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From Asahi Shinbun
Tokyo Electric Power Company (TEPCO) admitted to the possibility in its early March 28 press conference that the steel Reactor Pressure Vessels that hold nuclear fuel rods in the Reactors 1, 2, 3 at Fukushima I Nuclear Plant may have broken. TEPCO explained the situation "Imagine there's a hole." Because of this "hole", contaminated water that's been poured into the Pressure Vessels to cool the fuel rods continues to leak, it is assumed.
In the Reactors 1, 2, and 3, the water level within the Pressure Vessels are not rising as much as desired. TEPCO admitted in the March 28 press conference that the reason why the Pressure Vessels haven't been filled with water was "probably a hole near the bottom, that's the image we have". Asked why there was a hole, TEPCO answered they did not know.
The Reactor Pressure Vessels (RPVs) are the most important of the 5-layer protection against radiation leak (other 4 are the fuel pellets, cladding of fuel rods, Container Vessels, and the Reactor buildings). The RPVs at Fukushima I Nuclear Power Plant is made of 16-centimeter thick steel, and it has an outlet at the bottom to insert measuring instruments. It is possible that the leak is from that area.
TEPCO also admitted to the possibility of the exposed nuclear fuel rods overheating and damaging the RPVs. According to the nuclear experts, if the fuel rods get damaged and start to melt, it will fall to the bottom of the RPVs and settle. It then becomes harder to cool with water effectively, because the surface area is smaller. It is possible that the melted fuel rods melted the wall of the RPVs with high temperature and created a hole.
On the other hand, TEPCO said it didn't think the RPVs are completely broken, because the pressures inside the RPVs were higher than the atmosphere. "It is not like Chernobyl where the RPV exploded and the fuels were outside the RPV." TEPCO continued to believe in the integrity of the RPVs.