Thursday, December 01, 2011

On The Containment Vessel Damage at Fukushima Daiichi Unit 1

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

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.


Will Davis said...

I am very glad that NEI has made a post on this topic, because the melt damage is almost certain to be misinterpreted quite widely. I made a post myself including an illustration to show just how much more the corium would have to penetrate to reach the grade, much less exit the building and enter the ground or groundwater... essentially echoing NEI's post (which I admire for its clarity and brevity.)

Anonymous said...

I would like to know the release pathways from the PV. I read the one report from ORNL (I think) wherein it was noted that the most likely pathways out of the BWR PV during a significant core melting event were the instrument tube penetrations at the vessel base, not a large-scale breach of the vessel itself.

This is important because I've seen postings on various blogs from anti-nuke kooks showing shome of the building damage extending through the walls and claiming this was "corium". I've seen other kooks claiming that "no one has studied" the behavior of liquid corium, even though a cursory search using Google turned up over a dozen references to scientific studies of core melt behavior, transport coefficients, heat transfer rates, etc. Real, measured scientific data, although probably beyond the comprehension of most of the kooks.