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TEPCO has a simple diagram to explain the super-hot 3 sieverts/hour radiation on the drain pipe of Toshiba's SARRY, not the main pipe that feeds contaminated water to the vessels.
It was all too clear from the press conference that TEPCO didn't even know for sure. They were guessing that's what was, which was the fluctuating water pressure during the flushing caused the float to bob up and down (see the TEPCO diagram below), which in turn caused the water with highly radioactive sludge to be sucked out into the drain pipe.
Why did TEPCO think that was the cause? Because after they blasted the drain pipe with water the radiation level got lower, so it's got to be the sludge in the pipe.
Therefore, their countermeasure is going to be to close off the valve at the vessel so that the contaminated sludge from the vessel doesn't flow out to the drain pipe. Looks like a basic design flaw to me, but who am I to say?
Here's TEPCO's explanation (handout on 8/23/2011), if you can believe it:
