Update to the NYTimes story today:
In a further sign of spreading alarm Friday that uranium in the Japanese plant could begin to melt, Japan planned to import about 150 tons of boron from South Korea and France to mix with water to be sprayed onto damaged reactors, French and South Korean officials said Friday. Boron absorbs neutrons during a nuclear reaction and can be used in an effort to stop a meltdown if the zirconium cladding on uranium fuel rods is compromised.
Tokyo Electric Power Company, which operates the plant, said earlier this week that there was a possibility of “recriticality,” in which fission would resume if fuel rods melted and the uranium pellets slumped into a jumble on the floor of a storage pool or reactor core. Spraying pure water on the uranium under these conditions can actually accelerate fission, said Robert Albrecht, a longtime nuclear engineer.
Nuclear reactions at the plant had been halted soon after last week’s 9.0 earthquake and tsunami.