Sorry to alarm folks, but the BBC is reporting a third explosion at the crippled Fukushima nuclear power plant in Japan, this time at the previously intact, but failing Number 2 reactor.

The explosion is feared to have damaged the reactor's pressure-suppression system, Kyodo says. It adds that "radiation tops legal limit" after the explosion.

It was the Number 2 reactor whose fuel rods were twice exposed today, raising fears of a potential meltdown. A Tokyo Electric spokesperson says that the company has started to evacuate staff from the plant, but that 50 workers were staying on. More details as they come.

UPDATE: Initial reports seem bad. The BBC is reporting that "Radioactive materials are feared to be leaking at Fukushima," while Japan's Kyodo News agency says that Tokyo Electric detected "radiation of 8,217 micro sievert per hour, 8 times annual limit."

There are unconfirmed reports that the evacuation zone is about to be expanded to 75 kilometers.

UPDATE KYODO: "FLASH: Fukushima's No. 2 reactor container damaged, radiation leak feared"

UPDATE KYODO: "URGENT: Meltdown possible at Fukushima reactors: Tokyo Electric"

A critical situation called ''meltdown'' in which fuel rods melt and are destroyed is possible at the troubled nuclear plant in Fukushima Prefecture, Tokyo Electric Power Co. said Tuesday.

The cores of the Fukushima No. 1 nuclear power plant's three reactors are believed to have partially melted following Friday's magnitude 9.0 earthquake that hit northeastern and eastern Japan.

I can't be sure, but this sounds like they're distinguishing between the "partial meltdown" believed to have happened and the total meltdown they now say is "possible."