(New issues in bold)
Yesterday saw little progress in the Japanese attempts to restore some safety at the Fukushima Daiichi nuclear reactors.
March 17 satellite picture of the Daiichi reactors 1 to 4 by Digital Globe
North is to the left – the lower part of the pic shows the damaged reactor blocks 1 to 4 from left to right. Comparing with the March 16 picture (with no. 4 on the left), the roof of reactor 4 appears to have melted or exploded away during the last two days.
bigger pic
For lack of cooling, three of the six reactors have experienced a partial core meltdown. Uranium fuel rods in the primary reactor containment were only partly covered with water, heated up and melted. Steam reacted with the zirconium hull of the fuel rods, creating explosive hydrogen and additional heat. These reactors are currently cooled through fire lines by seawater and are regularly vented to release pressure. Each venting releases additional radioactivity. Salt from the seawater used will eventually disable the internal valves, gauges and other urgently needed internal equipment.
Reactors 1, 3 and 4 were damaged by hydrogen explosions outside of their primary containment. Reactor 2 experienced a hydrogen explosion within its primary containment. All four reactor are now radioactive scrap and will eventually have to be entombed in place.
Reactors 1 to 4 have an additional huge problem with their open spent fuel ponds which are located above the primary containment. With water circulation disabled the spent fuel heats up, evaporates the surrounding water and is radiating in dangerous doses directly into the environment. This makes human work in the area nearly impossible.
Attempts were made yesterday to cool and refill the spent fuel pond in reactor no. 3 with airfield fire engines. The dousing attempts were renewed this morning local Japanese time and steam could be seen emitting after water was sprayed. For unknown reasons the Japan Atomic Industry Forum in its March 18:00 10:00am status report overnight downgraded the reactor 3 containment vessel pressure from "stable" to "fluctuating".
There were no attempts made yet to cool down or to cover the spent fuel pond of no. 4 which is suspected to have run dry and is the most hot one with relative unused and now melting fuel. This melted fuel could accumulate at the bottom and restart a nuclear reaction producing addition energy. Such a hot and heavily radiating lava like mass would eventually burn through the surrounding concrete and react violently with any water below it.
Fixed line electricity was reported to have been restored to the relatively undamaged reactor 5 and 6 but according to the IAEA this has not been the case. Instead reactor 5 and 6 appear to receive some power from a partially restored no.6 backup diesel generator.
Restoring electricity to reactor 1 to 4 will be a priority. But even with electricity available it is unlikely that the cooling equipment in these explosion damaged reactors will be able to function.
Information by the plant operator Tokyo Power company TEPCO, the Japanese government and the Japanese media is scarce and incomplete or false. The decision making seems to be by committee and always too slow and too late. The urgency to act decisively to at least stop the situation from getting worse has not yet sunk in.
Additional resources:
AllThingsNuclear Union of Concerned Scientists
Bulletin of the Atomic Scientists
Atomic power review blog
Digital Globe Sat Pictures
IAEA Newscenter
NISA Japanese Nuclear Regulator
Japan Atomic Industry Forum (regular updates)
Japanese government press releases in English
Kyodo News Agency
Asahi Shimbun leading Japanese newspaper in English
NHK World TV via Ustream
Status reports for the German Federal Government by the Gesellschaft für Reaktorsicherheit in German language