Tepco has announced that there are 1,331 spent fuel rods in the pool of reactor 4.
However, 10/14/2011, Nuclear and Industrial Safety Agency announced that the pool contained 204 units of new fuel rods as well.
They assumed that once aftershock hits the pool and it’s damaged, the fuel rods get to 900 C, melting out of the zirconium cover in 2.3 hours later. As the zirconium alloy cover (Zircaloy) is destroyed, it emits hydrogen, which is likely to cause hydrogen explosion.
The melted fuel rods are estimated to have reached 2,800 C and started to melt down some 7.7 hours later.
Currently Tepco is reinforcing the fuel pool, which is now exposed to outside because it lost the wall from the explosion.
Today Hokkaido University stated that there is a possibility that another M9.0 hits north Japan again (soon? imminent?).
They say they caught the same earthquake echo of 89.9MHz as what they caught before 311.
According to their report, another M9.0 may hit from December to January, the epicenter may be from South Miyagi prefecture offshore to Ibaraki offshore, which is beside Fukushima plant.
Even apart from this report, it is rational to assume another major aftershock hits Fukushima plants anytime.
Fukushima is still on the edge.