fukushima #3 blacksmoke
continuing problems at #3, two days in a row there is fire and smoke, electrical short somewhere?
Possibility of criticality in the spent fuel pool in #4 is another concern at this point judging from high gamma radiation readings.
At approximately 14:30 on 16 March, TEPCO announced that the storage pool, located outside the containment area, might be boiling, and if so the exposed rods could reach criticality. The BBC commented that criticality would not mean a nuclear bomb-like explosion, but could cause a sustained release of radioactive materials. Around 20:00 JST it was planned to use a police water cannon to spray water on unit 4.
On 16 March the chairman of United States Nuclear Regulatory Commission (NRC), Gregory Jaczko, said in Congressional testimony that the NRC believed all of the water in the spent fuel pool had boiled dry. Japanese nuclear authorities and TEPCO contradicted this report, but later in the day Jaczko stood by his claim saying it had been confirmed by sources in Japan. At 13:00 TEPCO claimed that helicopter observation indicated that the pool had not boiled off. The French Institut de radioprotection et de sûreté nucléaire agreed, stating that helicopter crews diverted planned water dumps to unit 3 on the basis of their visual inspection of unit 4.
On 18 March, Japan was reportedly planning to import about 150 tons of boric acid, a neutron poison, from South Korea and France to counter the threat of criticality.
On 23 March it was reported that low level Neutron radiation (reported as "neutron beam") was observed several times, which may indicate damaged fuel reaching criticality somewhere at the plant[206
Unit #4 is the one to watch as it has over 1300 fuel rods in the pond, and wikipedia has it listed as damaged fuel rods in the pond..
That concentration is more than double the amount in other pools, and also double the safe number as per the design of the plant, but regulators did not act on this.