This morning Japan time, two CH 47 Chinook helicopters began dropping tons of water onto the used nuclear fuel stored in pools at the unit 3 and unit 4 reactor. The helicopters made four passes, and have claimed some success, though footage on NHK television shows most of the water dispersing before it reaches the reactor building. Fire trucks from the military and civil authorities have also arrived to douse the fuel pools using high-power water cannons.

So far the efforts have done little to reduce radiation levels around the plant: NHK reports that Tokyo Electric Power Company (TEPCO), which operates the plant, has seen no change in radiation levels 100 m from unit 3 (the broadcaster does not give a number). The Nuclear Industrial Safety Agency reports that radioactivity forced workers to temporarily evacuate the unit 3 control room yesterday.

NHK also reports elevated radiation levels around Fukushima prefecture. At Fukushima City, 65 km northwest of the plant and well outside the evacuation zone, authorities reported levels as high as 13.9 μSv/hr (0.0139 mSv/hr), according to the broadcaster. That is well above the background, and equivalent to 120 mSv/yr in rough terms, but it will only pose a threat to human health if it continues for a long period of time (see this post for more about the numbers).

Meanwhile, recent updates from the International Atomic Energy Agency and the Japan Atomic Industrial Forum have shed light on why the spent fuel pool at unit 4 was particularly vulnerable to an accident. According to the IAEA, plant operators removed fuel from unit 4 on 30 November of last year and transferred it to the pool as part of routine maintenance. When the quake struck, the pool’s cooling system was knocked out, and it began to heat up.

Under normal conditions, the spent fuel would have been too cool to cause much trouble (temperatures have been very slowly rising in the unit 5 and 6 fuel pools , which are in a similar situation). But in unit 4, the fuel was much fresher, and hence more radioactive. This may explain why the pool lost its water, and why the release of radiation has been particularly severe.

The always helpful JAIF chart shows that little else has changed since yesterday. Some instrumentation has been lost on the unit 2 reactor and sea water continues to flood all three of the stricken cores. A NISA spokesman that efforts are now underway to restore power to the site, which could improve instrumentation and allow for cooling to be restored to the spent fuel pools.

For full coverage of the Fukushima disaster, go to Nature’s news special.

Image: Digital Globe, 16 March 2011