fuel rod exposure at Fukushima Daiichi number 2 reactor is potentially the most serious event so far at the plant.

fuel rod exposure at Fukushima Daiichi number 2 reactor is potentially the most serious event so far at the plant.

A local government official confirmed the fuel rods were at one point largely, if not totally exposed; but we do not know for how long.

Without coolant around the rods, temperatures can rise to levels hot enough to melt metallic components over a prolonged period.

This opens the possibility of a serious meltdown - where molten, highly radioactive material from the reactor core falls through the floor of the containment vessel and into the ground underneath.

However, engineers appear to have restored some water flow into the reactor vessel and if they are successful, temperatures will begin to fall again rapidly.

What the incident illustrates is the ad-hoc nature of the operation being mounted at Fukushima.

An official with the Tokyo Electric Power Company (Tepco), which runs the site, said seawater was being pumped in both by fire engines and via the system installed to extinguish fires in the power station's turbine hall.