I'll throw out an idea and watch it get shot to hell.

If the venting to reduce the pressure causes a decrease in the level of heavy water which leads to the exposure of some of the rods which in turn causes a temperature rise (and this will accelerate with subsequent temperature rise) thereby allowing the nickel to melt and the ??? (can't recall the name of that stuff) to ignite which in turn ignites the hydrogen separated from the heavy water by the extreme temperatures causing a massive explosion which expels radioactive material into the atmosphere... why not pull the drain plug and remove the explosive fuel from equation? Is it because it's better to expel the radioactive material into the atmosphere than have it melt into the earth?

Start shootin.