A layman's guide to the situation at the Fukushima No. 1 nuclear power plant

From the Asahi Shinbun

Q: What does this talk of a "meltdown" at the No. 1 reactor of the Fukushima No. 1 nuclear power plant mean?


A: A situation in which nuclear fuel rods have melted, fallen and accumulated on the bottom of a pressure vessel is called a "meltdown." The May 16 report from Tokyo Electric Power Co. indicated a high likelihood of meltdowns not only at the No. 1 reactor but at the plant's No. 2 and No. 3 reactors.


Q: We may be dealing with multiple meltdowns?


A: Nuclear fuel rods produce intense heat for some time after power generation has halted. The No. 2 and No. 3 reactors have not been supplied with sufficient cooling water since the tsunami triggered by the Great East Japan Earthquake crippled the plant. That is the same as the situation at the No. 1 reactor. It is very likely that most of the fuel rods at these reactors have melted.


Q: The incident happened two months ago. Why did TEPCO not know it was facing this situation much earlier?


A: When TEPCO repaired a water-level gauge at the reactor on May 10, it found little water was left in the pressure vessels. Nevertheless, the temperature inside the vessels was only about 100 degrees. If the fuel rods had been fully exposed, the temperature would have been much higher. That indicates that all fuel rods fell and have been submerged in water on the bottom (of the vessels).


Q: What happens if fuel rods melt?


A: Melted fuel rods cannot be easily cooled by water because they are completely deformed. They become like a chunk of concrete. Cooling them requires the continued injection of water. In this case, intense heat from melted rods on the bottom made holes in the metal pressure vessel, causing the contaminated water to leak.


Q: What are these holes like?


A: There are several of them. Their total size is about the same as a circle with a diameter of a few centimeters, according to TEPCO. As well as contaminated water, the melted fuel rods themselves could have passed through holes of that size.


Q: What if parts of the melted fuel have leaked?


A: In the Chernobyl nuclear accident in 1986, nuclear substances were scattered over a wide area, carried in smoke produced by the large fire at the plant. The Fukushima incident is not as grave as the Chernobyl accident, but it is much worse than the Three Mile Island nuclear incident in 1979. In the case of Three Mile Island, meltdown also occurred, but the nuclear fuel rods were contained in the pressure vessel. That prevented substantial amounts of nuclear material from leaking.