In the video below, Arnie shows and discusses the problem with the fuel rack in reactor building 4 being exposed to air. You can see in the video, the "boxes," the at top of nuclear fuel racks. Those are supposed to be under 30 feet of water. But they are not, they are in air. So at least the top of the nuclear fuel is exposed to air, perhaps more. The water that is being sprayed into the pool is creating steam, but not filling the pool.
Water is for the purposes of both cooling and shielding. The lack of water means the nuclear fuel is unshielded. Its gamma rays are rising into the sky and bouncing off air molecules through a phenomenon called "skyshine." (That sounds prettier than it is - "scattered radiation of a primary gamma radiation source generated by aerial dispersion.") The gamma rays rain back on the site as background radiation, which is much higher than normal, making work on the refueling pool potentially lethal. In addition, the nuclear fuel is extremely hot and the plutonium inside can become volatile. The fact that the nuclear fuel pool doesn't have water means there might be a clean path for the heavy elements to escape from the building. Arnie would recommend increasing the evacuation zone. - Ilene
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Fairewinds Chief Engineer, Arnie Gundersen, analyzes new video from the Fukushima Daiichi Plant. Gundersen determines that the fuel rack in reactor building 4 is exposed to air. Original Ustream Video: http://www.ustream.tv/recorded/13684184