Gee, I must be prophetic or something: a day after my blog post on Criticality, the discussion of ongoing fission reactions has begun to surface major media sources. First from a Bloomberg Article
...melted fuel in the No. 1 reactor building may be causing isolated, uncontrolled nuclear chain reactions, Denis Flory, nuclear safety director for the UN’s International Atomic Energy Agency, said at a press conference in Vienna. Nuclear experts call these reactions "localized criticality," which will increase radiation and hamper the ability to shut down the plant. The reactions consist of a burst of heat, radiation and sometimes an "ethereal blue flash," according to the U.S. Energy Department’s Los Alamos National Laboratory web site. Twenty-one workers have been killed by "criticality accidents" since 1945, the site said. Radioactive chlorine found March 25 in the Unit 1 turbine building suggests chain reactions continued after the reactor shut down, physicist Ferenc Dalnoki-Veress of the James Martin Center for Nonproliferation Studies in Monterey, California, wrote in a March 28 paper.
The paper cited above has a detailed analysis, and suggests that Chlorine from salt water has been irradiated with neutrons during a criticality incident to produce the radioactive isotope: Cl-38. This isotope isn't a big threat; the point is that if criticality is going on continuously or sporadically workers are at a serious and unpredictable risk of extremely hazardous radiation.
Second, the Guardian
interviewed the US expert (on GE's BWR's) who installed the Fukushima reactors who
radioactive core in a reactor at the crippled Fukushima nuclear power plant appears to have melted through the bottom of its containment vessel and on to a concrete floor, experts say, raising fears of a major release of radiation at the site.
In case you missed the movie, this is basically the China Syndrome, where the reactor fuel becomes a radioactive lava that melts a hole in the containment vessel and drips wherever gravity will take it.
While we still haven't seen any close up photos of the reactors or the spent fuel ponds, two videos of the plant, from the Ground
and from the Air
give us a pretty clear idea that the spent fuel pools at least (which have no containment vessels) are seriously thrashed.
Also this moving first hand account
from one of the workers on the scene contains the following interesting quote.
Most of the Fukushima complex is still too irradiated to stay in for any length of time, the workers said. In their two hours off – and whenever radiation spikes – they retreat to a “safe haven”, a two-storey earthquake-resistant building at the centre of the complex constructed in July last year.
Its crazy that the safe bunker was just constructed last year! If this quake had happened 8 months earlier, its likely that the entire plant would have been abandoned and left to meltdown after only a week or so! (NOTE: this may yet happen, and is Prof. Michio Kaku's 'worst case scenario'). (NOTE2: The story above contains an apparently untrue claim that 6 workers were killed when #3 exploded. No fatalities so far, just injuries, and radiation sickness)