The reports are that the top one and a half meters of the fuel bundles in the reactor's core are no longer immersed in the cooling water as the level of critically needed cooling water has fell despite frantic efforts to pump more coolant into the reactor. With the tops of the fuel bundles exposed they can be expected to super heat possibly reaching 5,000 degrees leading to a meltdown.
This is taking place in Japan's oldest reactor, now in operation for 40 years.
Possible Meltdown At Tepco Reactor
TOKYO (Nikkei)--Tokyo Electric Power Co.'s (9501) Fukushima No. 1 nuclear plant in Fukushima Prefecture may be experiencing a meltdown in the wake of Friday's massive earthquake, the Nuclear and Industrial Safety Agency said Saturday.
The agency said cesium, a radioactive material produced by nuclear fission, was detected near the facility.
Fuel rods were exposed to the air as the water level in the reactor fell, said one observer.
Japan investigates possible nuclear meltdown
Ryohei Shiomi said that officials were checking whether a meltdown had taken place at the Fukushima Daiichi power plant's Unit 1, which had lost cooling ability in the aftermath of Friday's powerful earthquake.
Shiomi said that even if there was a meltdown, it wouldn't affect humans beyond a 10-kilometre radius.
Most of the 51,000 residents living within that radius have been evacuated, he said.
Earlier Saturday, Japan declared states of emergency for five nuclear reactors at two power plants after the units lost cooling ability.
Some 3,000 people within three kilometres of the plant were urged to leave their homes, but the evacuation zone was more than tripled to 6 miles (10 kilometers) after authorities detected eight times the normal radiation levels outside the facility and 1,000 times normal inside Unit 1's control room..
Explosion at Japan's quake-hit nuclear plant
Jiji news agency said there had been an explosion at the stricken 40-year-old Daichi 1 reactor and TV footage showed vapor rising from the plant, which lies 240 km (150 miles) north of Tokyo.
For some background see:
Why Worry? Japan's Nuclear Plants at Grave Risk From Quake Damage
Unless radical steps are taken now to reduce the vulnerability of nuclear power plants to earthquakes, Japan could experience a true nuclear catastrophe in the near future.
In my anti-nuclear activist days back in the 1980, I gained a fair understanding of the way reactors work. This could be the beginning of a catastrophic meltdown, leading to massive releases of radiation that could be sustained over a long period.
Officials now say the reactor building was destroyed but the reactor's containment vessel remained intact, averting massive releases of radiation. There's a desperate effort underway to maintain cooling with sea water along with all its corrosive salt.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~