According to NHK, NISA has reported that the coolant level in the Unit 5 reactor at the Fukushima Daiichi nuclear power plant "had fallen." Apparently at the time of the earthquake, fuel rods has already been installed in the reactor and water circulation was required to kep them cool. But when external electrical power and power from backup diesel generators were lost, cooling systems stopped and pressure in the reactor began to rise. Venting was used to reduce the pressure. (I don't remember this being reported.), which further reduced the water level. As of 9 PM JST (08:00:00 a.m. ET) on Tuesday, the water level was 2 meters above the fuel rods. That was 40 centimeters lower than 5 hours earlier.
This is the first time to my knowledge that a problem with the Unit 5 reactor has been reported.
It is hard to come to any other conclusion than that the single biggest problem at Fukushima Daiichi is the lack of off-site power. Reactor cores at Units 1,3, and 2 overheated, increasing pressure, leading to venting, and resulting in hydrogen explosions. Spent fuel pools at Units 4,5,6 are overheating. At Unit 4 this has caused two fires. The temperatures of Unit 5 and 6 spent fuel pools are rising. Unit 5 reactor is overheating, pressure has increased, and apparently there has been venting, but without an explosion. (Discussions were reported about removing cladding at Units 5 and 6 to prevent the buildup of H2.) Then March 15 14:10 UTC, the IAEA said laconically under the circumstances that "Attempts to return power to the entire Daiichi site are also continuing."
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