APR: your source for nuclear news and analysis since April 16, 2010

Tuesday, March 22, 2011

Interesting point on the timeline for site cleanup....

While examining some sources for a post I'll probably make later tonight, I ran across a reference to a time frame I'd forgotten about...

After the Three Mile Island accident, of course once the accident phase was over it was time to try to determine what to do. The TMI accident resulted in a serious core melt, which destroyed roughly the uppermost five feet of the fuel rods (and the control rods too) and a large amount of this material slumped in the core; fuel pellets were scattered all over.

This exact status wasn't found out until two years later when the reactor pressure vessel was finally opened up.

The containment of TMI was not damaged, much less covered by a demolished external structure as at several plants at Fukushima Daiishi.

The active region of the core at TMI was about 12 feet, with total fuel assembly length slightly longer so that we might say about a third of the core was demolished in the core melt event.

Jumping aside for a moment, we note that "source term" or the amount of radioactive material in a core (which is used to estimate exclusion area distance for licensing, among other things) is based on rated core power. The rated core power at TMI-2 was, at the time, the same as at TMI-1 or 2568 MWt although its Westinghouse generator was rated much higher at 956 MWe gross / 904.5 MWe net. (Normally MWe is stated as net, and that's the convention we follow here.) So we have a roughly 2500 MW reactor with about 1/3 core damage.

At Fukushima Daiichi, we have three reactors with varying power ratings and varying amounts of core damage. At No. 1, a 460 MWe net plant, about 70 percent of the fuel may be damaged; at No. 2, a 784 MWe plant perhaps one third, and at No. 3 which is just like No. 2 and is 784 MWe with no recent guess as to core damage but it's unlikely any better than No. 2.

Simple math indicates that there could well be more fuel mass damaged here at these three plants than at TMI, and if we add in the damage in the spent fuel pools it's hard to imagine it isn't a LOT more.

Back to our initial point, to bring this full circle ... Considering the state of the reactor buildings and the contamination outside it might be longer than two years before anyone gets a first look inside the reactor vessel of any of these plants.

3:25 PM Eastern Tuesday 3/22

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