We still don't have a good theory of why sudden water loss at unit 1 which was the first to melt down and the first to explode. Many people think evidence is that there was a pipe break because of the earthquake long before the tsunami took out the power.
from ANS: http://fukushima.ans.org/report/Fukushima_report.pdf
from CNIC report "In the case of Reactor Unit 1, the reactor water level reached a meager 45 cm above the fuel rods by 7:30 p.m. on March 11th, only 4 hours and 44 minutes after the earthquake. This means that the reactor water level decreased by approximately 4.5 meters within this relatively short period of time. We have two questions about this rapid loss of water from Reactor Unit 1: Firstly, what was the cause of this abnormal and sudden drop in the water level, and secondly, where did the water go? There are only two possible explanations for these questions. One possibility is that one of the pipes connected to the pressure vessel (such as the main steam pipe, water supply pipe, and re-circulating pipes) was damaged or broken by the earthquake vibration, and as a result, the coolant gushed out of the pressure vessel. Another possibility is that the "safety relief valve" of the main steam pipe opened automatically (or was manually opened by an operator), and from there, steam under high temperature and pressure in the pressure vessel gushed out into the suppression chamber (suppression pool). As a result, the water level in the reactor fell." http://cnic.jp/english/topics/safety/earthquake/fukukk19may11.html
from ANS: http://fukushima.ans.org/report/Fukushima_report.pdf
from CNIC report "In the case of Reactor Unit 1, the reactor water level reached a meager 45 cm above the fuel rods by 7:30 p.m. on March 11th, only 4 hours and 44 minutes after the earthquake. This means that the reactor water level decreased by approximately 4.5 meters within this relatively short period of time. We have two questions about this rapid loss of water from Reactor Unit 1: Firstly, what was the cause of this abnormal and sudden drop in the water level, and secondly, where did the water go? There are only two possible explanations for these questions. One possibility is that one of the pipes connected to the pressure vessel (such as the main steam pipe, water supply pipe, and re-circulating pipes) was damaged or broken by the earthquake vibration, and as a result, the coolant gushed out of the pressure vessel. Another possibility is that the "safety relief valve" of the main steam pipe opened automatically (or was manually opened by an operator), and from there, steam under high temperature and pressure in the pressure vessel gushed out into the suppression chamber (suppression pool). As a result, the water level in the reactor fell." http://cnic.jp/english/topics/safety/earthquake/fukukk19may11.html
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