(UPDATED With New Aerial Video of Plant) Focus at Fukushima Daiichi Now on Spent Fuel Tanks as Helicopters and Water Cannons Work to Quench Unit 3

(UPDATED With New Aerial Video of Plant) Focus at Fukushima Daiichi Now on Spent Fuel Tanks as Helicopters and Water Cannons Work to Quench Unit 3

UPDATED 11:44 a.m. -- A Japanese TV station has obtained this video of the damaged Fukushima Daiichi nuclear plant taken from a nearby helicopter:

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Japanese defense forces enlisted lead-shielded helicopters to drop water into the damaged roof of Fukushima Daiichi unit 3 Wednesday night, with water cannon trucks also standing by to replenish the reactor's spent-fuel tank.
Government and Tokyo Electric Power Co. officials said that securing the spent fuel pools in units 3 and 4 has become the top priority at the nuclear plant, which has struggled with a loss of cooling systems and potential core damage in three reactors following a magnitude 9 earthquake last week. Two helicopters made four attempts to drop water into unit 3 with mixed success. Radiation readings taken from a smaller helicopter beforehand indicated levels of 4.13 millisieverts per hour at 1,000 feet above the unit and 87.7 millisieverts at 300 feet. Although crews were equipped with protective gear, radiation levels limited the amount of time the helicopters could work in the area, and they did not hover directly above the unit.
Tepco and government officials decided to focus on the tank in unit 3, where reports indicated water levels were extremely low. Nuclear Regulatory Commission Chairman Gregory Jaczko told a congressional panel on Wednesday that the spent-fuel tank in unit 4 was completely dry. But citing observations from a helicopter the day before, Tepco officials said the tank did have water in it, and that refilling unit 3's tank took priority.
 White steam or smoke could be see rising from unit 3 Wednesday. It was unclear whether the substance came from the reactor coolant suppression system or from the spent-fuel tank, both of which may have been damaged in a blast Monday.
Some promising news also emerged Wednesday, as Tepco reported an electric line to supply AC power to units 1 and 2 was almost complete. Japanese regulatory authorities at a press conference late Wednesday said workers would begin installing temporary pumps in the reactor cooling system, as the original equipment had been damaged by seawater used to cool the cores of reactors 1, 2 and 3. Once the pumps and the electricity come online, officials are hopeful the cooling systems will be able to bring temperatures and pressures within the reactors under control.

Below is a status report for each reactor at Fukushima Daiichi as of midnight, Wednesday, based on reports from Tepco, Japanese government officials and international media. Unless noted, times and days of the week are EST. Today's updates are in bold italic:

Unit 1

As much as 70 percent of the fuel within the reactor may be damaged, according to a report Tuesday from Kyodo news. Japanese regulators said at a press conference Wednesday night that crews are now able to inject seawater into the core.

Unit 2

Tepco officials said they were not sure if increases in radiation observed at the plant Tuesday originated from a breach reported in unit 2’s suppression chamber.

As much as 33 percent of the fuel within the reactor may be damaged according to Kyodo news.

Japanese regulators said at a press conference Wednesday night that crews are now able to inject seawater into the core.

Unit 3

Late Wednesday night, Japanese security forces attempted to drop seawater into the spent fuel storage tank using two CH 47 helicopters. Four attempts met with mixed success, as footage of the operation showed much of the water missing the opening in the roof. Radiation readings on the ground of around 3,800 microsieverts did not decline significantly after the water drop. Trucks with water cannons usually used to control riots were preparing to spray water directly into the tank next.

 Also on Wednesday, Chief Cabinet Minister Yukio Edano said there was a "possibility" unit 3's primary containment had been breached.

Nuclear Regulatory Commission chairman Gregory Jaczko, in congressional testimony Wednesday, said he's received reports that the unit 3 spent fuel tank may have a crack in it that could lead to a loss of water inside. Unit 3 had been using mixed-oxide (MOX) fuel made from spent uranium and plutonium. The fuel’s lower melting point and added toxicity would make vapor in contact with the spent fuel potentially more harmful.

Unit 4

Firefighters extinguished a fire in the outer building Monday, and a second sighting of flames reported Tuesday were no longer visible after about 30 minutes.

Water temperature in its spent fuel tank reached 84 C Tuesday, twice the normal reading. Wednesday afternoon, Nuclear Regulatory Commission chairman Gregory Jaczko was quoted as saying that the tank was dry. Tepco denied that claim. When asked later, Jaczko stood by his statement, saying that the information came from NRC staff working with their counterparts in Japan. At a press conference late Wednesday night, Tepco officials said observations from a helicopter a day earlier indicated there was water in the tank, although they did not know how much or whether the fuel rods were exposed. Tepco said the observation of water in unit 4 influenced their decision to prioritize unit 3 for the helicopter operation.

All of the non-spent reactor's fuel had been transferred from the reactor core to the pool before the earthquake, making it more prone to heating. Plans were forming to add water and boric acid to the tank, although elevated radiation levels had been preventing access.

Unit 5

Nikkei news said Tuesday spent fuel tank water temperature was reported at 58.7 C, compared to 24 C normally. On Wednesday night, Japanese regulators at a press conference  said the water level in the tank had decreased, and earlier reports Wednesday indicated the temperature had risen slightly.

Unit 6

Nikkei news said Tuesday spent fuel tank water temperature has been reported at 57 C, compared to 24 C normally. New reports Wednesday indicated the water temperature had risen several degrees but remained well below boiling.

Fukushima Daini

Tepco reported Monday that all four reactors at its Fukushima Daini plant nearby have achieved cold shutdown.