At 11:57 JST (Japanese Standard Time), Tokyo Electric Power Co (TEPCO) released a photograph of Fukushima No.4 reactor showing that "a large portion of the building's outer wall has collapsed." Technicians were reported to be considering spraying boric acid on the building from a helicopter.
The new Wednesday fire cast into doubt the earlier hope that the Tuesday blaze in the Unit 4 housing was caused by lubricating oil pumps; instead at approximately 14:30, TEPCO announced its belief that the storage pool may have begun boiling, raising the possibility that exposed rods would reach criticality. BBC commented that criticality would not mean a nuclear bomb like explosion, but rather some enhanced and sustained release of radioactive materials would be a possible scenario.
Around 20:00 JST on 16 March it was planned to use a police water cannon to spray water on unit 4.
Julius Andreev, former director of the Soviet Spetsatom clean-up agency involved in Chernobyl clean-up, as well as Laurence Williams, professor of nuclear safety at the University of Central Lancashire, speculate that the Fukushima management could have been engaged in an unsafe industry practice of re-racking spent rods in the pool well beyond its rated capacity, in effect heightening danger of melting and pool boil-off.
On 16 March the chairman of United States Nuclear Regulatory Commission, Gregory B. Jaczko, said in Congressional testimony that the NRC believes all of the water in the spent fuel pool has boiled dry. Japanese nuclear authorities and TEPCO instantly disclaimed that this was the case and stressed that the situation was stable.
----Early reports from Japan on Wednesday say all the workers at the crippled Fukushima Nuclear Plant have been withdrawn after radiation spikes. Before the worker evacuation a fire was observed through the 8 by 8 meter hole in the outer shell of reactor unit #4. That fire apparently extinguished itself. There was also white smoke seen billowing out of Fukushima reactor unit #3. However, since all the workers had left the plant it was unclear if another fire had ignited.
However, the spent fuel storage pool at Unit #4 at the Fukushima Nuclear Plant is currently the biggest danger as Rachel Maddow of MSNBC explains in this clip. Units #5 and #6 also have spent fuel storage pools that may pose a future problem.
In Fuel-Cooling Pools, a Danger for the Longer Term -- Even as workers race to prevent the radioactive cores of the damaged nuclear reactors in Japan from melting down, concerns are growing that nearby pools holding spent fuel rods could pose an even greater danger...
Tokyo Electric Power Co (TEPCO) / Press Release:
Fire occurrence at Fukushima Daiichi Nuclear Power Station Unit 4 (2nd Release)--
At approximately 5:45 am, a TEPCO employee discovered a fire at the northwest corner of the Nuclear Reactor Building while transporting a battery to the central control room of Unit 4 of Fukushima Daiichi Nuclear Power Station.
TEPCO immediately reported this incident to the fire department and the local government. In addition, TEPCO also contacted related parties about this incident and began immediate preparations to extinguish the fire. (previously announced)
However, during an inspection at approximately 6:15 am, TEPCO staff found
no signs of fire. The area will be kept under strict surveillance.
more TEPCO News: here