There is new news today of note -- the single most potentially dangerous situation at Dai-ichi, the spent fuel pool at reactor #4, has achieved an increased margin of safety, putting us largely into the cleanup phase. That will be plenty of work. Here's the update:
Tokyo Electric Power Company (Tepco) started setting up an alternative circulation system, utilizing a heat exchanger, at unit 4 in mid-July. The system is designed to pump warm water out of the pool, cool it, and then return the water to the pool. The company said that it began a test run of the system at 0:44 am on 31 July. The temperature of the pool at that time was between 86°C and 87°C. Tepco said that following the successful trial run of the system, it had now put it into full operation.
5/4 -- TEPCO has managed to get a remote controlled camera to inspect the spent storage pool at building 4, and it appears to be holding water reasonably well, which significantly reduces the risk from that particular pool. The pool is holding the offloaded core of reactor #4 and had concerned engineers and raised speculation of high risks --highlighted by the US NRC's Jaczko in a dramatic March 16th news conference.
Unlike the reactors themselves, spent fuel pools have no hard containment, and normally hold only spent fuel. But when the tsunami struck, the pool #4 had the recently offloaded core of reactor #4, which was undergoing maintenance. Seeing that the pool holding this relatively potent and still very heat-producing core is holding water, instead of only being sprayed with water, allays a major concern. A pool that holds water can have cooling water circulated through it, and it's just a more stable situation. Concern is reduced for instance because a pump failure wouldn't lead to a rapid increase in emissions because the residual water in the pool would only lower gradually, allowing time for another pump to be brought to bear.
Concern is reduced among the well-informed, that is.
Paranoia-driven blogs have speculated all manner of frightening (and melodramatic) reasons to expect disasters, a new one every few days. One widely read blog cross-posted speculation suggesting that the EPA was hiding data on radiation levels in the US, no less; and later that tiny levels of extra radiation were quite dangerous (while in fact this amount will cause little harm; and here are precise reasons (2nd half of article)).
In contrast this blog was concerned about precise aspects of the Dai-ichi situation.
Superior information and understanding aren't just a matter of background expertise (although that helps). They come more reliably from a dispassionate seeking out information that is different than one has guessed or believed. Instead of scouring the internet to find speculation to support a pre-ordained conclusion, one must read widely in independent sources when the issue is complex and of great concern. 'Independent' here means sources that are not simply elaborating a set of ideas in common, but instead are using a truly searching and non-prejudiced method of seeking knowledge. In short, objective.
Of every aspect of Dai-ichi and radiation release, by far the most concerning was the spent fuel pool at #4, as we knew little about the exact state of the pool beyond gross related facts such as the explosion that damaged the building shell, and that two fires had occurred, and that the building was too radioactive for workers to enter and inspect the pool. The reactors were less concerning (definitely concerning still, but not as potentially dangerous) to experts as soon as water levels inside the reactors were increased, and even eventually due to the fact of the radioactive water leaks -- those leaks released water not as radioactive steam, but liquid water, quite radioactive and most of which could eventually be captured and filtered. Much or most of the emissions were going to be trapped in water and could be captured later. It was at that point simply logical that pool #4 was the largest remaining wildcard concern.
The possibility that "under control" might mean something less pleasant such as only spraying water onto a seriously damaged and exposed set of rods has finally been removed. Unlike the death of bin Laden, this is a real reason to feel a little safer.