Is there a physicist in the house?

This NHK story states "TEPCO says it found 220 becquerels of iodine-131 per cubic centimeter of water" in the spent fuel pool of reactor 4. I-131 has a half life of 8 days, so any produced while that reactor was functioning should be long gone.

http://www3.nhk.or.jp/daily/english/13_37.html

I know just enough about this stuff to know that the Uranium and Plutonium in the spent fuel undergoes spontaneous fission, but the article states that the levels are higher than expected. Does that mean that the damaged rods are somehow interacting with each other, or that the iodine and cesium are normally held within the fuel rods?