that the process of erosion – called coriuminteraction – has essentially ceased and no further damage is occurring at thistime. If that process is still continuing, it is doing so at such a slow ratethat TEPCO has more than enough time to develop a mitigation strategy.