Judging from the increased pressure within the containment structures, I would say that most, if not all, the loss of cooling liquid was due to evaporation (in essence, the formation of steam). The lack of circulation and cooling has prevented the removal of heat and the continual flow of cooler water over the core, so with the water stagnant, water near the core would heat up to boiling and turn to steam. As it "boiled off", the pressure increased and the liquid level dropped.
The reason I ask, is that the Japanese working with the reactors (at great personal sacrifice, I might add) have repeatedly stated that they are constantly pumping seawater to the reactors, and yet there is still a constant loss of water (you may answer me like some of my teachers years and years ago, "What did I just say?" in this case meaning that water goes in and because of the tremendous heat, much is converted to steam, thus the need for more and more water)..........which brings me to another question: Is the seawater being pumped into the metal vessel or to the outside of the metal vessel, or, to both areas?
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