(The following is an excerpt of a report by Ed Lyman of the UCS posted Sunday evening EDT.)
The nuclear crisis in Japan took a turn for the worse as serious problems developed at a second reactor at the Fukushima Dai-Ichi nuclear facility. Earlier concerns were focused on reactor Unit 1, but now the situation at Unit 3 is becoming serious.
Officials from Tokyo Electric reported that after multiple cooling system failures, the water level in the Unit 3 reactor vessel dropped 3 meters (nearly 10 feet), uncovering approximately 90 percent of each of the fuel rods in the core. Authorities were able to inject cooling water with a fire pump after reducing the containment pressure by a controlled venting of radioactive gas. As with Unit 1, they began pumping seawater into Unit 3. Seawater is highly corrosive and probably precludes any future use of the reactor, even if a crisis is averted.
However, Tokyo Electric recently reported that the water level in the Unit 3 reactor still remains more than 2 meters (6 feet) below the top of the fuel and company officials believe that water may be leaking from the reactor vessel. When the fuel is uncovered by water, it overheats and suffers damage. It is likely that the fuel has experienced significant damage at this point, and Japanese authorities have said they are proceeding on this assumption.
One particular concern with Unit 3 is the presence of mixed-oxide (MOX) fuel in the core. MOX is a mixture of plutonium and uranium oxides. In September 2010, plant operators loaded 32 fuel assemblies containing MOX fuel into this reactor. That amounts to approximately 6 percent of the core. MOX fuel generally worsens the consequences of severe accidents in which a large amount of radioactive gas and aerosol is released compared with non-MOX uranium fuel because MOX fuel contains greater amounts of plutonium and other actinides, which are highly toxic.
See the full report, and other updates on the situation in Japan at the UCS blog: All Things Nuclear.