According to Japan's Chief Cabinet Secretary Yukio Edano, they fear there has been a breach in the core vessel of nuclear reactor 3 at Fukushima Daiichi.
As a result, radiation levels have skyrocketed and Edano just announced that all workers—50 remained in the facility—have been evacuated from the
plant reactor buildings because of this new incident. Emergency containment operations appear to have been suspended temporarily.
The official explanation comes after a new cloud was spotted rising from the plant. Operators initially said that it may be steam, but apparently it's much graver than that. Glenn Sjoden, nuclear engineering professor at Georgia Tech University, says that, the new radiation levels indicate that fission byproducts are now venting from the building.
According to Sjoden, "there has definitely been some level of melting and breach of primary containment." Based on the new levels of radiation, he believes that only partial melting has occurred as "total meltdown" will result in much higher levels of radiation.
Update 1: The Tokyo Electric Power Company has released the image above, showing the state of reactor 4 after today's now-extinguished fire.
Update 2: Reuters reports that workers have returned to the facility to resume emergency operations.
Update 3: According to Japan's nuclear government agency, radiation reached 10 millisievert per hour at one point this morning, "possibly due to damage on reactor number 2 the day before". The agency says that it fell to 6.4 millisievert per hour at 10:45am, 2.3 millisievert at 10:54am and got up to 3.4 millisievert at 11:00am.
Update 4: The workers were evacuated from the reactor buildings following the spike, not the plant grounds.