In a further sign of spreading alarm 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 together 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…

Additionally, a senior Western nuclear industry executive said that there also appeared to be damage to the floor or sides of the spent fuel pool at Reactor No. 4, and that this was making it extremely hard to refill the pool with water. The problem was first reported by The Los Angeles Times.